London Power ad

Search the Forum

(Advanced Search)

Thoughts on variable NFB control in Fender Bandmaster57-Style PA
Hi All!

I'm currently working on my third amp project. Its basically a JCM800-type front preamp and tone stack coupled with a Fender Bandmaster57-style power section, with a few minor alterations. The Bandmaster has fixed NFB using a 56K into the cathode of the 1st PI tube, with a 5K-ohm and .1uf presence circuit. I'm using 3.3k on that cathode, and am going to try a variable NFB circuit using a 68K fixed resistor in series with a 50K variable resistor on a switch to allow either adjustment or elimination of the NFB. The presence control I've selected is using a 50K variable instead of the 5k used by Fender, basically for more range of control.

I'd like any and all opinions of those changes to the basic Fender circuit. Thanks in advance for all replies!
I think you may run into stability issues fi you use the original layout. With a layout with galactic grounding and such you won't have any issues with the variable negative feedback. However, I don't think a 50K presence control is a good idea. The range of the presence control in the stock model is fine plus because you are going to be varying the negative feedback you will be negating the effectiveness of the presence control at certain settings.
Hi Guys

There is some missing info from the original post. I believe the reference circuit is for the 5E7 bandmaster, which has a standard Fender gain stage with 1k5 on the cathode and 100k Ra as the front-end of the PA, followed by a concertina. The feedback resistor is 56k as stated, tied to the cathode and 5k pot dialing in 100nF for presence. The stock PA gain is 38 - higher than even a Marshall typically has.

Are you changing Ra? or leaving it at 100k.? If the latter, then the open-loop gain may be reduced.

Gain with 68k series and 3k3 shunt is about 22, similar to a closed-loop Marshall.

Gain with the 50k pot at max is about 38. Not a big sweep but noticeable. In my STUDIO amp the Vibe control does what this pot does, and is five times the value of the fixed-R to give closer to open-loop performance at its end.

The presence pot has to be about 10x the shunt-R value to dial out the cap, so 50k is okay here although some of the sweep will not seem to do anything.

Because of the PA front-end configuration, stability is better than with a Schmitt unless the layout is particularly terrible. Did you begin with a stock Marshall?

Have fun

Forum Jump:

Come in where it's warm!
A warm welcome to tube amp modding fans and those interested in hi-fi audio! Readers of Kevin O'Connor's The Ultimate Tone (TUT) book series form a part of our population. Kevin O'Connor is the creator of the popular Power Scaling methodology for amplifiers.
Please remember these three principles: respect, sharing, community.
Not familiar with The Ultimate Tone book series? See discussion topics, or click here to visit London Power/Power Press Publishing.

Tube Amp Forum Hosted by London Power
London Power logo