Vibro Champ Revival

[ampgeek]

Started subject on this thread but thought it better to move it over here for posterity.

https://tubeamparchive.com/viewtopic.php?t=23304

I wired up a loose 10" speaker after replacing the recto and power tube sockets, slapped in a 5AR4 and a new JJ6V6S and played it for a few minutes. Not bad at all! Definitely a keeper for the owner IMHO.

It had a glorious vibrato (tremolo actually?) effect for a few minutes but that shortly (after 10'ish minutes of play) faded away to just barely audible at all control settings unfortunately. Replaced the 12AX7 with known good tube to no avail.

I jotted down the voltages on the attached for consideration. I probably don't want to run it for very long given the excess plate dissipation. Hoping that a 5Y3 rectifier with its greater voltage drop will solve that issue.

Seeking advice on the following from our Fender afficianados:

Suggestions on what 8" replacement speaker would mate best with this circuit.

Where to start on resovling the low vibrato effect issue. This is a new circuit for me and I am coming up to speed on how it works. Any advice/guidance/experience is appreciated!

I am measuring ~25 mV DC up on the volume pot. The pot is only scratchy at the mid point of rotation. Thinking that only a good cleaning (with Caig Fader Lube) is needed. Would that measurement be considered excessive and warrant replacing caps? I hear no scratching on the tone knobs.

TIA,
Dave O.

[martin manning]

Got any DC voltage at the junction of the 0.02, 0.01, and speed pot? Hitting those caps with a little extra voltage may have started them leaking. It's a common failure scenario for this type of oscillator regardless, so three new 1kV caps would be a good place to start. Re the scratchy volume, it'd be worth swapping the 250p treble cap.

[ampgeek]

Thank you sir!

I should have seen sooner that you had provided the same sage advice in a similar thread ("AB763 vibrato issue") below. Nonetheless....I truly appreciate the re-guidance!

Yes indeed....~1VDC at that location!

My VC example is using a blue molded cap at the 0.02 uf location. Any tonal or performance benefits or hazards in ceramic or other replacement cap types there? I wouldn't think that tone, per-se, would be in the equation given the function of this part of the circuit. But...who knows what Leo and his gang might have observed/learned along the way!

Sincerely,
Dave O.

[martin manning]

The vast majority of Fenders have ceramics in those three locations, so I expect there was just a shortage of 0.02 ceramics the day that amp was made. Put any 0.02 you have in there with 600V rating and see if it comes back.

[echuta13]

I've seen that a couple time where the .02 was a film and the .01's were ceramic... could have been a cost issue as well. Not sure... Fender was the the Ford of amps after all.

[ampgeek]

Thanks guys!

Awaiting the delivery of a few parts to put this one to bed. Not the least of which is a new WGS 8" speaker.

I will report back on the performance of the finished product when complete.

Cheers,
Dave O.

[ampgeek]

Finally got the WGS 8" speaker yesterday and finished this one up.

Wow! I have never played a Champ circuit before and am very impressed with it's tone and feel. No wonder it can make for a VERY satisfying first build!!

Sparkly clean up to ~5 on the vol and onward into some nice bluesy grind up to the end of the scale. It is actually very useable at max volume which surprises me for some reason. I suspect that it will get even better as the speaker breaks in. Too bad that will occur in the owners hands.

The vibrato is killer as well. Very rich and full with a tremendous range of both intensity and speed.

Thanks as always for the help and guidance on this one guys!!

Sincerely,
Dave O.

[xtian]

Did replacing that .02 cap fix the vibrato?

[ampgeek]

The oscillator tube pin 3 voltage went from 2.1 VDC to 0.3 VDC right after replacing all three of the high voltage capacitors with 1 film and 2 ceramic. The vibrato didn't work at that point.

I suspected that the 25 uf cathode bypass cap was leaking DC so I replaced that as well.

The cathode voltage went back up to 2.1 VDC and the vibrato effect came back in spades!

Although I can't see how replacing the HV caps caused a LV cathode bypass cap failure, I have to believe that replacing the HV devices is what ultimately solved the problem.

I have to admit that I didn't go back and measure for DC around the pots though.

Cheers,
Dave O.