STEP-BY-STEP HOW TO FOR MODDING A VACUUM TUBE PA FOR HARP




Turn any Tube PA into an Awesome Harp Amp


INTRODUCTION
Vacuum tube PA amps have long been a favorite of blues harp players, and with the escalating cost of vintage tube guitar amps, harp players are seeking out the PAs wherever they have been hiding. From eBay to garage sales to flea markets to dusty attics, the PAs are coming out of retirement to wail the blues. The first of the old guitar amps to be priced into oblivian was the Fender's, then the Gibson's, and they were soon followed by the bargain-basement amps such as the Silvertone's. A recent eBay search found a ratty '59 Champ, with no guarantee it would work, that was bidded up to $700. Next up on the soon-to-be priced out of reach are vacuum tube PAs, so let's get them while we can.


Whatever PA you find, you must first do an assessment; these are the RED FLAGS that you might want to avoid:
  1. Is the amp rusty? It could have been kept in a damp environment 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? With some power tubes costing 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 capacitors.
  2. Replace any can capacitor that is swollen or leaky.
  3. Replace and add 1/4" jacks.
  4. Replace scratchy controls.
  5. Replace coupling 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, to use a volt meter, to read a schematic, to solder, and to 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, soldering 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 a scope and a 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:



MOD DETAILS
Get a game plan consisting of the changes you want to make, how, and the purpose of making them. Then you need to make a parts list. Consider the number of gain stages: 2 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 them:
  1. Install one 9-pin miniature tube socket and remove the miniature 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 to use the high gain half for the second stage and the phono input. (NOTE: most amps use the second stage of a dual triode as the mic input, which is pin 7.) If not, the phono input would be rendered useless with insufficient gain. Using the 12DW7 is not necessary, but it is 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 I want to 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 completely, 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 in-series with the coupling cap and the grid leak resistor of the next stage (or the volume control as we have with this amp). Adding 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 use test leads to 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, to lower 60Hz hum, and to reduce the possibility of sporadic oscillations. This is optional, but it will improve the performance if the amp has minimal 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-friendly. 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 0.1uF with a rating of 400 to 600V. Although you can go higher, you do risk oscillations that produce a "motorboat" sound (putt-putt) if you do not have sufficient filtering and isolation for the preamp section. I will use 0.47uF caps in this amp, and if I have any trouble, I will replace them with 0.22uF 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 impedances so that we can match up with different speaker configurations.

  6. Replace both the existing 1/4" mic jack and 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 both it and the phono jack with a 1/4" jack.

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

  8. Replace the 2-prong power cord with a 3-prong power cord:
    Lastly, but quite possibly 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 choose). 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 within the components tolerances (even if it was barely within) 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 its 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 differently or has a different mic, cup, etc. If you chase every mod recommendation 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 wear. 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 sine wave more than the other.

GET'R DONE
Check the heater windings; they should be twisted and laid 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 PAs 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, replacing any cap can that is swollen or leaky, as well as upgrading the coupling caps. This PA had paper capacitors in the power supply as well as cathode bypass caps for the push-pull finals and the second stage of the preamp; all will be replaced. It is also a good idea to replace any can caps even if they look good, but this is optional. The coupling capacitors are easy to find: just locate the plates of the preamp tubes, and the coupling capacitors 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, the ones used were often too low in wattage rating. Follow the voltage from the rectifier as it passes through each resistor and check the resistance with an ohm meter. If it is 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 are two 150 ohm resistors in-series that, instead of measuring 300 ohms, are measuring 3.35k. A close look will show that they are discolored (darkened) in the middle, which is a result of overheating. These resistors appear to be 2 watt resistors, and since they are in-series, the total rated power they can dissipate is 4 watts. Because they overheated and 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 both old 1/4" jacks with new ones and 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 to the spade connector on the speaker terminal strip, and it can be moved to allow for different speaker impedances. The sleeve on the jack is wired to speaker ground.





Bad solder joints and vintage amps are synonymous; seek and ye shall find. Look very closely at every solder joint using a good light and 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; this will not do. The reason may be because the transformer is not the amp's original and had been replaced sometime in its past life. You will sometimes see this in vintage equipment, but we can fix it. The amp is as quiet 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 that 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. 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:



AFTER THE SMOKE CLEARS
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, 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 what's 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.



BELL PACEMAKER MODEL PA20 MODIFIED FOR HARP BY LONE WOLF BLUES COMPANY:


    

    

    


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 usable 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


Not an electronics guy? Here is another option:
"Hi Randy, Buck Mooneyhan here. I have your harp break, harp tone +, harp delay, and harp shield.... use the harp delay all the time and the harp tone + A LOT. Great products, thank you. What I really want to thank you the most for though, is how you shared your expertise in your article on modding the vintage PA amp the Bell pacemaker. I took that article to a local guitar amp technical wizard and he followed your basic modding theory on a Challenger CH-30 I just got from Ebay. He had worked on a Bogen H-15 for me before as well as a1948 Knight 14S. They turned out pretty good but we were guessing here and there. Following your basic method (different number of stages) on this other PA head yielded the best thing I've ever played through, and the best PA head I've heard period..... Many usable tonal effects and tested at full 25 watts. Thanks again for allowing us to learn something good.

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