Reverb Power

[loudthud]

Has anyone ever measured the power output of a Fender Reverb Driver ? I'm thinking a Blackface Fender with a parallel 12AT7 would make about 1 Watt. The 6G15 with a 6K6GT maybe 2 Watts ? Is that much power necessary for a real Surfy Reverb ?

[Leo_Gnardo]

Haven't measured, and my guesstimates match yours. In each case I think there's well more than enough power to achieve an acceptable degree of surfiness and go far beyond into cowabunga territory. Less is plenty enough. Geeze, I'm soaked! I mean stoked! Hang ten dude. Or eleven if you got 'em.

[Mick Bailey]

When I designed my low voltage standalone mainly tube reverb I used a MOSFET constant current driver capacitively coupled to the tank input. A tank needs to be spanked pretty hard to get a good surf sound and the tank is more efficiently driven throughout its frequency range when using constant current (and gives a better response). I like an adjustable dwell to be able to drive the tank into distortion if desired - the input transducers are pretty robust and hold up exceptionally well. I've always thought of a regular 12AT7 driver as being about 1W.

[g1]

Perhaps less would do it, but how much less? I find the op amp driven circuits lacking, and note that there have been solid state versions with mini power amps instead (Yamaha), or paralleled op amps for drive (Yorkville).

[Helmholtz]

Obviously Leo thought/found the power capability of a single small signal triode not to be sufficient.
As reverb transducer impedance increases proportional to frequency, drive power strongly depends on frequency (and input vol. setting).

[Tom Phillips]

Has anyone ever measured the power output of a Fender Reverb Driver ? I'm thinking a Blackface Fender with a parallel 12AT7 would make about 1 Watt.

Yes. I did that once and saved the following note.
I used a 1966 fender Pro Reverb amp to test the actual drive signal to the reverb tank.
When driving the amp to produce 40W into a 4 Ohm speaker test load, the signal out of the reverb drive into an 8 Ohm test load was 800 mV RMS. This is only an 80 mW drive signal.

The result really surprised me. I'll look for any measurements that I made on a 6G15.

[Chuck H]

On the couple of reverb amps I've built I didn't concern myself with really high drive capability. Perhaps a bit less than a typical BF circuit and they sound just great for a normal reverb effect. Most BF amps will actually get that surf sound (or damned close) adjusted for max clean volume so it seems that more is better for THAT tone. I did build a "reverb only" amp for a customer that filches power off the host amp and targets tank drives at about 1W with more of less being possible (dwell adjustment). The customer adjusts it for about 1W as per my instruction and says it's the best reverb he's ever had. So, opinions are subjective but for my experience you can get good reverb sound at lower drive levels but best results mean more drive is better up to about 1 watt and perhaps a bit more if you want it really surfy.

[Tom Phillips]

I found some old notes I saved, circa 25-30 years ago, when I measured the spring drive power of a vintage Fender 6G15.

Applying a 50mV, 2KHz signal to the input with the dwell control maxed out, I measured 1.09V across an 8 Ohm resistive load in place of the reverb tank. The calculated drive power is therefore, 0.15W which is about twice the power I measured using the Fender Pro Amp. This is still much lower than the power levels talked about in on line discussions so my curiosity is peaked.

I also realize that the overall sound is dependent upon the actual impedance curve of the reverb tank's drive transducer. I'll plan on making more detailed measurements the next time I have a 6G15 in the shop.

Cheers,
Tom

[Helmholtz]

At 2kHz the reverb transducer impedance will about double.
At 1kHz the transducer impedance is about 10 times the DCR, so almost completely inductive (even more so at higher frequencies).
At lower frequency impedance is lower, drawing more power from same voltage.

A traditional single coil PU can put out around 100mVrms. Of course there are much hotter PUs.
Doubling input signal means 4 times reverb drive power.

So I think drive power capability or headroom should be larger than your measured numbers.

[Tom Phillips]

It was a long time ago and, now, I have no idea why I used those test values. I've put it on my list to make a better set of measurements some day.

[pdf64]

I recall the Mk series Mesa Boogie amps have a good rep for their reverb, however the different models use different arrangements, from a Fender similar dual 12AT7 to a single 12AX7 section (MkIII).
I assume it's a regular Fender spec reverb transformer.
The transformer primary typically gets its HT feed via a non decoupled 10k or 22k resistor.

https://el34world.com/charts/Schemat...oogie_mkii.pdf
https://el34world.com/charts/Schemat...ogie_mkiii.pdf
https://el34world.com/charts/Schemat...oogie_mkiv.pdf

[Merlinb]

The concept of output power doesn't really work for reverb. The tank is an inductor, so the voltage and current are not in phase and the power in the tank is theoretically zero. What matters is drive current, which is 'just enough' in the Fender driver.