The modifications might look intimidating at first, but are actually suprisingly easy to accomplish. Most Integrated or PA tube amplifiers follow the same basic layout; once you've relaced one, you can pretty much do them all. The good news is that most of what you need is already there; it just needs a little re-arranging to sound good with an electric guitar. This usually consists of the removal of some stuff, de-soldering of wires and resistors from tube sockets and the like, and adding in the new connections in the right places. No different than, say, the triode modification you did one time on an amplifier's output stage.
The results often turn out to be quite good, especially in the area of getting that 'tube sound. This of course depends on the initial piece of iron you have to work on; its operational condition at the time you start working on it - the condition of its tubes and other components. However, a lot of how well things turn out - and this is the fun part - depends on the choices you make in installing the modifications. There's ample room for experimentation, and, maybe you'll come up with the golden arrangement of circuit stages and connections that just kicks Marshalls right off the stage! "Kick butt on a Marshall amplifier?" you ask? Yes, because all a Marshall amplifier has over your amplifier is the way the circuits are arranged - they're both still just tubes and iron. If this interests you, by all means, read on!
As I alluded to above, this all can be quite a learning experience. This happens in several contexts, from the theory of amplifier operation, to how a typical amplifier is hooked up, to the practical experience of doing the physical changes to the circuitry. But most of what you'll learn is from making the necessary decisions and choices in the archetecture of your new amplifier. In this respect, it'll be your amplifier, quite possibly with your own unique signature sound to it. Not to mention it's appearance, arrangement of controls, and special functions - all of which you can choose. This can be extended to modifying amplifiers professionally, building guitar amplifiers completely from scratch or having a better idea of what to do in the area of repairs.
To accomplish this, the usual battery of electronic hobby equipment is essential - things like a soldering iron, voltmeter, electric drill with bits and center punch, tools for undoing screws, nuts and bolts, cutting and stripping wire - and a fair stock of common electrical components is very helpful. (Of course, you can determine exactly what you need at the time you assess the amplifier you intend to relace and order them from an electronic part source like Digi-Key or Mouser Electronics.) A notebook to jot down your findings and plans is also most helpful.
In this article, I wont be considering transistor amplifier types at all. Although the general principles might apply, it's not my intent to suggest that a transistor amplifier can be modified in the same ways as described here. What you want to look for is exclusively TUBE electronics.
Most of the iron that's suitable for doing these modifications was built in the 1950's or 1960's, during the heyday of tube amplification. Amps that work best for this are monaural and are either an "Integrated" type or a "PA" type. The PA types offer a special advantage that I'll get into in a bit. An Integrated amplifier means that the amplifier contains at least the following; a volume control, some sort of tone controls (usually bass and treble) and an input selector switch. When considering an Integrated amplifier, check the input selector switch to see that it has a microphone or a magnetic phonograph input. This will ensure that the unit already has enough gain stages to serve you well as a guitar amplifier. Note: if the unit does not have these markings, or if it clearly can handle only a crystal phono input, pass it by - or you may be dissappointed with the results after all your hard work.
Of the two types, the PA type amplifier is the one to get. This is usually characterized by having multiple microphone inputs, a phono input, some sort of tone controls and a master volume. The advantages of this type are that the master volume is already present (it has a spot on the control panel) and that there's lots of gain stages. This gives two advantages; one, there's lots of tube stages already in this type of amplifier; two, there's lots of control panel positions for potentiometers, which will give you the most flexability for controlling the sound.
Of course, there's some specific stuff to avoid - besides transistor amplifiers. One thing to definitely avoid is tube stereo amplifiers. There's two very basic reasons for this. One, is that a stereo amplifier would require you to do twice the amount of circuitry relacing - once for the left channel and once for the right channel. The second reason is a bit less obvious. There's quite a market for stereo tube electronics these days. You could easily blissfully hack up a very valuable piece of gear - something you could sell or trade for perhaps a bunch of amplifiers more suitable for this type of work. Certainly, if you run across a McIntosh, a Marantz or a Dynaco amplifier at a giveaway price - buy it - and set it aside from this type of endevour.
