Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ymi5150? on January 29, 2020, 08:17:47 pm
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hey there,
i'm building a 2 channel rig. V0 feeds the crunch channel, V1 is cascaded and feeds the lead channel. relay used to switch input to each valve grid.
there is cross talk from the channel not engaged (60 cycle hum), so I used the relay to break the signal between the crunch gain pot and the grid to V2 when the lead channel is used. that solved the problem when lead channel is used. However, lead channel pot can add 60 cycle hum to crunch channel. I can get the hum to go away while on crunch if I turn lead pot to 0. if the relay had 2 more poles I could break the lead signal when not in use as well and solve the problem, but i'm curious what's causing the hum rather than using the relay as a band aid.
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Welcome.
It might help to show a plan or a sketch. There must be many ways to do "a 2 channel rig".
I could *tell* you two ways to get to the book shop from here, via Red Rock or via Crooked Road, but a map is a lot better.
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figured that request would come. let me sit and draw it up.
more information:
both channels work fine if I bypass the relay. here's where it gets interesting. if I delete the V0 crunch channel altogether, the lead channel works fine when toggled on through the relay (with LED indicator lit). when I turn the channel off with foot switch, the hum is introduced. how is 60 cycle hum getting into the signal when there is no input (or output) with the relay open? I put a 1M Rg resistor in the relay closed position to ground, no change. I've swapped outputs on the relay and the problem follows, so it seems to be a design error, not hardware.
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Are you doing this project without any plans? That's pretty gutsy. :huh:
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Please just draw a simple sketch of the schematic, photo with your phone and post here. It's impossible to help you unless we can understand what you've built.
relay used to switch input to each valve grid.
Although this sounds a bit suspicious. There's usually no reason to switch on the input side if the channels have separate first tube stages. Keep all switching as late in the circuit as possible to minimize noise.
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the hum got worse over time so i'm suspecting the relay but here are a few sketches.
i'll need a second relay to avoid cross talk at the V2 mixers
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That’s a start. However we need to see the whole amplifier circuit, at least the whole preamp.
EDIT: If you are modifying some well known amp, you could take a schematic of that and use a simple paint program to draw your mods.