Cathode Follower Reverb Circuit

[printer2]

Late night back of a napkin idea, not too up on FETs so don't know if there is something obvious I am not seeing. Think the reverb tank has an 800 ohm input.

[printer2]

Impedance of the tank is too low, will use low voltage to power it.

[uneumann]

Impedance of the tank is too low, will use low voltage to power it.

Don't know how your FET idea will fair, but a simple transistor circuit at this link is working very well.

https://sites.google.com/site/string...ts/Home/reverb

There are many SS reverb options posted if you do some searching.

[Gregg]

I think you'll need to attenuate the signal before you feed it to the CF. Instead of the IRF you can use a Darlington made from 2 pcs of MPSA42 or similar.

[nickb]

Feeding the reverb signal into the far side of the PI will give you an unbalanced output. Not a huge deal but probably not what you intended.

[printer2]

Lot of ways to skin a cat, thought I could do it simply, actually might be able to with a step down transformer of the source follower, need to see what parts I have laying around. Nickb, not much of an imbalance and it is just reverb.

[pdf64]

I can't see that the global NFB loop will work (or at least work in the normal manner), as it isn't connected to the inverting LTP input?
The third input, at the tail, having rather low gain The Long-Tail Pair

[printer2]

I can't see that the global NFB loop will work (or at least work in the normal manner), as it isn't connected to the inverting LTP input?
The third input, at the tail, having rather low gain The Long-Tail Pair

Handy link. Need to look at it again when I have more time. Thanks.

[Wombaticus]

Late night back of a napkin idea, not too up on FETs so don't know if there is something obvious I am not seeing. Think the reverb tank has an 800 ohm input.

I'm not at all convinced that a cathode or source follower is the best way to drive a reverb tank! As reverb tanks are inductive current-driven devices it is (at least in theory) important that they are driven by a stage that has current-source-like characteristics in order to maintain some semblance of consistency with respect to frequency. I know that it is somewhat counterintuitive, but this implies that we want a driving stage with a relatively *high* output impedance, yet which retains the ability to source sufficient current into the tank. The low impedance output of a follower is not a guarantee that sufficient current can actually be sourced, and moreover acts much more like a voltage source than a current source. Check out this link over on AX84 where I (Paul Fawcett over there) discuss some of the trade offs in play. The discussion is in the context of tube drivers, but the same principles apply.

[pdf64]

Yes, this https://www.amplifiedparts.com/tech_...d_and_compared addvises that the driver source impedance should be >5 x the nominal 1kHz drive transducer impedance.
But a suitable series resistor could do that.

[Mike Sulzer]

i agree. As long as it has enough current capacity, a common cathode stage meets the requirements much better. As the impedance rises with frequency, this stage provides the necessary increasing voltage level. And yes, it is certainly true that an equivalent cathode follower (same tube, same DC current) provides no more AC current, but tries to do it by holding the voltage constant with frequency.

I'm not at all convinced that a cathode or source follower is the best way to drive a reverb tank! As reverb tanks are inductive current-driven devices it is (at least in theory) important that they are driven by a stage that has current-source-like characteristics in order to maintain some semblance of consistency with respect to frequency. I know that it is somewhat counterintuitive, but this implies that we want a driving stage with a relatively *high* output impedance, yet which retains the ability to source sufficient current into the tank. The low impedance output of a follower is not a guarantee that sufficient current can actually be sourced, and moreover acts much more like a voltage source than a current source. Check out this link over on AX84 where I (Paul Fawcett over there) discuss some of the trade offs in play. The discussion is in the context of tube drivers, but the same principles apply.

[Mike Sulzer]

Yes, this https://www.amplifiedparts.com/tech_...d_and_compared addvises that the driver source impedance should be >5 x the nominal 1kHz drive transducer impedance.
But a suitable series resistor could do that.

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.

[printer2]

I think the idea of using a source follower as I intended is not the way to go as if you pass enough current in order to drive the coil you are dissipating half your amp's VA's. Just too high a voltage. As far as acting as a current source rather than a voltage source, the resistor in the follower circuit as shown is roughly 100 times the tank impedance of 800 ohms at 1 kHz. I have some 25V line transformers that may help out getting the voltage down and the current up. I'll have to breadboard something together to see if it is viable, will not be for a few weeks as I have some house repairs to get done before winter hits.

Also thinking of changing the amp circuit. Thinking of an all octal with a 6SJ7, 6SL7 and a pair of 6W6's and a paraphase PI. Was thinking of doing a Bassman/Marshall circuit using some 12BK5's. I'll never use them for anything else and it might be interesting to see what they will sound like being run overvoltage. I have 30 of them so if I fry a couple I won't cry all that hard. Just a couple of bedroom amps to while away the hours.

[nickb]

...I got thrown off the forum.

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

[nickb]

..and then someone comes along and does it all wrong anyway...



The tank is an high impedance type (4FB3C1B). With the feedback, it looks like they went out of their way to get a low impedance i.e. constant voltage drive.