Notes on Converting PA Amps to Guitar Amps

(Notes and Responses to Jasniewski's Excellent Work)

Jasniewski's original paper

Overview

This is not a stand alone document; rather, it builds upon Joe Jasniewski's Converting PA/Integrated Amps into Guitar Amps, the seminal work on the subject. While the original work is excellent, these notes are things I believe many interested in the topic will also find useful.

Input interactions

One topic not really discussed in the original paper is that of the interaction between multiple inputs. Any amp with multiple inputs acts to some extent as an audio mixer. If two or more input jacks serve the same preamp, you have "passive mixing". The worst thing you can do is simply wire two jacks to the same circuit; in this case the two inputs will interfere with each other. A stronger guitar signal will foul up a weaker signal by forcing its signal into the weaker pickups. If one of the guitars is played through a pedal, the pedal can seriously ruin the other guitar's signal. (In theory, a high output pedal could destroy another pedal's output stage.)

So at the very least, each input jack should connect to a common preamp through a resistor. Common values are 47k, 68k, and 100k. Higher values give better isolation, but also reduce highs somewhat, as well as input volume.

Ideally, each input jack can feed its own preamp stage (such as 1/2 of a 12AX7). This "active mixer" will eliminate the problems discussed above. This, of course, means more tube stages are needed.

The trade-offs here are component count and circuit complexity versus sound quality when multiple inputs are utilized. Of course, if you only use one input jack, this is all moot.

Integrated Amplifiers

Jasniewski suggests sticking with PA amps, and in general I tend to agree. But the "Integrated" amplifier can be used to good effect as well.

One thing that won't work too well is to use an old phonograph (record player) amp as is. The problem here is that vinyl phonograph records were recorded with special tone compensation and the playback circuits have to undo this. This "playback curve" (defined by the RIAA) will change an input signal drastically, especially in the treble arena. You might like it, but most likely you would despise it. So if you want to use one of these as a guitar amp, expect to modify at least the RIAA circuit.

Jasniewski also suggests not reworking old stereo amps. While I agree that a good tube stereo amp is probably best left as is (if nothing else, a name brand will fetch you enough money to buy almost any guitar amp you want!), an off brand can be used just fine. This gives you the option of producing a guitar amp with two completely independent channels all the way through the power section. You can run these in stereo, treat them as A/B channels, or run a Y cord and mix two different amp settings. They offer a lot of possibilities.

Does size matter?

It's completely up to you what size amp is the right size. If you really need volume, because you will be practicing in a garage with your band and no PA, or because you'll be playing a lot of clubs, or whatever, you'll probably want a 30 to 100 watt amp, with at least two 6L6 or equivalent output tubes. Dual 6V6s might work as well, but four 6V6s would be much better.

On the other hand, if you're mainly going to practice by yourself at home, and don't want it that loud, or will always have a PA and can mic your amp, you can go as small as you want. A relatively small 5 - 15 watt amp well work fine. In fact if the amp has two 6BQ5 or 6V6 tubes, you could build a single-ended amp using only one of those tubes.

It's completely up to you - what do you want and need?

Sources

In addition to the sources mentioned in the original paper, you might check eBay, on-line want ads, music and for-sale newsgroups, musical instrument, PA and stereo dealers (they occasionally have old trade-ins they are tired of wasting space on), your grandparents, estate sales, churches and businesses that are remodeling, and so forth.

Know your tubes!

You really need to know your tubes. Preamp tubes can include single triodes, dual triodes, triple triodes (rare), and pentodes, as well as specialty tubes. Different tubes will produce different sounding amplification; a 6BQ5-based amp will sound somewhat different than a 6V6-based amp. You need to know the tubes likely to be found in the amps you run across. But most of us won't recall every tube, so it's a good idea to have a tube manual (or at least a cheat sheet) handy. You really should find a copy an old tube book such as the RCA Receiving Tube Manual. Earlier versions such as the RC22 will include more data on older tubes, whereas newer versions such as the RC-30 will include tubes not found in the earlier versions.

How many gain stages are enough?

This depends on your needs and the tubes used. For instance, most, common tubes used as preamps in PA amps have a gain of up to 100, but some may have gains of 70 or less. As far as how much gain you need, metal needs more gain than classic rock which needs more than blues, with jazz needing only clean tones. While gain actually refers only to how much the signal is amplified, most players think of gain also in terms of amp distortion; hence my "gain food chain" above.

You can compare the gain of various, common, dual triodes at http://www.rru.com/~meo/Guitar/Tubes/; look for "Dual Triode Specs".

Most people would be happy with three or at most four preamp stages. But you may want more (or less). You also may want more isolation between controls. With two gain stages, you can place volume and tone together after the first stage, or move the tone controls after the second stage. With several stages, you can easily add a master volume as well, still keeping it isolated from the tone controls.

Kick starting an old amp

If the amp hasn't been used in years, it's a good idea to be gentle with it at first. The best way to do this is with a variac, or at least a current limiter. A variac is a special type of transformer that lets you vary the output voltage. You can bring the amp up at, say, half voltage, let it run a few minutes, up the voltage, let it run a while, and eventually bring it up to full line (wall outlet) voltage.

If you don't have a variac, you can make an inexpensive current limited by wiring a light bulb socket in series with a three prong receptacle and a line cord. A light bulb in this socket will not light unless something (like the old amp) is plugged in to the outlet and turned on. If the amp is working properly, the light bulb will come on, but will be pretty dim. Let the amp sit, turned on, like this for a couple of hours before plugging it straight in to the wall.

