Biasing up ST35 Rev E

for the DIY ST35, the Dynakit and every other PP EL84

Biasing up ST35 Rev E

Postby crcook » Wed Nov 06, 2013 4:57 pm

Just finished stuffing and buttoning up a Rev E board. A transformer malfunction damaged my Rev C, so I simply rebuilt the whole amp. I installed the Zener mod, using 1N4740a 10v zeners, which are 5% tolerance.

I am running some old Amperex EL84s and am having trouble biasing two of the tubes. Can only get down to .38 and .39 volts at the test points before I run out of pot. They are well used tubes, and will be replaced with current manufacturers in the near future. The tubes still test fairly good on my calibrated military tester, so I would like to use them if possible.

Would going to 11v or 12v zeners help? I am trying to avoid removing components like pots, as the pads tend to pull. I could also move the 10 ohms up to 100 ohms if that would work. Anything to avoid pulling out the pots.

Suggestions?
crcook
 
Posts: 10
Joined: Tue Aug 10, 2004 7:50 pm
Location: Northern California

Re: Biasing up ST35 Rev E

Postby Shannon Parks » Thu Nov 07, 2013 11:07 am

Yes, switching to 100 ohm resistors at the 10 ohm spot will be the quickest and easiest fix for you. Thanks for the feedback.
Also, read this thread for some of my replies to Tom:
viewtopic.php?f=1&t=6003

Shannon
User avatar
Shannon Parks
Site Admin
 
Posts: 3764
Joined: Tue Mar 18, 2003 5:40 pm
Location: Poulsbo, Washington

Re: Biasing up ST35 Rev E

Postby crcook » Thu Nov 07, 2013 12:10 pm

Would changing to 12v zeners work? May be easier than changing the 10 ohm resistors

Thanks for any advice.
crcook
 
Posts: 10
Joined: Tue Aug 10, 2004 7:50 pm
Location: Northern California

Re: Biasing up ST35 Rev E

Postby Shannon Parks » Thu Nov 07, 2013 1:05 pm

crcook wrote:Would changing to 12v zeners work? May be easier than changing the 10 ohm resistors

Thanks for any advice.


Sure. Use 78-BZX85B12-TAP.

Right now you are biased at around 14.2V = 10V + (110 * .038). So even a 13V zener would work, but not guarantee compatibility in the future with other tubes.

Shannon
User avatar
Shannon Parks
Site Admin
 
Posts: 3764
Joined: Tue Mar 18, 2003 5:40 pm
Location: Poulsbo, Washington

Re: Biasing up ST35 Rev E

Postby crcook » Sat Nov 09, 2013 10:20 pm

OK, I split the difference. Replaced the 10v zeners with 11v 1N4741s. The old Amperex tubes biased up fine and the amp sounds really good. I think I have enough adjustment room for the next set up tubes, which will probably be newer types.
crcook
 
Posts: 10
Joined: Tue Aug 10, 2004 7:50 pm
Location: Northern California

Re: Biasing up ST35 Rev E

Postby MacL » Mon Nov 25, 2013 10:41 am

Hi All,
Is there any reason not to utilize the zener diode bias option? I assume this is a improvement of the original Dynaco design. NICE DESIGN of Bias location-I like being able to set bias without flipping amp over and removing the plate.

Also, it appears that the bias test points connection pad is between the bias pots-correct?
So while setting bias I would use individual tp's and chassis ground (no tp for ground)-correct?
Sounds like JJ tubes are hard to set bias low enough... anyone else using another low price EL84 they can suggest?
With the individual tube bias settings is there a need to order a matched quad? Or are matched pairs ok?

BTW-found some nice nickel plated metal hole plugs on ebay (under $3)to cover the bias access holes-should be able to easily pry off when needed?

THanks!
MacL
 
Posts: 52
Joined: Wed Mar 06, 2013 11:01 am
Location: new to Austin, TX

Re: Biasing up ST35 Rev E

Postby TomMcNally » Mon Nov 25, 2013 12:46 pm

Since you can set the bias for each tube, no need to worry about matched pairs really.
Keep in mind they are only "matched" at the time the seller matched them. After they
are on for a few hours they do their own thing. They don't track each other !

