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Re: Westbury The Tube at GEO Function of diodes

4/12/1999 2:06 PM
John Greene
Re: Westbury The Tube at GEO Function of diodes
Roger: It would be positive feedback if it were connected to the grid of the first tube, but connected to the cathode, it is negative feedback... that is, if all of the signal wasn't going through the bypass cap to ground.  
You're right, silly me didn't think of the Cathode as a *non*-inverting input. I did say it was a little too early........ :)  
R.G.: John, you might want to do some sims with resistors of up to 10K there and possibly 0.01-ish caps just for grins.  
I tried it and it does result in some pretty interesting distortion. However, any signal that sees a somewhat high impedance from the diodes results in large voltage swings in a big hurry. With the diodes being mostly broad-band, the levels are kept down at a more reasonable level. The biggest tone shapers seem to be the 470k/470pF on the grid of the second stage and the .0022uF output cap.  
Also, if the 10uF cap is a 50volt as shown in the schematic, I don't think that it might have been connected to the plate. The voltage shown on the schematic at the plate is 65V.  
4/13/1999 5:54 AM

Let's try some variations that could be explained by a mistake in tracing the circuit or by copy-protection tricks.  
1) What if the cathode of the first triode is not bypassed and the cathode of the second triode is bypassed? Then the diodes are on the feedback loop like on a Big Muff.  
2) If you remove one of the diodes with the cathode of the first triode bypassed, then the signal from the output triode will charge the first triode's cap, changing the grid-cathode bias. This can work like a compressor or expander depending on what diode you remove.
4/12/1999 12:52 PM

"I wouldn't be surprised if it was done as an attempt to protect the design. If another company copied the design exactly, they could prove it by showing the diodes connected the way they are have no effect on the signal. The only reason someone else would put it in there is because they copied the circuit. "
That's a very reasonable assumption. In this case, I didn't even get to make my own mistakes (I do that enough as it is...) if they are mistakes, because the schematic I drew it up from does indeed show the cap polarity as it is on the gif.  
The schematic is a hand drawn one that was sent to me, but it has every appearance of being well thought out and carefully done. I see the possibilities as  
(1) there may have been a simple polarity mistake on the polarity of the 10uF.  
(2) there may have been a mistake and the 10uF may connect to the plate of the tube before the 0.0022, as one of you noted  
(3) Since reverse biased aluminum caps can act like a short, this could be another of the dirty tricks or circuit protectors that John refers to - the deliberate misuse of the circuit to be able to prove copying.  
(4) The dirty trick may have been more subtle. People see something about the right size with markings on them and just read the markings. What if the maker had some blue shrink wrap made up with the markings "+ 10uF 50V - " on it and put it on an appropriately sized resistor or film cap? It would take very close inspection to find the difference.  
A suspicious mind is a toy forever...  
John, you might want to do some sims with resistors of up to 10K there and possibly 0.01-ish caps just for grins.
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