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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