When looking for a suitable unit, a judicious application of the "silk purse from a sow's ear" theory is beneficial. In other words, the better the amp you start out with, in terms of size, quality and condition, the better your project will turn out in the end. I've made some real screamers out of a simple monaural 15 watt Heathkit amp, but...practically speaking, that little amp just would not cut it playing in a band. Look for at least a pair of 6L6 or 6CA7 types for output tubes; possibly a quad of 6BQ5s in an amplifier before you buy. If you come up with an integrated or PA amp that has a quad of 6L6's in the output stage, you'll be able to create quite a machine!
There's numerous place to check for amplifiers of this genre. One of the most fruitful sources I've found is the common town fleamarket. Often what happens is the church finially ends up replacing their old tube PA with some newer equipment, and that old amplifier ends up on someone's table for $25. There's also the electronic-specific oriented fleamarkets - these are good because the sellers recognize that amplifiers are part of the market - but maybe not as good because sellers also recognize the possible value of 'the old beast and generally ask more. Other sources might be the local Want-ads, Salvation Army stores or other used goods shops. One specific place I might mention is the "For Sale" ads in antique radio hobbiest magazines, such as "The Antique Radio Collector" (PO BOX 2, Carlisle MA 01741 - $27 yr)
What to pay depends entirely upon a number of factors, some of which have been mentioned already, like the output tube compliment. In my opinion, that and the condition of the amplifier are chief considerations in deciding how much I'd be willing to spend. People's asking prices seem to be fairly random from my experience; I've seen sellers want $100 for an amp that was just absolutely trashed, while others have virtually given away amps that were very nice units. I've recently acquired a 30W DuKane PA amplifier, in pretty much as new condition; shiny paint, dust free and unweathered - which I paid $40 for. I feel it was a fair deal for both the seller and myself. I'll be basing this article mostly on my experience with that particular amplifier. It's also my second one, so, I've had some experience with this model.
The first thing you'll want to note is the number of gain stages that are available in the amplifier you've chosen. Generally, this can be assessed by counting the number of triode sections in 12AX7 type tubes in the amplifier; each 12AX7 tube has two sections. A typical Integrated type has one 12AX7 providing initial gain, another 12AX7 associated with the tone control circuitry, and perhaps another 12AX7 or 12AU7 used to drive the output tubes. That last tube wont change much in the modifications described here; it's part of the stuff already taken care of by the original amplifier's circuitry. Therefore, I'd count 4 stages available to play with, in this particular arrangement.
A typical PA type amplifier is arranged differently. Most often, there is a 12AX7 type tube for each pair of microphone inputs; each section amplifies one channel. If this is the case in your PA type amplifier, it sure makes things easy to relace a multiple section "pre" gain arrangement. So, if it was a PA amp with 4 microphone inputs, that's 4 stages of pre-gain already. Likely, the amplifier has another 12AX7 to handle mixing the original 4 inputs and the tone controls, so, we're up to 6 stages to play with before we hit the output driver tube. Makes your ears ring just thinking about it, right?
Other things that would be useful at this point would be a general visual inspection - check the condition of the wiring, look for burned or overheated resistors - especially the cathode and screen grid resistors of the output tubes, and those of the power supply - most often located around the filter capacitor can. One thing that often breaks on an old amplifier is the power switch; now's the time to check its operation and think about ordering a replacement if need be.
There's one thing good to do before you tear into it; an operational checkout. You'll want to connect a speaker or suitable load resistor to the amplifier output because tube type amplifiers require an output load to work properly. You can ruin the output transformer by running a loud signal through the amplifier with its output unloaded. Remember, since this is an old used amp whose operation you're probably unfamiliar with, it's best to take this simple precaution the first time you turn it on. Plug it in, turn it on and observe the filaments glowing in the tubes and any abberant operation like the fuse blowing. If all's well after a few minutes, I'd recommend that you make some measurements with your meter around the power supply and output tubes, verifying the voltage values called out in the amplifier's schematic.
