Reverb Recovery Stage- Why are they so noisy?

[tdvt]

Going though an recently completed Hoffman AB763 that works well (but could always be quieter...) & while prodding around, the reverb recovery mixing stage still seems quite sensitive (V3B in this instance).

That side of the tube is all somewhat microphonic: leads to the socket, plate resistor, etc.

While researching an earlier noise issue when I first fired this amp up, there were countless threads describing issues centering around this area; from roughly the 10p/3.3M junction to that tube.


A month or so ago I had similar issues with a vintage BF Princeton, essentially identical circuits (other than a 220K), all the noise was in & around that tube as well.

With only a basic understanding of amplification, I would think that the guitar signal entering the amp would be the most sensitive point (& maybe it is) but best practice seems to eliminate most problems there.

How does the signal from the reverb tank compare in strength to the original signal from the instrument?  I know the tank doesn't put out a lot but wondering how big it actually is.


MOSTLY UNRELATED QUESTION but REVERB related:

Doug has a 3W cathode resistor for the reverb driver, which I only noticed (& I didn't use) Fender doesn't specify a power rating, so I am assuming it is a 1/2W. Best practice for that ?
 

[sluckey]

How does the signal from the reverb tank compare in strength to the original signal from the instrument?  I know the tank doesn't put out a lot but wondering how big it actually is.

The tank output signal is lower than the instrument signal.

Doug has a 3W cathode resistor for the reverb driver, which I only noticed (& I didn't use) Fender doesn't specify a power rating, so I am assuming it is a 1/2W. Best practice for that ?

I used a 1/2w resistor in my TDR. 1/2 watt should be sufficient but it does get warm. Doug has a lot of repair experience with Fender amps. He may have had to replace that resistor before. The reverb is a simple SE power amp and if things go wrong, that cathode resistor just may burn up. So, a 3W flame proof resistor is not a bad idea, even though it's way overrated.

[tdvt]

Thanks for the insights. Interesting the signal is that diminished.


RE: AB763,  Maybe I'll up the rating of the reverb driver cathode resistor out of caution, see what is in stock

[PRR]

> Interesting the signal is that diminished.

The attenuation through the tank is like 1,000:1. The maximum input is a few Volts. So it comes out a few milliVolts, against dozens/hundreds mV from a guitar.

[tubeswell]

The lower input signal means the reverb recovery stage has higher S:N ratio than the input stage, and therefore (as S and N all get amplified), the noise shows up more. You can experiment with reducing the proportion of noise, e.g by using a big (e.g. 200uF) cathode bypass cap, or ‘less noisy’ (e.g. metal film) load resistors.

[tdvt]

I guess I had always assumed that reverb process was happening at a more robust level, since it is pretty far into the signal chain.

[sluckey]

The drive signal is quite high, but the output signal is quite low.

The output signal from the tank should be about 1 to 5 mV. Use a preamp circuit with flat
frequency response for recovery.

I've attached the pdf for your reading pleasure. Explains a lot of stuff about reverb...

[PRR]

...the reverb recovery stage has higher S:N ratio than the input stage...

That may be backward?

[tdvt]

I've attached the pdf for your reading pleasure. Explains a lot of stuff about reverb...

Thanks, that is quite comprehensive,

Great reference

[Williamblake]

I would think this may apply to 12ax7s, too, but i do know that tube rolling to achieve minimal noise in the reverb recovery circuit makes a difference with EF86s.

[sluckey]

It may be worthwhile to point out that Fender used a low noise 7025 tube for the reverb circuit in all but the lowly Princeton Reverb.