Just biased my Silverface Champ...posting results; please post yours

[ron vogel]

First off, a little backround on this amp.

About 2 years back a local craigslist ad had a '72 Champ for $200...I jumped on it.

Got there, and the amp looked like it was under a porch for the last 30 years easy...it worked, but had issues. I offered $150, and walked away with it for $175.

Ordered all the parts, good caps, carbon comp resistors, cloth wire, quality turret board, Weber alnico, put in a new OT even though the orginal worked. Built the amp, through in a 1959 RCA 12ax7, 1960 RCA VT107 (JAN 6V6), and a mid 60's NOS RCA 5Y3GT. Sounded phenominal. I restored the cab and hardware, and have enjoyed the amp ever since. I moved last June, and ever since the amp has sounded "off". I use it for recording, and has produced some killer tones...but I've struggled with recording it since I moved.

Starting with stock values I upped the cathode resistor to 640ohms from 470ohms...plate voltage was 420v, 12ax7 was seeing 285v on one side, 255v on the other.

Added resistors in series one by one to the first power drop until I got it down to 390v at the plate, added a resistor between the screen and the OT to drop the screen voltage just below the cathode voltage. Stock value for the 1st power drop resistor is 1K -1W...after I got done adding resistors: 6K!

So now it's idiling at .039 (which sounds high...but that's what it seemed to like. I was also able to better balance out the 12AX7 by changing resistor values on one side...once I got voltage down on v+ it settled at 259/253...it's not getting any better than that! It also aided in getting rid of the brightness that was over the top on my humbucker guitar.
Came out to: .0398 x (390-25.5)= 14.5071 watts

We watched my sister in law's newborns last night so they could go to a wedding...couldn't test the amp at volume until this afternoon (and BTW I don't miss having babies at night)!

Cranked it up this afternoon for a stress test; amp started smelling funky. Immediately turned it off and checked for hot spots...found nothing. Mostly worried about the power tranny because it's original, the OT is good, but in a box to bring the amp back to stock if I sell it someday.

I fooled around with the resistors on the 1st v drop again, and upped the bypass resistor value a touch.

Came up with 27V across the bypass at 743 ohms.

Biased: .036 x (402-27) = 13.5 watts

The extra voltage brightened up the amp, brought up the voltage of the 12ax7 to 265. With the brightness, it brought out the fizziness. Took the 2nd voltage drop (10K) and put in jumper resistors until I got the 12ax7 to 280 volts...fizziness gone.

Amp was still pretty bright, and got a bit brighter when I cleaned up the gain stage...the 12ax7 and the 6V6 both are balanced within 1 volt.

I removed the 15K resistor from the bass pot to ground, put in a 8K resistor to a 10K pot to ground to create a mid control. Helped a bunch to clean up the brighness and the amp is real smooth now. Still a little crisp in the treble department, but I think I can change cap values in the tone circuit to play with the Q.



I've learned more about this amp in the past few days than I had in the 2 years I've owned it. Which is good, because I'm going to build an ultralinear version of the princeton reverb soon...and the experience is really helping my confidence about it!



I have to stress the importance of being safe while you are doing this though.

I got zapped in the process...and I'm fairly seasoned messing with tube amps.

I was taking off my guitar and reached over to turn off the amp (which is just the chassis on my bench). Got zapped BIG TIME...the guitar I was holding literally flew out of my hands...and slammed down on a tile floor. Fortunately I was OK other than the pins and needles feeling in my forearms...and somehow my guitar wasn't damaged other than a scuff on the headstock.


I would really appreciate if anyone would share their bias challenges and outcomes on the Champ circuit. While I was doing this, I found enough info out there on it, but it was very scattered; which took a long time to sort out and apply. Be nice for future reference if this thread can help someone.

[soundguruman]

Welp, you did a nice job building it.
One thing you might try is a full wave bridge rectifier instead of the half wave for the bias voltage. Of course it still needs to be adjusted correctly, with a voltage divider, to eliminate the crossover distortion.
BUT the full wave rectifier turned out to sound better, I think quite a bit better.

[Chuck H]

Some of the wording and terminology in your post is hard to follow. That said, it sounds like your chasing tonal issues by just trying things until your convinced it sounds right. FWIW here's what I would do. I would put the amp back the way it originally was and, assuming the move was responsible for the tonal change due to slightly higher wall voltage, bring the voltage back down with a zener diode or a small amount of additional resistance in front of the rectifier circuit. That would essentially put the amp just exactly where it was when you loved it and much closer to stock.

Nice looking work.

[Gtr_tech]

Welp, you did a nice job building it.
One thing you might try is a full wave bridge rectifier instead of the half wave for the bias voltage. Of course it still needs to be adjusted correctly, with a voltage divider, to eliminate the crossover distortion.
BUT the full wave rectifier turned out to sound better, I think quite a bit better.

What? D00d....its a Champ....there is no bias supply.

[Gtr_tech]

You should move the cathode bypass cap away from the K resistor....the heat will kill it.

[ron vogel]

Some of the wording and terminology in your post is hard to follow. That said, it sounds like your chasing tonal issues by just trying things until your convinced it sounds right. FWIW here's what I would do. I would put the amp back the way it originally was and, assuming the move was responsible for the tonal change due to slightly higher wall voltage, bring the voltage back down with a zener diode or a small amount of additional resistance in front of the rectifier circuit. That would essentially put the amp just exactly where it was when you loved it and much closer to stock.

Nice looking work.

My terminology is going to be a problem because I'm self-taught on just about everything!

However, I am very familiar with my room and equipment so placebo effect is at a minimum. The amp sounds better now than it ever did, so no going back for me!

[ron vogel]

You should move the cathode bypass cap away from the K resistor....the heat will kill it.

Old pic...already done!

[Chuck H]

The amp sounds better now than it ever did, so no going back for me!

Uh oh... He's got the bug... ron vogel has the bug.

The amp looks great ron. So, what's your next project

[Chuck H]

What? D00d....its a Champ....there is no bias supply.

If your on the hunt, how did you miss correcting this one?:

Of course it still needs to be adjusted correctly, with a voltage divider, to eliminate the crossover distortion.

Single ended crossover distortion is a very complex issue. We do have a resident rocket scientist that could probably explain it though I doubt he would do us the honor

[olddawg]

If you are finicky about tonal differences in amps do to variations in line voltage in different locations you might want to invest in a variac. Might save a lot of time and effort modding circuits and possibly your PT.

[Gtr_tech]

If your on the hunt, how did you miss correcting this one?:



Single ended crossover distortion is a very complex issue. We do have a resident rocket scientist that could probably explain it though I doubt he would do us the honor

Well I figger'd stating that there was no bias supply would take care of the need to adjust what ain't there....