Vacuum tube PA amps have long been a favorite of blues harp players and with the esculating cost of vintage tube guitar amps harp players are seeking out the PA's where ever they have been hiding. From ebay to garage sales and flea markets and dusty attics the PA's are comming out of retirement to wail the blues. The first of the old guitar amps to be priced into oblivian was the Fenders then the Gibsons and they were soon followed by the bargain basement amps such as Silvertone. A recent ebay search found a ratty 59 Champ with no garrantee that it would work that was bidded up to $700. Next up on the soon to be priced out of reach is Vacuum tube PA's so let's get them while we can.

What ever PA you find you must first do an assessment and these are the RED FLAGS that you might want to avoid.

1. Is the amp rusty? It could have been kept in a damp enviroment that would lead to bad jacks, pots, tube sockets, oxidized wire and solder connections.

2. Is the cord attached? A cord that has been cut off is an indication that the amp was not working.

3. Are the tubes common tubes? some power tubes are over $70, you want to see tubes such as 6L6, 6V6, 12A anything, 5Y3 and others. Make a list of the tubes and check for availability and price if you are not sure.

4. Are the power transformer and output transformer present? If not you should probably pass on the amp. Transformers are expensive and something in the amp probably caused the failure and that something is most likely still there.

5. Are there unattached or cut wires? If so someone had been trying to fix it and could not, anything could be wrong with this amp.

Here are the things that we know we will need to replace in any vintage amp, or at least should.

1. Replace all paper capicators.
2. Replace any can capacitor that is swollen or leaky.
3. Replace and add 1/4" jacks.
4. Replace scratchy controls.
5. Replace coupeling caps with harp friendly caps.
6. Replace the 2 prong power cord with a 3 prong power cord.


What should be obvious at this point is that you need some skills in electronics. You must have the skills to work around high voltages, use a volt meter, read a schematic, solder and perform basic troubleshooting. Make no mistake there are voltages inside of these amps that can and will kill you if you do not know what you are doing. You must have the proper tools such as a multimeter, solder iron and hand tools plus it is not necessary but extremely helpful if you have access to test equipment such as a signal generator and a scope. I know it is a stretch to have the scope and signal generator and you can get by without them but if you enjoy this type of hobby they are worth their weight in gold.

The amp I have for this project is a Bell Lab Pacemaker PA 20. This was a common amp back in the day and varieties are easily found.

There are not many amps out there that you cannot find a schematic for if you try but if you cannot then you should try to draw it out yourself, here is my drawing of the Pacemaker PA 20 preamp and power supply.

Get a game plan consisting of the changes you want to make, how and the purpose of making them and make a parts list. Consider the number of gain stages, two is optimum, 3 is too many and can be feedback hell if steps are not taken to lower the gain of the first stage.

Here are the mods I will do and why I chose to do them.

1. Install one 9 pin miniature tube socket and remove the min 7 pin sockets.
The only major design change for this amp is to replace the two 6AV6 tube sockets which are miniature 7 pin sockets with one miniature 9 pin tube socket to allow the use of any 12A * 7 tube. This will allow for many gain options with tube swaps. I actually have in mind to try a 12DW7 which is 1/2 12AX7 (100mu high gain) and 1/2 12AU7 (20mu low gain). If you want to do this it is very important to remember to wire the mic input to pin 2 which is the grid of the low gain half of the tube and use the high gain half for the second satge and the phono input (NOTE: most amps use the second stage, pin 7 of a dual triode as the mic input), if not the phono input would be rendered useless with insufficient gain. This is not necessary but something that will make the amp more flexible.

2. Lower the gain of the first stage.
I want to lower the first stage gain of the mic input because it passes through three gain stages and optimize the phono input which passes through two gain stages so that I can have a high gain input and a low gain input, this will be accomplished with the 12DW7 tube. If there are three gain stages in your amp consider bypassing one stage completly or adding a voltage divider on the output of the first stage or a tube swap to lower gain tubes. Voltage dividers work better than tube swaps and are accomplished by adding a resistor between (in series) the coupling cap and the grid leak resistor of the next stage (or volume control as we have with this amp); a resistor of equal value to the grid leak resistor will lower gain by 1/2 and should be about right. Break the connection and useing test leads experiment with different resistor values.

