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No output - question

3/23/2004 1:34 PM
Craig S
No output - question
My amp started sounding weak after playing for an hour or so. I opened it up, and played with the bias pot, and replaced the bias pot with a 25K instead of the 50K. Now there is no output at all.  
I first thought the OT could be failing. I opened it up and take some measurements. Out of the bias supply caps I measured -90V where the schematic calls for -30V. The plates are still about 460V, which was where they were while the amp was working correctly.  
Would this cause the bias to be way below normal operating range, and hence cause the weakening outupt, then no output? Or would having that much negative voltage make the tubes bias way too hot? (These are the only original electolytic caps - never found suitable replacements)  
If the first assumption is correct, I really want to buy caps from Radio Shack just so I can have the amp working, then get better quality caps. The only suitable ones they sell are only rated to 35V. With the proper values, these will see 30V. Can I get away with these?  
Thank you very much.  
Craig V
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3/23/2004 8:08 PM

I suspect your new bias pot is either defective or wired incorrectly. If you have -90 volts on the grids, the tubes will be effectively cutoff and will not pass a signal. Got -90V on the control grid pins of the output tubes?  
If you have more than 30V on a cap, it better be rated for higher voltage than that. If the -30V is what the grids should see, the voltage at the bias supply filters will probably be higher. Especially at the first cap.
3/24/2004 12:06 AM
Craig S

Hmm.. I put some RS caps in, and put the older pot back in, and have -60V now. Still no output.  
This may be a OT problem after all.
3/24/2004 12:47 AM
Bruce /Mission Amps

Does the plate voltage (CAREFULL) measured at the tube socket not on the power supply B+ rail... go up and down a bit with different bias voltages?  
It's possible you might have a power tube problem or, a cathode that has come undone from the power tube socket or gorund.  
3/24/2004 1:53 AM
Craig S

I went to the bias supply caps, and was going to check the resistor, and I measured no reading, so when I unsoldered it to replace it, it fell apart. I replaced it, and now the amp is back putting out signal.  
I still have this one problem. The amp sounds pretty crappy at higher volumes. I tried tube swapping and such, no dice.  
I guess it sounds pretty good a lower volumes, just that when I turn her up, it sounds very different from how it used to. It's tinny and kind of mid-scooped sounding. It sounds kind of like an overdrive pedal with a real low battery or a misbiased Fuzz Face.  
Maybe something with the OT? Maybe I should bring it to someone with experienced ears.  
Thank you for your help in getting it fired back up again. I'm pumped!
3/24/2004 6:46 AM

Quit with the OT, it is the last thing that is going to go. They do fail, but everything else in there fails more often.  
Make sure you don't have a burnt open screen resistor. Check for B+ voltage on the screen of EACH power tube.  
Now that the bias is working, did you set it correctly? Crossover distortion from improper bias does not sound good.  
Are the tubes fresh?  
Got an effects loop? RUn the preamp signal to another amp to see if your problem is coming from the preamp.
3/26/2004 12:05 AM
Craig S

There is no effects loop.  
The tubes are fresh.  
The bias is set good.  
I'll need to check the screen resistors and voltages. On an EL84, is this pin 6 or 7?  
Thank you for help.

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