Stock board bias balance question

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Stock board bias balance question

Postby dannyr » Tue Apr 08, 2014 12:24 pm

Hi, theoretically would borrowing the ideal of using 500k trim pot and 1meg for bias balance like done on a the DIYTube board work with a stock board setup? The grid resistors are much higher, 270k, so I'm not sure if it would work. Maybe they should be lowered with the coupling cap value raised to work with newer EL34's anyway though. Thanks.
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Re: Stock board bias balance question

Postby Shannon Parks » Wed Apr 09, 2014 7:49 am

dannyr wrote:Hi, theoretically would borrowing the ideal of using 500k trim pot and 1meg for bias balance like done on a the DIYTube board work with a stock board setup? The grid resistors are much higher, 270k, so I'm not sure if it would work. Maybe they should be lowered with the coupling cap value raised to work with newer EL34's anyway though. Thanks.


I think that would be fine for many circuits including the stock one, and you actually get a little more balance adjustment with the 270k resistors.

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Re: Stock board bias balance question

Postby dannyr » Wed Apr 09, 2014 12:38 pm

Shannon, Thanks. I read Dave Gillespie's write ups on a stock ST70 over at Audiokarma and I wondered if I had dismissed the stock circuit too fast but thought that it would be nice to balance the bias.
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Re: Stock board bias balance question

Postby Shannon Parks » Thu Apr 10, 2014 4:14 pm

dannyr wrote:Shannon, Thanks. I read Dave Gillespie's write ups on a stock ST70 over at Audiokarma and I wondered if I had dismissed the stock circuit too fast but thought that it would be nice to balance the bias.


The stock circuit is excellent. The pentode voltage gain section is biased perfectly and with very low distortion. I spent weeks trying to eke some more performance out of it and couldn't. The cathodyne phase splitter is limited in drive capability, but matched well to EL34s and their high gm. Only flaw is 7199 availability.

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Re: Stock board bias balance question

Postby dannyr » Thu Apr 10, 2014 11:22 pm

Shannon, Yeah the 7199 availability concerns me. I only have two 7199's and I have no idea how much life is left on them. Did you by chance ever try any subs in the stock board? I was thinking of just cutting some traces etc to use 6GH8A's or maybe 6u8's but i'm not sure if it would be a performance hit or not. Dave Gillespie recommended to ditch the GZ34 and add a 50ohm resistor between diodes and the choke to bring voltages down. Also to add a capacitor etc to the input so no DC gets through. I may take his lead but add a CL-90 or something. Seems like if nothing else it really cools the power transformer.

I looked at the datasheets for the above driver tubes and the triode section's amplification factor is 17 for the 7199 and 46 for the 6GH8. 40 for a 6u8. I had hoped that they were near identical.
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Re: Stock board bias balance question

Postby Shannon Parks » Fri Apr 11, 2014 8:40 am

There have been plenty of folks that have switched to the 6GH8/6U8 types and have been happy. It's probably the best compromise with the stock circuit (with some cuts and jumpers).

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Re: Stock board bias balance question

Postby Geek » Fri Apr 11, 2014 12:46 pm

If you want closer to the 7199 sonics, 6AN8 works too. But it's also becoming a rare puppy :(
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Re: Stock board bias balance question

Postby dcriner » Fri Apr 11, 2014 4:37 pm

Russian-made Sovtek 7199s are available for $12.95: http://www.rfparts.com/7199-sov.html I didn't search for any reviews of them.

Until a few years ago, there was a plentiful supply of NOS US-made JAN Phillips 7199s for, as I recall, about $10 each. I bought a pair to serve as spares, so I expect that I have enough 7199s to last well beyond my remaining lifetime ;)

7199s run lightly loaded: the heater draws just 2.8 W, the triode's maximum plate dissipation is a mere 2.4 W, and the pentode section's plate is a maximum of 3 W - for a total of 8 W, peak. The 7199s in my ST-70 get barely warm to the touch. I would expect JAN or name-brand 7199s to last almost indefinitely - certainly much longer than the rectifier and audio power tubes.

Personally, I would resist ripping into an ST-70 to modify it to accept substitute tube types as long as my 7199s were working.
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Re: Stock board bias balance question

Postby dannyr » Fri Apr 11, 2014 10:30 pm

I can across the below post which may be of interest. It seems simple enough to try. I don' t know if the theory is correct or not.

https://groups.google.com/forum/#!topic ... e2z0fSGyww

"I've read a number of threads on re-tubing the 7199 lately. I just want
to pass a message in regards to prolonging the life of the fairly
expensive (and getting rare) 7199s for the Dynaco ST70.
According to Mr. Matt Kamna who wrote a short artivle in the Sound
Practices #10 (vol3 #1) page 35. He stated that there was a design
error of the ST70 in which the 7199's heater to cathode was too high
(over +100 VDC, to as high as +135 VDC). This exceeds the maximum
heater to cathode voltage, resulting in "leakage as low as 100K ohms!
This leakage resistance is in parallel with the 1% matched 47K cathode
resistor, thereby lowering its value and reducing drive to one of the
EL34s. This results in a 12% loss of output power and a 25 dB increase
in second harmonic distortion!" ... "If you have to replace the tube,
you will want to protect the new one by elevating the filament to about
+75 volts Dc. This is most easily done by using a resistive divider
off the output stage B+ supply. Connect a 390K 1 watt resistor between
the B+ and filament center taps, and a 82K 1/2 watt resistor from the
filament center tap to ground. A 1uF 200 volt capacitor connected
across the 82K will reduce the divider output ripple and unwanted signal
when full power, very low frequancy notes are being reproduced."

Hope this helps and credits goes to Mr. Matt Kamna and Sound Practices.

regards...Kevin

My opinions are mine, they do not represent my employer. Be very
careful with lethal high voltages !"

I found the howto in the post below but I"m not sure what tap is Lug 3. I think it's the middle 20uf going to the grid of the triode side of the 7199 but i'm not 100% sure.

http://www.audiokarma.org/forums/showpo ... stcount=12

"I would consider the voltage divider an essential mod. I don’t know whether I can describe it adequately without a diagram, but what I do is mount a terminal strip along the left side of the power transformer and at a right angle to the 7 lug terminal strip where the filament wiring is connected. Run a wire from lug 3 of the quad cap to a 1 watt 360K resistor attached to the new terminal strip. From the other end of the 360K resistor, run a 120K, 1/2 watt resistor to ground (lug 6 of the existing terminal strip). Also from this end of the 360K resistor, run two 330K, 1/2 watt resistors to the filament center taps (one to lug 5 and one to lug 7 of the existing terminal strip). When you fire up the amp, you should have 70-75 volts DC measured at lug 5 and 7 of the existing terminal strip. Also, the dual 0.1 ceramic cap remains in place. "
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Re: Stock board bias balance question

Postby dcriner » Sat Apr 12, 2014 7:54 am

there was a design error of the ST70 in which the 7199's heater to cathode was too high
(over +100 VDC, to as high as +135 VDC).

The RCA Receiving Tube Manual (RC-30) shows this data for the 7199's heater-cathode voltage:

Peak value +/- 200V max
Ave value 100V max
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Re: Stock board bias balance question

Postby kheper » Tue Apr 22, 2014 6:07 pm

In the link below is a 9 pin socket adapter - allowing the substitution of 6GH8As for 7199s without modification to the stock board. Dynakit includes these adapters with their st-70 amps. How well it works, I have no idea. It could pick up some weird oscillations.

http://www.dynakitparts.com/dynakit-pro ... ET-ADAPTER
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