Vacuum Tube Guitar Amplifiers – Part III

The tough part about doing anything with this much detail, the first time, is that every time I thought I had every part listed in the bill of materials, I found a few more parts that I needed but are not on the list.  The BOM Im listing below is most of what I need, short of a few hardware items and tools.

Resistor BOM

 

The following is a complete (I hope) list of resistors, with the component value on the left in Ohms, a note as to where it is in the circuit, if needed, and quantity.

I don’t know yet how to edit a table in WordPress, will fix up the table with headers at some point. All resistors not marked are 1/2 watt, metal film, with 350v/500v peak ratings.  The higher wattage resistors are at the bottom of the list.  I am going to use higher wattage value resistors in the “power rail”, than were specified on the original schematic.

 

47 End of LTP 1
470 1
820 Feedback 2
1500 3
2700 1
6800 2
10k 1
22k long tail pair 1
27k
68k 4
100k 10
220k 4
1m 6
2.2m 1
10m 1
10K 1 WATT 1
27K 1 WATT 1
82K 1 WATT 1
100k 1 WATT 1
470 3 WATT 3

 

Since these are mostly very inexpensive components,  around 15 cents each and up, I will order a few  more than I need for this amp build since I know I will be building another amp and most are common values.

Ordering 10 of each resistor, which will leave me with some extra,  is about $20.00 plus shipping.   I have these resistors in a web shopping card at Hoffman amps.

You can find a little cheaper resistors on bigger electronics warehouses web sites, but the pain of sifting through literally tens of thousands of parts, and not finding the exact value you want, is not worth saving a few dollars.

Resistors: cost about $20.00.

 

Capacitor BOM

This is the complete (hopefully) list of capacitors for a Fender Deluxe AB763 style amp build, less the big electrolytic filter caps.   You can find exact values for all of these capacitors.    I decided to buy Sozo Blue, pricey boutique signal caps, of which there are only 8, and the rest are as specified on a fender chassis (using photographs and blog posts).

0.00025 250pf 2 Ceramic
47 PF 1 Ceramic
0.001 1 Ceramic
0.01 2 Ceramic (tremolo)
0.02 1 Ceramic (tremolo)
0.047 3 mallory150 or Sozo
0.1 5 mallory150 or Sozo

The total cost of all capacitors, not including the big filter caps, is about $45.00, if I use pricey Sozo or similar signal caps, and about $20.00 if I use Mallory 150’s.    There is only one distributor for Sozo Blue,  Antique Electronic Supply:

Antique Electronic Supply

In addition to capacitors, they sell just about everything you would need to build a vacuum tube guitar amplifier. I have not dealt with them yet, will probably buy the signal caps and some other components.

All caps minus filter caps: about $45.00.

Filter Capactors and other electrolytics

Since electrolytics are harder to source with exact values specified in the Deluxe AB763 schematic,  I listed them separately.   As mentioned in other amp posts, I know I don’t have the electrical engineering chops to decide how far a given component can be from the design value, so I will use parts as close as I can find.

25uf 25v  Cathode cap 5
25uf 50v Bias cap 1
16uf  450v  Filter cap 5

The 16uf main filter caps are about 5.00 each total cost 25.00 + shipping.    I bought these from Amplified Parts.com  The remaining 6 24uf I bought from Mouser, more or less exact replacement axial lead 3.00 each, total cost about $43.00.

Electrolytics: about $43.00 + shipping.

Pot BOM

Even potentiometers are difficult to source.   There are literally thousands of different  styles, various shaft styles some knurled, split, solid, plastic, with varying lengths, different lead configurations.     The best I could find is that CTS makes the best potentiometer for these amplifiers,  they have the voltage ratings, are well made, but a bit pricey.    They sell for about 3.50 each, best price I could find was Amplified Parts.com.     Each channel pair has volume, bass, treble, and the tremolo input has speed and level for tremolo as well so total 8 pots, at 3.5 each, or about 28.00.

The black ‘witch hat’ knobs are about 2.5 each, so about $50.00 for pots and knobs.   I decided to put a mid pot in the reverb hole in the front panel, so total 56.00 for pots and knobs (ouch).     The only place I could find that sells CTS pots with all metal shafts, at the best price is amplified parts.com.   The other sites sell plastic shaft, no good for a music amp that gets beat around a little.

Pots and knobs:   $56.00 + shipping.

 

Total so far, about $170.00 for components, more to come

Transformers

Since I decided to go with Hamond transformers, and the prices are all fairly close,

the stock transformer, output transformer, and choke set from Mouser:

291BX power trans 89.91
194A Choke 18.49
1750H Output Trans 39.99
 Sub total 148.39
ship 22.2
 Total 170.59

 

 

 

Misc components

Everything not covered someplace else.

