reverb tank in/out power?

[Chuck H]

Hi all.

I don't have a "Fender" type reverb in an amp to do my own testing right now. I'm in a remote location (remote from my bench anyway) and I need to do some design work.

Can anyone tell me how much power is commonly used to drive a reverb pan in a typical BF Fender type circuit? Also, what would the pan output be?

I checked the Accutronics site but they say that the nominal power to drive their 8 ohm pan is 28mA (which would be only 6.3mW!?!) and the output would be between 1 and 5 mV at this drive power. But it seems to me (by looking at the circuits) that Fender drove the tank much harder than this. Any one happen to know this obscure info?

Thanks

Chuck

[Merlinb]

Reverb driving is a bit of a black art. I tried to do a little justice to it here:
The Valve Wizard

[Chuck H]

Excellent tutorial. Thanks, I think it will get me through this (though it also raises new design considerations that I must now deal with). Imformation is power as they say... So, how much information does it take to drive a reverb pan?

Chuck

[imaradiostar]

I've often wondered about driving verb without a transformer. I built a few circuits using high Z or med Z tanks from solid state amps but I was never totally happy with them- probably because I was trying to build them ampeg style with no series resistor between the plate and the tank.

I've been toying with a few other ideas recently but haven't had a chance to build them. After my successful mosfet phase inverter I really want to build a mosfet verb driver.

The output Z of a mosfet is more or less 1/gm so reeeeeeally low- surely low enough to drive a verb tank. Is there any reason why we can't just put a voltage divider and a cap at the gate and drive the tank from a big cap at the source? The gate could be driven from a higher voltage point in a circuit (output of the preamp in a fender, cathode of the Cf in a marshall) or the plate of a preceding triode gain stage.

Initially I thought this will never work because you'd end up having to use a low value for the source resistor to get sufficient load for the driver to work correctly. Now I think it'll be fine because you can use a series resistor to connect to the tank and if you've got 200-300 volts to play with an a mosfet that can handle all kinds of power without breaking a sweat, why not just use your 1k or whatever source resistor but bias the mosfet so the source is sitting at 1/10 to 1/5 of supply voltage.

once again...need to build it and see if I'm smart or just crazy.

jamie

[Chuck H]

The thing I'm designing is an outboard reverb for a friend. He wants to crank his amps for good overdrive and still be able to get reverb. So this will be a seperate reverb only amp. I thought it might be clever to just filch a little power from the dry (host) amp to drive the reverb tank. He has amps of many wattages so this circuit needs to be flexible in this regard. This is the input circuit I came up with. Does anyone see a problem with it or have improvement ideas?

Thanks

Chuck

[imaradiostar]

that's a cool idea!

I had an amp that I tried to do this with. I wanted to try a cap into a rheostat for a fixed bass cut but the ability to vary the dwell. Never got around to it.

jamie

[Enzo]

The power needs of the reverb pan are minimal. A simple voltage divider off the amp speaker will drive it.

This is not a new idea, Hammond Organs and others did this exact thing decades ago. Separate amp for the reverberated signal, pan driven from dry speaker.

There is this:
converting Hammond reverb to guitar

http://www.captain-foldback.com/Hamm...atics/AO35.JPG
Check the note on the schematic about the spring drive.

And a different drive scheme
http://www.captain-foldback.com/Hamm...atics/AO66.jpg

http://www.theatreorgans.com/hammond...cs/122vb-a.pdf

[olddawg]

Originally Posted by Enzo

The power needs of the reverb pan are minimal. A simple voltage divider off the amp speaker will drive it.

This is not a new idea, Hammond Organs and others did this exact thing decades ago. Separate amp for the reverberated signal, pan driven from dry speaker.

Didn't the original Maestro and Magnatone reverb units of the 1950s do this? You attached the input of the reverb amp to a combo amp's speaker with aligator clips on the ends of a longish length of speaker wire. The voltage divider was in the reverb amp driving a pan, small tube amp, and speaker.

[Enzo]

I don;t recall myself, but I wouldn;t be surprised. The approach has occured to various companies over the years.

[Chuck H]

Thanks Enzo, I knew that Hammond and Gibson had both done it this way. I was going to look those schems up just for reference, so you saved me the trouble . The only reason I'm not going with a simple voltage divider is that I know that when these pans fail it's usually because of too much current in the low end that the pan can't use anyway, but the inductors hate. That and this needs to be adjustable. So it follows that I need circuit protection in case it is mis adjusted by the user. I won't complicate it too much, but I guess I am trying to build a better mouse trap to some degree. My initial research is showing that I may have trouble finding zeners with a ZV low enough though.

Thanks

Chuck

[Chuck H]

No, it's obvious that I can't get zeners with a low enough ZV. Anyone have an idea how I can protect the transducers and VU meter? Do you think the "lamp" circuit will be effective over a wide adjustment range without having an audible effect on the host amp? This thing does have to be adjustable...Sigh. Always a rub.

Chuck

[olddawg]

Originally Posted by Chuck H

No, it's obvious that I can't get zeners with a low enough ZV. Anyone have an idea how I can protect the transducers and VU meter? Do you think the "lamp" circuit will be effective over a wide adjustment range without having an audible effect on the host amp? This thing does have to be adjustable...Sigh. Always a rub.
Chuck

Couldn't you use a simple clipping circuit with two diodes in opposite directions to clamp the input signal for 700mv for silicon and 300mv for germanium? Then use a voltage divider after that if you need a lower signal level. I used to use these "crowbar" limiters on the inputs of active crossovers and other stuff I built and didn't want some fool to blast them. It will let you know if the input exceeds the propogation voltage though by clipping the signal like a fuzz. You can even incorperate a limit indicator LED. If you want more headroom use more diodes. 4 will give you about 1.3v headroom for silicon.

[Chuck H]

I'll look into that. I'm not familiar with diode circuits (other than zener function and power supply circuits) because I've only ever done simple tube circuits. I've never built a diode clipper. I wouldn't even know which diodes to use. But some searching will probably reveal a tutorial or some example circuits.

Thanks

Chuck

[olddawg]

Originally Posted by Chuck H

I'll look into that. I'm not familiar with diode circuits (other than zener function and power supply circuits) because I've only ever done simple tube circuits. I've never built a diode clipper. I wouldn't even know which diodes to use. But some searching will probably reveal a tutorial or some example circuits.

Thanks

Chuck

Just about anything will work. I've used 2 legs of 2 small bridge rectifiers for a 1.3 volt clamp. Small cheap power supply rectifier diodes work fine.

[Chuck H]

Yeh, I've done some searching and what I'm finding is really generic as far as "which diode". I'm interpreting that the foreward voltage drop exhibited by any typical diode doesn't even start to conduct until that voltage is reached... Is this correct??? My concearn was how to figure the voltage I need PLUS the voltage drop and how any given part # would effect that (since I'm working with such small voltages). But I guess that's not the case. Seems pretty simple at this point if I understand this right.

Chuck

[olddawg]

Originally Posted by Chuck H

Yeh, I've done some searching and what I'm finding is really generic as far as "which diode". I'm interpreting that the foreward voltage drop exhibited by any typical diode doesn't even start to conduct until that voltage is reached... Is this correct??? My concearn was how to figure the voltage I need PLUS the voltage drop and how any given part # would effect that (since I'm working with such small voltages). But I guess that's not the case. Seems pretty simple at this point if I understand this right.

Chuck

Yeah pretty much thats it. The diodes won't conduct until they reach their propogation voltage. For example if you want to clamp the input not to exceed 1.3v you use 4 diodes, two in series with the pairs biased in both directions across the input. A simple limiter for about 20 cents.