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**MWJB** · 03-08-2011, 12:21 PM

Cathode bias doesn't need to be exact, if you wanted to subtly adjust bias you could sub power tubes? If you were obsessed with running to the exact mA then it would make sense to switch to fixed bias (if your amp is a genuine Fender then fixed bias will only allow you to run JJ/EH6V6, NOS won't take the raised voltage), tapped from the rectifier B+ winding, like a Princeton, with an adjustable trim pot.

Most cathode biased champs run a 470ohm cathode resistor which sees them dissipating 20 odd Watts, and plate currents of 50-60mA...more plate current = more gain, but I wouldn't go lower than 470ohms unless your champ was very low voltage (under 330vdc on the plate?).

The resistor configuration does not equate to a specific voltage gain, this varies according to plate voltage & the amount of signal you hit the tube with. When you say "more gain" do you mean more voltage gain, or a more distorted tone? A simple way to increase gain on this amp is to disconnect the NFB loop. If you wanted a more driven tone you could experiment with smaller value cathode resistors at 12AX7 pins 3 & 8, I'd start at pin 8 & try 820ohm/1K?

Gain will drop with a 12AT7 in comparison to a properly functioning 12AX7...there are "high gain" selected 12AX7 available from TAD for example.

In short, it would be useful for you to describe how you want the sound to change. For example, raising the preamp plate voltage, by reducing the value of the preamp dropping resistor (10K 1W), will increase voltage gain...but make the amp sound cleaner & brighter. Dropping the preamp voltage by enlarging the preamp dropping resistor (to more than 10K 1W...like 56K or 82K) will decrease voltage gain, but make the amp browner/grindier, more distorted. Which is it that you think you need?

The grid stopper is 34K, but the grid load if you are plugged into #1 input is 1meg.

**MonkHouse93** · 03-08-2011, 08:02 PM

I forgot to say that I wanted to put the 250 ohm resistor and 500 ohm pot in place of the 470 ohm cathode resistor on the 6V6. I wanted to be able to make a small adjustment to the bias without having to replace a resistor everytime.

By gain I mean more driven but at the same time smoother. . . perhaps that leans towards reducing that 10k 1W resistor you mentioned. The amp sounds "grindier" to me, but it doesn't sound like it's being driven very much.

**MWJB** · 03-08-2011, 08:12 PM

Reducing the 10K (raising the 12AX7 plate voltage) will make the amp brighter, increasing the 10K (lowering the 12AX7 plate voltage) will make the amp browner, grinder & smoother.

I would have the pot & fixedresistor at the cathode the other way around...larger value 470-500ohms for the fixed resistor at 5W, then a 250ohm to 300ohm pot, so that the pot is dissipating the smaller share of the cathode voltage...in fact I'd just fit a 680ohm resistor, no pot & be done with it.

**MonkHouse93** · 03-08-2011, 09:30 PM

Hmmm. . . I suppose I'll just see what happens, thanks for the tips.

**Enzo** · 03-09-2011, 01:06 PM

I'd agree, instead of a pot, just try a different resistor. I really doubt you will be adjusting this much once you have it set. But if you go with the pot, do some math. Calculate how much dissipation will go on in the pot. You don;t want a little pot trying to dissipate several watts. And further keep in mind that the pot powr rating is for the entire pot, if you are running only through the last 10% of the pot, then the entire dissipation is within that hot spot.

**MonkHouse93** · 03-09-2011, 10:33 PM

So before I say anything else, I came across a thread about pet peeves on the forum and I saw people talking about the people who are ignorant and are probably unsupervised teenagers, and some people who claim to have "golden ears" for lack of knowing what they're doing, and then I came back to this thread and realized I sort of came across like that, I'm sorry. I'm just here to learn from more experienced people, because I am that teenager.

With that said. . . I went for pin 8 of the 12AX7, took out the 1500 and put a 1k in there and that was exactly what I was looking for. I tried bringing up the value of the 10k 1W preamp dropping resistor. Radio Shack didn't have anything like 50k 1W, all 1/2W and 1/4W besides the 1k's. I had a 22k so i put it in series with the 10k, I didn't notice much difference in sound besides an overall decrease in volume. I got 6 1k resistors that i tought I could put in series to make 6k in place of that 10k, but I deicded that would be too messy so I'm sticking with the 1k on pin 8. Thank you!

As far as the variable bias goes. . . I did that, brought the resistance up to about 600 ohms, it smoothed the sound out a bit, made it a little cleaner, but that pot in there was a bit much so I put the 470 ohm back. Enzo, the pot Iwas at about 350 ohms, I measured a 16 volt drop across it, and I calculated it to be disipating 0.7Watts, I think that might be alright, but either way I dropped idea. Thanks for the heads up though.

**MWJB** · 03-10-2011, 01:12 AM

Monkhouse93 - opinions, designs, & theories are like raindrops - ten a penny. Well executed circuits are like gold nuggets , or diamonds, rare things. The ONLY way to know for sure is to get your hands dirty & kiss a few frogs. Learn the theory & back it up with critical listening tests...& bear in mind the experience of countless generations...we've all thought that adjustable cathode bias made sense at one time or another, then practicality waded in ....

Preamp tubes, especially when you only have one of them, don't draw much current (2-3mA), so subbing 10k for 32k sounds like a big change in resistance, but from the tube's point of view, it is in the "just about perceptible" region. you really need x5 or definitely x10 in ohmage to really hear a difference...especially after the time lag involved with heating up an iron & changing parts, switching mods in/out for instant A/Btests is always a good idea, if you can do it.

