Interested in a ST-70 driver with Mullard 5-20 circuit?

knowledge base for the classic Dynaco ST70

Anyone interested in a ST-70 driver with Mullard 5-20 circuit?

Yay!
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Nay!
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11%
Variation of (please describe)
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Total votes : 9

Interested in a ST-70 driver with Mullard 5-20 circuit?

Postby Geek » Tue Aug 18, 2009 1:59 pm

Or variation thereof?

Cheers!
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Postby burnedfingers » Tue Aug 18, 2009 4:39 pm

yes
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Postby crispycircuit » Wed Aug 19, 2009 7:08 pm

Absolutely... I built a copy of the Dynaco recommended circuits with potted A-420 xmers. Discribed as " Improved Mullard 30 watt amplifier using Dyna A-40 xmer" An EF86 to a 6CG7 to EL34s.... It tuned out to be one of my best sounding amps ever..... My friend made an irresistable offer on the amp. So I'm ready to build another...
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Postby Ty_Bower » Wed Aug 19, 2009 7:18 pm

Silly question... isn't Shannon's ST70 board a variation of the 5-20 circuit?
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Postby Geek » Wed Aug 19, 2009 9:33 pm

Ty_Bower wrote:Silly question... isn't Shannon's ST70 board a variation of the 5-20 circuit?


Shannon's is all triode and closer related to the KTA-HiFi board I believe (???)

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Postby justinsweber » Thu Aug 20, 2009 11:09 am

I think any new variation would be nice to try. Many of us here have tried or built new ST70s to try different drivers... I like the idea... as long as its not just a variation... Maybe GZ37 rec instead....
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Postby Ty_Bower » Thu Aug 20, 2009 11:38 am

A little reading suggests the Mullard 5-20 circuit used a EF86 pentode in the front seat to get boatloads of gain. Something like 50 mV for full output, even with feedback applied. It was suitable for use with your "gramophone" without the need for an expensive pre-amplifier circuit. Alternatively, you could opt for the passive tone control network and still drive the thing with a 600 mV input.

I'm not sure I need that much gain in my driver circuit, and I don't have a spot for bass and treble knobs. What's wrong with using a triode instead of the EF86?
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Postby Geek » Tue Aug 25, 2009 10:44 pm

Hi,

Sorry for the delay, was out of town.

I agree that the original 5-20 has a LOT of gain. However, in triode, it doesn't :))

A board with a 5-20 circuit can easily come with "trioding" instructions and part values.

However, some sources could benefit from 50mV sensitivity.... my big screen TV for example has 100mV for a maximum output (sick)

As a side note, the 6EJ7/EF184 is darn close to the EF86 in performance and sonics... would it be too confusing you think to have a "jumper-programmable" socket? Therefore eliminating the need for an adapter.

Cheers!
Last edited by Geek on Wed Aug 26, 2009 2:17 pm, edited 1 time in total.
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Postby burnedfingers » Wed Aug 26, 2009 4:44 am

How about a 6sj7/5693 as the input tube?
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Postby Geek » Wed Aug 26, 2009 12:49 pm

6bg6ga wrote:How about a 6sj7/5693 as the input tube?


Indeed, that is one gorgeous little tube! :))

There is plans for an octal board. For the first batch or two, I'd like to keep them minatures (and so I can afford to do more ;) ).

Cheers!
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Postby burnedfingers » Fri Nov 13, 2009 6:03 am

Are you thinking of something similar to the board that Triode electronics sells that uses a EF86/6au6 and 12Au7/12BH7?

I recently purchased a board and got the components soldered in and the board installed in one of my Dynaco Stereo 70's. It sounds great and opens up the possibilities of a lot of different 7 pin tubes for some fun tube rolling. Another upgrade to the board is changing the 12AU7 to a 12BH7.

Something along this line would be great in my opinion.
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Postby Geek » Fri Nov 13, 2009 5:53 pm

Hi,

EF86/6EJ7/6AU6 pre with an option for any B9A basing PI and flexible heater pad assignment for either 6V + shield or 12V split heaters.

Cheers!
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Postby EWBrown » Fri Nov 13, 2009 6:28 pm

EF184 / 6EJ7 is "da bomb" as they say, especially in triode mode. A largely unappreciated tube, and Amperex made the Bugle Boy EF184s which show up for low cost.

It would also be nice to make the PI "programmable" for either 9A or 9AJ (9DE or 9LP) type pinout.

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Postby Geek » Sat Nov 14, 2009 5:49 am

Hi Ed,

EWBrown wrote:It would also be nice to make the PI "programmable" for either 9A or 9AJ (9DE or 9LP) type pinout.


Well, that's what I mean.. both those are P-G-K-H-H-P-G-K for the first 8 pins like any 12A_7, just pin 9 is either a shield or heater CT.

All my PCB designs for these so far have pins 4, 5 and 9 brought out to their own pads so you can do as ya will with 'em :-)

Cheers!
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Postby EWBrown » Sat Nov 14, 2009 4:36 pm

Sounds like a "weiner" to me, he said, with a hot dog stuck in his ear ;-)

5687s would make schweet LTPIs but the darn things have a werid non-standard, non-compatible pinout.

BTW did the Z565 autopsy sample arrive there yet? It's been over a week... I used PP airmail, as it was the best combination of cost and speed (at least from the lower 48)

Perhaps the Poste Canadienne Dog Sled Team and Mule Express is running just a wee bit slow these days ? ;-)

/ed B
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