Cathode Follower Reverb Circuit

[Mike Sulzer]

Open forums, closed minds... You're a smart guy, we're lucky to have you around.

Thanks. Hope I do not wear out my welcome.

[Wombaticus]

Yes it can, if you have both sufficient voltage and current capability. I once suggested on the MIMF electronics forum that if you had a suitable power supply voltage available, you could use a one chip audio power amp as a general purpose tank driver, adjusting the series resistor as required for the particular tank impedance. I got thrown off the forum.

I've done this with the good old LM386 chip amp running off a 12V supply that was also used for relay switching and DC heaters. Worked just fine, although 12V supply is about as low as you would want to go..

[Enzo]

Thanks. Hope I do not wear out my welcome

Well, if you ain't done it yet in 8 years and 2000 posts....

[printer2]

Sort of what I had in mind once I realized I have too low an impedance tank. Wonder what the impedance ratio of the transformer was? Well that was an easy search.

[66 Kicks]

Back to the original schematic, an IRF820 Source Follower can be made to drive an 8E tank.

The 470pf / 1M input combination is taken from the classic Fender design. If you connect it ahead of the tone stack, you may want to go with a smaller input capacitor.

The 4.7K resistor in series with the tank alleviates the rise in current as the frequency decreases below 1KHz, but it also makes a voltage divider that requires 25Vp on the Source to get 4.0Vp at the tank at 1KHz. A smaller resistor in series with the tank would require a Source resister smaller than 30K which would cause the IRF820 to draw more than 8ma. I wouldn't want my SS reverb driver to draw more than 8ma.

This driver will supply an 8E tank with more than the recommended nominal current, but not a whole lot more before the signal at the tank starts to distort.

[printer2]

Back to the original schematic, an IRF820 Source Follower can be made to drive an 8E tank.

The 470pf / 1M input combination is taken from the classic Fender design. If you connect it ahead of the tone stack, you may want to go with a smaller input capacitor.

The 4.7K resistor in series with the tank alleviates the rise in current as the frequency decreases below 1KHz, but it also makes a voltage divider that requires 25Vp on the Source to get 4.0Vp at the tank at 1KHz. A smaller resistor in series with the tank would require a Source resister smaller than 30K which would cause the IRF820 to draw more than 8ma. I wouldn't want my SS reverb driver to draw more than 8ma.

This driver will supply an 8E tank with more than the recommended nominal current, but not a whole lot more before the signal at the tank starts to distort.

Sold! Going to have to try it. Thanks.

[printer2]

Did an amalgamation of some past ideas. In tweed position of the first switch you get a 5E3 type input and the second switch goes between a Tremolux 5E9A (paraphase PI) or 5G9 (Long Tail Pair fixed bias). The other position for the first switch would be for a Blackface amp, LTP for a cleaner sound, Paraphase for a little unbalance. Power supply values need to be worked out, originally was going to go 6W6GT for the power tube but may do a 6K6GT. Guessing the reverb as shown might be a problem for the Tweed position and anticipate putting it across the 1M resistor between the two switches for BF mode. A little more complicated amp than I originally was going to do, I wanted to replace the BF/Tweed Champ I gave to my brother and have reverb added.