Extra gain stage

[Diverted]

Hi all, a friend of mine has a CBS Champ clone (treble and bass tone stack) and wants to get more out of it - more volume. We’ve decided to just experiment with it.
It’s in a decent size chassis so there is room for a 6AV6 as the third gain stage. What do you all think of this?
I’m sure running a 12AX7 in V1 will make it way too gainy to be usable, but maybe a 12AU or AT there, and the option to use a 6AT6 in V2? I think with the different tube options he should be able to get more out of it while keeping it at least somewhat clean.
What do you think of this schematic?
NOTE: disregard tube names on schematic. First two stages are 12A-7, third is 6A-6.

[bmccowan]

Are you trying to get more volume or more gain? And you say it's a clone - are the voltages about the same as stock Fender. Before adding a tube, you may want change the rectifier to SS and see how you like it. And how efficient is the speaker? If its an 8". is their room for a 10?

[Willabe]

Is it a clone, or is it a real Fender, just a CBS Fender?

What speaker is in it, an 8"?

Try a 12" speaker, 1 that has a more efficient db rating. You'll be surprised about the volume difference.

Then you'll probably have to add an extra B+ filter stage node to kill the new louder humm because of the new speaker. You just put it before what's already there.

[pullshocks]

If you are lookking for a beefier Champ, dial 1-800-Dummyload

[Diverted]

Thanks for the replies.
The amp was built with a 12" and solid state rectifier. About 350VDC on the 6V6 plate. I'm using an oversized 10W OT.
The issue is the treble/bass tone stack. It really just decimates the gain — massive difference between stock and lifted tonestack, which I have on a switch. It's fine with stack bypassed but is quiet and too polite with tone controls in. My friend likes the tone controls but wants more of everything out of it.
I ended up putting in that gain stage and rolled tubes around. Apart from a little increase in hiss it sounds great with a 12AY7 in V1 and a 6AT6 in V2. Lots of extra volume and louder clean headroom with the lower gain tubes, which is what he hoped for.
Good suggestion on adding another filter node to feed the extra triode. I will do that. Not much hum at all, but quiet is better.
I may also try to find a place to put a choke...

Happy New Year everyone!

[bmccowan]

So it has a bigger speaker, a SS rectifier, an oversize OT, but other than that its a clone? 
I'm glad it worked out.

[HotBluePlates]

The issue is the treble/bass tone stack. It really just decimates the gain — massive difference between stock and lifted tonestack ...

Yes, because the tone stack Fender uses imposes a 20-decibel scoop, centered on 460Hz.  Turning Treble and Bass "off" results in a flat frequency response with everything down -20dB (which is the same as saying "one-tenth" with respect to voltage reductions).

    - To simulate the Champ in Tone Stack Calculator, turn the Middle control full-up in the link I provided.  Treble/Bass controls will respond like the amp.

Volume control right after the tone controls reduces signal level further.


Customers already got "all the mids" back in the 1950s.  The mid-scoop thing seemed to be the sound for the 60s... Standel was doing it, Ampeg was doing it, Fender started doing it.

Fender's new circuit with separate Treble & Bass controls cut the mids a lot, and then let the player adjust how much to reduce Bass and Treble on a complementary way (but still leaving an overall mid-scoop that gave the "new sound").

Today's user/builder might say, "The tone stack has a bunch of loss," and look for ways to minimize that.  But in the 1950s, Fender already did the "low-loss tone stack" in the 5F6/5F6-A Bassman and the 5F8/5F8-A high-power Twin amps.
More Loss = More Mid-scoop = "Treble and Bass controls that have more range of adjustment."


Meanwhile, a Tubescreamer pedal intentionally boosts midrange right in the same range the Fender tone stack scoops mids (100% by design).  One might want to consider the sonic goal of an amp-build, because changes to add a capability (multiple adjustable tone controls) typically detract from another capability (high signal-level to overdrive the output tube).