Just biased my SF Champ...FYI for comparing numbers

[Ron Vogel]

Ever since I moved my little champ has been "off". I chalked it up to wall voltage, and this 6v6 was HOT!

Anyhoo...I rebuilt it with all new components and turret board last year...put in some good tubes ;1959 RCA 12ax7, 1960 RCA VT107 (6V6 JAN). Sounded great....went down the shitter once I moved though.

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 .039Ma and should have a dissapation of 14.2W (which sounds high...but that's what champ's seem 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.




This was my first attempt to bias a champ, took the better part of an afternoon for me mostly because I had to run out twice for more resistors. Now I just have to order the exact values I need to clean up my mess. Amp sounds great again.


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.

[AndyFromDenver]

Glad all were unscathed.
Ouch!

[Ron Vogel]

Originally Posted by AndyFromDenver

Glad all were unscathed.
Ouch!

And here I thought the feild coil amp I got zapped with 3 years ago hurt at 278 volts...400+ was much stronger!

I got 5000v to the face when I was 16 (virtually no ampage) and I don't even remeber that feeling as powerful (was a PS to a medical laser)

[idylldon]

Originally Posted by Ron Vogel

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 .039Ma and should have a dissapation of 14.2W (which sounds high...but that's what champ's seem to like.

I'm assuming you meant dropping the screen voltage lower than the plate voltage since the cathode should be right around 19-24 volts.

It's actually idling at 39mA, which is .039 amps, right?

Dissipation is .039 * 390 (plate voltage) = 15.21 watts.

That's way too hot for a 6V6, which has a maximum rating of 14 watts in Class A. My guess is that if you power this thing up and move it into a dark room you will start seeing some reddish glow on the plates and your NOS tube will self destruct at some point. It might hang in there because Fender always pushed their designs, especially the ones with 6V6s, well past published specs., but since the tubes were of high quality they got away with it most of the time. I think where this tube is idling is past even what Fender would have done way back when.

I would put it back to stock and find out just where your AC line voltage is. If it's high, and chances are it will be since AC line voltages have been creeping up over the years since that amp was built, you can either get a variac to lower it or make one of these:

Vintage Voltage Adapter

What is the voltage on the heaters?

Cheers,
--
Don

[Ron Vogel]

Originally Posted by idylldon

I'm assuming you meant dropping the screen voltage lower than the plate voltage since the cathode should be right around 19-24 volts.

It's actually idling at 39mA, which is .039 amps, right?

Dissipation is .039 * 390 (plate voltage) = 15.21 watts.

That's way too hot for a 6V6, which has a maximum rating of 14 watts in Class A. My guess is that if you power this thing up and move it into a dark room you will start seeing some reddish glow on the plates and your NOS tube will self destruct at some point. It might hang in there because Fender always pushed their designs, especially the ones with 6V6s, well past published specs., but since the tubes were of high quality they got away with it most of the time. I think where this tube is idling is past even what Fender would have done way back when.

I would put it back to stock and find out just where your AC line voltage is. If it's high, and chances are it will be since AC line voltages have been creeping up over the years since that amp was built, you can either get a variac to lower it or make one of these:

Vintage Voltage Adapter

What is the voltage on the heaters?

Cheers,
--
Don

For a Champ that's normal...That's why I put a VT 107 in it! Voltage on the heaters is up there, not yet to 7 volts (which is the magic number to start modding that part).

Stock it was running MUCH hotter than that. Typical stock Champs run 18-19 watts typically. My first measurement showed 31volts through the 470 ohm cathode resistor @420+ volts!...no way it's going back to where I started.

Amp sounds very full and balanced now, I'm not concerned as much for the tubes as I am for the PT.


This conversation is EXACTLY why I posted this thread too. I am familiar with biasing the push-pull amps, but never a Class A Champ. There is plenty of info out there on the subject, but nothing with a concrete procedure on what to set it at. I read up on both camps to decide to run it with a hotter bias. The amp didn't start to wake up until I got in the high .03's, and many people bias them in the .042-.045 range.


I should also mention that it is VERY tube dependant, I tried the orginal good RCA 6V6, my VT-107, and a couple of newer Sovteks. I have a VERY strong RCA 5Y3GT in there, I suspect swapping around the recto would also be wise to get you in the ballpark quick, I just didn't have any spares here to try. I did a test on several different rectos in my Princeton a while back and found a pretty wide range of voltages:

(excerpt)

"I baselined the 6V6 tube match with the Copper weber since it is solid state. I got 408volts with .024 on tube 1, and .0237 on tube 2 (like I said, close match!)

Here are the results:
Tube / Voltage / Bias at 9.8 watts
GZ34 - 412 - .0237
Weber WU4GB - 408 - .024
5V4GB - 403 - .0243
PW 5U4GB - 399 - .0245
EH 5u4GB - 396 - .0247

I have looked up references on the web, showing much more drastic voltage losses for some of these tubes. I do see that the 5U4GB's are current hungry and really have earlier breakup and loss of bass at higher volumes.

As for the winner in my combo? No contest; the Amprex GZ34...it was the cleanest, most bass, best sounding distortion.
Second place...5V4GB
Third....Weber
Fourth...the Phillips 5U4GB (this was a close to a tie with the Weber)
Fifth was the new EH 5U4GB, wow did this thing make the amp sound like crap!

This wasn't the most scientific test, but at least I was able to hear quite a difference with such a subtle difference in voltage."

[idylldon]

Glad you're happy with the mods and I hope it works out in the long run! As long as it sounds good, it is good, IMHO.

Cheers,
--
Don

[Ron Vogel]

Originally Posted by idylldon

Glad you're happy with the mods and I hope it works out in the long run! As long as it sounds good, it is good, IMHO.

Cheers,
--
Don

Ehh, not so happy now...

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!

[idylldon]

Glad to hear you got it sorted out!

You've peaked my interest in my '65 Vibro Champ so I think I'll pull it out later this week and see where it's biased.

Cheers,
--
Don