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Vox V941 Valve Effects Loop Buffer.

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2/26/2006 12:42 PM

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

2/27/2006 11:06 PM

Hi
Have a look at "Tube buffered FX Loop for Marshall amps from Kurt's Amps"(fx_loop.gif[66k])at
http://blueguitar.org/schems.htm under MiscAmp Mods.
My train of thought would be what is the level going into the return jack?
If its a stomp box they are designed to feed "high gain stage" or guitar input but if its pro +4 gear obviously less gain is required.
A guess based on Kurts circuit
1k & 1k5 to ground?

2/28/2006 1:55 PM

Hi Peter,
Thanks for the reply and the info. Your suggestions for values seem to correlate with the remains of the resistors, one of which seems to have a brown band on the end, suggesting a value of "1-something".

As far as the use of this unit goes, it seems to be intended to be placed in series with an existing amplifier's effects loop in order to provide a "tube sound" to the overall loop. Regardless, I see the two stages as providing a buffer for each end of the effects loop, rather than overall gain, and certainly not distortion.

I'm not familiar with the way the cathode is set up on the return stage of this unit. I was curious to know if the effect of the 25uF bypass capacitor (presumably to increase gain) is negated by the use of another resistor in series with it. Certainly I could change the circuit to a more familiar configuration, but I'd like to keep this unit as original as possible, if only to hear what it it supposed to sound like.

Thanks again,
Paul

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