If the light doesn't turn on, the amp could have a blown fuse, broken line cord, bad power switch or bad power transformer. (It could have other problems as well, but these are by far the most likely.) If the light bulb comes anywhere close to its normal brightness, the amps has a short or similar problem. In either case, you need to look further to see which component[s] are bad.

The possibly more important issue is that this will allow the power supply capacitors, or caps, to have a chance of surviving intact. Old caps, especially if they haven't been used, get into a state where they can easily be destroyed if full power is applied. Using the variac or current limiter allows the caps to be "re-formed" and possibly last a while longer. At the very least, they have a good chance of working long enough to let you evaluate the amp as it is currently.

Cleaning components

Jasniewski advises a shot of "Cramolin or tube cleaner" to clean up scratchy pots. I suggest you get some Caig DeoxIT 5. This is the best thing going for cleaning scratchy pots, switches, and so forth. I suggest Caig ProGold GxL for cleaning tube sockets, etc. I tend to just replace oxidized input and output jacks unless they're really top notch, but you can clean these as well.

To clean pots, just put the spray tube at the slit where the leads come out, and spray a brief shot in. Turn the controls back and forth about 10 times. Sealed pots can't be cleaned like this. Switches can be cleaned if there's any opening to spray the cleaner into. Switches that piggy back onto pots often have no hole you can spray into. On the other hand, you can sometimes remove them easily for cleaning.

Schematics

If your amp doesn't have a schematic with it, there are a number of sources to check. There are web sites with lots of schematics. SAMS Publications has a huge library of books full of schematics for sale. If all else fails, and you are so inclined, make your own schematic and make it available. There's always someone looking for a schematic of almost any tube amplifier. You can give these away or sell them.

As you are designing the amp you will build, consider the work of those who have gone before. Look at schematics of amps whose sounds you like, or with similar features. This can save you a lot of work.

Layout notes

In case you are using this as an exercise in learning about tube amps or electronics in general, you need to know a few things about layout.
Again, it's a good idea to learn from others. If you can look under the chassis of a really quiet amp to see how it's built, do so.

Tone controls

The less complex your circuits are, the easier it is to keep them noise-free and keep all the tone your guitar and fingers produce. If one tone control is enough, stick with that! If you want more, consider using a James or Baxandall circuits (bass and treble only); you can get more flexibility from this circuit than most bass/mid/treble circuits. Excellent resources on this topic are available at http://www.duncanamps.com/ by selecting the "Technical" link. The Baxandall circuit (as well as the similar James circuit) is not the same as the typical Fender style two control circuit such as is found on the Deluxe Reverb. Fender uses a variant of their three control circuit with a fixed "mid" setting. (Fender did make a couple of amps with James tone controls, but these are fairly rare.)

At the same site, Duncan has made quite a bit of excellent and extremely useful software available, including a "Tone Stack Calculator" that lets you choose from a variety of tone circuits, graph the output values, and substitute components. There are also a power supply design package, an anode load calculator, and other items, all FREE. Duncan also provides a lot of tube data, in case you don't have a tube manual. You can even download your own copy of his tube data browser (with data 8^) for use on Windows based computers.

Safety

The original paper mentions a three wire line cord, but doesn't give any details. If you're not sure how to do this, you can read about it at http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/Mods/safe.html. The original paper lists this under ``Additional nice things'' but it really belongs here. An amp with a two wire cord usually has a cap on one side of the cord to the chassis; as these caps age they often short out so that a plug inserted backwards or a miswired poower outlet can turn the amp into a death trap.

Glen Otto sent me an email reminding me of another hazard, the old ``AC/DC'' amps that had no power transformer at all, so that everything in the amp, including all metal parts, were potentially hot:

``Although this configuration was more common with tube radios, there are some AC/DC audio amps floating around, often sitting at a flea market after being pulled out of the original (insulated) cabinet. With their nonpolarized line cords, people can end up poking around on a piece of powered-up equipment that has a HOT chassis!

``You can let people know that if they find a piece of equipment with tube designations that don't begin with 6 or 12 (such as a 50L6 or 35W4) it is not a good tinkering choice for anyone without experience.

These can turn into neat amps, but you need to know what you are doing. I have an old Lafayette AC/DC audio amp that I equipped with a built-in isolation transformer to address the line voltage issues and it can generate some sweet tones!''

All I would add is that it's easy to add that isolation transformer; this makes the amp both safer to play and safer to work on, especially for those with little or no experience.

Power supply capacitance

The original paper also mentions the possibility of beefing up power supply capacitances. But it doesn't address the fact that too much capacitance, especially in the first position after a tube rectifier, can destroy the rectifier. (This is not a problem with solid state rectifiers.) Check the data for any tube rectifier you plan to use for the maximum recommended capacitor value. If you need more filtering on the plate than a single capacitor can reasonably provide, use a PI filter before the plate:
        from rect. >---+---/\/\/\/---+---o to B+ and rest of poser supply
                       |             |
                      ---           ---
                      ---           ---
                       |             |
                       _             _
                       V             V
The resistor is typically from 100 ohms to 500 ohms; be aware that this will drop the voltages throughout the amp. Estimate or measure the currents in the amp and do the math (E = I * R) to see how much voltage a given resistor will drop for your current load.

Also, increasing the power supply capacitance on cathode-biased, push-pull amps, especially those with tube rectifiers, will decrease sag-- the compression/sustain effect you get at higher volumes. You can try various values of power supply capacitance in such amps to find the best compromise between hum reduction and tone.


Last updated: 03 March 2008

Copyright Y2K, 2005, 2008 Miles O'Neal, Austin, TX. All rights reserved.

Miles O'Neal <meo@XYZZY.rru.com> [remove the "XYZZY." to make things work!] c/o RNN / 1705 Oak Forest Dr / Round Rock, TX / 78681-1514