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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