Another good reason for doing the operational checkout is that you'll be assured your amp worked before you make your modifications to it! If repairs are needed, its best to do them at this point and verify that your amp works in its original form before proceeding. For a PA amp, simply plug in your guitar into the 1/4" jacks for each microphone channel and check that the sound comes through. For an Integrated type amplifier, you may need an adapter cable to connect your guitar's 1/4" jack to the RCA type input connector. Select "Microphone" or "Mag Phono" on the selector switch and verify the operation of the volume and tone controls. Note if the controls are "scratchy" or intermittant, a shot of tuner cleaner or Cramolin might be in order. Check for an undue amount of AC hum from the loudspeaker; with the volume control turned all the way down, this is an indication that the filter caps need to be replaced.
For an Integrated amplifier, your map might look something like the following: (crudely drawn using the ascii characters...)
|---------------|
Input First Second Tone Third Volume
Selector ------ 12AX7 --- 12AX7 ----- Control ------- 12AX7 ----- Contol --->
Switch Stage | Stage Circuitry Stage
|---------------------|
Phase ----- Push Output Tube ------
>------ Splitter Output Transformer ----> Speaker
Driver ----- Pull Output Tube ------
What's important at this point is to note the locations of the various
functional components, like the tone control circuits and the volume control.
Secondly, note how intimate the circuitry is between the 1st amplification
stage and the input selector switch. This may take some looking at to figure
out, but the good news is that you dont have to figure it out, because in the
modifications you'll make, the entire input selector switch is removed; only
the 1st 12AX7 amplification stage needs to be identified.
If you have a schematic with your amplifier, you should mark down which tube and section you'll call the "1st" and "2nd" amplification stages, etc. If you dont have a schematic, decide which section of the tube is physically closest to the input selector switch and pick that one as section "1". Of course, the corresponding section of that tube will be section "2" and so on. If you're having trouble finding the circuit sections at all in the tangle of wires, the best thing to look for is the plate resistors connected to the 12AX7 sockets, which typically range from 100K to 1Meg. (One idea to keep things in order would be to tag these resistors with tape.)
For an PA type amplifier, your map might look something like the following:
First Volume
12AX7 -- Control --|
Stage |
|
Second Volume |
12AX7 -- Control --|
Stage | Fifth Tone Sixth Master
|--- 12AX7 ----- Control ------- 12AX7 ----- Volume --->
Third Volume | Stage Circuitry Stage Control
12AX7 -- Control --|
Stage |
|
Fourth Volume |
12AX7 -- Control --|
Stage
Phase ----- Push Output Tube ------
>------ Splitter Output Transformer ----> Speaker
Driver ----- Pull Output Tube ------
Again, if you have a schematic, you should mark down which tube and
section you'll call the "1st" and "2nd", etc. If you dont have a schematic,
use the channel 1 section as section "1". Of course, the next channel will be
section "2" and so on...
With your amplifier sufficiently and hopefully, accurately mapped out, you can make some preliminary modification archetecture decisions. It's best to think about this some before going into it with the wire cutters and de-soldering tools. The first and most basic choice is where in the circuit to place the pre-volume control. In most amplifiers, including those built by both Fender and Marshall, this is placed between the first and second 12AX7 gain stages. Other choices might be prior to the first stage, but this would simply be duplicating the control already on your guitar. After the second stage could give you a degree of distortion controllable from your guitar's volume control, because two stages of 12AX7 gain is quite a bit. Generally, you can assume the best spot for the pre volume is wired in between stage 1 and stage 2.
Of course in the PA type amplifier, we have an entirely different situation and hence, options. There's a number of gain stages with associated volume controls already connected. One idea that I like is to wire all of them in a cascade arrangement; the output of volume control 1 feeding into the input of stage 2 and so on. It's what I call a "multi-variate" arrangement, because by adjusting the 4 pre-volumes in various ways, different tonal characteristics can result.
The second most important choice is the location of the tone control circuitry. If your amplifier was originally a HiFi Integrated amplifier, the tone controls are basically useless for electric guitar. This is because the turnover frequencies are in all the wrong places in the frequency spectrum, considering the appx 80 to 10,000 Hz frequency range of an electric guitar. Even with the PA amplifier type (unless the unit you're working on happens to be a Fender or a Traynor or something...) I've found that the tone controls are just placed all wrong. I've found that a good solution is to replace the entire circuit with the classic Fender / Marshall Bass-Mid-Treble tone control circuit which will be described later.