Tube Socket 8 pin chassis 3
Tube Socket 9 pin chassis 4
Mounting offsets for circuit boards + screws.   1″? 14
Pilot light and jewel (6.3v) 1
Lugs for grounding (think about solder point on chassis for grounding) 2
 Solder, and Flux
main eyelet board 1
filter cap eyelet board 1
bias eyelet board 1
Jack Shorting spkr 1
Jack Non shorting ext spkr 1
Jack shorting input 4
RCA jack for Tremolo Pedal 1
Switch SPST Paddle Standby 1
Switch SPST Paddle ON/OFF 1
Fuse Holder for 1 amp. 1
Fuses  1amp slo blo 5
 Braided ground wire.
AC Power Cord 1
AC Cord Strain Relief 1
 Heat Shrink Tubing.
Pack of washers for pots and jacks 20
 ‘Dog House’ cover for filter cap board.
Self Tapping screws for Tube Sockets
Screws/bolts for transformers
Hookup wire: 22ga for ckt board (at last 5 colors)
Hookup wire: 18ga for heater wiring (solid core)

That covers most of the “I know I need” small parts, from looking at the chassis photos, and schematics and layouts.

 

Chassis + face plate

As mentioned in a previous article,  about $67.00 from Zachmdhunter’s ebay store, includes shipping.  This is the “Fender Deluxe reverb AB763” chassis, although I am not building the reverb circuit.

 

Combo Cabinet – 1 x 12

I will source this from TRM Guitar Cabs.   I bought the unfinished bare bones cabinet.  The price is $190.00 which includes shipping.   I will also buy casters for about $25.00 from amplified parts, so I can roll the amp around without having to lift it.

 

Final Review

I have web carts setup at

  • Mouser – the transformers and small electrolytic caps
  • Amplified Parts.com – pots, knobs, signal caps, lamp, jewel switches, casters.
  • Hoffman amps – remaining eyelet boards, full set of resistors, diode, some of the wire tremolo opto isolator.

Already purchased:

  • TRM Guitar cabs – purchased cabinet already.
  • Zachmdhunter – chassis
  • Main eyelet board – Hoffman.

 

Only need to review the entire list again, check for errors recording part values and counts, make sure I didn’t duplicate anything, then send the orders.

 

Next on the todo list after the parts review is to look for a good soldering station, and better meter than I have now, and get some practice soldering, and make a plan for assembling and soldering the amp together.

 

 

 

 

 

 

 

 

 

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Vacuum Tube Guitar Amplifiers – Part II

Components

Once I found a design I liked,  the schematics and layouts and parts available, the next step is figuring out what parts to buy.  Before that,  its a good idea to have a complete list of every part in the build, or a Bill Of Materials or BOM.

I started by reading across the schematic, noting every part and its value.  Resistors, capacitors, tubes, transformers.   Next,  I started looking for photographs of Fender Deluxe blackface or silverface amplifier chassis, showing the internal construction.   Luckily, there are plenty available on the web.

The Fender schematics available on the web, have transformer part numbers, and there are several manufacturers who make replacements for these transformers.

Finally, all the small parts, like wire, screws, knobs, switches, etc. need to be added.   Before starting this project, I had actually thought someone would have a complete bill of materials, posted someplace.   No such luck, or at least I haven’t found one.

The basic list:

chassis ( the metal case that the amplifier guts go into)

cabinet (combo or head cabinet)

speaker (or speakers)

Transformers

Discrete components:  resistors, capacitors, potentiometers,, tube sockets, tubes, band aids, solder, wire, etc.

Hardware:  screws, bolts, washers, etc.

Tube set.

The first task is to  go through the schematic, recording components and values and types.  This will give us the complete (hopefully) list of all of the electronic components required to build the amplifier.  But it doesn’t tell us what type of component, and there are many.

Next, go through photographs, recording parts not on the schematic and layout like screws, power cord, pilot light, etc.

Output: a complete BOM.   That’s the idea anyway.

The hard part:  The value and component is listed on the schematic, e.g. “820 ohm resistor”, but there are many types of each component.  For example, there are ceramic disc, polypropylene, mica, and a dozen other types of capacitor, with various voltage and current ratings.  There are carbon composition, carbon film, metal film, metal oxide and many other types of resistor.   Which one to use for each item in the schematic?

The hard part is figuring out which type to use for each component in the circuit.   The schematics don’t tell you what type of capacitor or resistor, it just says the value, and most of the time, the tolerance.

So, its off to the blogs, and searching the web for photos of real chassis.   Lots of blogs have experienced folks who build and repair amplifiers who are happy to answer questions.

Resistors

These amplifiers originally came with carbon composition resistors.   Carbon comp’s are very good at handling voltage spikes.  But they absorb moisture, and with age, literally crumble apart.   They are noisy.    If you’ve ever turned on an old tube amp, and heard a loud crackle through the speaker, it is likely a failing carbon comp resistor.

If  you’re not careful soldering in a carbon comp resistor, you may change the value of the resistor permanently.   And the tolerances on these types of resistor is very wide, wider than other types of resistor.

