Tube Amps / Music Electronics
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|5/29/2000 6:06 AM|
||Adding MV (or PC?) to cathodyne PI|
In converting my Electar 30 into a Wreck Express with 2 cathode-biased EL84's I was wondering about adding a MV to a cathodyne driver similar to the Princeton AA964. Any suggestions?
How about a crossline MV similar to the Matchless Spitfire? A single-ganged 1M pot wired as a variable resistor between the outputs of the PI coupling caps... Possibly on a push-pull pot to switch in a cap for a Vox/Matchless Cut control instead of the MV?
What about the type that replaces the bias resistors with a 100k or 250k dual-ganged pot- will that work with cathode bias (with the "bias" resistors connected to ground instead of a bias voltage)?
Or just use the type with an extra pair of coupling caps and a dual-ganged pot?
Actually, I'd rather have a Presence Control but I have no idea how to wire it up to a cathodyne driver.
|5/29/2000 6:18 AM|
Here's how I wire up a prescence control using a cathodyne PI........run your feedback from the OT thru the feedback resistor and tie it into the top of the cathode resistor on the stage preceding the PI....at the same point in the circuit(top of the bias resistor in the stage preceding the PI) also wire a 50ohm pot with a .1uf cap in series from the pot to ground. The leg of the pot that is connected to the cap gets strapped to the wiper terminal. If you want....I'll draw you a schematic and e-mail it to ya.
|5/29/2000 6:59 AM|
The stage immediately before the cathodyne driver is the 3rd stage of the Wreck Express preamp (which uses an Rk of 10k) so I'm not sure that I want to mess with it... But I will draw in your mods on a copy of the Princeton schematic so that I can picture it better.
BTW in the BFPR AA964 schematic, the preceeding stage uses 1k5 Rk and 25uF Ck, but they are floating on top of a 47 ohm resistor to ground. (The NFBL connects to the top of the 47 ohm resistor.) So I would remove the 47 ohm resistor and replace that with a pot in series with a 0.1uF cap? Any adjustments to the 2k7 NFB resistor?
|5/29/2000 7:47 PM|
You could probaly do it that way, or just parallel the 50 ohm pot to the 47ohm resistor. Some experimentation would be in order here.
|5/29/2000 2:24 PM|
It strikes me that with the phase inverter you're using, yet another method for master-volume control may be possible; a large-value (2 to 5 meg) pot/capacitor/resistor combination connected between the plate and cathode of the PI tube. The series resistance would prevent going to zero volume, the cap would block DC, and the pot would be wired as a variable resistor. Unlike the crossline master, this one would achieve its effect mainly by negative feedback and not (for the most part) loading. I've never tried it myself, but it might be worth a shot. It will be more of a 'volume trim' then a max-to-silence master, but that may be enough.
Also, why not connect a similar network (all different values, of course) across the 10K cathode resistor of the previous stage? There are many ways to skin the 'presence' cat; no rule says it must be in the power amp negative feedback loop, and with a 10K resistance to work with, I can pretty much guarantee you will be able to get way more boost than you probably need.
|5/29/2000 7:27 PM|
Steve, look at the Princeton driver tube section and see the 1m grid load resistor on the triode ahead of the cathodyne driver?
Change that to a 1Ma or 500Ka pot and you have the master volume on my modified 15w Soulkicker amp.
It is NOT a major distortion producing MV but works pretty good and cheap!
|5/30/2000 6:40 AM|
I think that putting a MV ahead of the 3rd stage on the Wreck Express preamp would drastically effect its sound so I'm adding an extra eyelet after the coupling cap that goes into PI in case I want to try adding a MV there later. I'll file your suggestion away for future use on a BFPR-style amp...
P.S. Many cathodyne drivers use a 1M resistor between the grid and the tap on the cathode... any way to replace that fixed resistor with a pot or variable resistor to use as a MV? (I figured that it might unbalance the two phases...)
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