Bulletproofing a Vibro Champ

[thumbs]

I've been working on a SF Vibro Champ for a friend who plays harmonica. So far, I've completed Gerald Weber's "tweedifying" mods and added a 5F2-A style tone control so he can roll off some of the highs. IMO the amp is really sounding good with my guitar through it. I've played through it at all levels and can't get anything to fail. My friend, however, has gone through a couple of 6V6's recently, playing his harp through it (a much hotter signal).
Looking through the archives, I found a thread where a guy damaged his Champ playing through a distortion pedal with the amp on "10". Among other things, MWJB recommended changing the 470r bias resistor on the 6V6 to an 800r 5 watt to reduce current draw. Does anyone have any other ideas? I don't want the amp to be eating tubes when I give it back to the owner.

Thanks,

thumbs

[captntasty]

What kind of tubes have been used in there? New, NOS?

[thumbs]

The ones that went bad were a Groove Tubes 6V6 and an EH 6V6. I've got a jj in it now.

[captntasty]

Have you measured the current? If something is off there it could indicate a problem elsewhere. Putting in a higher value resistor will certainly help tubes run "colder" thus prolonging life, but it will change the amp's character - it would be "cleaner" - that or even use say a 200R resistor and after that a 2W pot wired as a variable resistor for adjustable bias...

Another thought is to use NOS tubes - not even anything that great... I've gotten in the habit of using mostly military tubes - for a 6V6 I go with a VT107A - they're nice sounding and ruggedized. The ultimate nearly indestructable tube would be the Bendix 5992 - expensive but built to an absolutely ridiculous standard... built for ballistic missiles. I have a few sets and they are insanely good. Or the industrial 7408 although they are hard to come by.

Lastly, because you have no standby the power tubes get hit with full voltage before having a chance to warm up. I use a Bendix 6106 - another insanely engineered tube is a pin for pin replacement for a 5Y3. It is indirectly heated so it has a slower warmup time and consequently so do the output tubes. These are actually not horribly expensive and will probably outlive your client. Another indirectly heated rectifier is the 5V4 (5V4GA is straight sided) but you might have to tweek some components as it doesn't drop the voltage as much as a 5Y3.

[thumbs]

Captntasty, thanks for the replies. You're right; there could be a lot of things going on with it. I kind of wonder about the speaker. It looks to be the original and he's hitting it pretty hard. He's coming by tomorrow to blow through it. Guess we'll see how it holds up. thanks again,

thumbs

[d95err]

What about screen resistors. The Vibro Champ (and many other vintage amps) have no screen resistors. From what I hear, a screen resistor can do wonders when it comes to improving tube life.

I don't know if it applies to this particular amp, tube and voltage levels though.