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 Post subject: 4S Universal tube preamp
PostPosted: 20 Oct 2012, 07:22 
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:cop: UPDATE - [Dec 2013] - 4S Universal Preamplifier (12A_7 Tubes) project page ready. Also, see Mark's 4S Universal Valve Preamps.

Sometime back I started the 4S tube preamp thread. From the original posting, with the original cct. four new 4S preamps evolved. I still feel that the best of the 4S series was "The Black" which was evolution MKIII.

The Universal (4S MKIV) is a most different breed. Once more it is designed by Suncalc (as was the MKIII) and is one himself is to build. What is special about this tube preamps is the fact it will take ANY 12A?? tube. I have built two, one as a stand alone preamp (since sold) and the second as the preamp section of a 6EM7 truly integrated amp, once more designed by Suncalc.

I am part-way building another Universal. This time it is a commissioned job. I am listening currently to a chip amp, the "SynergyII\Cu" and feel it needs a tube preamp. This will give me a chance to evaluate whether this is true.

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PostPosted: 21 Oct 2012, 16:31 
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I guess after building six or so of these preamps and associated PSs I would have to find it easier. Definitely neater this time around. Still lots of room to drop the tube socket (ceramic) in the middle or up away from the tranni. The uni-board construction of PS and amp makes for a neat and efficient solution. Having the power supplied by an AC wall wart provides double filtering of the mains. First the wall wart tranni then the on-board step-up (step-down used backwards). I like to think that by the time the HT has been filtered the power is nice and clean. All I have built so far, using this method, have been completely dead quiet so it must be working.

The rec. bridge for the HT is up behind the 24\240V tranni. a 24VAC wall wart suppliers the initial power stepped-up to about 240VAC. Filtering: 47uf-150ohms-100uf-150ohms-100uf. The HT rec. bridge is fully snubbed. The heater rec. bridge filtered by a 1000uf low ESR cap, then regulated (7812). Limiting resistor is 100ohm. I have been destroying regulators lately so this time a protection diode across the reg. Usually I test the regulated heater voltage and the regulator never works again. Hopefully the diode will stop this.
Attachment:
4SPS.jpg


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PostPosted: 25 Oct 2012, 02:00 
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Cct. brd is now complete apart for the wiring which links it to the outside world (RCAs and pot). Not so neat looking now but apart for only two short pieces of wire wrap wire in the signal path the rest is short links and components.
Attachment:
CctBrd.jpg
Attachment:
CctBrdII.jpg


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PostPosted: 26 Oct 2012, 16:00 
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Hi Matt,

What are the specs of the 24V wall wart you are using? Essentially what is the maximum current providing capabilities of the wall wart? I have several 12V@1A, can I use these instead as I also have several 12V/240V transformers that I could use to step the PS back up to 240V or will there be too much of a voltage drop across the bridge/caps/resistors in the heater circuit?

Otherwise off up to Jaycar to purchase a 24V/240V transformer :)

Also, I purchased the caps etc and noticed that rather than a 0.01uF/630V the mail order company sent 0.1uF/630V. Will this cause an issue using this as the last capacitor on the HT 4SPS? (see diagram attached)

Again off up to Jaycar if there is an issue.
Attachment:
SSSSPSa.JPG

Cheers
Chris


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PostPosted: 26 Oct 2012, 16:58 
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The wall wart is rated 24V @ 1A. I'm sure it does need a 1A rating but it is what we sell where I work. At staff prices the wall wart is cheap and barely gets warm. It supplies 24V to the step-up tranni (step down in reverse) and 24V then rectified directly and regulated for the DC heater current. It's a simple and very effective solution.

Please note the last snubbing cap on the end of the PS HT should be 0.1uf and not 0.01uf.

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PostPosted: 26 Oct 2012, 17:33 
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Off to Jaycar I go then for the 24VAC wall wart and transformer and the right snubbing cap


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PostPosted: 26 Oct 2012, 18:14 
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nodz wrote:
Off to Jaycar I go then for the 24VAC wall wart and transformer and the right snubbing cap

The Jaycar high voltage brown caps, I think, are very good. I have built amps, preamps and phono preamps using these as snubbers, filters and inter-stage coupling caps, output and input caps. They are cheap and I believe of an audio grade equal to Sprague Orange drops. Use them with confidence. I have even used them in the equalization network for phono preamps and they sound very, very good. Only the smaller values are rated 630V the bigger ones are rated 100V so don't get it wrong.

On the other hand the green caps are low quality and low voltage (100V) and should not be used in the signal path but are OK for snubbing. For the difference of a few cents - go for the browns (brown tur#ds*).

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PostPosted: 26 Oct 2012, 18:31 
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Thanks, will look at that. Have plenty of the green caps for snubbing. The mail order company suppied brown polys for the larger values at 630V, just they supplied 0.1uF rather than 0.01uF for that final cap.


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PostPosted: 05 Nov 2012, 13:14 
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i found the datasheets for those 'brown/dark red' caps - have also used them in my 6L6GC mono block builds and, the 4S build - no issues -

the 1A wall wart is ample for this circuit. total draw including that of the heater is <500ma anyway according to the JJ datasheets (i checked the 12AU7) - i love these little valves, less gain but the sound is wonderful.

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PostPosted: 05 Nov 2012, 17:15 
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@Grega: I favor the 12AU7 over the 12AX7 these days. I mainly use Tung-Sols.

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