Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: John on April 20, 2018, 01:16:06 pm

Title: 2 channel amp...
Post by: John on April 20, 2018, 01:16:06 pm

Is it possible to use NFB to just one channel? i.e. run it to the cathode of one of the preamp tubes instead of the (shared) PI? The harp channel is wonderful, the guitar channel (plays acoustic) is too mushy.


I would just try it but the amp's not in my possession yet. Thanks in advance!

Title: Re: 2 channel amp...
Post by: sluckey on April 20, 2018, 01:24:41 pm

The kind of NFB you are thinking of is for the power amp and will affect both channels. You can use local NFB in a preamp (usually from plate to grid) without affecting the other channel. Here's some info...

     http://www.valvewizard.co.uk/localfeedback.html

Usually, an amp that sounds good for a harp ain't gonna sound too good for acoustic guitar. You need a good clean amp for acoustic.

Title: Re: 2 channel amp...
Post by: 92Volts on April 20, 2018, 03:33:37 pm

Besides sluckey's suggestion, you can raise preamp (sometimes PI) voltages to keep things cleaner. Lower power supply resistor values to raise voltage, if that increases noise you can counter it with larger power supply caps.

You can apply poweramp feedback earlier up to "global" feedback into the input, but you don't want to. It's not great for guitar tone (behavior during overdrive) plus, as signal is delayed (phase shift) at different frequencies, you can get positive feedback even if polarity "looks" negative. That causes oscillation/squealing.

The concept is used in some hi-fi designs with flat frequency response that are never meant to be overdriven, especially solid state amps. It's still a design challenge because you can get phase shift and oscillation above audible frequencies where the amp no longer has "hi-fi" performance.

Title: Re: 2 channel amp...
Post by: John on April 20, 2018, 08:04:41 pm

Thanks both for your replies. Confirming what I suspected. When first built, the guitar channel did sound pretty good with acoustic -- not classical by any means of course. I went to the gig this past weekend and it sounded awful with the band, I figured because of having to crank it. Have it on the bench now and it turns out a coupling cap started leaking, throwing 2.5 volts into the audio path. Pretty sure that's not so good.


I'll replace that Sunday (these damn jobs).


He mostly plays harp through it, and I must say that sounds fantastic. *pats self on back 3 seconds before amp bursts into flames*


Thanks again!