1961 Bassman

[pnut5150]

All of the date codes says this is a '61 model..which is a 6G6 with 5881's. This one has 6L6's with a 5ar4 rectifier. The schematics I've found is a different bias circuit which this has modern parts so i know it's been replaced. Shouldn't the bias cap be larger? It is not breaking up like it did when I first got it, and having to run it harder now. I put a new set of TungSol 6L6's in it recently no help.

[Leo_Gnardo]

Do my eyes deceive me? Whoever put in a 470 uF 35V cap, must have been the last cap in the shop. "I hope this works . . . for now!" Typically bias voltage runs 45 to 55V for 6L6/5881. Given that, install a 63V rated cap at least. If all you can find is 100V, that will do nicely. And . . . you sure don't need 470 uF. Anywhere from 22 uF to 100 will do nicely here. I generally use 47 or 100 uF.

[pnut5150]

That's what I was thinking 100v 20-30mf

[Chuck H]

It is not breaking up like it did when I first got it, and having to run it harder now. I put a new set of TungSol 6L6's in it recently no help.

First, what Leo said. Then...

Check your bias current. It's possible the bias has drifted cooler. That could be making the amp more efficient than how you received it. You probably want about 40mA per tube.

[pnut5150]

I put a 100v cap in 220uf, I have a 22uf 50v I'm gonna put in I found afterwards.
My bias tool only does one tube, I have it set for 55ma.
I found a .25 cap (Astron) last night and changed it last night. But it seems like if I pull one tube out, volume is still the same, checked voltages and replaced resistors on the power tubes, still no change. All voltages seemed correct.

[mozz]

Your bias cap should be smaller value than the filter caps. You want the bias to come up sooner than the rest of the tube. The one schematic even shows a 50v cap and 54v bias! Use 100v to be sure.

[Leo_Gnardo]

I put a 100v cap in 220uf,

Well that is heading in the right direction

I have a 22uf 50v I'm gonna put in I found afterwards.

What voltage do you measure across this bias filter cap? If it's over 50V why would you install an underrated cap?

My bias tool only does one tube, I have it set for 55ma.

A bit to the high side but still relatively safe.

I found a .25 cap (Astron) last night and changed it last night.

Where, why? IIRC that's in the bass channel preamp. Was it leaky?

But it seems like if I pull one tube out, volume is still the same, checked voltages and replaced resistors on the power tubes, still no change. All voltages seemed correct.

Removed one output tube? With the bias high as it is, you're close to class A, you might not notice much volume change with an output tube removed. Maybe a lot of hum instead? What problem are you trying to resolve by removing tubes?

[Chuck H]

I agree with Leo. I don't understand the resistor changes, the seemingly random, or at least unexplained cap change, the too hot biasing, etc. What is the goal? Quicker breakup? It's possible the amp wasn't working "correctly" when you got it, but later it did because of a cold contact reconnecting or something. It might be tough to duplicate an anomalous tone from a memory of an amp that may not have been working correctly by adjusting the circuits. But if you loved it the way it was it seems reasonable to try. Still, unless you KNOW where you are going it doesn't make sense to just 'change some stuff' and see what happens.

[pnut5150]

I didn't do the resistor and cap change in the bias section. I did change the 470uf cap to a 220 100v last night. It broke up really nice around 5-6, now it's clean all the way thru. My question originally was what had been done to the bias circuit as it does not look like what I see on the schematic.
The .25 cap I changed last night was on the normal channel, it did pump the volume back up to par along with a 100k resistor that has been changed to a 57k by somebody else that was not making a connection.

[pnut5150]

Lack of attention of the bias cap voltage, I'll keep the 100v cap in and see if I have one with a lower MF.
The .25 cap was on the normal channel that I use, which really pumped the volume back up to the range it used to be. There is a 100k resistor around it that somebody previously has changed to a 57k that was not making a connection as well.
The reason I pulled a tube was just to simply see if there was a difference in the volume with one tube out, that was only reason to remove a tube.
However, amp is as loud as it once was, but is still stays clean all of the way
from 1-10. Amp was biased around 38ma with old tubes. Was trying to get a little breakup

[Leo_Gnardo]

Was trying to get a little breakup

The bass channel on these white/tan/brown early 60's Bassmen is unusual low gain for Fenders, not so easy to get the amp to break up. Note the eq drive is taken off the 2nd pre stage cathode. Have you tried playing thru the "normal" channel?

[pnut5150]

Yes normal channel is what I use.