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Old 13th October 2006, 10:29 AM   #1
bembel is offline bembel  Europe
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Question Dynaco ST70 clone, tubes substitution question

Hi all,

I've just purchased an ST-70 chinese clone (Dynavox VR70E-2).
And I wonder which tubes are valid substitutes ?

* power stage : fixed bias, B+ = +420V EL34. ---> 6550 ? 6L6 ? (suppressor pinout ? bias ?, filament supply should be enough with 3.5A per pair)

* driver chinese 6F2 tube should be exact match to 6U8/ECF82 (do you confirm ?) what about ECF80 ? (I got some of them) ECF 802 ?
It seems also that filament supply of the driver is too strong (2 tubes alone on a 4A supply, they are glowing too much, maybe two magic eyes can help )

Wath bias will you suggest for JJ E34L ? it's actually breaking in @35mA (haven't hooked an oscilloscope yet)

thanks

PS: for the price this amp is a real killer.
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Old 13th October 2006, 10:51 AM   #2
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Hi Bembel,

Had two of these myself for about two months and i agree with you...they are amazing for the money....can't help with your query though....i'm a tube newbie but i'm finding the experience very interesting.

The two signal tubes do seem to glow rather brightly....but with the lights off it looks lovely.

I'd be interested to hear what anybody says about them and any improvements i can make....at the moment i have EL34 Groove tubes in them.....and i'm bi-amping with them.

Tim.
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Old 13th October 2006, 12:23 PM   #3
jmateus is offline jmateus  United States
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Hi guys

Could you please give a link for these chinese boards?
I have a ST70 I would like to modify and I'd be interested in a
good modification.
Thanks
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Old 13th October 2006, 12:48 PM   #4
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Here it is Jmateus,

the model rev 2 (already including mods)---> http://www.tomwip.de/tmp/neueDynavoxPlatine.jpg

the modelo rev 1 (with mods pictures) ---> http://www.retro-optonica.de/Dynavox/Dynavox.htm

but I think, there are more sites refering to original ST70 mods.

hope this helps
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Old 13th October 2006, 01:24 PM   #5
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I've gone for 37mA Bias. Sounds great.
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Old 13th October 2006, 02:03 PM   #6
jmateus is offline jmateus  United States
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Hi Bembel
Sorry I must have misanderstoood your posting, I tought Dynavox
was a chinese company that sold those boards.
I was mostly interested in buying the boards.
Thanks anyway
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Old 12th February 2016, 09:03 AM   #7
disco is offline disco  Netherlands
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Default Stabilising bias on a Dynavox VR-70E II EL34 amplifier

The latest incarnation of this popular 2x35W EL34 amp is on my workbench. In my opinion the build quality is very good, besides the cheap stacked PS caps.

Click the image to open in full size.

I have added 1K grid stoppers to the EL34 tubes and now I am looking at the suggested modification of the bias circuit. See the circles in red. Some basic questions emerge.

Click the image to open in full size.

The addition of the 10uF to ground is intended to stabilise the bias voltage. I'm wondering what the effect will be on the bias at loud operating levels. Will the bias voltage drift because of charging caps?

Even so in the situation of blocking, would the addition of these caps add to slower recovery? Does connecting the negative lead of the 10uF to the cathode instead to ground worsen or better recovery?
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Old 12th February 2016, 09:17 AM   #8
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Blocking will be dominated by the RC coupling- the 100k resistor will pretty much take the 10u cap out of the equation. The 10u caps reduce the ripple a bit and provide decoupling, all good things. The only downside I can see to them is that the idle current adjustment will have a bit more "lag."
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Old 12th February 2016, 09:38 AM   #9
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Thanks SY. Now the main power supply is floating with mains fluctuations. Would adding a low noise / low source impedance regulator to the bias supply be of benefit?
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Old 12th February 2016, 09:45 AM   #10
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Quote:
Originally Posted by disco View Post
Thanks SY. Now the main power supply is floating with mains fluctuations. Would adding a low noise / low source impedance regulator to the bias supply be of benefit?
Counterintuitively, no. In fact, that will almost assure idle current fluctuations- if the mains varies, so do the heater voltages and hence cathode emission. If the bias moves in the same direction the effect somewhat cancels, providing more idle current stability.

Re: low impedance bias supply, remember that you have a 100k resistor.... Low noise is the main thing, and that's super easy since the draw on that supply is minimal.

edit: are the phase splitter resistors unbalanced? It's hard to read. If so, you have to wonder what the "designer" was thinking.
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