Biasing stock ST70 at 1.2?

Otis · « **on:** 10 Dec 2009, 11:35 pm »

Hi guys, a long time ago, I read that biasing a stock Dyna 70 at 1.2 instead of 1.56 would make the tubes last longer with only a negligible reduction in performance?

Is there anything to it?

Otis

avahifi · « **Reply #1 on:** 11 Dec 2009, 04:26 pm »

Low biasing will essentially be partially turning off the output tubes and likely result in muddy sound. I think. Any others out there doing this?

Frank

gjs_cds · « **Reply #2 on:** 11 Dec 2009, 07:31 pm »

I bias mine to 1.5 mV...and even then, it takes a good half-hour for it to really begin to perform. Then again, it's not an AVA amp. Without question--my biggest mistake in audio was buying (my) particular amp. Frank--if you're ever interested in a modded ST-70 for a rebuild or upgrade, let me know. (For example, The silly thing only has 8-ohm outputs, even though the transformers clearly state 4,8,16 ohm taps!)

avahifi · « **Reply #3 on:** 11 Dec 2009, 08:51 pm »

I can probably install an Ultimate 70 circuit set in an old Circio modded unit without any problems.

The only issue might be that we do not own any hole movers or undrillers if needed.

Regards,

Frank Van Alstine

rlee8394 · « **Reply #4 on:** 12 Dec 2009, 12:20 am »

This is actually a function of power. The original Dynaco ST-70 had 415 Vdc on the primary of the center tap of the output transformer. 410 Vdc appear on the plates of each output tube. 1.56 vdc across the 15.6 ohm across the cathode resistor equates to 100 ma of current for the two tubes together, 50 ma per tube. This equates to 20.5 watts of plate dissipation per tube.

It is possible to change the bias based on your plate voltage. My U70's use the newer Triode Electronics power transformer, which puts out a bit more voltage. I think I am running the bias based on my plate voltage to yield the same quiescent plate dissipation of 20.5 watts.

Plate dissipation is the product of plate voltage and current. P=IE. I believe that a rule of thumb is to run the tubes biased to about 70% of there maximum plate dissipation. This is common with musical tube amplifiers. Actually, I believe that it is based on accurate class AB.

David Manley would run his KT90, 6550, KT88, and other large power tubes at around 35 ma, but, his plate voltage was over 500 Vdc. Most other amps, like the Dyna MK III run the tubes at 60 ma at 465 Vdc.

I would measure the plate voltage of your amp and then adjust your bias to yield the same 20.5 watts of plate dissipation as a starting point. Listen for a while and then reduce the current by 5 ma. Listen some more. You can go the other way as well. But you will reduce tube life. You can keep going lower until you hear a marked reduction in sound quality. Then go back up. You should be able to hear the difference, especially with louder dynamic music. You probably won't need to go any lower than 35 ma of bias current. Hope this helps.

Ron

gjs_cds · « **Reply #5 on:** 12 Dec 2009, 01:03 am »

Quote from: avahifi on 11 Dec 2009, 08:51 pm

I can probably install an Ultimate 70 circuit set in an old Circio modded unit without any problems.

The only issue might be that we do not own any hole movers or undrillers if needed.

Regards,

Frank Van Alstine

Humm.... I may take some pics of it at some point and send them up your way. See if it's even worth the effort/cash. It's a good looking amp; it just sounds rather... er... mediocre.

rlee8394 · « **Reply #6 on:** 12 Dec 2009, 03:16 am »

You know, those bias resistors fromDynaco still measure pretty damn accurate. It probably wouldn't be a bad idea to measure the resistors true value. If you find that yours are not to spec, replace them with say a 10 ohm 1% resistor. With todays modern DMM's, there isn't a need for a 15.6 ohm value. A measurement of 1.0 Vdc across a 10 ohm resistor will set the bias to 100 ma. The original value of 15.6 ohms was used to provide an accurate 1.56 volts, which was the voltage of a fresh "D cell" battery. Accurate meters were beyond the reach of most hobbyist back in the 50's and 60's. So Dyna chose the 15.6 ohm resistor so you could use any old meter, measure the voltage of the fresh D cell, note the voltage, and then use the same meter to set the voltage to the same reference. Pretty damn clever on Dyna's part if you ask me.

Ron

Dawkus · « **Reply #7 on:** 17 Dec 2009, 10:26 pm »

Hi Otis,I had a stock St-70.I had it modified by a freind who installed the Purist Audio Dyna St-70 modification.With this mod,the 7199's tubes are replaced with the 12AX7's on the circuit board.The output tubes,EL34's are recommended to be biased at .78 volts,half that of the original 1.56 volts that the stock asks for.I use the amp a lot with Siemans tubes from Germany.The amps top cage never gets close to being hot,just a tad warm at worst.I have not needed to change a tube since 1990.The front end of the amp,which has replaced the 6AN8 tube with a 12AX7 has seperate bias controls,other than the bias for the output tubes is recommended to be set at .35 volts.It was this bias control on the front end which I was told could alter the sound of the amp.I was told to use my ear to go a little higher or lower on that bias setting to see what sound I liked best.I think Purist Audio still is around making cables.The original owner and modifyer is Jim Aud,Clute Texas.You could look him up and see if the offer still goes if you were interested,the mod came as a kit with a new circuit board.It did quite well in a Glass Audio 1990 Dyna ST-70 shoot off,but any chance you have to pick the brain of Frank VanAlStine,do so,because I don't know (Jack @#$%)compared to Frank,and he has been helpfully answering my questions for years...Mark Korda(mark.korda@myfairpoint.net)

Dawkus · « **Reply #8 on:** 18 Dec 2009, 08:41 am »

Hi Otis....thanks for the reply.The Purist Audio Design Dyna St-70 literature that I read when I answered to your question said that this mod made it so you did not need matched tubes.....Mark

Dawkus · « **Reply #9 on:** 18 Dec 2009, 08:59 am »

Hi Otis,I also forgot that I saw on E-bay,when you go to electronics/dynaco,a tube converter that changes the 7199 to a more affordable and better tube,I can't recall it right now...I'm listening to Triumvarate ...Pompei..great disc...Otis I have an old stereo salesman and musical freind that said I did the best (Floyd) immitation from Andy Griffth......this is a tube socket that just plugs into the same socket to let you use the better tube...e mail me and I'll forward it to you for my next computer project...sincerely....Mark(Dawkus)

Brett Buck · « **Reply #10 on:** 18 Dec 2009, 09:20 am »

Quote from: Dawkus on 18 Dec 2009, 08:59 am

...this is a tube socket that just plugs into the same socket to let you use the better tube.

Well, cheaper and more available, anyway. Same as the all of Frank's versions.

Brett

Listens2tubes · « **Reply #11 on:** 20 Dec 2009, 05:38 am »

I bias my MkIV's at 1.25v. Before replacing the ouput tube sockets the bias was less than stable. I would notice the sound become hard and the bias would be high, 1.62v. If I set the bias lower than 1.56v I heard no difference. I know that Dynaco ws pushing the envelope with my 40 watt monoblocks and had read on another forum that lower bias was a smart move. So since my last working on them I've biased at 1.25 and they hold it steady. Now I only check bias where before I was adjusting and trying to preset for the evening drop. Damn loose old sockets, good riddance.