Black Face Bassman Head problem?

[jamme61]

I have a 67 bassman head had it for years no issues Today when I power it on, it lights up then when I take it off standby I hear sound come through for a second, then go away for maybe three seconds and then the sound comes back like your turning up a volume control up slowly? The amp works fine after all this startup stuff. The power light stays on the whole time- I checked connections, tubes, and even the filter caps? Could it be the power switch? they're original. They seem to be fine when I check them with my meter (continuity test) Thanks for any help on this.

[jaysg]

Something is dying. It could be as simple as a cold solder joint or a bad filter cap in the power supply. It looks like you are capable of working on electronics safely:

If the amp's tube chart says it is AA864 and you know it's a '67, then it's an AB165 with the wrong tube chart....unless it's modded.

Do you have more tools than a DMM? This is a situation where I would crack out the oscilloscope, set a load and do some power up testing. I would remove the output tubes and start with the power supply....check the diodes and filter caps, B+, etc.

[jamme61]

you're right it is a AB165 that was converted to a AA864. It was converted years ago by me and I did many gigs with the amp since then. I only have the DVM. I did check the tubes again (cleaned the sockets to). Then I measured the Filter caps voltage- one of the 100uf caps in series measured 234 volts while the other one measured 434 volts? I thought these two should measure the same voltage right? all the other caps measured normal from 434DC down to 295DC. I changed the first filter cap and the 220k resistor and got the same measurement on the first filter cap(about half the voltage of the other. I didn't have time to change out the second one could that be the problem? Thanks for the help on this

[jaysg]

I only have the DVM. I did check the tubes again (cleaned the sockets to). Then I measured the Filter caps voltage- one of the 100uf caps in series measured 234 volts while the other one measured 434 volts? I thought these two should measure the same voltage right?

I forget whether you can read the 220Ks in the circuit. If you have a 20 to 40uF/350V filter cap you can clip it or tack in across each totem cap. It might help you isolate the bad one. The one measuring 434Vdc is probably shot, but it's easy to get things backwards.

[jamme61]

So- sounds like I changed the wrong one- I'll try the clips on the bad one. Thanks

[Firestorm]

When caps are in series, the junction between the two will have roughly half the voltage of the "top" one.

[jamme61]

Thanks I didn't know that- has to be something else then.

Update I'm an idiot sorry guys - I had replaced the PI input cap with a huge Value 225UF and it was the problem- the stk value is 500pf which I had moved up to .001 and some how put in a 225uf - going blind in my old age. Put the right value in there and no more problem- really dumb of me sorry to waste your time on this. thanks for trying to help.

[Stevem]

Wow,the phyical size of the cap was not a give away, or is it small for its value?

While you are pookin around in there you should split the filter grounds up so that the grounds for the filters that deal with the preamp section only are tied to each other ( they should be if stock ) but then run a new ground and land it at the input jack.
The factory grounding layout is not optimal!

[jamme61]

Yea it is bigger in size but not that much- I was messing with the value because I have the bass channel set up as a Marshall 2204 layout. I had both channels going through the one input cap but split it to two caps - one for the clean normal channel (.001) and one for the bass channel Marshall side (.022) it works fine now with the right .022 value. Again stupid mistake . Thanks for the grounding info I'll check that out.