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Old 18th August 2012, 05:32 AM   #1
zoi is offline zoi
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Join Date: May 2012
Default Trouble with homebuilt amp

Hi all,

I am working on my first amp build and I am running into trouble that I can't seem to solve. It's a guitar amp with a two stage preamp and single power tube in the main section (shown in the upper half of the schematic). This section works great, and I am very happy with the sound. The problem is with the reverb circuit.

When I connect the reverb circuit, the amp cuts out after a few seconds. If I disconnect terminal C from the reverb driver tube, the amp recovers and plays normally without reverb.

With terminal C disconnected and the amp working normally, the top of the cathode bypass capacitor on the power tube of the main section reads about 64V. As soon as I connect terminal C, the voltage at that point drops to 0 (implying no current through the power tube, and hence the loss of output). I do not understand why this is happening.

If the amp is played at a very low volume with everything hooked up as shown, the reverb works and the output is maintained. But as soon as the volume is turned up >10% or so, the output is lost until the volume is turned back down and the power to the amp is turned off and back on.

I suspect the problem is in the way I have connected the reverb circuit. I'm sure it's an obvious problem but I am at a loss.

Thanks a lot.
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Old 18th August 2012, 05:55 AM   #2
dgta is offline dgta  United States
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Both of your pentodes have the grids floating,
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Old 18th August 2012, 07:29 AM   #3
zoi is offline zoi
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Join Date: May 2012
Does this mean I need to put a resistor between the screen and ground?
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Old 18th August 2012, 12:28 PM   #4
MelB is offline MelB  Canada
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Location: BC Canada
Between G1 and ground = yes.
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