That said, there are plenty of 40 or 45 or 50 year old amplifiers with carbon composition resistors that work and sound fantastic.   Many amp builders replace only the components that are bad, and try to replace with as similar component to original as possible.

For a new amp build, what should we use?    Since carbon composition resistors are good at handling spikes we want that feature.   Unfortunately, the best resistor for a new amp build, metal film, do not handle spikes as well, so we need to use higher wattage resistors in certain parts of the amp, than originally designed.  The plate resistors, and resistors around the power supply are candidates for using higher wattage value metal film resistors, in place of using the original spec carbon comp. resistors.  You can find great discussions by professional amplifier builders on their web sites on this topic.   The rule is to use the minimum wattage necessary for a given component, but no lower.

Many new amp builds use 1/2 watt metal film resistors throughout most of the circuit,  then use higher wattage values around the power supply, and power tubes, since they are most susceptible to turn on and turn off spikes, when you turn the ‘standby’ switch on and off.

For most tube amplifiers, you should turn the power on, with the standby switch off, and let the tube heaters warm up the tube for half a minute or so.   Then, when the amp has warmed up, turn the standby switch on.  The power supply in many tube amps, feed high 300 volt dc range to near 500v dc to the power tubes, and lower voltage down the power rail to other tubes.   When you snap the standby switch “on”, a large voltage spike gets sent through the amplifier.   The new components need to be of sufficient construction to handle these spikes.   For example, if the original carbon composition resistor was rated at 1 watt, a 2 or 3 watt or even 5 watt metal film resistor may need to be used.

The problem is that the consequence of one or more of these resistors failing could be quite severe.

Capacitors

Capacitors in the tone path.

Capacitors have internal construction that give them many properties outside of the standard ratings of voltage, capacitance, and resistance.   Some capacitors are not very good tolerance but handle high voltages very well, and last a long time.   You’ll find ceramic disc capacitors in certain places in old amplifiers, such as the tremolo circuit, and some of the tone stack.   Back in the day when these amplifiers were designed and built, very small value capacitors, e.g. in the 50 nanofarad range, there were very few options.   Today, we have mica, polyester, polypropylene, etc.,  many with much wider range of available capacitance values than were available in the 1960’s.

Due to the low cost, high voltage handling capability, and availability, I decided to use ceramic disc capacitors in most places in the Deluxe amp build, that were in the original circuit, with one exception.

Due to the way vacuum tube amplifiers work,  high voltage DC is applied to a vacuum tube,  the signal is amplified, and the output signal has a large DC component, with the output signal superimposed on that.    The signal coming out of each tube is fed through a capacitor to filter out the DC component and leave the audio signal to apply to the next amplification stage.   These capacitors do more than just filter out DC, they do color and shape the signal.  So, these capacitors are likely candidates to use construction closer to those used in the original amplifier, and even replace some with capacitors that better shape the audio signal.

OK I’ll have the real electrical engineers cringing again with my explanation, but the general idea is valid.     Amp builders and technicians are sometimes musicians themselves but often not.   There is enough evidence that guitar amplifiers built with capacitors in certain positions in the signal path sound noticeably better than those build with other types of capacitor.   And its hard to argue that the pre-CBS era Fender amplifiers were some of the best sounding amplifiers ever built, as well as period Marshall’s, Ampeg’s, Vox’s.

The number of these ‘signal caps’ is fairly small.   I counted 7 in the original amp, and I will include one more expensive signal cap in my amp build that was a ceramic disc in the original fender build (the one cap leading into the phase inverter from the preamp).

SignalCaps

There are many heated discussions on many amp and guitar blogs between camps that say the type of capacitor doesn’t matter, and you can’t hear the difference vs the camp that says it really does.  Both sides tend to go overboard a bit.    Many musicians are not technical, so they might be looking for appearance,  but many musicians are also very technical and many say they hear a difference.

The price difference for 8 capacitors between a cheap (but very high quality) replacement is $1.00 to $1.50 for the less expensive ones, to about $6.00 for the high end boutique capacitors.   So, using the expensive ones adds about  $35.00 to an amp build, and a little more for a bigger amp, e.g. a Fender Twin style.  Not a huge amount considering this build will end up costing around $1,000.00 to $1,100.00 when completed.

Capacitors not in the tone path.

Capacitors not in the tone path, e.g. in the tremolo circuit, the power supply, and ‘self biasing’ caps on the cathodes of the small tubes, as best as I can ascertain, have much less effect on the tone of the amplifier, if any at all.     The main issue with these capacitors is that they do the intended job, are sized to handle voltage and voltage spikes, and are built with good enough quality to hold tolerance.   There are some amp builders and technicians who measure the value of every component put into an amplifier.   There is good blog information out there, on what brand capacitors are under/over value and what to watch out for.