The location of the tone controls in a classic Fender or Marshall amp is just after the second stage, feeding into the third. You can experiment with the locations in both the PA and Integrated amplifiers; where you choose to place this circuit in the amplification string will undoubtably determine much of the sound character of your amplifier.
Now, the whole reason to think about this and perhaps make some decisions around where you want to place things, is because you're now at the point where you're going to go in and start cutting out things you dont need. In an integrated amplifier, you definitely dont need the input selector switch; in fact, a good place to locate your 1/4" input jack is right in the hole left by its removal! When you start cutting, it makes sense to do so in a way that most helps you relace the existing circuitry into your new arrangement. This means leave certain wires intact, cut component leads in such a way that they are still useful in that they can be re-wired according to your new diagram.
I'd like to make a comment here before I go on with the wiring details. The quality of your wiring job will directly effect the performance of your amplifier! I suggest that you use a coaxial cable wherever possible; a good source of this cable is from an old defunct cassette tape deck - which you could possibly pick up at the same fleamarket you find your amplifier at. This type of wire is small, easily stripped and easy to solder. A typical deck contains enough of it to easily do all the wiring needed for your amp. Of course, you can probably order some of this stuff from the same electronic supply house you get the necessary potentiometers, capacitors, jacks and resistors from.
To connect the input jack to the 1st amplification stage, simply wire it with coaxial cable (a must at this point) to the grid pin of the tube socket and the ground to the circuit ground connection of the cathode or cathode resistor. Be sure to keep any exposed inner conductor as short as possible, so this highly sensitive connection is well shielded from noise pickup. Make sure you wire up the contactor lug to ground also, if you've obtained a jack of this type.
Likewise, install the pre-volume control by wiring it with coaxial cable. Here, the high side of the potentiometer (corresponding to "11") connects to the output decoupling capacitor of the first stage via the inner conductor of the coax, while the wiper of the potentiometer connects to the grid pin of 2nd amplification stage. The ground of both coaxs connects to the low side of the potentiometer and the cathode resistor ground connection of the 2nd amp stage only, to avoid ground loops.
Another suggestion I'd like to share is the consideration of the circuit loaded by the potentiometer used for the pre-volume. Usually, it's most convienent to use the same units that came with the amplifier wherever possible, to be able to use the same or similar matching knobs on the control panel. This is of course provided that they're in good shape and restorable with a contact cleaner as mentioned earlier. If this is all true, you'll want to consider changing the value of the decoupling capacitor from the 1st stage. The reason for this is because the value typically used in a PA or Integrated amp is too large for use with electric guitar (Ignore this advice if you're building an amp intended for Bass guitar use).
A good choice for the low frequency rolloff point of the RC circuit formed by the decoupling capacitor feeding into the impedance of the pre-volume control is 60 Hz. It's a good choice because at this point, the circuit has a lot of gain and lower frequency signals coming from the guitar pickups when you strike the strings can modulate the sound, knocking the stability of the amplifier's sound around as you play. This ultimately produces an annoying "pumping" effect which is undesirable. The following table shows, for various common values of potentiometers, appropriate values of decoupling capacitors. If you're ordering these caps, you'll want to order 400V values, or the same voltage values as is already in your amplifier.
Potentiometer Value Capacitor Value
100K .025
250K .01
500K .005
1Meg .0025
2Meg .001
For the case of a PA amplifier, another problem presents itself, if
you're going to use the multi-variate circuit arrangement I prefer. All
those gain stages presents too much gain to simply connect up directly in a
cascade. Yet, if you introduce a series resistor to attenuate the signal by
making a voltage divider where the lower leg is the potentiometer impedance,
the total impedance seen by the decoupling cap of the preceeding stage is
really huge, therefore the turnover frequency is really low. A better solution
is to load the decoupling cap of each preceeding stage with a value which
will set the crossover frequency to around 60 hz - and then tap across that
resistor with the voltage divider. If this sounds confusing, the following
diagram of the Load / Attenuator will hopefully clear this up some.
