Volume pot between triodes?

[Captn Dave]

I'm working on a early 50s guitar amp. It uses a 12AX7 for preamp, 6SN7 for a phase splitter and then two separate 6L6 pushpull channels for the twin 12s. One channel has tremolo.

There are two volume control pots. One is conventional. The other (labeled "Amplification) is placed in series BETWEEN the two triodes in the 12AX7. Any idea why it would be designed this way? It sure seems to introduce alot of noise.

Thanks

[jjman]

Do you have a schematic or the brand and model name? Many if not most guitar amps have a volume control pot "between" the 1st and second triode. This is often involving only one tube (dual triode.)

For example. 1959 Fender Champ:

http://www.schematicheaven.com/fende..._5f1_schem.pdf

[Captn Dave]

It's a Silvertone from the early 50s. Sorry, I can't find a schematic for it but the front end is very similar to the Champ.

My question was poorly worded. I am wondering what is the point of having a pot before the dual triode and another between the triodes, like the Fender Champ you pointed out. Seems like one would do the job nicely.

Both pots are off chassis so it's alot of low level signals running about picking up noise even though shielded wire is used. They seem to be in the habit of grounding both ends of the shielding so I'm going to lift one end and see what difference that makes.

[Captn Dave]

And perhaps more to the point, it added complexity and cost to put the volume pot between the triodes rather than locate it next to the jack. It must provide some sonic benefits. I am at a loss to understand those benefits.

[ray_moth]

I suppose one pot acts as a "master sensitivity" control, so that you can always use the full range of the volume control, but why the two controls should be situated where they are is a mystery.

[Teh Gav]

A Danelectro Challenger?

[makeb4break]

it's so you can overdrive one of the preamp tubes while still having a reasonable (low) volume. quite essential in gt amps

cheers

[Captn Dave]

Originally posted by Teh Gav
A Danelectro Challenger?

Yes Indeed.

Originally posted by makeb4break
it's so you can overdrive one of the preamp tubes while still having a reasonable (low) volume. quite essential in gt amps

cheers

And Makeb4break, now that I have it up and running and have fooled around with it a bit, I can confirm that you are spot on. Cranking the amplification and turning down the volume makes for a very cool over driven sound. The opposite is quite clean.

These things are fun. The tremilo circuit sends the B+ into oscillation. On the scope, the 2 or 3 Hz wave form looks like a sine wave complete with all the typical rectified power supply wave forms riding along on the sine wave. I should get a photo.

The tremilo feeds into just one channel but the B+ oscillation causes the second channel to ride in sympathy with the other. It's hard to tell a difference between them.

I wonder if the designers anticipated that?

[kevinkr]

Sounds like you need to replace some power supply filter caps, and possibly a few dropping resistors as well if you have not done so already.

What you describe sounds like classic motor-boating, i.e. low frequency positive feedback through the power supply.

[Captn Dave]

OK, thanks for the advise. I'll clip on some big caps and see if it still happens. The dropping resistors look to be on the high side but just within specs, but I haven't measured them warm. I can read up on the subject, but would high resistor values and the reduced current and voltage contribute to motor boating or just the opposite?

I note that this only happens when the tremilo is turned up. I guess that's not too suprising because that would tend to load the power supply.

Thanks for all the comments.