carlsbro 100 top, with problems

[per111]

Hi.
I recently got one more of these great amps, but this one was not in good shape. the output was very low, but not distorted. I changed the tubes, and all the filter caps, but still way too low. I tested bias (with biasprobe) and it read around 48 ma, just like on my other 100 top, but when you play through it, the ma drifting up to 100 ma, and then back to 48 ma. Anybody got a clue? Other strange thing: in about 2 minutes after I`ve shut the amp down, the filtercaps are almost drained ( from about 250v, to 20 v, or less) but on my other 100 top (with old caps) this takes aprox. 48 hours to drain, unless you do something.
Any help apreciated
Regards:
Per

[Jazz P Bass]

The power draining so fast may be a hint.
The only 'real' path to ground (fault wise) that the B+ could take is :filter caps & maybe the 0.1uf coupling cap on V1B.
You could try to measure the 0.1 on the grid side & see if it is leaking.
If it is, that would throw off the next section.
Did you replace all of the filter caps?
I count (6) 68uf & (2) 20uf caps.

[per111]

Thanks for your reply
hmm, think I need a litle help here, (not skilled enough): on the grid side, do you mean on the side connected to v2 pin7? and how to measure it, to ground? and what do I look for?
All 8 caps, were replaced.
regards.
Per

[Jazz P Bass]

Yes, on the grid of V2, pin 7.
The purpose of the 'coupling' cap is to pass the varying (ac) voltage that the plate is producing on a signal, while blocking any of the plate voltage.
On one side of the cap will be the plate voltage.
On the other side you want to see (ideally) zero volts.

[per111]

Thanks again. I did try that, and zero volts.....seems like the volume is ok, in 4-6 seconds, and then decreasing....further suggestions?
regards.
Per

[Jazz P Bass]

Do you have any spare tubes that you could 'Easter Egg'?

[per111]

I do have a few spares, whats on your mind?
Per

[Flotingerth]

Did you change the bias cap too?

[per111]

I did change all filter caps 6 *68 uf 450 v, 2* 20 uf 450 v and 1*68 uf 25 v, which I think must be bias cap?
One more strange thing: when I switch on main power (not standby switch), v2 gives a shines like a ligth bulb for a second, and then turns in to "normally" glow, like the other tubes. I`ve tried with different tubes, but its the same ...

[per111]

litle more testing: Measuring p7 på v2: when switching standby to on, the readings are 4-5 v dc, for a short time. Within a minute, the reading is zero. switching off the reading are aprox -0.4 v....

[Flotingerth]

I think that may be the v1 cathode bypass, look for a 47uF 100V (ish) nearer the power valves or Power supply end
I make 8 filter caps, 1 bypass, 1 bias.
make sure you replace it the right way ie positive to earth!

[per111]

done, but no change....

[per111]

Do you think, the 200 pf cap around v2, could be the problem? and if, how to check it? otherwise. I`m a litle lost here...
Thanks in advance.
Per