From Plate 0.005 500K
of previous >---||---.---/\/\/\----> (ground)
12AX7 stage |
|
| 5Meg 500K Pre-Volume Pot
|---/\/\/\-----/\/\/\---> (ground)
|
|-------> To Grid
of Next
Stage
This circuit may, of course, be used in the Integrated amplifier
modification should you find that the overall gain is too much to be usable.
That sometimes happens, because the type of tone controls used in a typical
Integrated amplifier have a lot of attenuation and when you replace them with
the Marshall / Fender tone control circuit (which I like), the overall gain of
the amplifier is very high.
The following diagram shows the classic Marshall / Fender tone control circuit. In a Marshall or Fender amplifier, this is usually installed between the 2nd and 3rd 12AX7 amplification stage. You might choose to place this somewhere in the "multi-variate" pre-volume arrangement, or just before the phase splitter stage. Experiment! Probably the only completely inappropriate place to install this circuit is just after the input jack, because the gain there is so high that you'd never be able to wire it in a way that wouldnt pick up a lot of noise. Note that the Bass control is wired counter-clockwise from the schematic.
|------> to next pre-volume control (1 meg)
| _________ ________
| | | | |
From Plate >-------/\/\/\------/\/\/\-------/\/\/\-----------> (ground)
of 12AX7 | R1 | R2 | R3
--- Treble --- Bass --- Midrange
Fender: --- --- --- Marshall:
| C1 | C2 | C3
R1 = 250K ----/\/\/\----------------- R1 = 250K
R2 = 250K R4 R2 = 1M, log
R3 = 10K R3 = 25K
R4 = 100K R4 = 56K
C1 = 250 pf C1 = 250 Pf
C2 = 0.1 uf C2 = 0.022 uf
C3 = 0.047 uf C3 = 0.022 uf
To install the post-volume control, there's basically two choices for
its location in the circuit topology. One is just before the phase splitter
tube, and this requires only a single potentiometer metering the level into the
grid input of that stage. An arguably better choice is to place this control
just before the two output tubes. However, you'd need a dual potentiometer in
that case, in order to meter the voltage to both tubes. This arrangement is
considered better because it includes the sound of the phase splitter tube.
There's no reason you couldnt do both, ending up with a multi-variate
arrangement of two pre-volumes. In an Integrated amplifier, you'd only be
limited by the number of control locations on the front panel.
Let's take a look at a couple of the circuit topologies possible. The first one I'll present assumes you're working on an Integrated type amplifier:
--------- --------- --------- --------- --------- ---------
| 1/4" | | 1st | | Pre 1 | | 2nd | | M/F | | Pre 2 |
>--| Input |---| 12AX7 |---| Volume|---| 12AX7 |---| Tone |---| Volume|-->
| Jack | | Stage | | Contr | | Stage | | Contr | | Contr |
--------- --------- --------- --------- --------- ---------
--------- --------- --------- ---------
| Phase |---| Master|---| Power |---| Output|
>--| Split | | Volume| | Output| | Trans |--->
| Tube |---| Contr |---| Tubes |---| former|
--------- --------- --------- ---------
This one shows the multi-variate arrangement made up from cascading
4 microphone inputs that were perhaps found on a PA type amplifier.
--------- --------- --------- --------- --------- ---------
| 1/4" | | 1st | | Load | | Pre 1 | | 2nd | | Load |
>--| Input |---| 12AX7 |---| Atten |---| Volume|---| 12XA7 |---| Atten |-->
| Jack | | Stage | | Circ | | Contr | | Stage | | Circ |
--------- --------- --------- --------- --------- ---------
--------- --------- --------- --------- --------- ---------
| Pre 2 | | 3nd | | Load | | Pre 3 | | 4th | | M/F |
>--| Volume|---| 12XA7 |---| Atten |---| Volume|---| 12AX7 |---| Tone |-->
| Contr | | Stage | | Circ | | Contr | | Stage | | Contr |
--------- --------- --------- --------- --------- ---------
--------- --------- --------- ---------
| Phase |---| Master|---| Power |---| Output|
>--| Split | | Volume| | Output| | Trans |--->
| Tube |---| Contr |---| Tubes |---| former|
--------- --------- --------- ---------
As you can see, the variations in the circuitry are only limited by your
imagination and experience with experimenting in the placement of the different
functions. As always, it makes sense to plan things out first, arranging how the
sections will be connected, obtaining the parts you'll need and deciding how
you'll secure things physically. One of the limitations is, of course, the
original control panel of the amplifier you're building from. I've eliminated
indicator lamps and relocated power switches to the rear panel in my efforts
to get just one more spot up front for another control, or for the input jack.
