1964 Super Reverb NFB rotary switch

[Darkpr0]

I recently rescued a 1964 Fender Super Reverb chassis that I am converting into a head. It required a new output transformer, so I installed a 1965 Twin Reverb OT. It sounds great, but I wanted to add a rotary switch in place of the AC courtesy plug in the back, with an adjustable NFB circuit so I can accurately replicate the NFB of a Super Reverb, Twin Reverb, and Deluxe Reverb. I also am going to add additional values, including off. I have designed a new backplate (attached a photo). but I am a little stuck on achieving the exact resistor values to replicate these models.

I intended to replace the 820 ohm resistor in the NFB circuit of the Super Reverb with appropriate values. Unfortunately, my electrical engineering skills are not up to par and my calculations for these values come up wrong. I stumbled upon this post regarding NFB voltages https://el34world.com/Forum/index.php?topic=16315.msg159903#msg159903, but still hasnt clicked. Any help would be amazing!

[Darkpr0]

Photo of the Output transformer:

[tubeswell]

The closed loop gain from all three types of amps is going to be different, not just because of the resistor values in the NFB voltage divider, but also because of the combinations and types of tubes and the different reflected loads of the OTs. So it’s not a simple matter of copying different resistors.


Basic lesson in global NFB design on Aiken site http://www.aikenamps.com/index.php/designing-for-global-negative-feedback

[Darkpr0]

The closed loop gain from all three types of amps is going to be different, not just because of the resistor values in the NFB voltage divider, but also because of the combinations and types of tubes and the different reflected loads of the OTs. So it’s not a simple matter of copying different resistors.


Basic lesson in global NFB design on Aiken site http://www.aikenamps.com/index.php/designing-for-global-negative-feedback

Thanks for the Aiken link, there's a lot of content there. I guess I can clarify that I am not looking for perfect NFB circuit match, but rather the closest approximations to the NFB voicings in these amps.

I suppose I chose amps that have additional circuit design changes. I suppose a better option would be:
* Super Reverb - 2 6L6 @ 2 ohms
* Pro Reverb - 2 6L6 @ 4 ohms
* Vibroverb - 2 6L6 @ 8 ohms.

[sluckey]

Don't mess with the 820Ω resistor. It's used in all three of those amps. All you need to do is connect a 100Ω in parallel with the existing 100Ω to cover the DR.

The TR has a 4Ω OT. The NFB voltage divider is 820Ω and 100Ω.
The SR has a 2Ω OT. The NFB voltage divider is 820Ω and 100Ω. (same as TR)
The DR has a 8Ω OT. The NFB voltage divider is 820Ω and 47Ω.

I think a better option would be to use a 2.5K pot in series with the 820Ω. Then just tweak by ear. NFB is kinda a personal thing. No right or wrong. Use a pot and let your ears tell you what is correct.

[Darkpr0]

Thanks for the reply Sluckey. The potentiometer doesnt really work for me, as I want the switch to also be able to completely cut the negative feedback.

Since this amp has a twin reverb output, its running 8 ohms with a 820/100 ohm NFB configuration. That doesnt match any production Fenders, so I am looking to get as close as possible to stock configurations.

[sluckey]

Since this amp has a twin reverb output, its running 8 ohms with a 820/100 ohm NFB configuration. That doesnt match any production Fenders, so I am looking to get as close as possible to stock configurations.

The stock Fender Twin Reverb has a 4Ω output, not 8Ω. ALL the Fender amps that have an 8Ω output use the 820/47 voltage divider.

This ain't rocket science. I'm sure you'll find something that works for you.

[Darkpr0]

Since this amp has a twin reverb output, its running 8 ohms with a 820/100 ohm NFB configuration. That doesnt match any production Fenders, so I am looking to get as close as possible to stock configurations.

The stock Fender Twin Reverb has a 4Ω output, not 8Ω. ALL the Fender amps that have an 8Ω output use the 820/47 voltage divider.

This ain't rocket science. I'm sure you'll find something that works for you.

The Twin Reverb output transformer runs @ 4Ω with the 4 6L6. With only 2 6L6 in the Super Reverb, it runs at 8Ω

[PRR]

...The potentiometer doesnt really work for me, as I want the switch to also be able to completely cut the negative feedback.

So wire the pot to ground, and take NFB off the wiper. You can take 100% down to zero % as you like.

[mresistor]

That's a cool new backplate - are you considering making more than one? 

[Darkpr0]

That's a cool new backplate - are you considering making more than one?

I will probably make one for myself, and see how it turns out. Didn’t really plan on more. If it turns out well, I can share the illustrator file for anyone who wants it