Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bbaug14 on February 23, 2015, 07:38:21 am
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I have a Plexi build where a rotary switch is wired to go between lead and bass spec (split/shared, V2a 0.68u bypass, tone stack resistor/cap added in parallel). On the bass setting (shared first cathode, no V2a bypass, 250p/56k stack) everything is fine. When on the lead setting, I get scratch BMT pots only. All the other pots are fine with no noise. I tried swapping the tone stack cap I switch in (270p added in parallel with 270p), as I just figured it was a leaky cap, but it made no change. I also swapped the existing one to be safe even though bass setting is fine. Since it works fine in the bass setting, only the tone stack cap/resistor, V2a bypass cap, and the split 2.7K/0.68u change. I tried both hi/lo inputs in lead setting and the issue is still there, telling me I can rule out the 2.7/0.68u, not that I thought it could be this anyway I wouldn't think. I also don't see how it could be the V2a bypass. I lifted that in the Lead setting as a check, but no change. Anyway, if anyone had some thoughts, I'm all ears. Maybe lead dress? Both the resistor and cap are on the rotary, so I don't think lead dress can be any issue.
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Can you show us a schematic?
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Here's a chassis photo. Its a stock 68 spec 1987 50W Marshall except for the Lead/Bass switch and "Larry" grounding. Thanks. PI cap can can't be seen. Its a 32/32 (64u) grounded to node 3 from the left.
(http://i58.tinypic.com/2u6pver.jpg)
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Nice looking amp. We still need to see a schematic of your switch mod so we can tell exactly what you are switching. Then we can maybe figure out what is not wired correctly. If you can't provide a schematic then try to describe the switching circuit in as much detail as you can.
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Here is a quick pictorial of the switch. This added to the chassis photo should be clear on how it works.
(http://i60.tinypic.com/2vcfc0p.jpg)
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Pots should not be scratchy if the switch is actually wired as drawn. I suspect you have two leads crossed on the part of the switch that goes to the tone stack. That would put dc on the pots. Have you actually measured any dc voltage on the pots? Recheck your wiring. Should be a very simple fix.
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I've quadruple checked the wiring and its right. You can see so in the photo I think. Yellow wire goes to Resistor/Cap junction on board and switch. Orange wire goes to other end of resistor. Purple wire goes to other end of cap.
There is definitely DC on the pots, though I have not measured it. In Bass setting the pots move full range with no noise. Switch to lead and its scratchy. I can't see what could possibly cause this other than DC.
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I've quadruple checked the wiring and its right. You can see so in the photo I think. Yellow wire goes to Resistor/Cap junction on board and switch. Orange wire goes to other end of resistor. Purple wire goes to other end of cap.
I can see where the wires connect on the board, but I can't tell how they are connected on the switch. If they are wired correctly then you have a defective switch or something is touching something on the switch that it should not be touching.
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What I'll do tonight is check my connections again, post a picture, and also check continuity and isolation of the switch in both lead and bass. I'll report back.