Stereo 70/ mods

knowledge base for the classic Dynaco ST70

Stereo 70/ mods

Postby leadtower » Fri Apr 08, 2005 6:30 pm

Hi all; I have found my ST 70 is now all new. I don't know how this happened. I just started with some sockets, then the Triodeel PS mods, then the 6gh8 board from one of the mod companies, then the transformer set from Handmade, and now Shannons driver board. It crept up on me. What I need to know now is exactly how to individually bias the EL 34's. I am pretty happy with it and just built a pre-amp from 5687's and 6ca4 rec tube with motor run oil PS and filter caps. The 5687 is CCS'd and the PS is choke loaded. I bring this up because the two pieces sound darn good together. I use Altec Valencias. I was running Roy's EL 84 amp on the Altecs and that was pretty nice also. All this by way of introduction; this is a good site. And Shannons driver board is a fine piece of work.
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Re: Stereo 70/ mods

Postby Shannon Parks » Sat Apr 09, 2005 6:57 am

leadtower wrote:What I need to know now is exactly how to individually bias the EL 34's.


Hi Leadtower,

I tried to make my board as 'stock friendly' as possible, so that the power supply and bias supply can stay at the same values as before. But one change I suggest making off the the board is removing the jumpered 15.6 ohms resistor and adding a separate 10 ohm resistors to each cathode. I then run a 'current sense' line to the front octal, just like in the original, but now there will be two sense lines per side alond with a ground. Using your DMM on at these test points, use the wirewound pot on the unit to raise the current up to the ballpark - like between 400mV and 500mV. Then adjust the blue pot on the board as a balance between the pair. After tweaking this pot, got back and set the wirewound to 500mV exactly. Repeat on the other side. After finishing the other side, you might have to ping pong between sides to home in on perfection.

BTW, glad you like it!

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Bias on ST 70

Postby leadtower » Sat Apr 09, 2005 3:28 pm

Hi Shannon; Thanks for the reply. I got you; run two wires up front one from each cathode and then set the bias with your adjustable pots on the board. Can you tell me; since you seem to have experience with these, are there any other changes you might suggest pursuent to my set-up?
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Handwound transformers

Postby TerrySmith » Sat Apr 09, 2005 6:24 pm

Go ahead and order a new power trans from Uncle Ned to replace that Handwound unit that will fail! Yellow_Light_Colorz_PDT_23

I got a ST-70 power trans from Ned, Absolutely NO noise, NO hum, NO vibration! Also, it runs very cool. Yellow_Light_Colorz_PDT_11
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Handwound

Postby leadtower » Sat Apr 09, 2005 7:10 pm

Thanks Terry for the warning. The power trans is actually pretty beefy, it is 30% larger than the stock 060. It measures well and has a decent reputation. Do you know of any problems or outright failures, and if so, was it a short or something? Thanks; J.R.
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BUZZZZZZZ

Postby TerrySmith » Sat Apr 09, 2005 10:15 pm

Mine started humming after about 3-4 months, then it buzzed louder like a cheap battery charger. It got so loud you could hear it over the music! It also caused a hum in the audio that went away with Ned's transformer.

The Handwound outputs are pretty good, i'll leave them alone for now.
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