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Vox V941 Valve Effects Loop Buffer.

2/26/2006 12:42 PM
Paul Drexhage	Vox V941 Valve Effects Loop Buffer. I realize this isn't exactly an amp, but it's tube and most amps modern amps have effects loops, so this should be a pretty familiar circuit. My friend got this unit for free, didn't work, gave it to me to fix. I opened it up and the bypass capacitor (25uF/25V) on the cathode of the return stage had blown its guts all over the inside of the chassis. In addition, the other two resistors on the cathode of the return stage were compeletely smoked. Finally, the 10 ohm reistor that lifts the circuit ground from earth ground (chassis) was reading over 1K and had a nice little burn mark in the side. I traced the schematic and whipped up the following in MS Visio: http://www.rit.edu/~pfd6632/temp/Vox%20V941%20Valve%20Effects%20Loop%20Buffer.gif My question is this: What values of the two cathode resistors should I use? I tried the loadline approach for cathode bias as outlined on Aiken's site, but configuration of the two resistors in the cathode make me wonder how to approach it. I don't believe I'm looking for a high gain stage as typical in the pre-amp of most guitar amps. I almost would have assumed to see another cathode follower. Any help from you knowledgeable gents would be greatly appreciated. Thank you, Paul

2/26/2006 12:42 PM

Paul Drexhage

Vox V941 Valve Effects Loop Buffer.

I realize this isn't exactly an amp, but it's tube and most amps modern amps have effects loops, so this should be a pretty familiar circuit.

My friend got this unit for free, didn't work, gave it to me to fix. I opened it up and the bypass capacitor (25uF/25V) on the cathode of the return stage had blown its guts all over the inside of the chassis. In addition, the other two resistors on the cathode of the return stage were compeletely smoked. Finally, the 10 ohm reistor that lifts the circuit ground from earth ground (chassis) was reading over 1K and had a nice little burn mark in the side. I traced the schematic and whipped up the following in MS Visio:

http://www.rit.edu/~pfd6632/temp/Vox%20V941%20Valve%20Effects%20Loop%20Buffer.gif

My question is this: What values of the two cathode resistors should I use?

I tried the loadline approach for cathode bias as outlined on Aiken's site, but configuration of the two resistors in the cathode make me wonder how to approach it. I don't believe I'm looking for a high gain stage as typical in the pre-amp of most guitar amps. I almost would have assumed to see another cathode follower. Any help from you knowledgeable gents would be greatly appreciated.

Thank you,
Paul

Replies:

Peter HiHave a look at "Tube buff... -- 2/27/2006 11:06 PM