Blackstar HT Studio 20 power transformer

[dwhutchens]

This amp was brought to me with a blown HT fuse and a mangled fuse holder. I managed to straighten out the fuse holder to install a new fuse. When I power up the amp I get a loud hum with guitar and speaker plugged in and all volume controlls at 0. I see about 30mv at 60hz on the speaker output. When playing the guitar I can here a distorted tone over the notes like maybe 60hz. I'm getting about 250vdc on the output tube plates, I saw a reference in this thread Bias Circuit Question of about 380 on the plates. I get about 275vac unloaded on the power transformer. Can someone confirm the correct plate voltage and the correct power transformer output.
Thanks,
David

[The Dude]

I can't help with your actual question, but if I had low B+ and loud hum I would check for AC on the B+ supply and/or filter caps first.

[dwhutchens]

Filter caps was my first thought but I want to make sure the PT is ok before I make an attempt to change the caps, I know it's going to be a pain. I'm used to tube rectifiers and center tapped power transformers and the resulting voltage drop. I know some solid state power supplies output higher dc voltages than the ac input but I'm not familiar with this one, the schematic is in the referenced thread in my op.

[mozwell]

i am by no means an expert on valve amps, there are many more here way more knowledgeable then me, but here is my 2c
There must be some reason the mains fuse went.

1. unplug the two EL34 valves, and the two pre-amp valves
2. measure the AC voltage between SP1 and SP2
3. measure the AC and DC voltages on the power supply nodes, ie from each end of R200 to 0V, and from each end of R204 to 0V
4. Check that F1 and F2 (near CON6) are both ok
5. Measure the AC and DC voltages from TP9 to TP14, from TP11 to TP 14, from TP12 to TP14, from TP13 to TP14
If all these check ok, reinstall the two pre-amp valves & recheck TP11 to TP14, and TP12 to TP14 are still +/- 12.6VDC
6. With POWER OFF & the power supplies discharged, check the resistors around teh EL34 valves, R219, R220, R172, R173, R207, R221

It may be one of the low voltage power supplies that is missing, causing your loud hum

[Enzo]

The transformer doesn't matter. Even if your transformer put out 100v less than it should, your rectifiers and filters should still smooth it. So regardless of the transformer, you can still measure for excess ripple on the power supply. And if you have a bad cap, that could result in unfiltered voltage, which would measure very low. Note we are suggesting Testing for bad caps, not simply replacing them in hopes.

275vAC should rectify and filter to about 385vDC. Don't measure at the plates, measure at the supply. Right at the center tap connection. If you are getting 100v less, AND loud hum, that is a real strong clue that you have lost a filter cap. Either a bad part, or a broken connection. Transformer failures are the least common of all failures, and when they do fail, it would be extremely unusual for the voltage to be way off.

[dwhutchens]

I'll try to check that out this weekend, thanks.

[dwhutchens]

275vAC should rectify and filter to about 385vDC

Thanks Enzo, that's exactly the information I was looking for. It's been a long time since I studied power supply theory. I'll try to check out the caps this weekend.

[Enzo]

You either have tons of ripple or you don't.

[dwhutchens]

OK, I got in front of this thing with a clear head this morning. I only have a x10 probe for my scope so I can barely read 400vdc but I was able to get it on the screen good enough. Yes, I have lots of ripple when the tubes heat up. With the OT center tap unplugged I have about 388vdc and when I plug it in it drops to around 250 or so. This thing has some kind of crazy auto bias circuit but it has 2 cathode resistors in parallel with a 100uf cap. Those were reading 0 ohms with the tubes in. I pull the tubes and now I get about 113 ohms, more like what I would expect. Upon further inspection one of the tubes has the cathode shorted to the heaters. My tube tester didn't catch that. I don't have any EL34 tubes so my next question is can I safely sub in a pair of 6v6 or 6L6 to check out the amp.
Thanks,
David

[Mick Bailey]

A pair of 6L6 will be fine to test - just make sure it biases ok.

[dwhutchens]

I powered it up with the 6L6's in it and no hum. I have 340vdc on the OT center tap. I have 11.5 or so across the cathode resistors. It sounds kind of dry and brittle but I suppose the 6L6's are running pretty cold in that circuit. I guess it's time for new tubes.