DIY Audio Projects Forum

1930 171-A SE amp
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Author:  mwhouston [ 24 Feb 2013, 20:01 ]
Post subject:  1930 171-A SE amp

I just purchased a couple of 71-A tubes for a uuP power (lol) amp. I intend to use a microphone tranni 10k\10k for the input interface and Edcor 5k OPT for output. Has any one made an amp with these tubes and do you have a schematic?

Author:  mwhouston [ 25 Feb 2013, 17:02 ]
Post subject:  Re: 1930 171-A SE amp

I have ordered another pair of 71-A tubes. Also here is a link to a schematic. I would rather use a 6SN7 to drive the 71-A.

Author:  Suncalc [ 25 Feb 2013, 20:20 ]
Post subject:  Re: 1930 171-A SE amp

A couple of quick points. I would use a 6SL7 instead of the 6SN7. As your going to be running the tube at about -40v bias I think you'll want the extra amplification in the driver that the 6SL7 will provide. I would also replace the two 22Ω resistors with a 100Ω hum pot and then subtract 50Ω from the required bias resistor.

Looks like a fun little project.

Author:  mwhouston [ 26 Feb 2013, 03:32 ]
Post subject:  Re: 1930 171-A SE amp

Matt: I am considering a few (yes two) "general" schematics. The one I am leaning towards which uses no driver tube but an inter-stage tranni. I was was hoping to use the Hammond 124B 10K\90K partial nickel core tranni. What do you think? I'm hoping my Oppo-95 will drive the 71-A via the inter-stage with no driver tube. Once more what do you think?

Part of the project is to try to get away from (possibly) preamps and caps!!??!! With a HT of 135V, -27V bias is required. A Rk of about 1700 will produce this but then I must by-pass Rk with a 100uf cap. To get away from the cap I could use 3 X 9V batteries or tap my PS for that bias voltage. With these parameters the 71-A should pass about 17mA to produce a speaker smashing 0.5W! Thank God I have a pair of 96db efficient Fostex speakers.

Any comments will help.

Author:  mwhouston [ 27 Feb 2013, 02:03 ]
Post subject:  Re: 1930 171-A SE amp

I have had two sleepless nights trying to figure out how I could make this amp build look "old" and quirky, a little Steam Punk a little Dune. I may have found just the right base for the job. Take a look at this pot holder. I'm not saying it is from the 30's but it is old. The amp will sit on a plate of copper or timber and sit down on the top lid. Tubes and OPTs (Edcor's ) erect on the top. The rest of the amp will hang underneath the top plate hidden in the base. I may put power trannis in the base or use an external plug pack (18V 2.2A). Also I may sand and re-varnish the base, have it sand blasted and re-varnish of leave as is.

I will start the PS soon.

Author:  vortchun [ 27 Feb 2013, 09:07 ]
Post subject:  Re: 1930 171-A SE amp

Hi when I want wood stuff look old, I varnish it. Then sand the edges down and put lacquer over it. the result looks like this. This I made for my sister.
If you want I can make some sketches of amp design for you.

Author:  mwhouston [ 27 Feb 2013, 21:31 ]
Post subject:  Re: 1930 171-A SE amp

Vortchun: thanks for the offer but I'm trying to keep costs down on this build. I was just wondering if I should leave the finish as is after a cleanup or strip it back and start again.

Author:  vortchun [ 28 Feb 2013, 09:17 ]
Post subject:  Re: 1930 171-A SE amp

Better to leave it as it is.

Author:  mwhouston [ 28 Feb 2013, 15:45 ]
Post subject:  Re: 1930 171-A SE amp

vortchun wrote:
Better to leave it as it is.

I will just give it a cleanup and rub in some cedar oil.

Author:  mwhouston [ 05 Mar 2013, 08:52 ]
Post subject:  Re: 1930 171-A SE amp a little bit of info. about the tube. One set of my tubes came today. I pickup another pair from the post office tomorrow. No reason not to start the amp now.

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