One-Bottle Preamp

[skipwave]

[quote author="skipwave"]Here's my box:
Click here for larger image.

Click here for output options (notice the hi-tech labeling system).[/quote]

I replaced my pics with new ones. Links in the quote above.

This thread has turned into a novel, but there is so much good information in it. It's worth the time.

 

[tk@halmi]

Allow me to add a couple of comments here. The tubes used for this amp are out of production so the chances of this exact circuit being reproduced for profit are really low. Dave did not invent the two triode global feedback topology. There are books from the 1950s that show the same thing using a variety tubes. Granted, the exact resistor values are the result of his hard work and should be respected. While I do support Dave's move to protect his intellectual property I see very little reason to take this down. Removing exact component values from the schematics would have sufficed.

 

[Brown Note]

i finished the one bottle version last week and recorded for the first time with it this weekend. it sounds great, but i have one problem. when using the pre on a close mic'd guitar cab (57), even at minimum gain the output is WAY too hot for my DAW's inputs. i know adding the output transformer will drop the level, but i don't really feel like paying for another transformer. any suggestions?

could i place a 10k pot after C4 to attenuate the output?

thanks

 

[pstamler]

[quote author="Brown Note"]i finished the one bottle version last week and recorded for the first time with it this weekend. it sounds great, but i have one problem. when using the pre on a close mic'd guitar cab (57), even at minimum gain the output is WAY too hot for my DAW's inputs. i know adding the output transformer will drop the level, but i don't really feel like paying for another transformer. any suggestions?

could i place a 10k pot after C4 to attenuate the output?[/quote]

You could, but it's probably too low an impedance for the circuit to be really comfortable driving it. Assuming your DAW has a 10k input Z (probable), you could build a 90.9k series resistor into the DAW end of a cable, in series with the input, and thereby drop signal level about 20dB while keeping the One Bottle happier about the load.

Oh, I forgot to ask...is your DAW a nominally -10dBV input? And can you set it for nominally +4dBu instead?

Peace,
Paul

 

[Kingston]

Hey good timing with the attenuation,

I'm doing the unbalanced bridged T 600-ohm stepped attenuator for both of the channels to solve this problem as I'm using a 4:1 output transformer. Newyorkdave posted the schematic ealier in the thread.

I'd like to make absolutely sure how to wire it correctly.

The 'in' of the attenuator comes from C4/R10, right? And goes straight into the output transformer?

Or am I completely off the mark here?

Thanks,
Mike

 

[skipwave]

[quote author="pstamler"][quote author="Brown Note"]i finished the one bottle version last week and recorded for the first time with it this weekend. it sounds great, but i have one problem. when using the pre on a close mic'd guitar cab (57), even at minimum gain the output is WAY too hot for my DAW's inputs. i know adding the output transformer will drop the level, but i don't really feel like paying for another transformer. any suggestions?

could i place a 10k pot after C4 to attenuate the output?[/quote]
Oh, I forgot to ask...is your DAW a nominally -10dBV input? And can you set it for nominally +4dBu instead?[/quote]

Did you put the 20db pad in front of your One-Bottle? With that you might not have a problem, since it reduces the low end of the gain setting to just 18db.

 

[NewYorkDave]

Brown:
Best solution: call Edcor and ask for a WSM15K/600 transformer. Cost: about $9. Or if money's really tight, ask for PC15K/600, about $5.

Next best solution: build a switchable L-Pad. Design it for the same value as the input impedance of your DAW (which must be 10K or higher or the One-Bottle won't be happy driving it). Here's a handy online calculator:
http://www.sengpielaudio.com/calculator-Lpad.htm

But by the time you're done, you'd have spent as much or more than the transformer would have cost you. At any rate, a simple pot really isn't the way to go.

Kingston:
The attenuator goes after the transformer. And if the transformer is permanently installed, you can eliminate the 220K "tiedown" resistor on the output coupling cap.

At any rate, that switchable 20dB pad on the input is a must-have unless you're always using low-output mics on quiet sources.

 

[Kingston]

thanks for the info,

I was always going to put the 20db pad on the input, but then I found out about the added flexibility of output attenuation.

And now I'm even more confused. The unbalaced attenuator only has in, out and ground on the schematic. But the transformer outputs balanced lines 2 and 3, and then there's ground. What am I missing here? Do I just throw away the balanced signal?



I'm such a newbie.

 

[Kingston]

Bump

just building this,

would somebody please enlighten me with an answer... I read earlier in the thread that unbalanced attenuator is rarely a problem in practise. Does that mean I should just omit the balanced out?

