Vintage Fender Bassman Tube Question!

[taskforcestudios]

Hi,

I have a question for all you amp techs out there. I have a '74 SF Fender Bassman 50 head, which I recently modded with a master volume & prescense control which allows me to drive the amp harder at lower volumes.

I'm getting some pretty nice overdriven sounds, but wanted to tighten up the sound a bit. Basically tighter lows and smoother highs. I really dig the tone of the amp, but wouldn't mind messing with some tube arrangements.

Would it be a very bad idea to put 12AX7s in place of the 12AU7s (I believe they are, or 12AT7) in this amp? Do 12AX7s and Fender amps not mix well?

I was also considering changing my power tubes to 6L6GC. Would this be the best way to go for the sound I'm looking for?

Currently running the head through a Marshall 1960A cab with G-12T75s.

Thanks alot!
Brian

[Adicted to Tubes]

The bassman 50 came stock with 6L6GC tubes and uses 12AX7's except the phase inverter which is a 12AT7.There is no real advantage to changing the 12AT7 to a 12AX7.Maybe a slight increase in gain,but hardly worth it.
To tighten up the bass try swapping the preamp cathode bypass caps from 25uf to 5uf.It makes less bass that way and gets less flubby on lower notes.You lose a little gain but not much.Are you using a post phase inverter master volume?
Marshall amps use .02uf caps on the drivelines instead of Fender's .1uf.This also tightens bass.
The lower value caps don't add bass,they lessen it because Fender's have too much already for distorted sounds and it can be muffled.
Most Marshall's sound pretty tight on bass notes.
The GT75 speakers are best for metal sounds and don't translate well to Fender amps.

www.claramps.com

[Glasstone Amps]

Agreed on the cathode bypass cap reduction. You could even go lower than that - down to 0.47 uF and you'd still keep your mid and high end gain while EQ'ing the bass down a bit. Play around with the values a bit to see what suits you.

[taskforcestudios]

To tighten up the bass try swapping the preamp cathode bypass caps from 25uf to 5uf.It makes less bass that way and gets less flubby on lower notes.You lose a little gain but not much.Are you using a post phase inverter master volume?
Marshall amps use .02uf caps on the drivelines instead of Fender's .1uf.This also tightens bass.

Thanks alot for the responses so far!

How can I find the preamp cathode bypass caps in the circuit and how many of them are there? I haven't had a ton of experience with working on amps, so I'm still kind of learning. If I change the values, is there a certain type of cap you would recommend I use?

I'm a little unsure of whether my master is post phase inverter. I had to replace 2 of the .1mfd caps with 4 .047s and the whole thing ended up working really well. Surprisingly one of the better master volume circuits I've heard, apparently better than the ones that Fender actually used.

Regarding the speakers, they actually sound pretty good with this amp surprisingly. Its kind of a different sound but I dig it.

Thanks again,
Brian

[Glasstone Amps]

The cathode bypass caps are usually 25uF/25V on those amps. They'll be marked with the value and voltage, and are in parallel with the resistors that connect to the cathode pins on each preamp tube. The resistors are usually 1.5k on those amps, I think.