First post/First Design!

[loudthud]

Back to the design, NFB to the phase inverter would a better place. With all the tone controls on the front end, variations in source impedance will change the tone. A better place would be between the 12AT7 and the phase inverter. That way you are not attenuating the signal 20 to 40 dB and then try to amplify it out of the mud. There is no ground reference in the heater circuit, a DC voltage of around 50V should reduce hum to a minimum.

Bite me lamps ??? ROFLOL!

[Satellite Icarus]

I'm considering using the 12AT7 as the long tail PI and use the two PCL86 triode halves as a buffer+preamp stage, which solves the NFB loop issue (I can just feed it back in the preamp gain stage grid as expected), the tone stack uncertainties, and the 12AT7 not being a great preamp tube issue. thoughts?

Otherwise, I think running the NFB loop in the PI with the uneven plate loads would create a pretty unavoidable imbalance, no?

ground reference to the heater is being worked on as well, will be adding a stage at the end of the B+ to serve as heater bias.

Pin 5 is indeed a 55V tap on the B+ secondary.

SI

[mooreamps]

Bite me lamps ??? ROFLOL!

That's what Dana used to call them, and at the time I was not laughing...

-g

[Satellite Icarus]

Still not getting the Bite Me lamp thing, but jokes usually loose their flavor if you have to explain them... I guess I'll just wait

[mooreamps]

Still not getting the Bite Me lamp thing, but jokes usually loose their flavor if you have to explain them... I guess I'll just wait

A few years back I introduced the concept of fixed bias on a couple of the other guitar amp building forums ; and I was burned at the stake for being a heretic.. Even got threw off a couple of them forums. But, it was also when I was picked up by the two sisters in the music recording industry, and they came to my defense.. When folks found out "who they were", then they all got real nice to me after that, including Dana....

-g

[loudthud]

Still not getting the Bite Me lamp thing, but jokes usually loose their flavor if you have to explain them... I guess I'll just wait

There have been a number of high spirited debates over the virtues of LED biasing on this and other boards. I hope this thread does not degenerate to the "bite me" stage, but will focus on the technical aspects. Since the LED has a non-linear voltage vs current curve, I would expect to see the generation of some distortion. (They might actually cancel some distortion!) That may be good for guitar amps, not so good when some old professor is looking over your shoulder. If you have the equipment available to evaluate a single stage with and without LED biasing, I would like to see the results. Try as many different LEDs as you can find.

[Satellite Icarus]

I was planning on getting an LED grab bag from Sparkfun and going LEDmad. Will depend on the amount of time I can manage for testing my different options, which is always less then one would like. Rest assured though that if LED there is, thorough testing there will be.

[J M Fahey]

As a general rule, in tube power stages the less amplification stages involved in the NFB loop the better , because unfortunately you have the output transformer there, a complex impedance device which introduces *lots* of phase shifts and resonances in the response.
You no longer have a 180 degree NFB signal by a long shot, and this obviously complicates everything.
You have phase shift problems even in Op Amps, but they can usually be compensated by a simple capacitor or RC network; none of that is possible with a very complex real world Transformer.