For example, say your front panel has an input selector switch, a volume control, and bass and treble controls. The AC power is part of the volume control. That leaves you with 4 available positions, but, considering the above diagram for the Integrated amplifier, 7 positions are required!! This shows another logical reason why the PA amps are the ones to look for, because generally they have more open front panel positions to work with. This is not to say that with some effort and creativity, you couldnt retrofit your amplifier with an entirely new front panel, cut and drilled to accomodate your new design.
Another option is that there's nothing hard and fast saying you have to use the full implementation of the multi-variate control system. You could easily replace the panel potentiometers with trimmer potentiometers which are mounted internally, and adjusted to taste such that all you really need up front is a "pre" and a "post" volume control. Another trick would be to find a 4 or 5 gang deep potentiometer and turn all of the sections of the multi-variate pre volume control simultaneously. There's lots of options and possibilities to try, and any one specific combination of choices could lead to your own signature sound.
Most amplifiers of this nature offer several impedance taps on the output transformer secondary. An easy way to handle the inpedance selection is just to have a flying lead, which is connected internally to the output jack, come through the chassis so it can be connected to the proper impedance tap. By keeping this wire short, dressing the chassis hole with a grommet and tying it off behind, will give this setup a "well attended to" appearance. A crimped on terminal lug at the end of the wire finishes it off nicely.
In all these type of amplifiers I've built, a three wire cord; line, neutral and ground is a must. I dont bother with the "polarity" switches found on the older tube guitar amps. Adding a three wire line cord will reduce the noise pickup and interferance when operating the new amplifier at high levels of gain. The best place to connect the ground is through a bolt directly to the amplifier chassis.
A Standby Switch is also a nice addition if you have room for it, preferably next to the power switch. This allows the amplifier to remain warmed up, but with no B+ current flowing, which can help to save the output tubes. The place in the circuit to put this switch is just after the rectifier tube and just before the filter capacitors. If you insert it downstream of this point, the filter capacitors stand to possibly charge up beyond their rated voltage values when in standby mode, which could stress them to the point of failure. That's no good, especially when considering that Standby mode is supposed to help things last longer!
While we're talking about the amplifier's power supply, another useful addition is to beef up the total supply capacitance. You can order 1100 uf 450V caps (which should be a sufficient voltage rating for most applications) from Mouser Electronics. They're expensive, about $30 a pop, but placing one of these across the main B+ decoupling capacitor (the one the center tap of the output transformer is connected to) gives a marked improvement in the dynamics of the amplifier and the amount of hum and noise coming from the power supply. New Sensor Corp has smaller value caps, in the 100 uf range, at more realistic prices - like $3.50. This is still enough cap to give a good effect in this area; you can always double or triple up at that cost!
(Note: Some players like the compression effect resulting from the B+ supply collapsing under heavy (ie: cranked) amplifier output, so, be sure to consider your preferance in this area. An option would be to include a 50K 5W resistor in series with the cap connection to the B+ rail, bypassed by a SPST switch, so the guitarist could choose to have the cap bank "in" or "out" of the power supply circuit)
In conclusion, I hope this article helped you sufficiently in going about restoring and relacing an old tube PA or Intergated amplifier for use with electric guitar. Following the same guidlines and process I've described here, I've successfully modified tube PA amplifiers and sold them in the used marketplace, to the complete satisfaction of the guitarist making the purchase. While we might not make a living doing this, it is a satisfying and rewarding endevour to pursue, at very little risk. After doing just a few generic amplifier relacings, it becomes easy and you can then apply what you've learned to any tube amplifier, be it a Grommes or a Gibson. If you have any questions regarding this article, please feel free to contact me at the e-net address shown. Enjoy!
Joe Jasniewski, Jasniewski@elesys.eng.pko.dec.com