Finished the ST-70 from Blair

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Finished the ST-70 from Blair

Postby DeathRex » Sun Jan 24, 2016 4:48 pm

Here's the, well not really, ST-70 I got from Blair. I originally made a board with 2 6AU6s as pre amps, and a 6C4 concertina, but I got a lot of oscillation with feedback. I couldn't get any more than 8db of feedback and POW. I switched to a LTP with the 6AU6s in triode mode, they seem to work pretty good.
I don't think there was any Dynaco parts left on the chassis, even the sockets say "6K6".
I don't know what the output transformers are rated for, but I'm getting 27 watts out at 1KHz. At 15KHz I'm getting a sawtooth out and that's at the most 15 watts.
It did have a voltage doubler using both 6.3 windings to make negative bias. Unfortunately it put -12 on the filaments, so I used one filament and a voltage quadrupler for bias.
Maybe in 20 years someone will get ahold of it and say "What was this guy thinking?" and rip everything out.
It does sound very good. Got to love those Russian toobs.
Attachments
807 6AU6 -2_zpstwc4tucx.jpg
807 6AU6 -2_zpstwc4tucx.jpg (85.69 KiB) Viewed 1480 times
807 6AU6 -1_zpsbpxcf3zx.jpg
807 6AU6 -1_zpsbpxcf3zx.jpg (51.46 KiB) Viewed 1480 times
807 6AU6_zpssqeawzni.gif
807 6AU6_zpssqeawzni.gif (23.01 KiB) Viewed 1480 times
Last edited by DeathRex on Sat Jul 15, 2017 11:29 am, edited 3 times in total.
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Re: Finished the ST-70 from Blair

Postby SteveH » Mon Jan 25, 2016 9:09 am

Looks good! Any hum issues with that input wiring ?
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Re: Finished the ST-70 from Blair

Postby DeathRex » Mon Jan 25, 2016 10:52 am

Nope. The input wires are only about 2", should be the same as a real ST-70.
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Re: Finished the ST-70 from Blair

Postby EWBrown » Mon Jan 25, 2016 3:27 pm

One could make the "economy" version, using 12AU6s and 1625s, with same results $) [:) (love) (banana)


6AN6 is a quad plate, common cathode rectifier.

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Re: Finished the ST-70 from Blair

Postby Geek » Mon Jan 25, 2016 3:49 pm

Love the topology used (y)
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Re: Finished the ST-70 from Blair

Postby DeathRex » Mon Jan 25, 2016 9:43 pm

I fixed the schematic. Thanks Ed.
I wanted to use 1625s, but the sockets wouldn't fit, the 7 pins barely fit, and I didn't want to widen the holes, in case someone wanted to revert back to octals.
The 5 pin socket on the 807 didn't quite fit either. I had to order a set of phenolic sockets and file them down.
I started with a crazy idea of using phenolic boards to mount the sockets, with components on a separate board underneath, so there are no components and high voltage on top.
With the 2 kinds of feedback, I ended up with more wiring. There is even some wiring between the boards; the 2 grounds, and wiring to the capacitors. Like the original, you can check the bias from the front.
The 0D3 is a pretty purple.
I'll have to check background noise on Wednesday or so. I just got a Philco 39-119, 38-7 and a wire recorder to work on.
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Re: Finished the ST-70 from Blair

Postby DeathRex » Tue Jan 26, 2016 7:21 am

SteveH wrote:Looks good! Any hum issues with that input wiring ?


I measured hum and background noise with my oscope, and there's no hum. I have a Tektronix 2215 that goes down to 2mvpp, only thing I see is a 7Mhz triangle wave at 1.8mvpp. At the speaker (Infinity RS5B), I can't hear anything out of the woofer and a barely audible hiss out of the tweeter.


Edit: Come to think of it, the 7MHz might be from the Zune plugged into it. I don't have a good working preamp. I might just need to fix one of the PATs I have.
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