Help with High Voltage in Blackface Vibro Champ

Josey, all I know is what I have read. There is a design maximum rating and there is an absolute max rating. Most tube cognoscenti consider 6v6’s from the late ‘50’s on to be 14 watt tubes.
At any rate, it takes something way beyond 14 watts of plate dissipation to redplate one of them in a cathode biased circuit, ime.....way beyond.
Fwiw, in a cathode biased amp, the tube is working its hardest at idle. When signal is applied to the control grid, the plate dissipation figure decreases. It is just the opposite in fixed biasing.....applied signal increases the current draw above the idling situation.
Again....I have never seen a Fender Champ type amp that was operating as low 100% of MPD. It is always more..usually above 16 watts. The tube sets that biasing...unless one changes the resistor. Only in the last couple of years have I seen mention of people running higher resistances to try to get that dissipation down. I already know what that sounds like because I did the experiment....out of curiosity when I observed my SF Champ sound better than any .champ I had ever heard...including the BF Champ my poor buddy had beside it. I measured the two amps. His amp, which sounded very good but not as lively as mine, was dissipating right at 18 watts. My Sf was about 20 watts. As I noted....it had been running that way for about forty years and was going strong the last time I talked to the young fellow who received it as an engagement present from his fiancé.
Based on what I heard from that SF .champ when I put an 820 ohm bias resistor in it and read 14.7 watts of plate dissipation, I would hazard a guess that IFLeo Fender put the Champ out with such biasing, we would all be playing little solid state amps and saving our money by not needing a higher dollar tube amp. (;^)
Leo’s little Champ used that 470ohm resistor from the late ‘40’s until the end of the production in 1982....and surely the reissues use the same resistance???? That said, if a person wants a little tube amp that sounds like a sterile solid state Fender Frontman, neuter the little amp with a big bias resistance number. For my money, I want a Champ!!!! Or actually, I want my 1964 Rickenbacker B9A...solid copper chassis and an OEM Jensen C12Q!!! Single 6V6 and trem....best amp of its type I have owned. A C12Q ina 5 watt amp. Leo didn’t even use a Q in the 25 watt DR. Rickenbacker put some money into those amps.

 

Duh, I wasn't even thinking about the 5y3 tube... I was just thinking of changing rectifier tubes to drop the B+.

I wouldn't run a 5u4 either Wally. But, as you describe on solid state rectifiers, a copper cap doesn't draw the current like a rectifier tube. The nice thing about the CC rectifiers, is they are designed to react like a tube rectifier... A little sag here and there. As you know, Fender used something similar in it's first reissue Bassman and built it into the circuit of the Vibroverb you and I have had so many conversations about.

 

Axis, I don't see anything except solid state rectification in those two amps. Where is the 'emulation' of a tube rectifier?

 

I had a first year reissue bassman which had a sst rectifier which plugged into the tube rectifier socket. I don't think it had anything other than diodes in it. If you wanted sag you could plug in a tube rectifier. I think the Weber copper caps have other components beside diodes to simulate sag. I never had the Vibroverb RI but I do have the CVR first year in white which is based on the VVRI. One of these days I will do the Moyer mods and either install a Weber CC or replace the transformer and install a tube rectifier. But I don't think stock it has any components for sag either unless I am missing something here.

 

@Wally - I probably fell for the marketing hype! LOL I was under the impression they had done more than just SS rectification in the VVRI. Not the first time I was wrong. The Bassman has a tube socket with a plug in SS rectifier (I so wish they had done this with the VVRI as well). I also thought it had more than just a few diodes. Oh well....

@tweedman2001 - The CC's need a tube socket, which neither of our amps (VVRI and CVR) have. So, a Copper Cap won't help.

 

I don’t know, Axis, what they did other than I see diodes and no resistors in the rectifier positions when I look at the schematics. However, if there is a socket with a SS plug in rectifier, one can run any solid state emulator that one wants as long as voltages and bias are understood. One could run the GZ34 emulator..?WZ34?...or a 5U4 or even a 5R4 emulator...whatever the designation for those might be. The GZ34 would drop voltage from the ‘straight’ solid state and a 5u4 emulator would drop even more voltage.

 

@tweedman2001 - The CC's need a tube socket, which neither of our amps (VVRI and CVR) have. So, a Copper Cap won't help.[/QUOTE]

There is a copper cap module that does not require a socket. Also it would be easy to install a socket without changing the transformer as the cc tube version does not require the 5 volt filament wiring. Of course you would not be able to install a real tube rectifier without changing the transformer too.

https://www.tedweber.com/wbr

 

Leo knew what he was doing, didn't he???
RE: out of spec resistors..... I would replace any and all resistors that were seriously out of the range that was spec'd. One would want to make sure that one was reading the resistance of just one particular resistor. That might necessitate lifting one end form the circuit. That 2700ohm resistor is the negative feedback loop resistance. IF it really is only a 50 ohm resistor now, it would cause the amp to be loosing a fair amount of the signal due to phase cancellation. You may be reading something other than just that resistor if you are reading it in circuit. A 2 watt bias resistor should suffice.
IF you still have the original electrolytics on the board, imho those need to be replaced. I have found that fresh e-caps plus good general service solve a lot of problems.

 

On the resistor, I've never seen a resistor drift low - they always go high, so there is likely something else going on with that too.

 

At this point, a full voltage chart would be usefull.

Voltages at each pin for all three tubes.

 

1700 vs 1500 on a cathode resistor wouldn't bother me at all unless you really wanted to replace it.

See if you can scoot over the bypass cap away from that power resistor a bit. That sucker will generate heat and many Champs have cooked bypass capacitors because of that.

No need to bother going with the 2W - that 5W you put in will last forever.

From a quick visual inspection everything looks good.

As far as the resistor, look at it in the circuit. That negative feedback resistor goes across the entire output stage, so there is going to be other resistances in play. Sometimes those just don't measure what you expect in circuit.

 

Voltages look good


Thanks

 

Thanks for the pics. Love the vintage caps and resistors -- looks good.

You asked above if there's a resistor type for the power resistor that 'is favored.' Although I love CC resistors for their authenticity in this simple circuit, 'authenticity' will include several downsides in the power resistor. I agree the wirewound resistor is just about ideal; they're quiet, stable, and extremely robust. I have CC dropping resistors in mine -- but I don't have a CC power resistor. If you can move its bypass cap further right, as Corliss1 suggested, your amp oughta be about perfect.

BTW, I'm glad you went with Wally's advice and went back to stock on that 470 (instead of a 1M?). Increasing the 6V6 cathode resistance in cathode bias amps is a popular way to try to cool bias, but it has the unintended consequence of actually increasing effective B+ -- especially not a good thing in a VibroChamp. Your description of the sonic changes you got is great.

 

You'd have to lift the caps from ground in order to take that measurement. Anything that small wouldn't worry me at all.