Vibro Champ recap job-questions on cathode bypass

[mfreqmaster]

Hello all,

I scored a '66 Vibro Champ yesterday, it appears to be all original. The unit is in non working condition. I figured while troubleshooting & have the chassis open, I would go ahead & replace the filter caps since the cap can appears to be original & 45+yrs old. I read an earlier thread where Enzo recommends leaving the original cap can in there for posterity's sake & just fitting 3 20uF caps to the 3 power supply points, tying the negative leads together & attaching them to the ground lug with the TF centertap ground. I like this idea since it involves the least amount of intrusion into this old chassis. My question is, what about the 4 cathode bypass caps? Are these generally replaced as well? My prior experience involves only building stuff from the ground up & everything was new. Thanks.

[bob p]

Congrats on the new amp!

my experience with DOA champs is that Ck on the power tube is almost always bad. the heat from Rk shortens the life of Ck. It's a common failure mode. Be sure to check that.

The carbon comp resistors that take a lot of heat always seem to drift upward. While you have the amp open I'd measure all the resistors and replace anything that's out of tolerance. The 6V6 cathode resistor and the Pi resistor are prone to drift upwards.

The cathode caps on the preamp tubes are cheap and easy to replace. I can't think of any good arguments to keep them. Your time is more valuable than the caps so you might as well replace them while you're servicing the amp. Regarding the PSU caps: I'm not sure I'd replace the cap can unless I knew that the caps were malfunctioning. I'm still using the original can on my SFVC. I tested the caps for capacitance, ESR & frequency response and they remained in-spec. The supply rail had good B+ without improper amounts of ripple so I didn't replace the can. Kind of hard to believe, I know, but there are some old amps out there with PSU caps that are still good. Of course, most of those amps don't arrive as DOA.

[mfreqmaster]

Thanks for the reply. I'll be sure to check all resistors & the 6V6 cathode cap. I may try to see how it runs with the original cap can left installed.

[Enzo]

Yes. You can always go back later and install individual caps if they are needed, but if it works it works.

[mfreqmaster]

All it took to get the amp working again were 2 capacitors, the 6v6 cathode cap (bob P you were on the money on that one), and v1's pin 8 cathode cap. I left the cap can where it was. Looks like I'll be staying up late playing tonight.........

[bob p]

Congrats on your success! Have fun with your new amp.