Variable NFB in Fender Bandmaster57-Style PA. Opinons?

[dtbradio]

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 1.5K cathode resistor of the 1st PI tube, with a 5K-ohm/.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!

[LOUDthud]

What are your plans for an OT and load impedance ?

[dtbradio]

Thank you for your reply!

The transformer I'll be using is here: https://www.amazon.com/gp/product/B09PNVYX8Y it is a "40W 5K:0-4Ω-8Ω Push-Pull Output Transformer".

It's on its way, so I can't speak from any testing data point of view yet. My speaker load will be 8 ohms, and NFB will be tapped from the 8-ohm output. My general thought is to be able to switch between 6K6 and 6V6 by putting in a selector switch and using it to select the 4-ohm output when using the 6K6. My logic is that this will reflect a higher impedance from the 8-ohm speaker back to the plate, and be a bit safer for the 6K6. I understand it will change the tone color, but until I actually finish the circuit and test it, I won't know if that's a good or bad thing. I'm also considering a selector to take NFB from either the 4-ohm or 8-ohm output, but haven't added that yet. Oh, I will have a 10-ohm 20-watt load resistor connected to the output jack's shorting switch so that the PA will still have a safe load when no speaker is plugged in. I've done that with all my amp builds, and will continue to do so for the sake of the PA tubes.

Edit: I forgot to mention that the boost amp for the phase splitter (V3-A on the schematic I have) will be changed up between 12AX7, 12AU7, and 12AZ7 tubes as I experiment with different tubes' tone and gain differences.

[LOUDthud]

Looking at the 5E7 schematic, there is nothing to tell us if Fender had a special transformer with 2.66 Ohm secondary, or they just used something with a 2 Ohm secondary that they already had in stock. At any rate, you will probably want to go higher of the feedback resistor and have at least a 2:1 or higher range on the variable FB.

The one cool thing about that circuit is that the Presence control will still do something when FB is disconnected

I'm not sure swapping PI tubes will produce any useful tone options. The concertina will be all over the place in terms of balance and headroom on both outputs. (There is a fixed bias mod for the concertina that is popular on Princeton Reverbs. I can't remember the name just now.)

[dtbradio]

Thank you for your reply!

I'm giving myself the tube swap option because I have 2 good 6K6's, and 1.2 to 1.3 good 6V6's. One 6V6 tests good, the other tests in the bad range but still gives decent volume, hence the mildly-humorous 1.2 - 1.3 reference. I'll have to look into the fixed cathodyne bias option.

[LOUDthud]

It's called the "paul c mod". Some adjustment of the resistor values may be necessary to get headroom in all cases.

[dtbradio]

Ok, thank you for the info!

[Reeltarded]

Looking at the 5E7 schematic, there is nothing to tell us if Fender had a special transformer with 2.66 Ohm secondary, or they just used something with a 2 Ohm secondary that they already had in stock. At any rate, you will probably want to go higher of the feedback resistor and have at least a 2:1 or higher range on the variable FB.

The one cool thing about that circuit is that the Presence control will still do something when FB is disconnected

I'm not sure swapping PI tubes will produce any useful tone options. The concertina will be all over the place in terms of balance and headroom on both outputs. (There is a fixed bias mod for the concertina that is popular on Princeton Reverbs. I can't remember the name just now.)

I sold 3x10 brownie just before covid. They used 4 and 8. This makes the 3x10s harder to find than they should be. A lot of 8Rs went to the dump, relative.

[LOUDthud]

I sold 3x10 brownie just before covid. They used 4 and 8. This makes the 3x10s harder to find than they should be. A lot of 8Rs went to the dump, relative.

I'm not sure what you mean by "brownie" and "They used 4 and 8". I'm finding two Bandmaster schematics, 6G7 and 6G7A that show connection to a single speaker. No note on impedance. The AA763 shows two speakers in parallel, since that's a Blackface, one would assume a 4 Ohm load. There is also a schematic titled "57 Bandmaster" dated 18DEC2012 that shows three 8 Ohm speakers in parallel and says 2.6 Ohm typical load, 2 Ohm minimum. I thought, perhaps wrongly, that dtbradio was referring to the 5E7 tweed Bandmaster which shows three speakers in parallel. My assumption is that those tweed amps used 8 Ohm speakers not too different from the 5F6A Bassman.

I don't remember ever seeing a vintage brown or cream tolex cabinet with three speakers.

Edit: After closer examination, 6G7A OT = 45217, AA763 OT = 125A6A.

[Reeltarded]

Brown 3x10. There never was a 2.63R OT. They used 4 and 8 ohm OTs in all of the Bandmaster combos.

AFCA015B-2F45-4480-9135-35B97E8792F0.jpeg

[LOUDthud]

Thanks, that's a very rare model.