One-Bottle Preamp

[Svart]

anyone? :green:

 

[CJ]

Talk to Bill Yacey, even though I have not seem him here for a while. Maybe his Johnson blew a head gasket somewhere in the backwaters of Lake Titicaca.

 

[Svart]

damn CJ.. you need some meds...

or need to get off em..

:green:

 

[tk@halmi]

[quote author="NewYorkDave"]No interest in my little 12AV7 jobbie?
[/quote]

Dave,

Is that 100K gain pot an audio taper one? I just found a few 100K rlog pots in someone's basement.

Thanks,
Tamas

 

[csonics]

Hey New York Dave,

Are C3 and C5 electrolytics?

-mike

 

[NewYorkDave]

are they the same that are in the schemo? I'll need to find some.. any suggestions?

Any good mic input transformer of 1:10 to 1:15 ratio should work for the input. Be prepared to adjust the compensation network on the secondary on test. Some input transformers might "prefer" to work into a solid resistive load on the secondary as well. I left it off on my prototype since the "Ouncers", like most input transformers of their vintage, were designed to work into an unloaded grid. Adding a grid resistor did not improve performance in my case, it just cut the amplitude a tiny bit.

If you want an output transformer, an Edcor WSM10k/600 should do the job for about 8 bucks :wink: I like the idea of building the preamp as shown and adding a 1/4" switching TRS jack between the coupling cap/tiedown resistor and the output transformer primary, to give you the option of bypassing the output xfmr (and its associated voltage loss) when unbalanced operation is acceptable.

Is that 100K gain pot an audio taper one?

Yes, audio taper.

Are C3 and C5 electrolytics?

Yes. It wouldn't hurt to use a film cap for C5, but good luck in finding one in that value and voltage rating. It should not be a lower value than that, since it will affect the phase/frequency characteristic of the feedback loop.

 

[csonics]

New York Dave,

Would you consider this a more transparent or colored pre? I really want to put a bunch of these together for a preamp stage on a desk. I know that the trannies I use will have an effect on that as well but I was just curious as to your thoughts.

 

[NewYorkDave]

The tube circuit itself is enclosed in a feedback loop and is pretty clean, in the relative sense, so the transformers used will have a huge bearing on the "sound" of the preamp, I'm sure. To my untrained ears, mine sounds "warm" but not "wooly", if that makes any sense at all. (That's why I hate these subjective descriptions, they're nearly useless).

 

[csonics]

Thanks NewYorkDave,

I'm thinking of slapping some Lundahls on the front end of these babies and whipping up a batch of 8 or so.

-Mike

 

[AnalogPackrat]

I've got both NYDave's 12AV7 and CJ's 417A pre on my "to build" list and I've been collecting parts (got tubes and some passive stuff already). NYDave: Would an Ouncer O-1 be preferrable to a Triad JO-3 for input on your design? CJ: what about yours? Do either of you have recommended output iron (something Sowter or CineMag would be most convenient).

I remember the discussion about CJ's design requiring a gapped output tranny which could take 20-30mA (big, rare, expensive). Can't you just AC couple it ala LA-2A? Or is there a fundamental thing I'm missing about impedance matching? I'd like to get a balanced output from the thing for a reasonable $$$. Insights much appreciated.

Analog Packrat

 

[CJ]

Try the 417 circuit with just a cap output and see what you think. It sounds good to me.
cj

 

[AnalogPackrat]

I don't doubt that it sounds good--just wondered about the tradeoffs in getting balanced out using a tranny. Why can't the output be AC coupled followed by iron to avoid DC in the output and its associated problems? I'm looking forward to building the thing.

 

[CJ]

Yes you could do either a cap into a transformer or a choke-cap-transformer parafeed circuit. You need some henries to match up with the plate but not as many as your typical tube, as the plate resistance is a lot lower, I remember someting like 5 k ohms.

 

[Svart]

CJ: what turn ratio on the iron do you suggest for balance out? I'm going to need balanced out on these..

 

[AnalogPackrat]

I'll have to do a little reading on the parafeed thing. Looks a bit like the V72 (another thing I've collected parts for). Interesting. Thanks for the help.

 

[rafafredd]

I guess a 3:1 iron would be good for this 417 circuit...

 

[CJ]

Yes, 600 times 9 equals 5400 ohms on the plate.
(3:1 = 9:1 imp ratio for the new guys)

 

[Svart]

thanks Rafa and CJ.

etching the psu board now.. i'll let you know how it works..

:thumb:

 

[NewYorkDave]

Since it's a triode, if the plate resistance is about 5K, you'll actually want to run it into 10K or higher. So 4:1 or 5:1 is more like it.

 

[Svart]

ok so my little power supply works like a charm.. +48vdc, +6.3vdc and +150vdc all there and steady as can be.. now to hook up the tube..


I need to work out some kind of attenuator setup, likely a bunch of 10db pads.. what's the best way to do this?