Silverface Champ raspy/distorted output

[Doug]

Silverface champ with blown factory speaker. Replaced with weber 30watt ceramic. Sounded harsh, played around a few hours, 6v6 went bad. Lighting up shorting out at higher volume. Replaced tube. Checked the bias/output of the tube. Calculated output at 22watts! I could see where the speaker and tube would go bad. Increased cathode resistor until output was about 10 watts. Output was harsh, clipping was muddy and distorted. Adjusted down to 8 watts, better, but still bad.
Could this be an output transformer problem? I heard these tiny little things were weak, could it handle 22watts? It sounds marginal up until about 5, then really goes south. Any tips on troubleshooting?
Thanks
Doug

[bob p]

are you familar with the tube amp debugging page at GEO?

[stokes]

You say "output was 10 watts".Do you mean output or idle watts?You should choose a cathode resistor that will idle the 6V6 at about 10 to 14 watts.Sounds like you are idling that tube way too high.

[Enzo]

Remember the CHamp is a single ended amp, and it is running Class A. Bias is not set like a push pull AB amp.

The power dissipated in the tube is not what is dissipated in the transformer. The transformer is about transfering power from the primary to the secondary. The transformer will have a DC current rating, but that is different from its power output rating. The idle power didssipated in the tube is a matter of current flowing through it and the voltage difference between plate and cathode. That is NOT the power output of the amp.

[Doug]

You're right, I'm measuring at idle.

At idle it was initially at 22 watts, then 10 now 8.

[Enzo]

Point was that class A amps conduct full current all the time, so they bias out at a lot higher current than the typical push pull amp we usually see. It is supposed to be higher current.

Look up 6V6 in a tube manual such as RCA. In a class A amp, you will see in their example 315v on the plate and then this:
zero signal plate current 34ma
maximum signal plate current 35ma.

Looking at a Champ AA764 as an example, the plate voltage is 350, the cathode resistor is 470 ohms, and it runs with 19v on that cathode. 19v across 470 ohms is 40ma. That means a dissipation of about 13 watts. One watt shy of the book max.

By CBS era, the Champ was up to 420v on the plate, and 24v at the cathode across the same 470 ohms. That is 50ma, and with 396v across the tube (420 - 24 = 396), that is a dissipation of 19 watts. 5w over the book max. Not to mention the plate being 70v over the max there.


Keep in mind that Fender routinely exceeded specs on the 6V6s. The book values are for commercial applications like TV sets, table radios, and other things designed to sit in the home and w0rk for a long time. In guitar amps we are more interested ijn power output and tone than long life. We expect to replace tubes once a year. Dad was not interested in retubing his TV set once a year.

[bob p]

Dad was not interested in retubing his TV set once a year.

you can say that again! i remember that it was a MAJOR EVENT every time that we had to call the TV repair man!

[slidincharlie (Carlo P)]

Point was that class A amps conduct full current all the time, so they bias out at a lot higher current than the typical push pull amp we usually see. It is supposed to be higher current.

What Enzo said.
My SF VibroChamp used to run at around 14W at idle. When I was fiddling inside it and was posing questions about such high dissipation, I was adviced by forum guys to use a 25/50 (instead of 25/25) cathode bypass cap, and to mount it high on its legs and away from the bypass resistor.
That amp had just the type and amount of saturation that was expected, and worked fine.
Yours maybe has got a different problem - other guys here will help you to sort it out.

[Jag]

How high would most of us be comfortable biasing a 6V6 in an SE amp, not by the rule book but by real world experience.

I will confess that I do push my own SE amps up to around 16W at idle if I'm using JJ's or other new production 6V6's which can handle a little more power. I like the sound better and, as long as the tubes don't go ballistic, I don't mind changing out the tube after a year.

I always stay within the general guidelines when working on other peoples amps or using NOS tubes. However, most people who try myt personal amps wonder why they soudn better than theirs.

[Doug]

Plate voltage is 470v

Might this present a problem? I lifted one end of the feedback resistor to check it out. Clipping a little smoother and sooner, but still lousy.
The initial specs were, 470v plate, 24v at cathode across 470ohm. So idling about 22watts? I had heard that idling at around 14watts was alright, but I just kept dropping the wattage looking for a better sound. I'll check out the troubleshooting site.
Thanks for all the replies.

[Bruce / Mission Amps]

Might this present a problem? I lifted one end of the feedback resistor to check it out. Clipping a little smoother and sooner, but still lousy.
The initial specs were, 470v plate, 24v at cathode across 470ohm. So idling about 22watts? I had heard that idling at around 14watts was alright, but I just kept dropping the wattage looking for a better sound. I'll check out the troubleshooting site.
Thanks for all the replies.

Just this week I refurbished a SF Champ.
I had to use three 12v 5 watt zeners in series with the B+ center tap to ground in order to get the absurdly high B+ down to the point that the power tube was idling at just over 15 watts!!
I also replaced the 1K screen node resistor with a 2k7 2 watter to keep the screen voltage closer to where it should be.

[Jag]

Lifting the feedback resistor won't cause a problem.

22W is too high. Try replacing the 470 ohm cathode resistor with something in the 800 range to get between 12 and 14ish watts.

Is the amp otherwise stock? Have you tried a new 12AX7?

Does this amp sound raspy in relation to other Champs? How does the speaker sound being driven by a different amp. How does the Champ sound driving a different speaker?

[Bruce / Mission Amps]

Lifting the feedback resistor won't cause a problem.

22W is too high. Try replacing the 470 ohm cathode resistor with something in the 800 range to get between 12 and 14ish watts.

Yes 22w is too high but, no, I don't think you should use a larger resistance in the cathode.... the problem is quite simply, the B+ plate and screen voltage is too high... if you keep rebiasing the class A power tube amp to cool it down to 12-14 watts, it will be very easy to drive the grid hard enough to shut the tube off into class B or into saturation.
Get the power tube's plate and screen B+ down with what ever method suits you. I use 5 watt Zener diodes since they are cheap.

[Doug]

Speaker comparisons

I did try a different speaker, not better. Don't have another champ to compare with. Have replaced 6v6 and 12ax7, no improvement. I'm going to go back and check every connection and voltage and get back to you if I'm still stumped.

[Liam]

When you say replaced 6V6, what did you replace it with? I've had bad experiences with trying to run "cheap and nasty" tubes in old cathode biased amps.

Oh yes, what Bruce said, although hiking up cathode resistor value has yielded good results for me too...