Tone stack question(s)

[Fred G.]

I am in the process of converting a Bandmaster to a 5F6-A clone. I am pretty much finished with the major stuff - all I have left to do is change the pot values and drop the preamp voltage a bit.

It appears, when comparing schematics of Blackface vs. Tweed/Marshall, that the tone stacks are wired slightly differently with respect to the midrange control.

Would someone be willing to explain to me why the difference, and what type of "behaviorial" differences (i.e. sonic) in the tone stack would be expected?

The reason I ask is, the amp sounds nice as is with the blackface pot values and wiring. I'm curious as to what I will achieve when I switch over the pot values and the mid-pot wiring to Tweed specs.

Also, this amp goes from 0 to LOUD VERY quickly on the volume pot. It's the original 1M audio pot that was there before. Thoughts, suggestions regarding this? Is this typical of this circuit?

So far, this has been a very fun project.

Thanks!

Fred G.

[Matt T.]

Try Duncan's tone stack calculator to see the difference. Fender's way affects the gain a bit more than the Marshall way (well, 'Tweed way' but you get the idea). Fender's way is a bit more 'interactive'.

It might help to consider that some amps have a 'gain boost' switch which just disconnects the mid pot from ground...thus 'lifting' the entire tone stack. Now consider that the Fender way is 'adjusting' that resistance to ground...that's why it affects the gain more than the Marshall way (with the Marshall way that resistance to ground remains 25K no matter where you set the mid pot).

Then there's the Vox way which is just all wrong.

Oh yea, that volume ON/OFF thing is common if there is a bright cap. It still happens without the bright cap but a bright cap will make it even worse. Also, the original 5F6-A used a 12AY7 input tube didn't it? Big difference.

[Bruce / Mission Amps]

"Then there's the Vox way which is just all wrong."

WHAT!!! No way!
VOX RULES.

[Fred G.]

Thanks, Matt, for the very understandable explanation. Good info.

So...hypothetically speaking, I would assume, then, that if one were to rewire a Blackface-style tone stack midrange control into the tweed configuration (pretty simple), then there would be somewhat of a lessening of the gain thrown away in the midrange control, right? Or am I leaping into an oversimplification here?

Thanks,

Fred G.

[isaac]

Just try the twon kinds and pick which you like best. Now I wouldn't lift the ground on the mid pot as it increases gain way too much, though if you put a resistor (about 33k) in series with the mid pot to ground instead of the just mid pot to ground u get a nice boost, BUT make it switchable by shorting it out with a spst switch. As I've said it is footswitchable witha simple circuit but without a schematic on hand its too hard to explain, if you want PM me other wise just do a standard switch to short it out, maybe a push pull pot. or just a switch on the back, whatever teh case that boost works really well. If you dont make it switchable it will reduce functionality of the bass and mid pots.