3. Replace the power supply filter caps and increase the values.
increase the values of the power supply filter caps to improve the low end performance, lower 60hz hum and reduce the possibility of speradic oscillations. This is optional but it will improve the performance if the amp has minimul filtering and most do. Check the schematic for the caps that I will use.

4. Replace coupling caps with harp friendly caps.
This is the number one mod to make any amp more harp family. The coupling caps in guitar amps as well as PA amps are designed for higher frequencies and we want to enhance the low end response so it is necessary to increase the value of the caps. The standard is .1uf with a rating of from 400 to 600v although you can go higher you do risk oscillations that produce a "motorboat" sound (put-put) if you do not have sufficient filtering and isolation for the preamp section. I will use .47uf caps in this amp and if I have any trouble I will replace them with .22uf caps.

5. Add a 1/4" speaker jack.
You will find screw terminals common on vintage PA amps for speaker connections, I will leave the screw terminals in place and use the wire with the spade terminal and connect it to the center contact of the 1/4" speaker jack. This allows us to connect to several different impedences so that we can match up with different speaker configurations.

6. Replace the existing 1/4" mic jack and replace the phono jack with a 1/4" jack.
The old mic input jack is oxidized and may not make a good connection so I will replace it and replace the phono jack with a 1/4" jack.

7. Remove negative feedback loop.
Most of the popular vintage amps for harp did not have negative feedback loops plus you can deffinately get earlier breakup by not having one.

8. Replace the 2 prong power cord with a 3 prong power cord.
Lastly but quit possible most important is installing a power cord with a chassis ground.

Not too bad, here is my new schematic with the changes.

The 6AV6 tubes are basically 1/2 of a 12AX7 each with a 6.3v heater although this amp had 12.6v applied to the heaters which is what we will need for the 12A * 7 (whichever one we go with). There is a lesson here and that is to never assume anything in an amp, weird things were sometimes done because it may have been withen the components tolerances (even if it was barely withen) to save money. A standby switch is not necessary because the 6AX5GT rectifier has a heater and will come up to the max voltage slowly.

A word about excessive mods here, I do not believe in excessive mods because I believe that an amp has a character or a natural presence, a tone that is fundamental to the components, their age and the design of the amp. Let an amp be itself and it will sound it's best, make simple, reversable mods and if they do not bring the tone into where you like it sell it and get another, what may not be perfect for you may be perfect for another who plays different, has a different mic, cup, etc. If you chase every mod recomendation on every forum you will never be satisfied, you will spend more time modding than playing and your amp will be the worse for the ware. The 80 to 90v plate voltage mod is the second worst mod out there, it causes excessive preamp distortion and muddies up your tone, I find 110 to 125v great. The technical reason that it is a bad mod is that the tube is operating in a nonlinear portion of the plate voltage curve and you are amplifying one side of the sinewave more than the other. The worst mod out there is to increase preamp gain for earlier breakup, all this does is decrease useable volume control range and greatly increases feedback, if your amp starts to breakup around 5 to 7 on the volume control leave it alone, it's perfect. We are not going plan any further mods at this point, we will stick with what I consider the minimum and see how it sounds.

For making the necessary changes to chassis, a unibit was used. I enlarged the min 7 pin socket to fit the min 9 pin and added a hole for a 1/4" speaker jack in the back.

Check the heater windings, they should be twisted and layed down next to the chassis, if not rewire them and you will greatly reduce hum. The voltage should not be floating, it should be at dc potential or ground potential. To place the windings at dc potential add a 100 ohm resistor from each side of the winding to ground, if there is a center tap just install one 100 ohm resistor from the center tap to ground. Some PA's such as this one will have a hum balance control which puts each side of the winding to dc potential through a variable potentiometer with the center leg connected to the 6V6 cathodes.

Recapping consists of replacing all paper capacitors should be replaced as well as any cap can that is swollen or leaky as well as upgrading the coupling caps. This PA had paper capicators in the power supply as well as the cathode bypass caps for the pushpull finals and the second stage of the preamp and all will be replaced. It is also a good idea to replace any can caps even if they look good, but that is optional. The coupling capacitors are easy to find, just locate the plates of the preamp tubes and they will be there. On a min 9 pin socket for 12A * 7 tubes the plates are on pins 1 and 6. Simply remove them and install your new caps.

