Need help diagnosing problem with silverface champ

[ron vogel]

I built up a Champ with new everything based off the blackface schematic. The only thing I kept were orginal trannies.

I worked great for about 8 months or so, then started cutting out with power chords hit hard. Turning down the volume to about 8 or so would fix it, and this went on for a few weeks. I used the amp for an evening recording a solo...next day tried it out and no sound. I tried swapping all the tubes, and it makes sound, but the volume was low. Ordered a new OT for it, got it in, and the same low volume...but when you turn it off the volume swells to normal as it drains the caps.

I suspect something with the rectifier, because I'm pretty sure the retifier tube got fried. Replacing it helped for awhile. I also have several speakers at my disposal, and tried that too.

Any help pointing me in the right direction would be appreciated!

[Jazz P Bass]

How about starting by checking all of the static dc voltages.
12AX7:
Plate pins 1 & 6.
Cathode pins 3 & 8
Heaters : pin 4 & 9 (Vac)
6V6
Plate pin 3
Grid pin 5
Heaters pin 2 & 7.

[ron vogel]

Actually, already ahead of you:
12ax7 .1 ac pin 1, 2.1V pin 8
6V6 pin 1 0v, pin2 6.96ac, pin 3 400v, pin 4 400V, pin 5 ov, pin 6 ov, pin 7 0v, pin 8 13v

But during checking I heard a little crackle...then the amp worked fine...low hum, good tone..weird. I killed the orgininal RCA 6V6 during this debockle, but the Sovtek I replaced it with sounds just fine. Got to do some tube shopping now!

I planned on selling this amp and am replacing it with a hybrid princeton of sorts with an ultralinear output, and a rowdy front end.

Love the Champ to bits, but can only afford one amp right now... Wanted to make sure this was top notch before listing it.

[MWJB]

Pins 1 & 6 of the 12AX7 should have 200vdc+ on them. What dc do you get at pin 3?

13vdc at pin 8 of the 6v6 is way off...should be twice that, triple check resistor value, should be 470 ohms, change the bypass cap, use a 100v cap mount it away from the resistor, double check polarity.

[ron vogel]

I measured it before the crackle I heard...so it probably is off, since it was doing the low volume thing. I didn't remeasure it after it started working fine again. It's probably just a bad solder joint, so I'm going to resolder a few things and measure again when I get some new tubes. I have some old JAN RCA's on the way going in it.

[ThermionicScott]

You could try replacing the 6V6 cathode capacitor again -- I think a bum cathode capacitor might be costing you volume. This is a total shot in the dark, but perhaps the lack of capacitance there allows the cathode voltage to plummet when the power is switched off, briefly allowing the tube's gain to rise.

Like I said, just a guess. I'll try unsoldering the cathode cap from my 5F2A when I get home to see if it does that volume burst thing.

- Scott

[MWJB]

A lack of bypass cap doesn't affect cathode voltage, cathode voltage is low because something is shorting the resistor (like a blown bypass cap), or the resistor is the wrong value.

[ron vogel]

I'm actually inclined to believe the tube sockets need to be replaced, but it's stll working great. The original RCA 6V6 was toasted, and I popped in a newer Sovtek I had to diagonose the amp. Got it working, but something was missing I couldn't put my finger on. I wanted all RCA's in it, so I ordered a RCA VT107...just got it in today....all I have to say is WOW!

Old Fenders deserve nothing less than old tubes.