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Old 21st August 2012, 04:35 AM   #1
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Default ST70 "Imposter"

Attached is a teaser picture of a new driver board that will take its place as one of the many variants of the venerable Dynaco ST70. The output transformers (original) were grabbed on E-pay, and will be the only thing left in the design that bears any resemblance to the original.

The chassis is a nice SS pre-punched number offered by Triode Electronics. The output transformers will be stripped and painted metallic cast-iron gray to go with the SS chassis. The power transformer will be an ugly Woodward-Schumacher monster scored on E-pay. This manufacturer had a habit of painting the transformer after the varnish dunk, so that the finish is ugly and lumpy, albeit durable. Hopefully, when I'm finished, it'll sort of match the output iron. Schematics will follow when all is worked out. I'm using 6L6GC instead of the original EL34 outputs, so I'll be working out bias voltages and screen supply from some bench observations. First up in the sockets will be "New Svetlana" 6L6GCs.
Guess the input tube if you can - the input is an LTP, and there are an obscenely liberal amount of Supertex depletion mode fets scattered about the board.
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Last edited by wrenchone; 21st August 2012 at 04:50 AM. Reason: sp
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Old 21st August 2012, 04:57 AM   #2
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Default Heat....

Sink those two remaining TO-220s........please!




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Old 21st August 2012, 05:04 AM   #3
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When I find some TO-220 sinks with fins not as long as the ones I had on hand (used on all the other devices), I'll sink the two remaining devices.It won't be important for initial investigations. In the long run, dissipation on those two devices will end up somewhere between 0.6 - 0.8W, so some nominal heat sink will be necessary. I may use some clip-on sinks I have on hand.
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Old 21st August 2012, 05:43 AM   #4
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Quote:
Originally Posted by wrenchone View Post
Guess the input tube if you can
6BL8?

jeff
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Old 21st August 2012, 06:07 AM   #5
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How do you do an LTP with a 6BL8? The input tube does have 9 pins, however.
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Old 21st August 2012, 06:14 AM   #6
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Another clue - it also could have been a 7-pin tube, but the transconductance was a bit too low.
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Old 21st August 2012, 06:24 AM   #7
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Originally Posted by wrenchone View Post
How do you do an LTP with a 6BL8? The input tube does have 9 pins, however.
I take that back, you need two per channel to do LTP.

6BQ7?

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Old 21st August 2012, 07:39 AM   #8
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That's more like it. I did some looking around at the various readily available dual triodes, and that one won out on the basis of transconductance vs. plate current for a cheap tube. You can get higher transconductance with other dual tubes, but at the cost of drain current per plate >>10ma - no thanks. I disqualified the 6DJ8/6922 from the start, as the trendiness/audiophool factor has jacked up the price of that tube way above sane levels. Besides, I already had some 6BQ7As on hand - a sleeve of nice NOS GEs. People that have experience with this tube, please comment on other brands that might work better for this application, if such is the case.

Last edited by wrenchone; 21st August 2012 at 07:43 AM.
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Old 21st August 2012, 12:50 PM   #9
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I have used the 6BQ7 for LTP. I used a CCS in the tail so I could tweak the current. The pinout is such that there are about 20 tubes that will plug in to the socket and usually work with a twist of the CCS pot. I remember running several 6BQ7's through this amp when I built it and don't remember enough differences to have noted anything.

Some 6BQ7's can be microphonic in high gain applications like a guitar amp, but that was another cheap tube project in a different thread.

My list of pinout compatible tubes. I have tried some, but not all of them. Yes the audiophool 6DJ8 will fit, and I tried them. The 6BQ7 seemed to work just as good.

6AQ8 6BC8 6BK7 6BQ7 6BZ7 6BS8 6BX8 6BZ8 6DJ8 6DT8 6ES8 6FW8 6HK8 6JK8 6KN8 6922 7057 7308 6CG7 6EV7 6FQ7 6GU7 ECC89 7058 7898

Schematic of screen drive test amp. It's from about 5 years ago. My latest experiments with the tubes on this list were all guitar amp related.
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Old 21st August 2012, 01:42 PM   #10
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Quote:
Attached is a teaser picture of a new driver board that will take its place as one of the many variants of the venerable Dynaco ST70. The output transformers (original) were grabbed on E-pay, and will be the only thing left in the design that bears any resemblance to the original.
So why call this a Dynaco ST-70 "Imposter"...?

If I hook an A-470 to the back of my refrigerator will that make it an ST-70 imposter, too? The clone chassis doesn't make it an ST-70 anything either.

You need to retitle this thread to something like... "My unique never before seen design with A-470 iron"...

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