Tube Amps / Music Electronics
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|6/4/2000 2:46 AM|
||Master volume for Deluxe Reverb clone?|
Finished an AB763 Deluxe Reverb (vibrato channel) clone(minus the normal channel, reverb, and tremelo at the moment). The amp is much louder than I expected. I want to try a master volume. Would a 1 meg pot between the 220k and 1000pF, just before the driver tube, work? OR what about a post PI stacked 500k I've read about?
|6/4/2000 6:30 AM|
If it is a single channel amp, you don't need that series 220K resistor.
You might be better off with a MV pot here with the wiper connected to the .001uF coupling cap,
but, I think you might like a smaller value pot, like a 250Ka there instead.
Try a 250Ka first or a 500Ka with a 510k resistor across the outside lugs of the pot.
If it sounds too constricted, then take the 510K resistor off.
Without the trem circuit and the other channel loading down the preamp , yours is probably too hot anyhow so I bet you like the 250Ka MV there.
|6/4/2000 10:39 PM|
I removed the 220k resistor and put in a 250K-L with a 50k resistor between the wiper and CW to simulate a 250Ka. Seems to work, but I've picked up a loud hum. The hum doesn't change with the volume control, but does with the new master. If I pull out tube 1 the hum stays, but if I pull either tube 2 or the driver tube the hum goes away.
What have I done?
There are only a couple of parts between the volume control and master control and I believe they all check out fine. Also I used shielded wire for the master control. I haven't been able to test the filter cap, which regulates both tubes 1 and 2 yet. Can't imagine that being the problem though. It was fine before.
|6/5/2000 1:56 AM|
You might check to see where you grounded the MV pot. You might also check the articles at DMZ and GeoFEX about adding resistors to a linear pot; I believe that a resistor between the wiper and CW terminal will simulate an anti-log taper. (For an audio taper the resistor goes between the wiper and the CCW (ground) terminal.)
--Hope that helps!
|6/5/2000 8:03 AM|
Yeah, possible ground loop with the MV pot.
And also make sure the wiper from the MV pot goes to small value DC blocking cap before it hits the grid of the phase inverter/driver tube.
|6/5/2000 8:24 AM|
For a pre-PI MV on a BF-style amp do you recommend that there be two coupling/blocking capsó one after the anode of the previous stage and one going into the PI? (I guess there is some DC on the input grid of the PI from the 1M resistor to the cathode connection- right?) With a Marshall-style amp the pre-PI MV is usually located after the tone stack so an extra coupling/blocking cap would not be required...
|6/5/2000 1:41 PM|
Thanks for the help. I thought I was beginning to understand this stuff, but I'm stumped again. BTW, there is a .1uF before the Master Volume, and a .001uF after. I also star grounded all the controls.
I now think I may have done something to the bias supply. I noticed the hum decreases when I adj the bias more negative. This clone is using a borrowed supply from pin 6 of the rectifier. It has a 33uF filter cap. The amp has 415V at the plates(to ground) and is set for -35V at the 220k resistors just before the 6V6's.
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