Need help with Champ tone stack...

[brewbaker]

...from the more experienced than myself.

First post here...

<b>EL34 Champ</b>
I'm trying to get more bass and more gain out of the preamp. I tried changing the resistor that hangs off the bass pot to 22K but it was way too much--impossible with HBs. So now I'm thinking of using the 10K (it was originally 6800) and changing the first cathode network. I'm thinking of using an 820 ohm and a 40uF there. Should I consider changing the second one as well?

Other possibilities I've read about are changing the plate resistors to 220K or higher--would that make sense?

Are there other areas of the circuit to focus on as I continue to experiment?

Any ideas on how to get this thing happening? It's going through a 12" ceramic Blue Dog.

Schematic attached...

[mhuss]

If you haven't already, check out the free Duncan Tone Stack Calculator (http://www.duncanamps.com/tsc/).

--mark h

[rockstah]

if u want more bass out of it try changing it from cathode bias to fixed/adjustable bias.

[Fretts]

I think it's the speaker more than anything. You can get more bass if the amp is in a "wall", i.e. has other amps all around it so that the sound waves coming from the back have to travel further to come around to the front. Kind of a baffle effect. You can get instant grind and punch by lifting the neg. feedback wire. I capped mine off with heatshrink and just left it inside.
I came across a Bronco that had been worked on by Red Rhodes at some point ages ago and he had changed the cathode resistor on V1 to 820 ohms and reduced the input 68K resistor to some other value, can't recall, maybe 22K. It sounded too dirty for my taste, but you might like it!
You should hear a Champ through a 4 ohm external cab, like a 2x12 Bassman.

[PRR]

> I think it's the speaker

On a happy Champ, the little speaker is usually the big problem for bass. And "external cab, like a 2x12 Bassman" is often very impressive. (But "4 ohms" is not the point... air-grab is. If you hang an 8 ohm load on a 4-ohm Champ, the amp has poor leverage on the speaker. But if that speaker is say a 8x12" Full Stack, the cone area has a whale of a lot of leverage on the air, even for large bass waves.)

BTW: You should note that you are responding to a question posted last year, by someone who may have never been back (at least not posting). But Champ-whipping is fun.....

> changed the cathode resistor on V1 to 820 ohms

Modest effect. Leo had good reason to use 1K5. There's good reason to use 800 instead. Neither should make a WOW! difference.

> reduced the input 68K resistor to some other value, can't recall, maybe 22K.

Affects radio reception. If taken below 5K, affects input clipping a bit, but you have to be pretty energetic to clip a stock Champ's input. Maybe you are, and the sum of low series grid R and lower cathode R did change the tone "dirty" in a way that some players would like.

> try changing it from cathode bias to fixed/adjustable bias.

Something can happen to a Push-Pull amp when you raise supply voltage and drop load past what the tubes can stand in self-bias, and re-engineer to good fix-bias. Such amps changed the face of Rock.

However a Single-Ended amp like Champ doesn't work that way. It is either biased as good as it gets, or it sucks. Self-bias is the most reliable way to get there.

> changing the plate resistors to 220K or higher--would that make sense?

Probably not, but it's too humid to prove that tonight.

> I'm trying to get more bass and more gain out of the preamp.

So why did you use that tonestack????

It is missing a capacitor. It has NO midrange dip. In a passive tone control, you can't get the effect of bass-boost if you don't lose some midrange first. See first diagram below. I've modeled your drawing in Duncan TSC. The change of connection on the bass pot makes no difference. Duncan does not let us omit a cap, but setting C3 to 1pFd (47,000pFd is typical) makes it ineffective like no cap. I've used your treb-cap but I think it should be smaller for less nose-flute.

Second diagram shows some rad bass-boom, but all the not-boost is between "1" and "0" (the yellow line is like "0.5" on a 0-10 dial). 1Meg bass pot is just too big for 12AX7 plate and 100K slope resistor. Go back to Fender's values. And when not sure, copy Leo's connections and values EXACTLY. That's not the only way (and Fender did a few variations to voice amps to speakers and fashions), but it is known-good. Start in 1963, then try variations. The Champ with chassis number starting in "AA" is a good reference for a 3-knob Champ.

> and more gain

Hard to reconcile that with your low-loss (no boost) tone control variant. It should have 10-12dB more average gain than a stock AA-Champ, which should be WOW! (In fact bypassing the similar tone stack on the bigger amps is a common WOW! trick. You lose tone-shaping but the naked raw sound of a well-played guitar will sing out.) Wonder if there's a innovation/mistake on the amp as well as the plan.

Your output transformer is CRITICAL to SE amp bottom end. Since the EL34 is twice the tube that a 6V6 ever was, there's a temptation to run it hotter. You need to do so, or it will sound like a sick 6V6. But then you can't run standard Champ/Princeton output transformers. If you increase plate current, the DC in the OT just saturates the core. The Champ OT is very marginal, the Princeton gets by on 6V6 but an EL34 can easily turn the OT's bass to jelly. Also to use the higher power possible with EL34 you either raise the supply voltage or lower the plate current. (Or use the EL34 like a 6V6 at 300V and 40mA, which should work, but is "a waste" of a big heater-hungry bottle.)