'67 Champ tone stack bypassed...

[11 Gauge]

Good lawd, what have I done...

I've got a screamer on my hands.

She starts breaking up at 3, and anything above 7 is unuseable...

I know the "standard" tweak is a voltage divider before the vol. knob - typically a 1 meg/1 meg setup - kinda sounds like overkill, though. Why remove the insertion loss, only to add it back in?

I'm guessing a simple 470K load prior to the vol. pot should tame things a bit, and shave some top end off (it's bordering excessive, but that's probably the stock 8" more than anything - I plan to run it into an open back cab with a single 12").

I know you guys have played with this circuit ad nauseum (5F1). Does that resistor value look like a good ballpark figure? I really don't want to dig into the board any more - I've left the cathode resistor & cap values stock, and NFB loop connected...

This little bugger is a beast, now.

I've asked it before, but I must ask again - why do so many modern amps have so many preamp gain stages? Other than to push a T/M/B, what's the use?

[JohnnyCrash]

Actually the 5F1 is a little different than the AA764 Champ.

As far as "Why remove the insertion loss, only to add it back in?" you've answered that - it seems a bit much to you.

Honestly, breakup on 3 sounds like a fantastic thing to me...

Anyway, you can tweak the Volume pot's value, you can throw in a resistor to bring things down, you can adjust how hard your input is hitting things (those 68k resistors), or you can tweak how hard the 6V6's grid gets hit (swapping the 220k resistor?).

As far as "why do so many modern amps have so many preamp gain stages?" - because most amps aren't running a lone 6V6 for output... I'm happy with the multiple stages in my 100 watt Marshall, and I think my neighbors are too HAHA!

At the same time, you're totally right, sometimes - on some amps, you just don't need the hair metal fizz up front.

[EMan]

If the amp in question becomes unusable after 7 on the volume knob...rather than mod the amp, why not just don't turn the knob any further than 7?

[Wally]

Originally Posted by EMan

If the amp in question becomes unusable after 7 on the volume knob...rather than mod the amp, why not just don't turn the knob any further than 7?

Or...I have to wonder what the difference is between having the tone stack usable and getting to turn the volume up all the way or lifting the tone stack and dealing with the increase in gain by turing the volume down or as noted by reinserting the loss somehow. I am thinking that I personally might like to run my power tube a bit harder without the preamp going to mush. IT would be good to having a switch to go back and forth to test the differences in each circuit. OF...I could simply apply a Bad Bob to my guitar's signal and hit the front end with a big signal for preamp gain increase AND still have tone controls.
So many options....I suppose the No. 2 hole would be a good place for a switch for a stack lift switch?

[JohnnyCrash]

Originally Posted by Wally

So many options....I suppose the No. 2 hole would be a good place for a switch for a stack lift switch?

Better yet, a push-pull switch on one of the pots - if it'll fit in that angled-front chassis (I have put DPDT P-P Pot/Switches in SF Champ chassis before).

I like hot preamps sometimes, but if it's too hot, do what you gotta do for your tastes.

[tpaul]

Personally, I like to defribulate the orgone causticator, bring the frappe stack up to 130, and replace the preconjugal triangulator with a couple of hard boiled eggs.

Beam me up, Scotty...

[11 Gauge]

Nah - don't need to go back to the stock tone stack - the overall coloration with it bypassed is nearly perfect.

I just need to tame this mofo a bit. I'd like breakup to start around 5, and not so much gonzo beyond 7 that it's just angry electrons emanating from every square inch of the amp. It's great for a few Queens of the Stone Age covers (Do It Again on Songs for the Deaf comes to mind), but that's not really my bag.

Probably a trimpot right on the input of the volume pot would work best.

Man - people who are amazed with the tones of their Valve Junior haven't heard a damned thing! I'm almost afraid to pop it thru to an external cab. It may be the only amp I use for awhile, if I can get the breakup points to my satisfaction.

Thanks for the ideas - I just don't want to alter things other than moving a couple of wires around (and adding a resistor or 2).

[EMan]

Okay then, since you're bound and determined -

Here's what you could do. With the tone stack out of the circuit, the 1 meg pot becomes part of the plate load for the 1st stage, and plays a role in setting the gain of that stage. If you mess with that 1 meg value, by adding some kinda path to ground, the gain of the 1st stage will get lower, and the sound will change. You can see this by - Av = (mu*Ra)/(Ra+ra). "Ra" is the total plate load resistance, meaning the plate resistor (100K) in parallel with the 1 meg volume pot.

With that in mind, you could dial the volume up to the point you feel is nirvana max. Then carefully measure the resistance of the pot where it is set. You may have to pull the ground connection to get an accurate measurement.

Next, find another pot that is as close as possible to that reading. Now, that value subtracted from 1 meg will get you the rest of the resistance needed to keep the 1 meg load intact.

For example, if the pot dialed to nirvana max measured 750K, you would need to get a 750K pot and a 250K ohm resistor.

The new pot would be wired as before, and the additional resistor would go from the 1st stage coupler to the pot terminal opposite the terminal connected to ground. In this manner, the pot at "10" will only go to what used to be 7 on the old pot..and your gain value will remain intact.

Voila!

[jjmantele]

Thru the open 12" things could change dramatically. Especially if the 8" is stock. I added the same switch on my '74 VC and '71 DR. The DR is a big monster in bypass mode. The VC a mini-monster.