how to bias my rebuilt dynaco st 70

vuthecuongjapan · 24th May 2012, 01:00 PM

Hi all
I want to replace my 4 el34 in my rebuilt dynaco st-70.
My question is: in the surface of my amp, there is 2 bias pots. So in order to bias my newly replaced tubes, I must connect my current multimeter into that 2 bias pots then adjust the current of new tubes, right? Please see my attached picture.
Thank you very much & sorry for my bad english.

Palustris · 24th May 2012, 02:32 PM

See the link: http://www.curcioaudio.com/st7_mnl.pdf

Original Burnedfingers · 26th May 2012, 09:41 AM

Check the schematic. Put your test leads across the 15.6 ohm resistor on the cathode pin 8 of the output tubes. With the meter on the DC voltage scale you will adjust the pot for that side until you get 1.56 volts DC. This will equate to 100 mA for the pair of output tubes. You will do the same for the other side.

rmyauck · 05:58 PM

They ran the tubes hot back in the day as they were low cost to replace, but with todays tube's it's better to run a lower dissipation by trying a lower setting to around 1.40 for better life.

Watch for any red plating in a dark room with stock or near stock settings.

See where they sound the best at the lowest bias setting you can achieve for the longest tube life..

The B+ influences this which can be a fair bit higher with todays line voltages being higher.

http://dynacotubeaudio.forumotion.co...ht=bias+st+-70

Diode mod is also great for rectifier tube life using UF4007 diodes. Also helps if the power flashes off and on in succession,

http://dynacotubeaudio.forumotion.co...fier-diode-mod

http://www.audiokarma.org/forums/sho...d.php?t=443917

Hope that helps!

Randy

Original Burnedfingers · 27th May 2012, 11:50 AM

Actually I did the diode mod here in 2002 and posted it and from there it got copied. The bias would have to down a lot before your going to hear anything.
The difference between 100mA for the pair and 90mA isn't going to make much if any difference in the tube life....been there. The diodes should be rated a minimum of 1000 volts to be safe and the "fast recovery" do not sound any better nor can any difference be heard between them and the standard 1N4007's. I believe I had posts on that forum before Bob banned me for posting that I had started a new forum. My suggestion for tubes is going to be either the Russian 6n3cE or the 6BG6GA that are available on the web.

I convert all my Dynaco 70's to individual tube bias because as the matched tubes wear they do not stay matched in most cases. This also allows one to purchase some $10ea 6BG6GA's instead of $50 on up matched pairs.

Just my .02

vuthecuongjapan · 28th May 2012, 02:56 AM

Many thanks for your response. But I meaned trange problem is that when I opened my front cover of my dynaso st 70, bias pin on front socket, to which I will connect the wire of my multimeter, is not there. My dynaco st 70 is not stock one but rebuilt by Will Vincent.
So how do I measer volt of my newly replaced kt77 tube?
I attached 3 images: front of dynaco, upper chassis and the back of my dynaco for your reference.
Please help me. I need your help. Thank you.

rmyauck · 03:36 AM

Look around those bias pots ,looks like just a fuse in the middle, but maybe he put test points around there. Yes, maybe those small black circles with holes in the middle in front of the pots are your new test points). ( All mounted on a plate between rect. tube and big black capacitor)

Also remove top cover for driver circuit and post pics.

Post some good under shots may help too!

Randy

Original Burnedfingers · 28th May 2012, 09:10 AM

It looks like those test points are the two turret terminals next to the bias pots. Put one test probe in one of the turret terminal test points and take the other meter test probe to a good ground and see what you get. I am only seeing one test point per channel so a common ground is what you need to complete your circuit to test the bias.

vuthecuongjapan · 28th May 2012, 09:12 AM

Quote:

Originally Posted by Original Burnedfingers

It looks like those test points are the two turret terminals next to the bias pots. Put one test probe in one of the turret terminal test points and take the other meter test probe to a good ground and see what you get.

thank you. I will test it

26th May 2012, 05:40 PM	#4
rmyauck diyAudio Member Join Date: Apr 2009	They ran the tubes hot back in the day as they were low cost to replace, but with todays tube's it's better to run a lower dissipation by trying a lower setting to around 1.40 for better life. Watch for any red plating in a dark room with stock or near stock settings. See where they sound the best at the lowest bias setting you can achieve for the longest tube life.. The B+ influences this which can be a fair bit higher with todays line voltages being higher. http://dynacotubeaudio.forumotion.co...ht=bias+st+-70 Diode mod is also great for rectifier tube life using UF4007 diodes. Also helps if the power flashes off and on in succession, http://dynacotubeaudio.forumotion.co...fier-diode-mod http://www.audiokarma.org/forums/sho...d.php?t=443917 Hope that helps! Randy Last edited by rmyauck; 26th May 2012 at 05:58 PM.

28th May 2012, 03:25 AM	#7
rmyauck diyAudio Member Join Date: Apr 2009	Look around those bias pots ,looks like just a fuse in the middle, but maybe he put test points around there. Yes, maybe those small black circles with holes in the middle in front of the pots are your new test points). ( All mounted on a plate between rect. tube and big black capacitor) Also remove top cover for driver circuit and post pics. Post some good under shots may help too! Randy Last edited by rmyauck; 28th May 2012 at 03:36 AM.

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24th May 2012, 02:32 PM	#2
Palustris diyAudio Member Join Date: Aug 2006 Location: Cape Cod	See the link: http://www.curcioaudio.com/st7_mnl.pdf

26th May 2012, 09:41 AM	#3
Original Burnedfingers diyAudio Member Join Date: Nov 2002	Check the schematic. Put your test leads across the 15.6 ohm resistor on the cathode pin 8 of the output tubes. With the meter on the DC voltage scale you will adjust the pot for that side until you get 1.56 volts DC. This will equate to 100 mA for the pair of output tubes. You will do the same for the other side.

27th May 2012, 11:50 AM	#5
Original Burnedfingers diyAudio Member Join Date: Nov 2002	Actually I did the diode mod here in 2002 and posted it and from there it got copied. The bias would have to down a lot before your going to hear anything. The difference between 100mA for the pair and 90mA isn't going to make much if any difference in the tube life....been there. The diodes should be rated a minimum of 1000 volts to be safe and the "fast recovery" do not sound any better nor can any difference be heard between them and the standard 1N4007's. I believe I had posts on that forum before Bob banned me for posting that I had started a new forum. My suggestion for tubes is going to be either the Russian 6n3cE or the 6BG6GA that are available on the web. I convert all my Dynaco 70's to individual tube bias because as the matched tubes wear they do not stay matched in most cases. This also allows one to purchase some $10ea 6BG6GA's instead of $50 on up matched pairs. Just my .02

28th May 2012, 09:10 AM	#8
Original Burnedfingers diyAudio Member Join Date: Nov 2002	It looks like those test points are the two turret terminals next to the bias pots. Put one test probe in one of the turret terminal test points and take the other meter test probe to a good ground and see what you get. I am only seeing one test point per channel so a common ground is what you need to complete your circuit to test the bias.