For example, the bias capacitors on cathodes of small tubes, can be larger value, but using smaller capacitance value in these positions in the circuit will affect the tone of the amplifier.   Since I don’t have the engineering design chops to know what the effect will be of replacing, say a 25uf cathode capacitor with a 22uf,  I will do my best to replace the capacitors with as close to original design center values as possible.

Some capacitor brands just don’t work very well.   Some brand capacitors, even rated and measure a certain capacitance value, just don’t operate as well as others in certain critical parts of the amplifier.   The power supply filter capacitors need to be able to maintain the charge level to keep the output voltage stable to the output tubes.  If they can’t do this well, it may result in terrible sounding distortion, as opposed to the smooth clipping we like to hear from tube amplifiers.

Brands that people like in guitar amplifiers Ive read, Nichicon, F&T, and Sprague.   The spragues have seen ridiculous price increases,  10x to 20x in some cases, so the other brands are getting more popular.

Unfortunately, in today’s electronics world, axial leaded capacitors (leads come out either end of the cylinder) are not used very much, so they are more expensive, and you have to hunt to find the exact value required.   As an example, some Sprague filter capacitors 16uf rated 450 or 475 volts,can be in the 15.00 price range.  But you can find 18uf 500v Nichicon capacitors for around 2.00, but they are not axial lead, so they won’t fit onto the circuit boards exactly the same way as the original axials did.

The short story is that you can reduce the capacitor budget from about 40.00 to about 11.00 if you use radial leaded capacitors, with values different from what the original schematic specified.

Many of these circuits were on the design edge of ‘parasitic oscillation’.   Basically it means that if you go fiddling around replacing components in an old tube amp, with values different from the original design center, you could inadvertently cause the amplifier to produce oscillations that may be outside the audible range.  These oscillations can be unstable, and could cause the amplifier to amplify signals it was not intended to process, which could overheat and burn out expensive transformers and tubes, and other components.

Without an oscilloscope, it is extremely difficult to diagnose and fix parasitic oscillation, so its likely your brand new amp build will last hours or days and you won’t know what killed it.    So, if you don’t know, then stick to as close to original values and construction components as possible.   If you like to experiment, then … good luck, and keep your fire insurance paid up.

 

Transformers.

As Einstein said, keep things as simple as possible, but no simpler (Paraphrased).  As Fender probably said, keep components as cheap as possible, but no cheaper.   Unfortunately, transformers for vacuum tube amplifiers are huge honking pieces of metal with big coils of wire inside them.   They are very expensive to build and purchase, and since there are so few applications for these components, that makes them even more expensive.

I have found a few companies that sell excellent transformers.

Mercury Magnetics builds and sells the cadillac of the transformer.  They are expensive, but perform very well.

Hammond makes very good transformers, less expensive than Mercury, but from what I have read, also excellent performing, very good value components.

Heyboer seems to make good transformers, but I have not been able to get as much information from builders and tech’s about these. They may be very good.   Their prices are a little lower than the the two top rated.

Cheaper no brand name.   Ive contacted some resellers of tube amplifier components, asking what the brand they sell.  One company that sells on the web told me “Well, we think that is proprietary, and we won’t tell you”.    Can you imagine purchasing the single most expensive component, responsible for the operation and tone of the amplifier,  buying a no brand name component that the seller won’t even tell you who makes it?  No thanks.    As best as I can tell, these much cheaper brand units over heat, cause hum and fail much more often than the brand name units.    Maybe ok for someone, but if Im going to spend around $1,000.00  I would like to know what the manufacturer is for every component in the system, and I would like to read ratings from amp builders and technicians about problems they have had with these components, if any.

I would suggest: if the component seller refuses to tell you what company made a component, they stay away.

For the first amp build, I chose Hammond for the power transformer, output transformer and choke.   Since this is not a reverb amp, there is no reverb transformer.    The total cost of all 3 for this amp is around $170.00 and that includes shipping.

Vacuum Tubes.

This is probably the most difficult part to source.  Back in the day when vacuum tubes were a commodity part, there were many vendors, and the parts were very cheap compared to the amplifier itself.   They have an extremely long shelf life, and there are many web sites who sell “new old stock”, at ever increasing prices.  Ive seen some new old stock output tubes go for 500.00 or more, than similar new manufacture tubes sell for 30.00.   Is it worth it? Maybe if you’re restoring museum quality original amplifiers, but for most of is, probably not.    Ive read that JJ tubes are good quality, handle the voltages well, but as far as sound goes, we’ll see.

This is probably the area I know least about, so when I get ready to purchase a tube set, I’ll have to do more reading.   Will post more information later.

Chassis

This is the metal box that the transformers, circuit boards, tube sockets, switches, jacks etc are bolted onto.   The original fender chassis were thick sheet metal bend into shape, with stamped ends.   There are a few vendors on the web who sell chassis more or less the same, but not exact replicas of the original fender units.   You can go through as much torture as you like trying to find an exact replica, but I decided to find a metal working company who has made these chassis, that has all of the holes cut and drilled, that I can find modern components to fit.

