Bias Question

[gtrplayr1976]

Hey folks. I was playing around with a SF Vibrolux (converted to BF) I have been trying to get working correctly for awhile. I had noticed I have some hum on the vibrato channel only when I turn the volume up ,and the bias goes up as well. Even more when I adjust the bass control. This is with only the power tubes, rectifier ,V2 ,V4 ,and PI ,and NO signal. When I put V3 ,and V5 back in the bias went even higher with control adjustment. Is this normal ?
With the normal channel I don't see this.

[g1]

As far as the power tubes are concerned, hum is the same as signal. So if they are amplifying hum, the idle current will increase.
So some 'bias' increase is normal. If it is a major difference you could have some oscillation.
Whether the audible hum level is normal or not is a different question.

[gtrplayr1976]

As far as the power tubes are concerned, hum is the same as signal. So if they are amplifying hum, the idle current will increase.
So some 'bias' increase is normal. If it is a major difference you could have some oscillation.
Whether the audible hum level is normal or not is a different question.

Thanks g1. I have other issues. No output on the vibrato channel ,and only a small amount on the normal channel when it's maxed out ,and it's distorted.

[Chuck H]

Did the amp work correctly at any time?

If yes, what has changed or what work has been performed since then?

Have you performed voltage measurements on all pins?

Do you have a signal generator, dummy load and oscilloscope?

[gtrplayr1976]

Did the amp work correctly at any time?

If yes, what has changed or what work has been performed since then?

Have you performed voltage measurements on all pins?

Do you have a signal generator, dummy load and oscilloscope?

Chuck it did work pretty well after the initial cap ,and tube change. Then the board went south ,and I had to replace it. Iv'e been working on it since. I have a few "stupid" moments I'm cleaning up now ,and will get back with results later.
I did check the voltages. They all look ok except the cathodes of the PI. They seem low. Should be around 80-100V ,and I have less than 50.
I do have all the instruments if needed. I'm just not very good with a scope ,and I don't have a lot of room where I am to lay all that stuff out ,but I'm working on a new shop.

[Chuck H]

Not to sound like like a douche, but you don't need to be good with a scope to do the tests as we may request them All test procedures would be explained.

I double hate those black fiber boards. All the repair guys here are of the opinion that they can nearly always be cleaned and/or dried out and do not require replacement. That hasn't been my experience on several. I think they should all be at the bottom of the ocean. So in that regard I think your decision to replace a bad board was a good one (even if you replaced it with another black board). But that's a lot of potential wiring errors. Incorrect voltages on the PI are first place to start. You're low, distorted signal may be a symptom of the problem there. Once you check the bias and affirm that it's in a safe range you can move on to the other matters. No point futzing with the bias now as long as it stays in a safe range since any wiring errors you fix may solve the drift issue along the way.

Please post the schematic you used and the layout you followed along with voltages taken with the amp as it now. No voltage measurements taken before more recent changes were made please. Those are of no use now.

[gtrplayr1976]

Not to sound like like a douche, but you don't need to be good with a scope to do the tests as we may request them All test procedures would be explained.

I double hate those black fiber boards. All the repair guys here are of the opinion that they can nearly always be cleaned and/or dried out and do not require replacement. That hasn't been my experience on several. I think they should all be at the bottom of the ocean. So in that regard I think your decision to replace a bad board was a good one (even if you replaced it with another black board). But that's a lot of potential wiring errors. Incorrect voltages on the PI are first place to start. You're low, distorted signal may be a symptom of the problem there. Once you check the bias and affirm that it's in a safe range you can move on to the other matters. No point futzing with the bias now as long as it stays in a safe range since any wiring errors you fix may solve the drift issue along the way.

Please post the schematic you used and the layout you followed along with voltages taken with the amp as it now. No voltage measurements taken before more recent changes were made please. Those are of no use now.

Thanks Chuck. I replaced the board with a fiberglass one from Doug Hoffman. The filter cap board as well. Once I get my "screw ups" fixed I will get that info ,and report back.

[Enzo]

Chuck, if you put the black fiber boards at the bottom of the ocean, they get even MORE conductive, not less...

[gtrplayr1976]

OK. I got a couple of things straightened out. The grid wire from the input jacks on the vibrato channel was hanging by a thread. Fixed that ,and a couple of " goofball errors". The PI cathode voltage is up some (62V) ,but not what it should be. The vibrato channel still has hum when I turn the volume up only a little (2),and when I get close to the input jacks, bright switch ,and volume pot with a chopstick the hum gets really loud. Also can tap the chassis in that area ,and hear it through the speakers.

[gtrplayr1976]

Think I got most of it straight. Had a bad grounding switch contact on a new input jack. I'll do some more testing.