1972 Fender Bassman sonic issue

[Chevy]

On the bass input, the cap to bypass cathode on pin 3, has to be 10uF -25 uF.
Otherwise the low frequencies won't pass that stage.
It looks like it says 1 uF on the schematic for the cathode bypass cap.

If that stage won't pass low frequencies, the deep switch won't work either.

Next V1B, same thing. It can't really be 5uF cause the lows will be lost...too much.

Cathode Bypass Capacitor Calculator

Ooops, forgot to mention that I upped those cathode bypass caps on the bass channel to 25uF each. So both channels are configured the same. Yeah, it did have 1uF on the bass channel input stage... go figure.

[Chevy]

OK, swapped out the output tubes with new Groove Tubes 6L6, happened to have a pair after all. Amp sounds pretty much the same as with the JJ 6L6's. No huge surprise there. (although the JJ's do put out more power). Not that I'm really familiar with the sound of many 6L6's... But the amp still sounds a little harsh, a little spiky for some reason; both channels about the same; one channel has an old stock RCA USA 12AX7, the other channel a new Tung Sol 12AX7.. and the driver stage a NOS JAN 12AT7. Sounds like an amp with bad EQ or something. I just don't get it... should sound as good as a Super Reverb or Bandmaster... both of which I've played and serviced quite a bit. But it just doesn't. Should have wide frequency response, smooth glassy highs and sweet mids.

Just for an experiment, I put an 820 ohm R in parallel with the existing 820 feedback R, to see what effect more feedback would have, and the spikiness was reduced (which is great!), but the feedback/compression kills the dynamics of the amp, and some high and low frequencies, it seems.

So what do I do from here ? I need this amp to sound awesome... for recording purposes... I've asked this before, and still wonder if the output transformer may somehow be involved in this weirdness... could 1972 been a bad OT year or something ? I don't even know if an OT could sound bad, but what else is there to check ?

PS The only other thing I found on this amp a little out of the ordinary was the B+, up at 478 volts, which is a little high per the specs. The power xformer appears to be original.

Any ideas?

[soundguruman]

Ooops, forgot to mention that I upped those cathode bypass caps on the bass channel to 25uF each. So both channels are configured the same. Yeah, it did have 1uF on the bass channel input stage... go figure.

If you look at it...seems like the normal input should have been the bass input,
and the bass input should have been normal.
Might have been a factory mistake.
The 1uF bypass won't be any good for bass frequencies.
But on the other hand 25uF might be "a little too" subsonic.
That why I wind up with about 10uF or less.

You see when it passes subsonic it's almost DC. And it sounds muddy that way.
Too much like a boogie amp. Almost fart like.

[Chevy]

bump

OK, swapped out the output tubes with new Groove Tubes 6L6, happened to have a pair after all. Amp sounds pretty much the same as with the JJ 6L6's. No huge surprise there. (although the JJ's do put out more power). Not that I'm really familiar with the sound of many 6L6's... But the amp still sounds a little harsh, a little spiky for some reason; both channels about the same; one channel has an old stock RCA USA 12AX7, the other channel a new Tung Sol 12AX7.. and the driver stage a NOS JAN 12AT7. Sounds like an amp with bad EQ or something. I just don't get it... should sound as good as a Super Reverb or Bandmaster... both of which I've played and serviced quite a bit. But it just doesn't. Should have wide frequency response, smooth glassy highs and sweet mids.

Just for an experiment, I put an 820 ohm R in parallel with the existing 820 feedback R, to see what effect more feedback would have, and the spikiness was reduced (which is great!), but the feedback/compression kills the dynamics of the amp, and some high and low frequencies, it seems.

So what do I do from here ? I need this amp to sound awesome... for recording purposes... I've asked this before, and still wonder if the output transformer may somehow be involved in this weirdness... could 1972 been a bad OT year or something ? I don't even know if an OT could sound bad, but what else is there to check ?

PS The only other thing I found on this amp a little out of the ordinary was the B+, up at 478 volts, which is a little high per the specs. The power xformer appears to be original.

Any ideas?

PS I've also tried putting a 100 ohm R in series with the B+ supply... to simulate a tube rectifier, thinking maybe it would reduce the spikiness a little bit, but it didn't seem to help.


Could there be some weird phase anomaly going on here ? Why doesn't this amp sound great ?

[Chevy]

One other thing I tried was to change bias current... went up to 45mA, and the amp seemed to sound just a hair better... a tad smoother, less harsh... went up to 50mA, and noticed no further change, so left it at 45mA, around 70% dissipation.

I just don't get it... this amp should sound every bit as good as a super reverb, but it doesn't. Doesn't have the magic... even when played thru 4 Jensen P10R's. A little harsh and spiky for some reason.

Tried everything I could think of, even lowering the preamp voltages by using a 10K R in the first R of the preamp supply B+.... saw some lower preamp voltages on some other Fender amps, thought that might do it... but no.

Any ideas?

[Chevy]

Problem solved....

Learn something every day... seems the problem I was trying to resolve was in the tone stack. Sounds much better now !

[Justin Thomas]

Care to be specific?

Justin

[Chevy]

Care to be specific?

Justin

Changed the slope resistor to 56K, which changed the tonal response a lot, and for some reason on this particular amp, got rid of the spiky nature of the beast.