The chassis I found is marketed as a “Fender Deluxe Reverb” type, so it has one more hole drilled in the front panel for a reverb knob, and 3 more holes drilled in the back for RCA jacks for the reverb tank and reverb foot switch, and extra tube socket hold in the top (bottom) of the chassis.     The price was very good, $65.00 which includes the face plate and shipping to my door.

This is an excellent chassis, well made, of thick materials.  You can find the seller on ebay, their store name is ‘zachmdhunter’.    If you’re going to do your own amp build, check out their ebay store:

Items for sale from zachmdhunter zachmdhunter

They sell a nice range of steel and aluminum amplifier chassis and stomp box cases. Check them out.  (Note:  I don’t know the sellers or have any affiliation with them.  They sell a good product, and I would recommend their company.)

This is a photo of the chassis with the eyelet board sitting about where it will be mounted.  I measured all of the holes for tube sockets, transformer, pots, switches, and they are all correct.   This is a heavy, well built chassis that will require little, if any drilling.

DSC_0485_v1

 

Eyelet Boards

The first component I purchased, even before the bill of materials was complete,  was the eyelet board.   I searched a long time to find a supplier that has a good price, and top quality product.    You can purchase ‘fiber board’, close to the original material used by Fender and other amp builders back in the day, but I decided to go with fiberglass.

Fiberboard absorbs moisture, and warps with heat.   You can find many old Fender and other tube amps that have very warped fiberboards,  and the fiber boards have absorbed so much moisture that they actually conduct electricity!  If fiber board degrades to this state, you have to remove the board, unsolder every wire and component, and resolder and require the amplifier, which is about 90% of the work to build an amplifier in the first place.

The best boards I could find, at the best price are from Hoffman Amps.

Hoffman Amps (parts and builder blog)

Doug Hoffman makes the eyelet and turret boards.  You can buy the eyelet boards completed, with eyelets or turrets already installed, or drilled and you can install the turrets or eyelets yourself.  You can also design a turret or eyelet board yourself and give the design file to Doug and he will make the board for you. He sells top quality products at good prices.    I will use his company for this and my next amp build.

(Note:  I don’t know the sellers or have any affiliation with them.  They sell a good product, and I would recommend their company.)

For the amp build I am working on, a Fender Deluxe AB763 style there are 3 eyelet boards:   1) main board (see the image above),  2) the ‘bias’ board, that has a capacitor, resistor and diode, and 3) the filter cap board that is under the ‘dog house’ on top of the chassis.   The total cost of all 3 eyelet boards is about $25.00 for this amp.   All top quality product.

Cabinet

This is the single most expensive part in the amp, requires the most labor, and is very important since it has to be well made so it won’t rattle and buzz when you’re playing the amp.

I found a few of the boutique amp parts supply companies on the web who sell a cabinet in the 300.00 range without a speaker.   These are covered with tolex, have handles and a speaker cloth.  Most don’t have casters.

After a lot of searching and reading I found two small business carpentry companies specializing in building amplifier cabinets, head cabinets and speaker cabinets.   I decided to go with TRM Guiatar Cabinets in New Hamshire:

TRM Guitar Cabinets.

since they are close, shipping is a little cheaper, and have a very good reputation.    Check out Tim’s web site if you’re considering an amp build project.   I just ordered a Deluxe style cabinet, will post photos when it arrives in a few weeks.

I found Tim’s web site through  Rob Robinettes awesome amplifier web site.

Check out Rob’s site, and the absolutely beautiful cabinet from TRM:

Rob Robinette’s amp site

Check out this beautiful amp build:

5e3_Complete_Back_Closed

 

 

Speaker

I’ll cross this bridge when I come to it.  Ive been reading reviews of new speakers, maybe will go with a used one from Ebay or Craigslist.  All I know at this point is that it will be a 12″ guitar speaker.

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Vacuum Tube Guitar Amplifiers – Part I

I got a guitar last year, but no amplifier yet.   I’ve been looking on Reverb and Craigslist, but the amps I want are way out of my price range.  I decided to build a vacuum tube guitar amplifier, and write a series of articles showing what I learned along the way.   Here goes…

Id like to get a Deluxe or Pro, but  the pre CBS units are in the $2,500 to $3,400.00, way out of my price range for a small practice amp.   The later 60’s models up into the 70’s are cheaper, but well known loaded with wiring problems.

After CBS bought Fender they lowered standards for the builds, and also changed to plastic insulated wiring around 1969.  Since it was the first plastic insulated wiring,  it had lots of problems, cracking, leaking electric signal, arcing, etc.   Its no wonder why these amps are 1/3rd the price of pre-cbs models.

One option is to buy an early 70’s model, that works, with good transformers, and rewire it.  I plan to do that eventually.

