Biasing KT66 in stock ST70

knowledge base for the classic Dynaco ST70

Postby mugsy » Thu Jul 31, 2008 2:18 pm

Hi Burnedfingers.

Just checked and it was page no. 2 from:

http://www.diyaudio.com/forums/showthre ... genumber=1

However just realised that was dated 2004 and I imagine alot of upgrades/gear has passed through your hands since then! I wasn't trying to catch you out by the way, just genuinely interested in your comments.

I happen to think my hearing is very good. I wrote for Hi-Fi World a few years back, and got the opportunity to listen to alot of hideously over-priced gear. Every now and again I hear something that - despite all it's short-comings - just makes me sit back and enjoy the music. It puts me in touch with its spirit and just makes me want to do NOTHING else but listen to music constantly - the ST70 does just that. I should add that all passive components are hand-matched, and high quality (Kiwame resistors, 3 watt metal oxides, Black Gate bias caps, Arcotronics film/foil caps and so forth.), the only original parts being the two output transformers. Currently using a matched quad of JJ KT77s. Sounds fantastic to me - the presence is probably what catches my attention the most. All instruments have real body and a natural tone. As mentioned, by benchmark is the Greg Ball GB150D solid state power amps, and they take alot of beating...

I also have the VTA board, and will construct this in a week or so, and then see how it compares with my new 'stock' ST70 driver board.

I may well try a new quad cap. However first I will shield the underside of the power transformer with copper and see if that solves the hum problem. I'm also constructing a 'DC trap' as we have alot of DC on the main shere which makes the trannie vibrate alot. That could be causing microphonics in my NOS 7199s.

It's all good fun this! :)

- John
mugsy
 
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Postby burnedfingers » Thu Jul 31, 2008 3:35 pm

John,

You may very well want to upgrade the stock coupling caps on the VTA board. The Orange capacitors sound ok but I believe there is some to be gained with premium parts with this circuit. Also, unless they have made a change to the board I believe the bias controls are reversed to the stock bias controls. A counterclockwire turn will increase the bias. I would suggest floating the VTA board from the chassis despite the claims. Also mount the board using plastic non-conductive mounting hardware. I do hope you enjoy the board but I also hope that you could sometime experience the liquid midrange and high frequency response of the Mapletree board. Having had a number of boards I have been able to experience the differences that each has to offer. Sometimes there are gains to be had in paying more for a board. Don't get me wrong I could get real excited if the VTA board had 6FQ7 as an input tube and a 12BH7 as a phase splitter for example. The VTA board is a good board. I'm just saying its not the king of the heap. Keep an eye on the plate resistors if memory serves me correctly they ran a little hot on me and started changing color. The voltages were spot on but I would have liked a heftier resistor.

To be perfectly honest here I find myself wishing I still had a VTA board so that I could tweek it to high heaven and see what it could do with some imagination. Some different tubes, good resistors,caps, and CCS might just wake it up.

I have a Dynaco board with quality parts that are hand matched and it does sound a world apart from the norm. As they came from the factory I have a hard time listening to them for any period of time.
burnedfingers
KT88
 
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Postby burnedfingers » Sun Aug 24, 2008 5:16 pm

Just finished adding a DPDT switch to my Dynaco Stereo 70's bias circuit.

One direction puts a 9K resistor into play for EL34 tubes. The other direction puts a 6.4K resistor into play. The two resistance choices allow for a number of different tubes to be used and properly biased.
burnedfingers
KT88
 
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