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Building tube overdrive...need advice.

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1/5/2000 4:36 PM
keith	Building tube overdrive...need advice. I'm considering building either a Westbury w-20, or a Guyatone Tube Overdrive. Has anyone used or built either of these, and can you give your opinions on the tone quality. I found the schematics at GEOFEX. Also, does anyone know whwere I can get the PT for either of these? Thanks! Keith

1/5/2000 5:37 PM

You can use two filament transformers back to back:

120V:12.6v ==> 6.3V:120V

The tube filament comes off the 12.6V and there will be 240VAC on the second trans. You can get these at mouser, etc.

A simpler approach for experimentation might be get a P-T442 from Antique Electronic Supply (www.tubesandmore.com) for less than $15.

Its secondaries are 125V, 15 ma and 12.6V, 0.3 amps which could power two 12AX7s. The B+ will only be maybe 160V, lower than those schematics, but this is enough for good tone.

1/5/2000 6:33 PM
Bill	The lower plate voltages will probably help with the amount of noise. Aside from the two 6.3V arrangement, Mouser does stock a transformer that delivers 230V in a CT package that can deliver 25mA. It's only 7/8" high and less than $12. Look at the bottom of page 259.

1/6/2000 12:46 AM
Keith	Thanks for the heads-up, guys. I'll look into these.

1/6/2000 1:03 AM

Re:trannies, hold up!

You can use two filament transformers back to back:

120V:12.6v ==> 6.3V:120V

I've put this idea up, RG amongst others warned me and they are right. If you use two 12V trannies back-to-back, you'll get your wall supply back, but going up (as in 115>12-6>230) won't work: these transformers are made to deal with certain amplitudes and wattages, and feeding 12V into a 6V "primary" will clip, and may fry the transformer...
I am stating this right guys?

1/6/2000 12:39 PM

Do you know of any single transformer that would supply both the filament voltage and the plate voltage (~230V)? On Hoffman's catalog page he sells something called a preamp PT. No voltages listed. Is anyone familiar with this item? Also, is anyone familiar with the two devices I mentioned in the first posting of this thread? Which is better, or are they similar? The schematics would suggest they are. Thanks!

Keith

1/6/2000 2:36 PM
R.G.	Hammond sells several that work. 262B6 7VA, 120Vac@43ma, 6.2V@0.3A 262B12 7VA, 120Vac@27ma, 12.6V@0.3A 262D6 12VA, 120Vac@53ma, 6.3V@0.9A and several others. There's a whole range of these. Stancor also makes the ??PT8914?? which is about one or two 12AX7's - it's what was in the Alembic preamp. These are all open frame, smallish tranformers.

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