The other problem with the wiring is that CBS-Fender had no quality standards in the late 60’s.   You can find images of the chassis guts on the web, the wiring in many of these amps looks like a rats nest.

Another option is buying a less desirable amp that works well, and sounds good, like a Fender Twin Reverb.  Way to much power for inside the house.  Maybe a model with a master volume, but, still too loud, way too loud.

How about building an amplifier?

I ran into a few blog sites a few years ago with lots of information about building vacuum tube guitar amplifiers.     It was surprising, to me anyway, that these 50+ year old amplifier designs are still relevant and so popular.    For a handful of the major brands, Fender, Marshall, Ampeg, Vox,  the schematics and layouts and plenty of photographs of the guts are available.

There are also a few companies on the web that sell kits and all of the parts.   You can get a schematic,  build a bill of materials, and source all of the parts yourself,  or buy various levels of kits.   Some places sell the entire kit, with detailed instructions,  and videos on youtube. Others sell a bag of discrete parts, resistors, capacitors, tube sockets, etc. and you source the cabinet, chassis, speaker etc.

Let’s build a Deluxe!

I like the Fender Deluxe design since it is a 20 watt range amplifier.   The ‘good ones’ sound great, (GREAT!) at reasonable volumes, smaller and a lot lighter than a twin.  You can mic it if you need to gig, and need more coverage.   And it is a fairly simple circuit compared to the bigger 4 output tube models, like the Twin’s or bigger Marshalls.

The power transformer in this model, is undersized, and that coupled with the tube rectifier, this amp breaks up at lower volumes, starting around 4 on the volume knob so you can get dirty sounds still at lower volumes than bigger amps.  Since it has a long tail pair phase inverter,  the clipping sound is much better than the smaller Fender Princeton Reverb.   I had a blackface Princeton Reverb back in the day, and the one thing I did not like at all about that amplifier is that the breakup sounded awful on that amp.   Years later I learned that it was due to the amplifiers design.

Where to start?

It took quite a while to locate readable schematics.    There are plenty of copies of schematics that look like they were faxed, then the copy faxed then that copy faxed.  You can’t tell if a resistor is 800, or 600, or 680 ohms, some component values are completely unreadable.    I found a good schematic and a fantastic layout.

Deluxe-AB763-schematic

There are a few versions around, but this one seems to be very popular among amp builders.   I chose this one since it had the required fairly low output, with 2 x 6V6 output tubes, and does not have reverb, since reverb adds complexity to the circuit and actually the reverb amp has lower output due to having to drive 2 more tubes with the same transformer.

This model, as all the old Fenders and their contemporaries, use eyelet boards.  This is an old version of a circuit board, that has holes drilled into a piece of thin fiber board with small metal rings stamped into the holes.

Discrete components, resistors, capacitors, and wires are set into the holes and solder is applied to the eyelet.   The benefit of this design is that it is incredibly simple, and components can be replaced without disassembling the amplifier and removing the circuit board, since components are soldered to the top of the board.

Here’s a photo of the chassis of a period Fender Twin:

15_underside_before_02_zps2ca503f3

As an aside, the wiring in this particular unit isn’t the worst of the CBS fenders, but its not very good either.  The heater wiring twist isn’t tight and consistent, and it looks to have had some work done over the years. Some of the capacitors have been replaced as some of the wiring as well.   But its also an incredibly simple thing, with maybe 65 or so resistors and capacitors, about 10 vacuum tubes, some pots, a few jacks, and transformers.  You’ll find these amplifiers, having been used for over 50 years, with a few repairs done over the years, working and sounding fantastic.

Danger High Voltage present even if the amplifier is unplugged!!!

This fairly standard warning Ive seen on all guitar amp buildings sites needs to be stated again:  vacuum tube guitar amplifiers use very high voltage throughout the circuit.    It is an absolute must to learn how to work with high voltage,  learn how to discharge filter capacitors, and how to safely set the bias, and diagnose these amplifiers.   The stored charge in the filter capacitors can kill or severely injure you, even if the amplifier is turned off and unplugged.      Please don’t even think about pulling the chassis out of your old Fender Twin or Marshall Plexi without learning what not to touch, and how to discharge the storage capacitors without hurting yourself, or someone else.

Also, please don’t even think about leaving partially completed amplifier projects where  children, other family members or pets can contact high voltage, even if the amplifier is unplugged after use.

For a sobering experience, search you tube for high voltage injuries.

Did I mention that vacuum tube guitar amplifiers are extremely dangerous to tinker with?

 

 

Learning curve

I guess I could have just bought a bag of parts and soldered them together, plugged the thing in and see if it worked, or threw out sparks and smoke.     But I wanted to learn something about the operation of vacuum tubes,  how the major stages of the amplifier work, the best way to assemble, test and adjust things and what kinds of precautions I needed to learn about before getting started.

This article is a list of some of the most useful web sites I have found for learning about vacuum tube amplifiers.

The Valve Wizard has great information on his web site, and has written books on the subject.

