Fender Bassman

[Rayme]

I have a 68' Bassman with very low power/volume but with no hum or noise. Has been tested with another set of new, known good tubes. Has two 6L6 tubes, no rectifier tube and 4 pre-amp tubes. All caps and resistors look to be good as far as visual inspection( no damage or burnt components). How can I determine if this is power or output transformer related? Amp has played great previously. Thanks for any advice, Rayme

[pdf64]

A faulty transformer is the worst case scenario, try to eliminate everything else first.
See if this helps
Tube Amplifier Debugging Page

[MWJB]

First step is to record & list all dc voltages at 6L6 pins 3, 4, 5, 12A#7 pins 1, 6, 3, 8.

Compare these to the Fender schem, but bear in mind that wall AC voltages have risen in the last 41 years and your actual voltages will be higher by 10-15%...you're really looking for a proportional comparison.

Also try disturbance testing, by probing dc voltages at the power tubes with you voltmeter, then the preamp tube next to the power tubes (should hear a slight "fsst" when probing pin 3?), then 2nd from the power tubes & so on...as you get nearer the inputs of the amp, noise will increase to a loud crack when probing pins 1 & 6 of V1 & V2. If you get a little noise probing the power tubes, but nothing at the next preamp tube you know the problem is at/after that preamp tube.

Make sure you are read up on safe practices regarding working on a live tube amp, even small tube amps can potentially kill. Use meter probes with clips/wire grabbers until you are comfortable with what you are doing, make & change connections with the amp turned off & unplugged from the wall.

Your amp should have 220K (red/red/yel) bleeder resistors in parallel with the first 2 filter caps (70-100uf @ 350v) in the cap pan, if so the caps will drain when you power down, unplug from the wall then put the standby switch in play mode (be sure to put it back to standby when powering back up).

[Rayme]

Odd Tube

This amp has been running 2 RCA-7452 power tubes for a few months and I have no data on these tubes. I doubt that these are 6L6 compatible and wonder if these are to blame for problems.

[Prairie Dawg]

This amp has been running 2 RCA-7452 power tubes for a few months and I have no data on these tubes. I doubt that these are 6L6 compatible and wonder if these are to blame for problems.

Sounds pretty strange. I was not able to come up with a data sheet.

Get yourself back to square one and work from there and do the voltage thing like another poster recommended. Start thinking about things that may have changed value-like carbon comp resistors that get a lot of heat. Also, you can't tell anything from looking at these components. They need to be tested with the right equipment for the job, and any electrolytic capacitor more than five years old could probably stand getting replaced.

[Rayme]

Found It!

Thanks guys, you pointed me right to it. I remover the filter caps pan and was going to do a omh check when I notice one cap loose. Looking closer one end on this cap had become unsoldered. Resoldered and tested. Back to full power and sounding good again. Not putting the weird tubes back in till I get more info on these. Thanks again guys!

[ThermionicScott]

"7452" might be the date code. Are there any other markings on them? Got pics?

- Scott

[Rayme]

Scott,
No longer have the tubes and didn't take pics. The "7452" and "RCA" were the only print on the tubes. Both were large print. Thanks, Rayme