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Super simple single stage tube preamp
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Author:  Suncalc [ 19 Jul 2011, 21:55 ]
Post subject:  Re: Super simple single stage tube preamp

Well Mark, how about the "4S MK IV Universal". Just remember, you asked for it. ;)
Quote:
In a future build (if Matt is not sick of charting my course) I may do a build which can switch from 12AU7 to 12AX7.

How about we do one better. A "Universal" 4S preamp for the dedicated "Tube Roller". This design, with no changes to the circuit whatsoever, will handle the 12AU7, 12AY7, 12AT7, 12AX7, and 12AZ7. Here is the design and calculated performance numbers.
Attachment:
USSSS Preamp.jpg

The full volume stage gains run from -9.9 to -28.1 (19.9dBv to 29.0dBv) with fairly low distortion numbers. (Remember that an audio taper pot at half volume is a 20dBv loss and these gains are a lot more reasonable looking, i.e -0.1dBv to 9dBv at half volume.) All these distortion numbers are quoted with a 1v peak to peak input voltage. The 12AX7 has the least input headroom but it also has a bucket load of gain so that shouldn't be a problem. I had to go to a 250kΩ volume pot to control distortion, but even with that pot, the output impedance numbers don't look too bad. So long as the amp (or whatever) you're driving has an input impedance greater than about 100kΩ the loses should be minimal (worst with the 12AX7; about -2.2dBv at about 8/10 on the volume scale).

I may build one of these with a 12AU7 cathode follower output stage just to drive prototyped power stages under test. So, let me know what you think.

Author:  mwhouston [ 20 Jul 2011, 01:00 ]
Post subject:  Re: Super simple single stage tube preamp

Matt: Thanks once more. This is all the more reason to knock up a kit for the tube junky who wants to roll his own. Think how much fun this would be with a hand full of tubes, a bottle of Bourbon and a stack of test CDs. This makes it nice and easy to use whatever tube you have around without having to scour the 'net for an appropriate schematic.

Add this cct. to my already easy to build from Radio Shack parts 280VDC PS from a 24 AC plug pack for your Mains and we have a winner.

Thanks a bunch. I might just make one.

Author:  mwhouston [ 20 Jul 2011, 02:54 ]
Post subject:  Re: Super simple single stage tube preamp

Matt: Which tube would work the all-over best in this cct?

Author:  Suncalc [ 20 Jul 2011, 19:22 ]
Post subject:  Re: Super simple single stage tube preamp

mwhouston wrote:
Which tube would work the all-over best in this cct?
That's a difficult question to answer.

I think I like the 12AY7 bias location the best. I think it represents about the best tradeoff between small signal linearity and large signal compression prior to saturation. Although it would be tough to make a case that the 12AU7 bias point is not just as good. I would put the 12AX7 next. It's a more typical bias point for the tube. It has a nice balance and plenty of both positive and negative swing headroom. In last place is the 12AT7/12AZ7. If we were really going to take advantage of this tube it should have a higher B+ and a higher plate load. However, of all the tubes the 12AT7/12AZ7 will definitely be the warmest and most mellow, so maybe that's good.

As you said, a hand full of tubes, a bottle of bourbon (I'm partial to Kentucky bourbons myself), and a stack of test CDs could make for an enjoyable evening.

Author:  mwhouston [ 20 Jul 2011, 19:43 ]
Post subject:  Re: Super simple single stage tube preamp

I may host a DIY night for the club and flick between U7s & X7s

Author:  mwhouston [ 21 Jul 2011, 11:49 ]
Post subject:  Re: Super simple single stage tube preamp

Ok so it is 3:45am. It works.The MATT Special now dubbed "The Black" 4S MKIII. This seriously understated suncalc designed single stage preamp sounded very good not cased and I assume will sound better now.

I have found the CONN tubes are mellow and lucid where the GD 12AU7 is bright and edgy. Next the Universal (MKIV) which I have renamed: 4S Universe.
Attachment:
4S MKIII Black.jpg
Attachment:
MattSpecialInside_IVss.jpg

Author:  Les [ 21 Jul 2011, 12:35 ]
Post subject:  Re: Super simple single stage tube preamp

I see you've employed what looks like duende criatura tube rings. Do you prefer them to good ole grommets, is it just for their dressiness?

Nice build!

Author:  mwhouston [ 21 Jul 2011, 16:43 ]
Post subject:  Re: Super simple single stage tube preamp

Les: These were the very first type of tube rignt I ever used. I have also used the thin neoprene band types. Others in the audio club like their tube rings too but always comment on mine. Yes a bit dressy and quite functional. I pay a premium, though, for them

I force these rings over 6N1Ps which stretches them. To get a snug fit on 12AXxx I pull the steel ring off and crush it down. They do appear to lose some of their spring after this though.

Author:  Suncalc [ 21 Jul 2011, 19:03 ]
Post subject:  Re: Super simple single stage tube preamp

I'm not wild about tube rings. They interfere with the tube glow as I sit in the dark listening to my amps. Now if someone made one that was clear... :up:

Mark, I was just loooking at your pics and wondered; are you input and output RCA connectors grounded to the case? This looks to be the case for both the MK II and the MK III from the pics.

Author:  mwhouston [ 21 Jul 2011, 21:58 ]
Post subject:  Re: Super simple single stage tube preamp

Yes. The case, RCA's ground and earth all share the one earthing. This is why I cannot ground the power socket or use a metal power connector. I am using a short tail which has a 6.5mm phone jack type connector. The male end leads from the amp. Female on the power pack. It works OK and looks fair. If I isolated the metal case from the earth then I could use a 2.5mm power plug.

My method produces no hum or noise. It just means I have to have an isolated connection.

Matt: There is clear tube rings. They are made from glass tube in the shape of a horse shoe. They add three neoprene rings so they will grip on the tube. Very expensive.

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