Change one perameter at a time & have fun, stay safe...and take all opinions, even mine, with a pinch of salt. ;-)

**MonkHouse93** · 03-10-2011, 02:53 AM

I have a quite a few 22k's. if I stuck two of those in series with the 10k I'd get 54k, just shy of the 56k you suggested. It would need to be around there to really hear much difference? I feel like that big of a jump could potentially damage something. . . but then again the 6V6 has around 440 volts on its plate and I'm pretty sure that's over its limit :-/ I suppose it's not the biggest deal in the world

**MWJB** · 03-10-2011, 07:57 AM

You won't damage the 12AX7 tube by running it at lower voltage, it will "function" well enough anywhere between 150vdc (or lower, but you'll get no clean tone) up to 300vdc. Back in the early 50's it wasn't unsual to preamp dropping resistors of 100K.

The 6V6 isn't seeing that full 440vdc, subtract the cathode voltage at pin 8 to determine what the tube is really seeing.

**riz** · 03-10-2011, 09:14 AM

Try dropping the B+ with zener diodes. My local store sells 10v 5w for a buck each, and I string them together for the drop I'm looking for. Search the forum for a good description of how to do it, and come back here if you can't figure it out. Also, ADD A SCREEN RESISTOR, even a common old school value of 470-ohms does a lot, others may suggest a higher value like 1K. I find the added screen resistance smooths the sound a little, but it will greatly extend the life/reliability of the tube. Watch the tube in the dark while you play the amp and you will SEE what it does. And hey, I've had my share of ignorant posts here, all in the process of learning. I never got that impression from you, FWIW! Try playing with cathode bypass caps in the preamp, too. You might find something you like. Values smaller than the stock 25uf will reduce the low frequency gain, making the amp sound brighter. Try values all the way down to 1uf on one or both of the stages, just to see what it does. Never know till ya try! A Champ is a great circuit to learn on.

**riz** · 03-10-2011, 09:19 AM

P.S. I like biasing these at 14-15w dissipation on the tube, the last one I upped the cathode resistor to 560R, after dropping B+ to something like 380v. Still evolving my take on that. Don't remember if it was mentioned elsewhere in the thread, but make sure to use a 50v bypass cap on the 6v6 cathode.

**Bruce / Mission Amps** · 03-10-2011, 08:14 PM

Originally posted by MonkHouse93 View Post

The last thing that really got me was changing the tube type. I just wanted to see what would happen there. With the settings set as I described above i had the 29dB gain, I change the tube type from 12AX7 to 12AT7 and the gain jumps up to 32dB. I always thought the 12AT7 was a low gain tube. . .

Any explanations for all of this? Thank you!
And any suggestions to bring up the gain just a little bit in the amp would also be appreciated!

Well it's a little stretch to say the mu, 60-70 vs 100, is the gain... There is more to it then that.
The impedance of a 12AT7 is different then a 12AX7, much lower, like around 10K-11K, while the 12AX7 is much higher maybe 50K-60K. Call that a about 5:1.
Also the tube ratings and impedances change with plate voltage.
As you were, with a common 1K cathode resistor for bias, try plugging in different plate load resistors with the two two tubes, like 20k on the 12AT7 and 100K for the 12AX7.... you'll see how things change around.
Now, mismatch the 12AX7 with a 470K plate load (about 5 times greater) and watch what happens.
Did this program correct for the impedance change and allow for a different voltage drop across the bias and plate load resistors..?? I don't know.
The problem I find with many of these computer generated programs is that I don't know if they are taking all the real world stuff into consideration.

**MonkHouse93** · 03-10-2011, 11:13 PM

I meant to say around 400V not 440V, from the plate to the cathode It said 390V but from the plate to the chassis it said 415. Is that the difference you were talking about, MWJB? It also has a cap from grid to cathode on the 6V6. The champ schematic (aa764) doesn't have that but the bronco (ab764) does:
http://www.thevintagesound.com/ffg/s...a764_schem.gif
http://www.thevintagesound.com/ffg/s...b764_schem.gif
but the Vibro Champ aa764 doesn't have that cap either. I have another champ and I took it out and it didn't seem to make difference.

The bias was at about 20 or 21 watts, and there is a 50v cap on the 6V6 cathode, I replaced those already. Did wonders for one champ, the other didn't seem affected, but those caps might as well be updated anyway.

I'm still not understanding the calculator, no matter what resistor whenever i choose a higher resistance it shows me more gain in dB. the 12AT7 with a 22k plate resistor shows me 32dB gain, and then i raise it to 100k and it shows me 40dB. Is that how it works cause I think it should be going down with more resistance, not up. . . I think I should just leave that alone too

**MWJB** · 03-11-2011, 08:19 AM

Fenders have 100K resistors at the plate (56K in a cathodyne PI, but your amp has no PI), it's part of the essential voicing of the amp, along with the 1.5K cathode resistors. Any 12A#7 tube will work in that stage as it is, but none will give as much gain as a 12AX7. Forget the calculator...it is not taking account of plate voltage, or signal applied to the tube - both of these ultimately affect the voltage gain. I would retain the 100K plate resistors, reducing them will make the amp brighter & harsher, not grindier & smoother. I would adjust the cathode resistor as you have done at the second stage...possibly at the first stage too if you liked what the first one did...I would also adjust the b+ supply to the 12ax7 plate resistors, by altering the value of the preamp dropping resistor.

If you remove the grid to ground cap at the 6V6 and all is well, then fine...if any issues arise, just put it back. If you don't hear a difference, does it matter whether it is there or not?

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