Next up are resistance checks. It is very important to check the resistance of all voltage dropping resistors in the B+ line (the rectified dc voltage supplied to the power tubes and the preamp). Carbon resistors do not hold up well under large voltage drops and as a cost cutting measure they were often too low in rating. Follow the voltage from the rectifier as it passes through each resistor and check the resistane with an ohm meter if it more than 20% off of the indicated value replace it, if it is discolored replace it. Be sure to check the preamp plate resistors, they can become so high in resistance as to keep the tube from amplifying.

Shown in the image below is two 150 ohm resistors in series that instead of measureing 300 ohms is measureing 3.35k and a close look will show that they are discolored (darkened) in the middle which is a result of overheating. These resistors apear to be 2 watt resistors and since they are in series the total rated power they can dissipate is 4 watts and since they overheated and they are power supply resistors we should replace them with a 300 ohm cement or wire wound resistor rated to at least 5 watts. You will typically find plate resistors to be 1/2 watt carbon resistors, always replace them with 1 watt carbon resistors.

Replacing old 1/4" jacks with new ones and replacing a phono jack with a 1/4" jack is very straight forward, just be sure to use a jack with a switch for ground on the center connector for all inputs. Here are images of how the speaker jack was added. The tip connection on speaker jack is wired the the spade connector on the speaker terminal strip and it can be moved to allow for different speaker impedences, the sleeve on the jack is wired to speaker ground.

Bad solder joints and vintage amps are senomonus, seek and yee shall find. Look very closely at every solder joint, have good light and use a magnifying glass. When you find them remove the old solder, clean the terminal and the wire and resolder.

When installing a new power cord connect the green wire to the chassis at one of the power transformer screws, the white wire connects to one of the pt primary wires and the black is connected to the fuse holder with the other side of the fuse being wired through the on-off switch to the remaining pt primary wire.

Next is the power up, voltage, noise and gain checks. As far as voltage is concerned the plate voltages on the 6V6's are 300v which is fine but my plate voltages in the preamp are in the mud, first stage is 47v and second stage is 65v and this will not do. The reason may be because the transformer is not the amp's original and had been replaced sometime in it's past life and you will sometimes see this in vintage equipment; but we can fix it. The amp is as quite as can be, no hum or oscillations. Even with the first stage of the mic input being the low gain stage of the 12DW7 I am still saturating the second stage, the phono input is good so I will have to reduce the first stage gain. To reduce the gain of the first stage I will replace the 100k 1w plate resistor with a 22k 1w resistor and to address the low plate voltages I will replace both 47k power supply voltage dropping resistors with 10k 1w resistors. Here is my new schematic

The final assessment and sound test is right on target. The amp sounds awesome with the microphone input being a high gain input and the phono input being a low gain input it is very versatile and works with any mic. The amp puts out about 15 watts of audio power which is plenty loud enough plus it can drive a pair of 8's, 10's or 12's or any single speaker size. Plenty bottom, good punch and plenty crunch, it overdrives and saturates the power section well. I could go on and on but whats the point! You have heard it all before; but to cut to the meat of the matter, a harp friendly vintage PA fits the bill. Enough said! Below is a sound test borrowing the 10" speaker in my Midnight Creeper.

Here is what the new owner of this amp had to say about it.

"Hey Randy, I was going to wait to play this at a gig as I didn't know what I was getting and really wasn't expecting what I got. To say I was blown away would be a huge understatement. I can't get over the sound coming out of this PA. I pulled the amp out of the box, packed very well, no broken tubes thank you. I grabbed the stereo speaker cause I didn't feel like fooling with a speaker in a combo amp. I let the amp warm up and brought the volume up slowly. I played at a low volume and got a nice sound, pretty much what I expected. So I cranked the volume and started to blow. I stopped immediately as I was amazed at what was coming out of my stereo speaker. It was a rich, full, beautifully distorted tone. I grabbed the mic again and pushed as much air through the harp as I could. I was rewarded with a sound I don't get out of my blues deluxe. Lots of bottom, lots of crunch and distortion unlike that of the reissue amps. this amp sings. I have smaller amps that have a great distorted sound, but they don't put out half the volume this does without feedback. I can't begin to imagine how great your class A amps must sound. I have never had an amp with so much useable volume, louder and without all the feedback normally associated with high volumes. The sound is also thicker and fuller than I expected, lots of bottom. Thank you for opening a new door for me in amplification.
John"