One-Bottle Preamp

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Well-known member
Jun 4, 2004
New York (Hudson Valley)
Here's a little something I whipped up during some lunch breaks and other stolen moments at work. This is based on a design idea that I presented originally over on the Drawing Board.



Hey Dave,

Looking good. I love the blue, reminds me of some old Pye pres and compressors I guy I know has. I think I need to build some of these!! What are you using for the power supply??

thats a nifty little box dave!

Did you design for maximum clarity or color? Looking forward to hearing that sometime!

Very nice little boxes. They remind me of those sweet guitar tube amps made by ZVEX.

Very nice design and beautiful build!

A thought and a question: Dave mentions the possibility of using a 4:1 output transformer. That would make the maximum output before clipping +18dBu (high gain) to +15dBu (low gain). I'd rather not; if balanced output is needed, I'd forego the 600-ohm drive and go with a 1:1 xformer.

The question: Dave, have you tried hanging a 500k resistor across the input xformer's secondary in addition to the zobel? And if so, how did it affect the response?

Nice boxes indeed !

Have done a quick look around for an European equivalent of the 12AV7 but didn't found anything yet in ECC/PCC/...-land. Different heating-requirements wouldn't be a problem. Anybody knows ?



This is for your ribbon, right ? Any testdriving with that or other mics yet ?
Is the 56dB gain enough for the ribbon ? Stop, enough question marks. :wink:
What do you think about this list as subs?


Found it here:

This looks really sweet!

I'd like to thank you again for all you contribute to the forum. This mere DIY mortal (the still learning kind) appreciates your straight forward, damn useful designs.


Thanks for the kind remarks, guys. I'll try to answer some questions...

Show us a pic of your point-to-point work

I built these into some little 2.25" x 4" Bud boxes I had laying around. If I had to do again, I would have used an enclosure about 1.5 times the size. Because of the small space, the internal layout is cramped and not up to my usual standards of neatness... so I didn't take a picture of the innards.

Did you design for maximum clarity or color?

I was aiming for most linear amplification within the limits of a one-bottle design with the particular tube, range of gain and operating voltages and currents I chose. Since it's a transformer-coupled tube circuit running with a relatively small amount of feedback--between 18dB and 36dB of feedback depending on gain setting--I figured the "color" would take care of itself.

have you tried hanging a 500k resistor across the input xformer's secondary in addition to the zobel? And if so, how did it affect the response?

Yeah... All it did was introduce about a dB of loss across the band
:wink: I found that this transformer performed well with secondary unloaded. This particular transformer (UTC O-11, connected as 200:50K) dates from the time when input xfmrs were usually run into unloaded grids. A more "modern" transformer might (or might not) do better with a resistive load on the secondary.

The Zobel tames a broad ~3dB rise centered around 16kHz. (The rise is correctly predicted by the curve on the UTC datasheet). Instead of trying to flatten out the rise completely, I compensated just enough to bring it down to 1dB, to keep my in-band response within the +/- 1dB spec. After compensation, 1kHz squarewave response looked very good, nice and square with only a slight overshoot. I didn't go for a "perfect" flat leading corner; I didn't want to clobber the high end.

equivalent of the 12AV7

Resources on the web suggest that types 5965, 7062 and E180CC are equivalent or at least close. Since those tubes were intended primarily for on/off service in computers, though, I would be prepared to sort for noise.

What are you using for the power supply??

Any well-filtered 300V supply capable of 7mA or more will do. Here's what I put together--it's pictured next to the preamp. The parts choices were driven mostly by what I happened to have laying around.


12.6VAC is NOT the best choice for the heater supply, however. 6.3VAC or DC, or 12.6VDC, would give lower noise with the 12A- family of tubes. I'll probably end up using this preamp with a DC heater supply most of the time. I didn't go with DC heaters on the little PSU because I wasn't sure if the transformer could handle the extra load of the rectification/filtering/regulation and I was too lazy to calculate it. :wink: The heater winding is rated to 300mA, and the 12AV7 draws 225mA.
Dave, This makes me rekindle my ideas of building a tube console, or at least a new front end for my old one. :guinness:

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