The Valve Wizard

he covers the main sections of  vacuum tube amplifiers, how they work, and shares his great insight.  I have not ordered his book, will do so this week.

Another great site is Tubebooks.org:

tubebooks.org Vintage info from the days of vacuum tubes

This site has many great books that the copyright has expired.  People have done fantastic job scanning these old books.    There are physics and engineering books with as much technical detail as you would like to go.

There are also many amp builders and technicians with fantastic technical Q&A on amplifier repair and modifications.   Aiken is a particularly good one:

Aiken Amplification

Also, youtube has many great instructional channels.  Uncle Doug’s Youtube channel is one of my favorites.  He is an excellent teacher, has many great instructional videos on amplifier building, and repair, and theory:

Uncle Doug – Youtube vacuum tube amplifier info

The major parts of a guitar amplifier.

I’m not an electrical engineer, so this will be a birds eye view of the major component sections of a vacuum tube guitar amplifier.   There are many great sources of information, a few above, if you want more technical detail.

The Power Supply

The power supply in a vacuum tube guitar amplifier is one of the more expensive parts to build, because they have very big, expensive transformers and filter capacitors.  The transformers are expensive since they have several output taps, supplying high current and high voltage.    A typical vacuum tube guitar amplifier power transformer has 115-120vac primaries (or 220-230 of you live in Europe, or other parts of the world),  and two or three secondary windings.

There is a heater circuit winding that provides high current at 6.3vac.  The power tube heaters can consume 1 to 1.5 amps each.   The Deluxe circuit I am working on has a 6 amp 6.3vac secondary winding to power heaters in all of the tubes in the amp.

Rectifiers convert AC voltage into DC.  Or, more accurately, the rectifier is the first part of the conversion process.    There are two types of rectifiers used in guitar amplifiers:  vacuum tube, and solid state.   That’s right,  some later model, late 60’s, early 70’s vacuum tube amplifiers had solid state diode rectification, such as the Fender Twin.    The reason the Twin’s didn’t use a vacuum tube rectifier is that the most efficient rectifier could not supply enough current for the 80-100 watt amplifiers so they designed in solid state rectifiers.

Amplifiers with vacuum tube rectifiers have built in compression, which is part of their sound character.   If you pluck the guitar strings very hard, the tube rectifier output voltage will drop slightly, so the output tube output will drop, and will increase as the notes decay, which is … compression.   These amps will compress more, the higher the volume is turned up, which as mentioned is part of their character.

Solid state rectified amps tend  to provide more consistent amplification character as the volume is turned up.  You can read more about the technical details on one of the great sites or youtube videos above.

So, the power supply has a big honking iron core transformer.  These are expensive to manufacture.  They have 2 or 3 secondary windings, depending on if they are used to drive a vacuum tube or solid state rectifier.   The high voltage winding, to the plates of the vacuum tubes, typically output 600 to 700volts AC (RMS).   So, you don’t want to inadvertently stick your fingers into these amplifiers, plugged in or not.

The AC voltage is rated at RMS, so the peak voltage is about 1.4 to 1.3 times that number, the lower number is for a loaded amp, i.e. you’re playing it with the volume turned up.

For my Deluxe build, the power transformer high voltage winding is 330 – 0 – 330vac (rms).   I.e. 660vac with a center tap.   The transformer has a 5vac tap that is used to drive the tube rectifier, and a 6.3vac tap to drive the tube heaters.

The transformer is hooked up to a rectifier that turns AC voltage that swings positive and negative,   to voltage that swings only positive, but is still very ‘bumpy’.

The transformer converts 110vac from the wall outlet to 660vac.   The rectifier, either tube or solid state, flips the bottom half of the AC wave positive, so that it looks like the second image in the picture below.   The 3rd stage is filtering, by capacitors and inductors (chokes), that smooth out the wave to look like the 3rd image at the bottom of the picture below.

AC_RECTIFICATION

The reason vacuum tube amplifier power supplies are so expensive is that they require high voltage and high current to operate, which require big expensive transformers, and filter capacitors.   The upside of this design is that, if you use relatively good components the amplifier will probably outlive its owner.  There are many early 1960’s Fender Bassman, and Marshall JTM45 amplifiers still in operation with many original components, that sound absolutely fantastic.

So, we need a big expensive transformer,  several big electrolytic filter capacitors, either a vacuum tube or solid state rectifiers, and on some amps an choke (a big coil of wire).   That covers the basic parts of a guitar amplifier power supply.

A little more detail

The pre amplifier tubes, (covered in a later section), take very low voltage input from your guitar or other instrument, and boost the signal that is fed to later stages of the amplifier.   The first stages of the amplifier, the pre-amp, are the most susceptible to noise, and hum.    So, the power used to feed the pre-amp tubes is filtered the most.   Each successive stage of the amplifier has power that is filtered less than the previous stage.  The output tubes, least susceptible to noise and hum, are filtered the least. Electrical engineers out there are cringing at my explanation, ok, its admittedly over simplified, but covers the main points in a descriptive way.

