Vibro Champ capacitor questions

[Vito]

Hi stokes:

Sorry for the delay in replying. The forum is not sending notifications of replies, so I didn’t know you had posted. But I was in no condition to reply anyway. (Got slammed by a kidney stone...if you've ever had one, I need say no more. If you've never had one, consider yourself among the very fortunate.)

Anyhow, my responses are inserted below:

The schem calls for a 2 watt cathode resistor on that 6V6.It is adequate,but I always prefer 5 watt for added safety margin and heat dissipation.

Several folks have recommended a 5W cathode resistor, as well as a beefier 50V cap for the 25µF cathode capacitor. I would have installed those mods had the original voltages in the amp remained the same, but the RC network was successful in knocking the voltages down to near the levels shown on the schematic, so I'm inclined to leave those new components alone now. The cathode voltage is 21.8V , which is within the rating of the 25µF, 25V cap (a Sprague Atom electrolytic), and the 470Ω, 2W cathode resistor barely gets warm to the touch.

Actually, that really surprised me, as the old cathode resistor ran very hot...so much so that the color-coding bands were discolored. More tellingly, there was obvious heat damage on the old (original) cathode capacitor. As you can see in the image linked in my last post, I've separated the cathode cap and resistor to prevent that kind of heat damage...but with the new resistor not getting hot I'm not concerned about it. I'm sure that's a result of the lower voltages that are now present throughout the amp.

I am trying to wrap my head around this R/C network you are describing.What it seems to me is that you added a series resistor of 500ohms between the rect and the first cap,and then lowered the resistor value to the screen tap to 400 ohms,from the stock 1K?Do I have that right?

Not exactly. The key to understanding it is in realizing that I used a 20/20/20/20µF, 475VDC four-section can cap. In the original wiring (which also used a 20/20/20/20µF four-section can cap), the first two sections were wired in parallel to make a 40µF cap for the first stage of the power supply (connecting to the 6V6 plate through the output transformer primary).

What I did was to use the first two sections of the can cap as a two-stage RC filter. Here's a thumbnail link to the schematic:




As you can see, I didn’t mess with the 1KΩ resistor, or anything else in the original design. I simply broke up the 20µF+20µF first-stage filter cap into two parts, and used those parts to create a two-stage RC filter. Not only did it kill the annoying 120Hz hum, but it knocked the voltages down to pretty much the same levels as in the schematic, with the result that everything sounds much better, and the amp is running much cooler. (Plate dissipation is ~13.5W...just below spec for the JJ Tubes 6V6-S power tube running in pentode mode.)

Changing the ac cord config as you describe with the switch and fuse isnt going to quiet anything,it is a prudent safety thing,tho.

Right…that’s exactly why I did it. Now with the power switch off, the only exposed thing that’s hot inside the amp with the power switch off are the terminals on the fuseholder. I think I can remember not to touch those if I have the chassis on the bench and plugged in.

I cant see in the picture,but is your filament string a single wire arrangement,with one heater pin connected to ground,or is it the twisted pair?If the single wire arrangement,change it to the twisted pair,that alone will quiet things then try my suggestion with the heater ground reference connected to pin 8 of your power tube and you should see a lot of that hiss,etc when you raise the volume disappear.

It’s the single green wire arrangement, which uses chassis ground as part of the heater circuit. The hiss I mentioned isn’t excessive, actually. It’s about what I would expect with an amp cranked up to 10. If I were actually playing at that level, the signal-to-noise ratio would be so high that the hiss wouldn’t be noticeable.

Anyhow, I do like the idea of running dedicated wiring for the heater circuit. Next time I get some time to play with it, I’ll put the amp back on the bench and float those heaters above chassis ground. But right now, I’m just enjoying the spectacular sparkle and twang of my Vibro Champ, which sounds better than it has ever sounded, is running much cooler (temperature-wise), and the 120Hz hum is utterly vanquished.

Thanks again for your help and comments, amigo.

[telenut62]

Congrats on the maintenance of your amp, You'll prob find some interest in this...

Service Diary: Silver Face Fender Champ Fun! - Telecaster Guitar Forum

[stokes]

Sorry to hear about the stone,Vito.Never had one myself,but I've seen many that had,not fun,hope you're feeling better.Bottom line is you have the amp performing where you want it and it sounds good,operating safely.Changing that heater supply to a twisted pair is always a good idea,but if its quiet enough for you,theres no rush,it is still safe the way it is.Glad you got it sorted out.

[clyde1]

+1
But I would suggest sticking with a modern production 6V6. NOS types can't take the high voltages but the newer ones hold up fine IMHE. I blew up a NOS 6V6 (RCA) on the last one I worked on while putting it through the crank and wail test. 423Vp! It just couldn't take it. The EH I put in held up fine and sounded good.

Had a Silverface Champ here last week with 494v on the plate Chuck!

[Chuck H]

Had a Silverface Champ here last week with 494v on the plate Chuck!

That seems too high for a properly operating unit. I would speculate a replaced PT, shorted turn or two on the PT primary winding or the bias was way too cold causing a lack of current.