Seems like a "waste" of the balancing output transformer.

 

[NewYorkDave]

The main purpose of the transformer is to match the high impedance of the tube plate to the low impedance of the line. Balanced output is just a bonus.

You don't have to ground the low side of the attenuator. It's an unbalanced attenuator, but that doesn't automatically mean that the "com" side has to be connected to ground. (Unbalanced doesn't have to mean "one side grounded", although that's often the case). You can let it float. It was done all the time in the old days: look at the "T" attenuators used on the inputs and outputs of old tube compressors.

 

[Kingston]

Ta,

back to soldering bench then.

:thumb:

 

[Kingston]

Whew,

metalwork done, and most of the soldering and wiring, too. Too bad I still have to wait for some of the parts, like the psu transformer. I wanna plug this thing in and record already.

 

[NewYorkDave]

Note: the "onebottle input/output" PDF I linked earlier in this thread contains an error in the drawing of the 20dB pad. I just noticed it now as I was emailing the link to Consul.

Anyway, the pad should look like this:
http://groupdiy.twin-x.com/albums/userpics/10031/20dBMicPad.png

I've removed the file for now to avoid further confusion. I'll correct the PDF when I get a chance.

 

[Consul]

Well, I ordered away for a large part of the parts I'll need from Mouser. I still need to track down most of the caps, since Mouser makes finding the ones I need a major chore. I hate their web site sometimes. I did manage to land the big 'lytics for the power supply, though, which were the ones I was most concerned with getting.

I need to take a trip to Ann Arbor on Saturday for some other parts, including chassis-mount tube sockets, and hopefully caps as well.

For the chassis, I got an aluminum bud box from Mouser. I hope 8" x 6" x 2" is big enough for building this project. I plan to do it "guitar amp" style.

 

[MRP]

Yeah, how come Digikey's website seems like it's so easy to use, while Mousers is mind-numbingly counter-intuitive?

 

[babyhead]

For Mouser, I just use the catalog and put in their codes manually. Too bad they are way cheaper on pots, resistors and switches.

:guinness:

 

[NewYorkDave]

Note: if replacing the output plate resistor and coupling cap with a gapped 15K:600 transformer like Tamas', you need to increase the value of the output tube cathode resistor. These values should get you in the ballpark:

1K (for One-Bottle pre)
510 ohms (for the two-12AV7 line amp posted on the Drawing Board).

In either case, the output tube plate resistance will be a good match for the 15K primary. Current draw of either circuit will remain about the same as the original versions. Output tube plate dissipation will increase, but will still be within safe limits, and maximum output power will increase by a fair amount. The feedback resistor can be increased slightly to compensate for the ~2dB loss of net gain from using a 5:1 transformer ratio (compared to the 4:1 specified originally).

 

[Kingston]

Here are some "waiting for the mains transformer" pictures of my two bottle.

A bigger version here, http://kingstondrums.bombsquad.org/nydtwobottlebig.jpg


Some specs:

12AV7 RCA for channel 1
5965A general electric for drop in replacement for channel 2
5879 sylvania for both channels.

heaters are 6.3VAC

jensen JT-110K-HPC 1:8 (a slightly older version) input transformers and OEP A262A2E 4:1 for output. Mostly high quality electrolytics by Rifa.

Phantom is still missing. I'll do an adaptation of the gyraf G9 voltage tripler to suit the unused 12VAC 300mA out of the mains transformer. I still need to figure out how to do it.

I don't know how appropriate the "ball of caps" straight in the tube sockets approach is, but at least the signal paths are short. Guess it's my take on tight right. Because of the amount of control in the front panel there's a bit too much "teflon decoration" there as well(as PRR would probably put it). There's 48v, pad, gain, hi-z input and attenuator for both channels.

Can't wait to start using this. I'll start a new thread with finished pics and sound examples as soon as it's fired up (if nothing goes wrong). Just hope I didn't screw anything up really bad.

Thanks a million NYD. This has been a tremendous learning experience.

 

[Consul]

HA! Diving around in my new parts stash (thanks to CJ) I was able to find some of the cap values I need. Still need to track down some others, but I don't think it'll be too hard.

I can't wait for my trip to Purchase Radio on Saturday...

 

[Consul]

I just ordered away for 2 WSM15K:600 trafos from Edcor. I'm looking forward to seeing this all come together.

I don't know why I'm giving you all a blow-by-blow, other than for the moral support. :green: