Tube Amps / Music Electronics
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|8/10/1999 9:16 AM|
||Crybaby GCB-95 buffer ?|
This is the first time I post a message. I'm a french student so, please, excuse my mistakes...
I have 2 questions:
1) On the very interesting Justin Philpott site, I found the GCB-95 but I looked in mine and it seems there is a 2M2 resistor in parallel with the 22pf condensator in the buffer stage. Is this correct ?
2) Could anyone explain me WHY this #$%! buffer sucks all the treble? I have written some (problably wrong ...) formulas and even at 10kHz, omitting the .01uF cap, the input impedance is 418k. Seems like a quite good buffer . Good enough to keep my treble. So WHY is this buffer so bad??? I think R.G.'s mod with a JFET should work but I want to understand why the stock buffer is not good enough.
I'd like to thank all those wonderful people who created those excellent sites (GEO,AMZ,...)
|8/10/1999 12:05 PM|
You should try putting in the true by-pass mod.
|8/10/1999 12:34 PM|
My amp-tech friend called me one day about one of these.
1) he found the 2M2 resistor there as well. The 22pf is there to shunt RFI to ground, otherwise the thing picks up your local radio stations.
2) I don't know why the treble losses are so high. The one my friend asked me about had no noticeable treble loss, just a huge amount of hiss. I think that the darlington had been damaged somehow.
You do know that the JFET buffer fits right in the PCB where the darlington buffer is now, don't you? Remove the 1M8 pullup resistor and put a low noise JFET in there. - that's all there is to it. That's what we did to the noisy one, and it got lots quieter.
It is possible that your darlington has been damaged as well. You could try replacing the darlington with another MPSA13 to see. You could also stuff in the JFET buffer.
|8/10/1999 9:22 PM|
Well, thank you, Joe & R.G.
I'd like to add 2 things
1) A friend of mine has the same Crybaby and the same problem. Even in french guitar magazines, the GCB-95 is known for its treble loss in bypass mode.
2) Actually, I've opened my Crybaby to do the JFET mod that I had seen at GEO. When I saw the MPSA-13, I asked myself "What is wrong with this ???". In fact, it's just an understanding problem. I will sure do this JFET mod but I'd like to understand why...
|8/10/1999 11:10 PM|
|8/11/1999 8:18 AM|
I had the same problem on mine and put in a true bypass (DPDT switch) which made a hell lot of difference.
|8/11/1999 4:14 PM|
Thank you for your help
In fact, I know that a DPDT is a perfect bypass (just a wire between input and output - no connection with the circuit) but I don't want to spend too much money (sorry, I'm a little bit mean...). I don't often plug my Crybaby and I think a buffer should be enough for me.
Moreover, I'd like to understand WHY all those darlington buffers are not enough to do a good bypass (input impedance is high enough THEORETICALLY )
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