My son has a Webcor 4905 converted to use as a guitar amp. He had the amp cabinet get damaged and it pulled the wires from the output tx, so I epoxy'd them back in place. The wires weren't broken, just the paper. But the amp wouldn't work anymore. I traced it down to the feedback windings in the output transformer, in series with the cathode bias circuits for both power tubes.
So I just put a short from the cathodes to the bias resistor and cap. But now the thing has a really loud oscillation in the several kHz range. I guess this amp needs the negative feedback.
I have seen other similar Webcor circuits that merely GND one side of the output transformer secondary and then feed the other side through a 47K resistor back to the 1st stage cathode resistor. I guess I just pick a side and if it oscillated try the other side of the output trans winding? Some feedback circuits have a cap, but they didn't go back to the cathode of a previous tube. I suppose the cathode of the preamp tubes are going to be referenced to GND just like the speaker, so no cap is needed.
So I just put a short from the cathodes to the bias resistor and cap. But now the thing has a really loud oscillation in the several kHz range. I guess this amp needs the negative feedback.
I have seen other similar Webcor circuits that merely GND one side of the output transformer secondary and then feed the other side through a 47K resistor back to the 1st stage cathode resistor. I guess I just pick a side and if it oscillated try the other side of the output trans winding? Some feedback circuits have a cap, but they didn't go back to the cathode of a previous tube. I suppose the cathode of the preamp tubes are going to be referenced to GND just like the speaker, so no cap is needed.
So you have the Attached? Or were *other* mods done??
> ...put a short from the cathodes to the bias resistor and cap. But now the thing has a really loud oscillation in the several kHz range. I guess this amp needs the negative feedback.
Tube amp does not "need" NFB. (Some wild cases, but not as mild as this.) As described, I don't see enough gain to oscillate, unless the OT primary lead runs around the preamp tube. Nor enuff gain to play guitar real easily. So I am wondering if other mods were done; and what.
Very pointed question: does the Volume control affect the squeal??
You can take NFB from the speaker side. You know what you are doing. However if you go to V1B there isn't enough gain for significant NFB; also no great way to inject it. If you go to V1A cathode, the Volume pot in the middle varies NFB along with volume.
> ...put a short from the cathodes to the bias resistor and cap. But now the thing has a really loud oscillation in the several kHz range. I guess this amp needs the negative feedback.
Tube amp does not "need" NFB. (Some wild cases, but not as mild as this.) As described, I don't see enough gain to oscillate, unless the OT primary lead runs around the preamp tube. Nor enuff gain to play guitar real easily. So I am wondering if other mods were done; and what.
Very pointed question: does the Volume control affect the squeal??
You can take NFB from the speaker side. You know what you are doing. However if you go to V1B there isn't enough gain for significant NFB; also no great way to inject it. If you go to V1A cathode, the Volume pot in the middle varies NFB along with volume.
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