Crate BV-6212 problem

[labb]

I have an crate blue voodoo (BV-60) that I am working on and can't seem to find the fix. This amp has a loud hum with nothing plugged in. Pulling the second pre amp tube kills the hum. I have checked Foot switch and effects sockets and they seem to be fine. Plugging into the effects return plays good and no hum. I am thinking that it is a power supply filter problem and would like to try to check for ripple on the B+ line. Can I check this with a fluke DDM or will I have to do it with a scope (which I do not have). Any suggestions as to what the problem could be would be appreciated. Thanks

[Structo]

have you tried substituting known good tubes in each spot?

[sluckey]

This amp has a loud hum with nothing plugged in.

Does it still hum with a guitar plugged in and guitar volume set to zero?

You can check B+ ripple with a Fluke set to AC volts but the reading probably won't tell you much unless you know what reading to expect from a good amp. Same with the scope. A good check of a filter cap that may be causing hum is to simply bridge a test cap across it.

[labb]

New tubes in each spot and rolled the pre amp tubes to be sure.

Guitar volume does not have any effect on the noise.

Problem is on each channel, clean and lead.

This amp is auto biasing and I have checked the biasing just to be sure it is not running too hot. They have it running at about 23 watts out of 30 watt max. (6L6) for each tube. little on the hot side but should not be a problem. I think that the power amp end is good and playing through the effects return I guess confirms this. Problem seems to be in the second pre amp tube position. I have tried 4 different tubes in that position and it does not change. I have chop sticked every component in this amp and get nothing. Touched up all solder joints that looked suspect. Amp has a hum balancing circuit (humdinger across the filament wiring) and it works. I have cleaned all of the jumper connections in the amp. It has spade lugs instead of ribbon cables. I am at a real loss for what this problem is. Probably something simple. Have a complete set of filter caps for the power supply coming and intend to change all of them out. Anything else you think of that I might check?

[Phil_S]

Can you be more explicit on your observation about the fx loop? What exactly did you do? It seems you did something and the hum disappeared.

[frankdrebin]

if its common to both channels its either the first triode or the reverb/loop section,it sounds like a filter cap gone,try to clip a good cap with alligators on the various preamp nodes.
If you plug in the fx return does the hum disappear completely or not?

schems:
http://www.ultimate-guitar.com/forum/sh ... ?t=1534810

[labb]

I plugged a cd player into the fx return jack and the hum disappeared and it played good. I have clipped a 20 mf 500 volt cap across each of the existing caps and there is no change. I have the reverb cables disconnected from the amp and the reverb pan. could be in the reverb jack. How would you check the reverb jack? This amp has cliff type jacks and the best I can tell they seen to work as they should.

When I pull the first pre amp(j13) tube the hum is still there. Pull the second (j14) and the hum goes away.

[Phil_S]

Run a jumper from the plate of the 2nd triode of V1 to the grid of the 1st triode of V2. That cuts out all of what's in between. If the hum goes away, you know the offending part is isolated to what's jumpered. Remove the jumper from V1 and place it into the circuit progressively closer towards V2. When the hum starts, you should have the location of the offending part.

Not sure what, if anything, to do about the CF attached to V1.

This shouldn't take long. You don't have much to lose giving it a try. I think I'd start to the right of the blocking cap on V1. If you still have the hum on first try, this isn't going to yield results. Look elsewhere.

My logic here may be oversimplified. The hum rides in on the grid of V2. It doesn't go away with only V1 pulled. Therefore it must be inbetween.

[labb]

Phil, I did the jumper test up to the coupling cap C11 and it still hummed. I had pulled the coupling cap to test and it tested good. Something that i noticed was that I could ground the grid of V2 and it till hummed. Not as loud but the hum was still there.

[Phil_S]

Does the CF feed into V2 grid also? I'm easily confused by the schematic...there is a line that suggests the grid is referenced to the CF, but I'm not really sure if it is simply two unconnected wires crossing. If so, then all that part of the circuit is in play for a problem, too. Since the jumper test did very little, I think I'd look at the CF. There's sand in the CF. I know nothing about transistors.

[labb]

The lines just cross, do not connect..Q3 just switches R17 in and out. Thanks for the help. This thing really has me stumped. I have talked with an authorized crate repair man and he was as helpful as he could be on the phone. Really though he didn't suggest anything that I had not already tried except for looking at it on a scope...Which I do not have. Any other ideas?

I have to say this about the Crate, it is a heck of a lot easier to work on than some of the amps I have seen. ie: Fender RI and Peavey Classic.