Silverface Champs - help!(some newbie content)

[porge]

Hey guys,

I have a small stable of champs (2 silverface Champs, one silverface vibro champ) that I love and try to care for. Two have issues...both of them champs. I'm "voltage aware" from years in the IT industry and can solder, so am not totally ham-fisted, but am nonetheless a total newbie to a degree . I promise to be careful and not fry myself in the process.

This hasn't stopped me fixing the odd problem here and there as I go along.

First one works, but is really low on output - in rough terms, about 25% of standard when compared to the vibrochamp. Tubes are known good ones, all resistors are within spec, speaker is known good as well. Any hints or ideas?

Second one is a little more complex. Was working fine, went to install another speaker, it flickered on, it died. No pilot light. Fuse tests fine. Bulb is fine. Tubes are known good set. Unzipped the chassis from the cab and had a poke around. Voltage to the rectifier from PT is fine (about 390V without 5y3 in socket), votage to heaters is fine. Resistors are seem in spec - one that "leads" to the hot side of the speaker out reads low, but in spec when lifted...could this be the problem?

Any help appreciated


Andrew

[Enzo]

No, the resistor from the speaker hot is the feedback resistor, and it has connections at eaither end that form parallel current paths that confuse your ohm meter when it is soldered in place.

Just for science, swap the tubes from a "good" amp into the low out amp. Now take voltage readings at the tubes. Is there B+ voltage at both plate and screen grid on the power tube? What voltage is at cathode and control grid? Now the preamp tubes. What is on the plates and cathodes of them?

Apply some steady signal like from a generator to the input and scope stage by stage, looking for any unexpected diminution of the signal level.

And as always, be alert for cracked solder to controls, cracked controls, etc.

As to the dead one, if the heaters light, then the bulb should too. You said the heateer voltage was OK, but do the heaters light? No heaters, no sound. And it is all well and good that there is HV AC at the recto socket, but put the tube back in. Now is there B+ in the circuit?

[porge]

Thanks Enzo - will unzip both of them and report back later

Much appreciated

[bob p]

i don't know if this is helpful, but IME the most common problem with champ type amps that sound weaker than they should and/or function intermittently is a malfunctioning cathode resistor bypass cap on the power tube.

fender used a 25uF 25VDC cap there, and the working voltage is near the cap's limit, which shortens its service life. in addition they're typically mounted right up against the cathode bias resistor, which gets pretty hot, and the cap's service life is further de-rated. the old carbon comp resistors often get hot and drift way out of spec as a function of heat and age. the change in cathode resistance changes the bias of the output stage and performance changes accordingly.

assuming you have a scope: when you signal trace your way through the amp, if you find that the distortion is occurring in the output stage, check the cathode cap and resistor. chances are one or both is bad. when replacing parts i'll opt for a higher wattage rated metal film or cabon film resistor (less prone to drift than a carbon comp) and a cap with a higher working voltage rating and temp rating, like a 25uF 50 VDC cap that's rated at 105C.

of course, this may not apply at all to your situation -- its just my idea of what the "usual suspects" are for the champ.

hth.

[porge]

Thanks Bob,

I don't have a 'scope nor access to one, so I'll just replace the cap as a matter of course. Thanks for the tip - it's greatly appreciated, and makes me think I'll replace it in the other champs in my "stable"

[porge]

Problem solved (not by me)

OK - I ended up chasing this one too long, and figured I'd drop them into my amp tech. He lives two hours away, but is great.

The low power one - ended up being a poor ground connection in the circuit. Apparently was a pain in the a$$ to find.

The No go one was (shuffles feet) a combination of a blown globe and a somewhat scared owner. I figured continuity between poles on the filiament of the bulb meant OK, but didn't figure on this not being the case

I guess I just thought "I don't want to totally screw this amp by leaving it on....." Sorry guys - me culpa and all that. Will test with known good bulbs in the future!

[bob p]

a bad tube? Enzo wins again!

[porge]

nuh...

Sorry - not another Enzo victory - blown pilot light bulb = victory for false logic