Bassman 100 tube amp overheating

[propmonkey]

after spending about 30mins searching the interent im still stuck. my friend has given me his Bassman 100 tube amp from the 70's to try to repair. ive taken an electronics class so im not an expert but i can figure it out after awhile. the problem is 2 of the 4 tubes are super heating. they heat up when the standby is off and it takes about 1 min to heat up. from one site i looked at they said it was a bais problem. but i measured the voltage of pin 5 of the tubes and i got around -50 dc volts and on the 4th tube im getting about -60 dc volts. and they described the bais problem would be showing 0 volts and in a working amp it should ideally read around -50. so as of now im stuck on what to do.

heres the schematic - http://www.drtube.com/schematics/fender/bassman100.gif

heres the other forum thing i read about it - http://www.fenderforum.com/forum.html?db=&topic_number=498145

edit: ill take pictures as soon as i get my camera back form my friend

 

[Mayhem]

1st thing to do is swap out the overheating valves with those that are functioning correctly and see if the problem persists.

I assume that you have checked for mechanical faults before electrical ones? Most faults tend to be mechanical in origin, such as dry solder joints, broken component legs or valves not seating properly. Check the valve bases and the pins on each valve as they can loose contact over time (as well as fill up with crap) and cause problems.

I had the 135W head version of this combo (from the schematics I assume it is a combo amp/speakers) and the only problem that I encountered with the valves was the 1W ceramic 470 Ohm resistors. Unfortunately, I cannot recall the exact problem that led me to them. This is going back a few years to my band days. Meter them and see what you find. Also look to see if there is any discolouration of them that would lead you to suspect heat damage.

When it comes to setting the bias, the easiest way that I found to do this was to turn the volume up on the amp (with nothing plugged into the input) until you hear the hum. Then adjust the bias control to get the lowest level of hum.

Also, be careful when working on valve amps, as the grid voltages are very high and they are DC voltages.