No need to obsess over bias settings, just set it when you change tubes.

The Triode stainless steel chassis makes it a bit tough to drill holes for a top access hole,
so I flip them over.

... tom
User avatar
TomMcNally
Darling du Jour
 
Posts: 2729
Joined: Sat Nov 19, 2005 2:19 pm
Location: Northfield, NJ

Re: Biasing up ST35 Rev E

Postby MacL » Mon Dec 16, 2013 10:03 am

Ok- just finished populating the PCB.
nothing wired to the board and no tubes installed.

Trying to set initial bias to 400 ohms per instructions (using zener option) -IS this possible without tubes installed? Don't have tubes ordered yet-so I can't check with tubes installed.

Using J1 pin 6 for ground and tube socket pin 3 and not getting any adjustable readings...

Thanks
MacL
 
Posts: 52
Joined: Wed Mar 06, 2013 11:01 am
Location: new to Austin, TX

Re: Biasing up ST35 Rev E

Postby Shannon Parks » Tue Dec 17, 2013 5:46 pm

MacL wrote:Ok- just finished populating the PCB.
nothing wired to the board and no tubes installed.

Trying to set initial bias to 400 ohms per instructions (using zener option) -IS this possible without tubes installed? Don't have tubes ordered yet-so I can't check with tubes installed.

Using J1 pin 6 for ground and tube socket pin 3 and not getting any adjustable readings...

Thanks


The instructions are for the regular cathode resistor. The zener mod is pretty much undocumented. :)

First, follow the parts list:
Code: Select all
Zener Mod:
Omit R17, R20, R23 & R26. Sub the following:
 4  R18,R21,R24,R27  10V zener 2%  78-BZX85B10-TAP  0.10
 4  R19,R22,R25,R28  100 ohm Pot   72-T93YB-100  1.58


Secondly, adjust the 200 ohm pots fully open (to 200 ohms). Measure this (amp off obviously) with a DMM set to ohms. You can measure on the inner meniscus leads at:
R24 & R45,
R27 & R46,
R21 & R44, and
R18 & R43.

Then you can fire it up and see where they bias up at, measuring the voltage drop across the 10 ohm resistors (same as the manual). Hopefully they all bias under 35mA. Then you can adjust the pots and the bias up to 35mA. Let me know where they end up.

Shannon
User avatar
Shannon Parks
Site Admin
 
Posts: 3764
Joined: Tue Mar 18, 2003 5:40 pm
Location: Poulsbo, Washington

Re: Biasing up ST35 Rev E

Postby MacL » Wed Dec 18, 2013 1:48 pm

Hi,
Thanks for the tips.
Any chance you mean 100 ohms not 200 ohms?
I rechecked the parts-to make sure they are correct- and check the inner meniscus it seems they max out at approx. 100 ohm readings.
It takes many turns on these pots and it seems like they continue to turn after maxed out-is this how these pots work?
My transformers just shipped and I still need to order tubes so it may be a month before I am able to power up to respond with real numbers.
Again I appreciate yours or any ones advice.
MacL
 
Posts: 52
Joined: Wed Mar 06, 2013 11:01 am
Location: new to Austin, TX

Re: Biasing up ST35 Rev E

Postby Shannon Parks » Sat Dec 21, 2013 6:31 am

MacL wrote:Hi,
Thanks for the tips.
Any chance you mean 100 ohms not 200 ohms?
I rechecked the parts-to make sure they are correct- and check the inner meniscus it seems they max out at approx. 100 ohm readings.
It takes many turns on these pots and it seems like they continue to turn after maxed out-is this how these pots work?
My transformers just shipped and I still need to order tubes so it may be a month before I am able to power up to respond with real numbers.
Again I appreciate yours or any ones advice.


Whoops - yes, meant 100 ohms. I'm considering changing it to 200 ohms so I projected there. Yes, they are 9 turn pots and will continue to 'click' at the end.
I think you are setup and ready to go once everything arrives. Good luck!

Shannon
User avatar
Shannon Parks
Site Admin
 
Posts: 3764
Joined: Tue Mar 18, 2003 5:40 pm
Location: Poulsbo, Washington


Return to stereo 35

Who is online

Users browsing this forum: No registered users and 4 guests