Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hesamadman on August 20, 2016, 09:50:33 pm
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This has been a pretty big project for me. I figured I would just start a thread focusing on the whole thing. I am doing channel switching using dougs 5v relays. I know this isn't the type of build that you see around here very often, but I have a lot of questions (some not regarding the actual switching as much as the channel layout) hopefully I can get some input.
I have a lot of noise in this thing, but thats not my issue. Im spending the night tidying up some wires. Hopefully that helps. The issue I am having is on the "clean channel".
It works and sounds nice. Noisy but nice. However, when I turn the gain up the gain increases as it should. As it reaches about 2 o clock the gain reduces. And starts to increase again at about 4 o clock. So between 3 and 4 o clock the gain reduces. Its very odd. I am about to switch the pot to see if it helps. Could this be a lead dress issue? Is it possible poor wire routing will create some sort of capacitance that kills some frequency from heading to where it should be going?
I will include some photos after I tidy it up tonight.
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My guess would be that you most likely are experiencing an oscillation when dialed at the point you mention. A lot of things factor into why this could be. Tracing it down could be difficult (especially without testing and/or photos) but tidying up your component layout, using shielded wiring, and assuring that there are no ground loop possibilities "should" all help or at least help eliminate the possibility of these things from the equation.
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I did a re-wire. Im just not getting the results I would like. The way that my original schematic is setup, It leaves too much stuff still connected. When using the clean channel, my high gain tone stack slightly affects the tone when adjusted and vice versa. The clean channel gain control just did not want to cooperate with the other components in the chain of that stage but they are necessary for the high gain channel. I think just keeping the gain controls side by side and swapping them out will make for a better design. I re-worked my schematic and it completely removes the unused components from the circuitry. I am curious if these components can affect anything at an idle even if they are not connected.
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Did try to replace that potentiometer ? I have here a pot that just mute the sound when at about 8. Very odd!
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I did a re-wire. Im just not getting the results I would like.
Relay 1A and the overall signal output is dependent on relay 2A's position. Not good if that's actually how you have it wired? You should re-think the design over again and take a different approach. Are you not getting (initial) switch popping noises?
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I have 10 meg resistors across relay contacts. I have ZERO popping on switching channels. High gain channel works great. Low gain channel works and sounds good BUT adjusting the high gain tone stack will affect the tone when low gain is engaged. It should not do that. Only the tone stack assigned to the channel that is engaged should affect the signal. Also the placement of low gain "gain" control is not a good spot. So I revised my first schematic to put BOTH gain controls in the same place, just being switched out when channel changes. I think moving the low gain tone stack to the new location will be better too. Both old and new revisions are above.
I have not tried second revision yet.
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Heres what I noticed. If I dont completely remove the tone stacks from the circuit, some of the signal is lost in the stack. My hi gain channel is actually not very high gain at all. So far I have not had to ground out the unused triodes when in high gain. However, wire routing VERY crucial here. Even using shielded cable, I have some unwanted noise. A quick flick with the chopsticks shows exactly where the issue is. I Have decided that if I can get this amp to function the way I want, I will rebuild the board. This will allow me to place components in better positions. Since this was a MOD of an existing amp, I had little room for relays and is causing some troubles.
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This is getting exciting. Heres a small clip of the channel switching in action.
Independent Gain/EQ/Master Volume on both channels
http://www.rdkelectricalservice.com/IMG_6874.mov (http://www.rdkelectricalservice.com/IMG_6874.mov)
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sounds good so far madman but much too short :icon_biggrin:
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Will get a nice demo soon. Still got some voicing to do.