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-   -   Negative feed back squealing (http://www.diyaudio.com/forums/instruments-amps/229177-negative-feed-back-squealing.html)

gkenne 2nd February 2013 09:44 AM

Negative feed back squealing
 
I have a 5A3 build, that I have working very well. I put in a negative feedback loop circuit to gain a little extra headroom which it does great. The only thing now is that when I turn the amp on, it squeals, have to turn it off then back on and it stops. It seems to only do it with the circuit off. I am not sure, should it go to ground if not on?? Or does having a ground before the cathode resistor bypass it completely?

jjman 2nd February 2013 12:12 PM

It is positive feedback if the wires to the output plates are "switched." This is a very common issue with new builds since the odds of getting it right are 50%. You can switch the output plate wires or the leads to the grids. 5A3 does not come with NFB. I'm not sure if NFB will truly provide more clean volume potential (headroom.)

gkenne 2nd February 2013 08:05 PM

Thanks for your reply. I'm not sure I understand what you mean by the output plates. This is the same feedback loop as in the Princeton and a variety of other Fenders. The circuit does indeed provide more clean headroom. In this article it shows exactly what is being done. The only difference is that I put a switch in between the 56K resistor and the output to speaker. To, turn it off, for lower volumes. http://www.google.com/url?sa=t&rct=j...41867550,d.b2I

The project is done on the 5E3, mine just has 6SC7 and 6SL7's but looks like the mods do very well in my amp, so far. Except this one little glitch. I may just hard wire it and leave it be. It sounds good with it there. Could you tell me, if there are any other things I should be aware of with this modification. It all seems to work very well so far.

gkenne 3rd February 2013 12:36 AM

It's funny, but when I hard wired it, it squealed the whole time. So, I put it back with the switch in, I just have to start it with the bypass engaged, then once it's warmed up, I can switch it to the regular cathode resistor, if I want, but it sounds good with it on the bypass route. Still don't know why the squeal though. ?? For now my project is working pretty good, I have a 6SC7, one 6SL7, two 6V6's and the 5Y3 rectifier. I still think I will eventually try to get the other 6SC7 tube hooked up, that would be almost exactly like a Bassman circuit (5E6-A) Except the Bassman, uses 12AY7's in the first two stages, then a 12AX7 to the 6V6's.

Printer2 3rd February 2013 07:59 PM

What you have is positive feedback rather than negative feedback and this causes your amp to squeal. jjman was saying that if you reverse the output transformer primary leads connected to the 6V6's plates you will have the right phase relationship between your signal and the NFB signal. You could also revers the output transformer leads to achieve the same result, do whichever is easier.


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