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Negative feedback loop mod on single ended amp (Epiphone Galaxie 10)

Started by Mprall00, February 09, 2019, 02:30:33 AM

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Attached is the schematic for an epiphone galaxie 10.  I have modded it heavily as a learning experience following another board's advice.  First thing I did years ago was cut the negative feedback resistor out (R15) but now thought I would like to add it back in on a switch to try values.

I wired it like the picture attached in place of R15.  I had tested the switch prior to hooking it up.  All good to go.  Attached it and fired it up - no change in sound at all between 33k, 10k, and negative feedback off.  That can't be right right?

So I took it back apart and used my multimeter to measure the switch on board and every setting (even off) measured 1.5k.

So I took the switch out entirely and measured between the pads - 1.5k.

So I wired a 33k in to r15 and measured on board - 1.5k.

Fired it up with 33k and sounded exactly like when I thought I had the negative feedback removed.

I mean it actually sounds really good after all the mods, but anyone know what is happening?  Can i not modify negative feedback loop in this amp?  Can I not even remove it?


Your meter readings sound about right.
You will always have R7 there, and it is basically in parallel with R15 because of the low resistance of the OT secondary.
Feedback can be very subtle and hard to notice.
Try open loop (no R15 installed), compared to a lower value like 5K for R15.
You should notice some difference, at least a bit of volume drop with a 5K for R15.  All other values between 5K and open, will be even less noticeable.

Also, with 5K installed for R15, it should read around 1K with your meter. (5K paralleled with 1K5)


Thanks!  Yeah it dawned on me that the 1.5k was the cathode resistor.  But when I had the 10k in there shouldnt it have read 1.3?  I guess with it being that close (and that late at night) it could easily be by own user error or mental rounding...

Also I thought higher was going to be more noticeable.  But now realize looking at it that the parallel resistor is just averaging it out to be basically no change.

Thanks - I will try 5k and see if that is noticeable.

With NFB having so little impact on this amp, is it even worth trying a presence control?  that was going to be next for me...


Perhaps the switch is defective or miswired.

Measure the switch in each position with your ohm meter, to verify the contacts inside are closing.


Switch was good.  Rewired with a 5k and 820r.  820r was likely too low but good to hear its working.  5k is a good sweet spot.  Off now sounds like there is no negative feedback.  10k was just high enough in this circuit to be barely noticeable so stock sounded almost like no negative feedback at all.

So I may leave the switch there.  Or I may solder 5k in there and experiment with the switch at other places...  like c4.  Or maybe an off switch for the tone stack...  Or a cold clipper switch.  Wonder if this amp is high enough gain to have that be worthwhile...


You were reporting that the switch made no difference in your resistance readings.  Is that no longer the case?


I have a feeling you are going low enough now with that resistor that it is affecting the bias of V1b (because it is in parallel with the 1K5 cathode resistor).
So it may be that what you are hearing is the change in V1b bias, rather than the change in NFB.
You can check this by measuring the DC voltage at V1b pin8.
With no signal, if the DC changes for the different switch positions, the bias change may be what you are noticing.
Ideally, the NFB should not affect the DC conditions of V1b.
If you really wanted to do a definitive test, you would then open the NFB loop, and put the resistors directly in parallel with the 1K5 cathode resistor, then compare the audible result.


Yeah the comment about altering the bias was correct.  The negative feedback in this amp does seem to apply some negative feedback with low resistor values but it has to go so low that it alters the bias of the second triode as well.

So i removed it entirely and put a switch in for bias resistors instead for cool, hot, or normal bias.  Much more interesting.  Then i drilled another hole and made the bypass cap switchable as well.  Not a real noticeable difference between 5uf and 22uf but the .68uf setting really tightens up the bass for drive.  That with the bias switch makes for some cool sounds.