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Beard P100 - Big Tube Amp Needs Help

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pich

Well-Known Member
Just got this non-working amplifier. Doesn't work now but sure hope I can get it going.

This was listed on CL for about the cost of the gas for the 1 hour trip to get it. The guy who sold it was more concerned about finding someone who would enjoy it than making a buck. He tried to fix the amp without success but told me it once sounded really nice driving his Maggies.

Here is what I know about the amp:
Prior owner and the owner before him replaced some resistors in the amp. (see metal oxide resistors)
The power fuse is blown.
One of the preamp tubes tests bad for emission and one of the KT88 tests a little weaker than the other 3. There is a bias adjustment for each of the four output tubes so hopefully they don't have to match.

Tomorrow I'll pull the tubes and bring the power up slowly. If that goes well I'll replace the bad preamp tube and see what happens. Even if I can get it going I probably pull the board and replace the metal oxide resistors and install new tube sockets.

Any advice or information on this amp would be appreciated.
 

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I suppose just do your basic power checks. Power transformer for shorts, diodes/rectifiers, power filter capacitors and shorted output tubes.
 
The printed circuit board looks to be carbonized ,around the resistors , this can cause the board to conduct electricity like a resistor .Clean the board with an exacto knife or similar tool ( part of the board will flake away) . After cleaning check with an ohm meter to make sure that the carbon trails are not conductive , if it is part of the board may have to cut out and filled with epoxy used to repair fiberglass automotive bodies . Also check for cracks in the in the foil traces and burned open foils .
 
Luckey dog :yes: I also was after it but I guess I was just a little to slow, Least it went to a good home :thmbsp: Steve
 
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Mark - thank you for the tip on repairing the board. It confirms that I'll have to remove it.

Steve - sorry I beat you to it and it does feel like I hit the lottery. Philadelphia CL has some amazing deals every now and then. About a month ago I saw a HK Citation II, a Leach SS amp, and great looking pair of DIY speakers go for $500. I was a little slow on that one and really wanted that HK to drive my Dahlquist DQ10s. Hopefully this Beard will do the job.
 
Mark All's fair in love and war , Nice to find an AK'er got it, There will be outher items up for grab so WATCH OUT :thmbsp: Steve
 
The amp has a bias issue on one channel. The amp has a board with pots to adjust the tube bias (see right side of picture in earlier post). I believe the board connects to the cathode and then there is a 10 ohm resistor to ground. When I first powered up the amp with full voltage both of the 10 ohm resistors on the bad channel burned out. I thought the problem could be a bad cap in the bias board. Today I replaced the caps and the burnt resistors. I powered the amp up and thought I solved the problem. I had the amp on for about 10 minutes and began adjusting the bias to the .5 volts dc documented on the bias board. When I connected my multimeter to the output side tube on the bad channel I noticed the voltage begin to rise. Before I knew what happened the 10 ohm resistor flashed and it was gone. Any suggestions what to try now? I have tested the tubes and they all test good.
 
If it has one good channel, can't you take ohm and voltage measurements of the good channel and compare them to the bad channel? Is the output transformer in the bad channel ok? Are the diodes in the bad channel ok?

Disconnect the B+ on the bad channel and then measure the good channel for referrance.

How about those red power filter capacitors?
 
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I believe both output transformers are okay and that the red power supply filter caps are good. For the 10 minutes when the amp was working there was no hum and the sound output was balanced on both channels. I like the idea of using the good channel as a reference point. I did measure the resistance of the bias board pots (when i had the board out) and they were the same on for all four. I'm hoping the problem could be a loose or dirty tube socket so I'll give them a good cleaning. I'm sure it can't be that simple. Thanks for the suggestions.
 
THe problem could be one fo the output tubes (or sockets, as you say) not holding bias... or the bias supply (I am guessing based on the description it's a "fixed" bias amp with its own bias power supply?).
 
its cathode bias and if there are any caps bypassing the cathodes those would be suspect as well. Those big red cans in there have bleeder resistors on them so they must be in series whats the voltage rating on the power supply cans ?
would like a closer look at the board with all the pots on it I might be wrong but I dont see any diodes on that board to make me think the amp is fixed bias.
 
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pull the output tubes, measure the bias voltage on the grids of the good channel, then the bad channel. try adjusting the pots on the bad channel and see if you are loosing bias due to bad flakey pots...

coupling caps to the output tube grids could be on the way out, creating the same problem.
 
Attached is a closeup of the bias board. I assume it is not fixed bias since I can adjust bias on each tube via the variable resistor. I suspected a bad cap so I removed the board and replaced the black caps in the picture with same value but rated for 250 volts instead of 100. The red power supply caps have a 350 volt rating.
 

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ya do wut bharper said could be flakey pots as well. Caps in SERIES in the power supply cool stuff the b+ must run around 550 to 600v in that amp
 
another thing it might be is a loose or fried control grid in that one output tube that tests low this is kind of a hard problem to spot. Some times an output tube with this problem will fire up just fine and work like it should then any vibration sets it off. If you have a tube tester leave that tube in the tester for awhile till its real warmed up then test it and while your testing it lightly thump it with the eraser end of a pencil then watch and see if the meter on the tester jumps all over the place. I have experienced this once or twice one time was on some tube mono's with fixed bias I would get the bias all nice and set and then one tube would just run off and no amount of control voltage would keep the tube in check.
 
Could very well be the tube. The socket where the resistor blew had the weaker tube in it. I tried the tap test but the needle didn't move with the emission or gird leakage tests. I'll try the other suggestions and also move the weak tube to the other channel and see what happens.

A replacement tube would be a simple fix but the KT88s seem to be expensive. I believe the current tubes are 80s production Genalex Gold Lions. The stickers are missing from the bases and the lettering has rubbed off all but one tube. That tube has the gold lion and made in England markings. If it is the tube, do I need to replace with a matched pair or can I just get a single tube? Eventually I'll spring for some good tubes but would first want to be sure the amp works.
 
Did some more testing tonight and think the problem is more than just the weak tube. I spent some time playing with the bias adjustment on the good channel. I started with the bias pots set at the lowest setting and then adjusted as the amp warmed up. It took about 20 minutes for the voltages to stabilize and I noticed that when I increased the bias on one tube the bias decreased on the other. Figured out that you need to make minor adjustments on one side and then the other.

I then moved the tubes to the channel that has the problem and went through the same process. As the amp warmed up the bias on the tube that previously blew the resistor went up quickly to about .6 or .7 volts instead of the recommended .5 volts. I tried to reduce the bias but there wasn't any adjustment left. As the bias in this tube increased the bias in the other tube went to .02 volts (basically nothing). I shut the amp down to avoid blowing the resistor again.

Earlier today I took voltage measurements on both channels with out the output tubes in. Everything matches well.

Any thoughts on where to look now? Could the problem be in the filter caps? They are multistage caps, 2 400 uf 350 volt caps in each of the four red caps. No hum but could a bad filter cap cause the problem I've described?
 
I think I figured it out. :banana:

I rewired the section of the bias board that controlled the "good" channel to the "bad" channel. I monitored the tube bias at idle for about 30 minutes and all is fine on the bad channel. Each channel of the bias board has a cap (which I already replaced) a diode, 4 resistors, and 2 trimmer pots. My guess is that the diode is failing when it heats up.

I think I'll replace all of the components and replace the tube sockets while I'm in there. Thanks for all the suggestions and help.
 
Please keep us posted on your findings. I dont have the capability to work on tube amps. So this is exciting to me to read about it. Thanks, David:thmbsp:
 
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