Reverb Noise

[Daver]

Dai,
I know this thread has gotten ridiculously long, but I did try replacing the 500pf cap. It's polypropylene. I also tried a 470pf silver mica. No help or change. Thanks.
Dave

[pdf64]

Just to confirm, are you still getting the 'farty, electrical kind of noise only when playing. It can sometimes sound ring modulator' thing going on?
Or is it the weird waveforms you are trying to get to the root of now?
Peter.

[Daver]

Peter,
Most of the farty, weird noise is gone except when the volume and/or dwell are near max. Then you can still hear a crackly, farty distorted sound. I don't really care how the waveform looks if I can get it to sound clean. Are the scope traces leading me down the wrong path? Thanks.
Dave

[Tom Phillips]

Dave,
IMHO I do think the scope traces are leading you down the wrong path. The traces you posted are what I would expect to see.

The “crackle” is puzzling but I think the farty distorted sound is to be expected with the volume and dwell both set high.

You have only posted the detail of part of the schematic and there are deviations from the standard Fender circuit in what you have shown. I suspect that there are other deviations and there is probably something going on that’s caused by the sum total of your complete circuit such as a little extra gain here & there. [I won’t repeat here but my comments of 9/5 still apply]

Later,
Tom

[pdf64]

Given the nature of the circuit / load, the scope traces on the plates of the reverb driver are never going to be nice sine waves at high signal levels, so you can just ignore it if they're not pretty and not causing a problem.
However, if that nasty, crackly blippy parasitic is still going on, then it's worth trying to identify what conditions cause it to happen, what the root cause is, and how to resolve it.
I would set up the sig gen and amp to push it into that fault condition, then try and observe what's going on with the scope, what the difference is between the traces with the signal level just below the onset of the distortion, and with the distortion happening.
And try a grid stopper on that second stage. There's a 360 degree phase shift between there and the reverb driver plate, and a whole lot of gain when the Dwell is set high, which is when the parasitic is occuring. Peter.

[jmaf]

Thanks jmaf

You might've missunderstood my original description of my circuit (schematic attached) - my fault probably for making it poorly worded. I think this because I can't quite figure out why you factor in a 47k plate resistor. If I am correct in my guess in this regard, and if I take that out of the equation, does that mean the two boundary load lines are 1k2 and 21k (assuming those specific 'boundary' frequencies)?

(Or am I missing the point completely, and is there some reason to factor in a 47k value in the load line function for the RT driver stage, which I as yet can't understand)?

Cheers

Tubeswell, on your original question you said you had a 47k load resistor on the first stage, without looking at the schematic I assumed the 47k resistor was in series with the inductive load.

In your case it's only an inductive load(with a lower than 1K DC resistance I would guess), I would just use those 2 reactive lines. It wouldn't work like that in practice, even though you have a bypass 25uF cap setting AC ground, you still have the cathode resistor setting a higher DC bias for the tube. So make sure you measure DC voltage from pin 2 to 3, not from pin 2 to ground, to check your operating BIAS.

The theoretical lower frequency load will be ridiculously off the chart(~ 300 mA). The 500pF high-pass cap will make your actual system will work closer to the 21K loadline. Once you have the slope of the line just scroll the ruler parallel to that line and you'll have a very close idea to where you're working.

Note that working with 435 V DC and -6.8 V DC BIAS you have very little linear space to work on and you'll be linear on higher frequencies mostly, you'll need to work only with the higher frequencies and attenuate the current via a cathode resistor OR an added anode resistor like I suggested for DaveR.

[Daver]

OK, I put this aside for a while and did other things. I dove back in and I'm now going to call this resolved. Here's what I did.....

1 Put away the scope and only used my ears.
2 Added a 1M resistor to the Dwell pot between the center wiper and ground to help lower the input signal a bit.
3 Raise the reverb driver stage cathode resistor to 3.3K from 2.2K.
4 Added a 100pf cap across the reverb transformer secondary/tank input to supress ringing.

Now if the Volume and Dwell are maxed and you're really strumming hard with a hotter pickup, you might hear a crackle of distortion. At "normal" settings it's fine and using an overdrive pedal works just great. So, I guess some of this is just the nature of the reverb beast. I'd like to thank all those who helped! You know who you are.
Dave