1968 Fender Bassman- Serious Volume Loss

[Lydian]

Hello all once again,

http://bmamps.com/Schematics/fender/bassman_ac568.pdf

Continuing to work on a buddy's amp, all repairs have gone well though I cant find the source of a serious volume loss.

Brief description- The amp on 10 compares to about 2.5 on my 40 watt Fender Pro Reverb AA-165 that I have. This has had volume issues since he bought it. The amp noise isn't horrible, but I'll describe it in case it is relative to the issue. Sounds like a mild ocean maker with intermittent light fizz crackles. The tone that comes through sounds great, or would be if I could get it at decent volumes.

Tube dissipation is around 14-16 watts, the electrolytics in the doghouse, and the bias capacitor have been changed. Ive gone through quite a few resistors that were also out of spec. The volume loss was the same before the swap. A new 3-prong chord has been installed. I have checked the 1 meg spec on the volume pot.

I had to change a cathode resistor (150 ohm 7 watt) on the outer output tube, it had shorted out completely.

Both guitar and bass inputs are suffering from the same problem, so I believe this rules out the earliest stages. I haven't changed the lytics to the pre-amp cathodes yet. I have also thrown in 'known' good tubes in all stages.

Anyone have a good bit of advise at a possible checklist they would do from here? I'll try to provide pictures if need be, just not very computer savvy. Any questions to help build towards some avenues to pursue, I would be glad to answer.

Thanks guys,
Dalton

[The Dude]

Not to "pick nits", resistors "never" short, but I'll believe it may have burned open. Some quick checks: Check all of your plate voltages. Maybe you have a plate resistor that's gone towards open. Check your actual idle current by measuring the voltage across the cathode resistors to make sure the output tubes are both conducting. Insert a test tone and follow it through the amp. See where the signal is low or not as it should be.

[Lydian]

Not to "pick nits", resistors "never" short, but I'll believe it may have burned open. Some quick checks: Check all of your plate voltages. Maybe you have a plate resistor that's gone towards open. Check your actual idle current by measuring the voltage across the cathode resistors to make sure the output tubes are both conducting. Insert a test tone and follow it through the amp. See where the signal is low or not as it should be.

Funny you point that out on the "short" I only used that term due to the local repair shop I happened to talk to. Scary to think that's the common vocabulary of the popular amp guys here.

I'll research all your suggestions and get on them accordingly. What device do you use for test tones?

-Dalton

[The Dude]

Since you found a burnt cathode resistor, I'd start there and measure the voltage drop across each and compare. That is a clue that something happened in that circuit. For test tones, you can use most anything if you don't have a generator- phone, mp3 player, computer, etc. Just google (for instance 1k test tone, or 400hz, or whatever) and download it to your device or just stream it. Then, if you don't have a scope just measure AC voltage tracing through the amp with your meter.

[Enzo]

I only use sine waves in my shop when I need to see the waveform. You are concerned with low levels, so I'd use something like an MP3 player or CD player or even an FM radio feed. Music is a lot easier to listen to while working than a steady tone.

[pdf64]

Regarding the stock cathode bypass caps, bear in mind that their common failure mode is high ESR ie towards open circuit, leaving the cathode effectively unbypassed, thereby losing ~6dB gain from that stage. So if they're all bad, each channel may have lost ~12dB gain, which is significant.
But as a 1st step, it would be a good idea to print out a schematic and with the amp on your bench, note down the measured voltage next to each typical voltage on there (amp on, idling, with a dummy load, controls full CCW).

[catalin gramada]

check the signal caps for leakage.Cheers

[TigerAmps]

Question: Does it have the white Mallory electrolytics and the brown bypass caps?

If so I would change them ALL. The white Mallories are garbage and the brown caps are also "junk" and make the amp sound "dead".
I have (am currently working on) a 1969 Bassman 50 that had them. I replaced the filters (two of which ruptured due to age)
and the bias cap (just to protect the new set of output tubes) and it still sounded "dead as a toilet bowl"!
I the replaced the white Mallories (because they test to drift after 40 years) and all the brown bypass with
new quality caps and the amp is vicious! I know people don't advise "shotgun" changes, but when it comes
to stuff that is 40+ years old and you want it to sound good; I see no other option.
I also change out the 470 ohm and 1.5K resistors on the power tube sockets and the 100 ohm filament resistors.
Fender amps, when setup properly, should sound loud, punchy and great!

Note: This assumes that both transformers are functioning properly!

[Lydian]

Update-

The issue ended up being the electrolytic caps of the pre-amp cathodes to ground. I've got 2 more to change tomorrow, as well as 2 1500ohm resistors. Hopefully this will get rid of the excess noice. Just about ready to rock the roof of!

Thanks guys.