The following image shows a clip of the Fender Deluxe AB763 schematic showing the power supply section. The rectified output from the GZ34 tube rectifier is first filtered through a 16uf 450v capacitor.  This output voltage of about 420vdc goes right to the output tubes.  Next, in the ‘power rail’, the voltage goes through a choke, and a second 16uf capacitor to the grid2 of the output tubes.    The next stages of the power rail go through a resistor/capacitor pair, and feed the phase inverter, followed by the preamp tubes.    The 230v line, that has had the most filtering, feeds the two preamp tubes.

 

PowerSupplyDELUXE

The idea is: early preamp tubes get the most filtering.  The later stages, and power tubes, get the least.

That’s the simplest, non technical description of the power supply.

Most vacuum tube guitar amplifier power supplies work this way.  The major differences are presence or absence of a filtering choke,  the size and number of filter capacitors, and whether the amp uses a tube or solid state rectifier.

 

Preamp stage

The preamp stage takes signal right from your microphone, guitar or other instrument, and amplifies this very small signal for use in later stages of the amp.  The smaller tubes are preamp tubes. In the image below, the preamp tubes are in front and toward the center and left and have metal covers over them. Since the preamp tubes are most susceptible to noise and stray electric/magnetic fields, they have metal shields to reduce noise and hum.    The ‘naked’ tubes are the output tubes on the right.   The power transformer is in back on the far right (the big black box),  to its left is the output transformer.   A reverb transformer is the silver one in the front between the small tubes.   The filter capacitor board is in back on the left.

OLYMPUS DIGITAL CAMERA

 

This is one of the preamp sections in the Fender Deluxe Ab763 circuit.  The half circle things are schematic representation of half of a dual section vacuum tube (the 7025).   The inputs 1 and 2 are on the far left.  The ‘tone stack’ is between the first two preamp stages.  As

PreampStage_Deluxe

As I learn more stuff myself, I will put up more articles with details about each amp section.    The basic idea, is that each preamp section takes an input of some voltage, and amplifies that signal to a higher voltage level that is fed into the next amplifier section.   This amplifier has bass and treble controls, and a volume control for each pair of inputs.    The 6800 ohm resistor is the “mid resistor”, which sets the midrange level.  Some users have had a midrange pot put into the amp in place of this resistor to have a variable midrange control.    For a new amp build this is fine, but most people don’t want to hack up a vintage amplifier.

 

 

Single ended vs Class AB and the Phase Inverter

Basically, the simplest tube amplifier has one output tube, that is always amplifying.   This is called a “Class A” amplifier.  This type of output circuit has the least amount of distortion.    Its called ‘single ended’ because it has only one output tube.   An example of this kind of output circuit is a Fender Champ.

Most other Fender amplifiers (in fact all, I think), have at least 2 output tubes that need to work together.   The bigger amplifiers have 4 output tubes, each pair works together.

After the preamp section, the sound signal is split into two signals, each one has opposite phase (mirror image).  One phase feeds one output tube, (or pair of tubes for a 4 tube amp)and the other phase feeds the other tube.     While one tube is ‘pushing’, i.e going positive, the other tube is ‘pulling’, or going negative, then when the wave switches direction,  the tube that was pushing starts to pull, and the tube that was pulling,starts to push current.

The benefit of this design, is that the components can be adjusted so that if there is no input signal, the amp is in quiescent state, and draws very low current.   The output tubes are almost off.   When a signal comes through, the output tubes turn on, and start amplifying.     There is a range of very small input signals that don’t turn the output tubes on. 

There are many different designs for the phase inverter.    The two most commonly used in guitar amplifiers are 1) cathodyne,  and 2)  long tail pair.   The smaller amps like the Princeton Reverb use the cathodyne, and almost all bigger amps, the Pro, Deluxe, Twin Vibrolux, Vibroverb, Bassman, use the long tail pair.  The long tail pair produces the best sound quality, but uses two tube sections.

After reading about how the two phase inverters work, and how they affect sound quality,  I decided on a Pro or Deluxe schematic for my amp build project.

Single ended amps don’t need a phase inverter, since they just amplify one signal and have one output tube.  Multiple output tube amplifiers, the ones I have come across, all use a phase inverter to split mirror image signals to output tubes (or pairs of output tubes).

Long tail pair phase inverters are used in the best sounding amplifiers.

Output section

The output section drives the output transformer that is connected to the speakers.  The purpose of the output transformer is to convert large voltage swings that the output tubes produce, into lower voltage but higher current, that speakers need.

The output tubes require the highest voltage and need to be biased to work properly.   If the bias is not set correctly on output tubes, the amplifier can sound terrible, if biased too ‘cold’ or the tubes can redplate and burn out, if biased too hot.

 

 

 

 

 

 

 

 

 

 

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