ST35 Rev E!

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

Re: ST35 Rev E!

Postby TomMcNally » Thu Sep 12, 2013 2:39 pm

separks wrote: I would shoot for 33 to 35mA across the 10 ohm resistor. Let me know if you have plenty of adjustment range and what brand EL84s you are using. With these 2% Zeners, it should be fairly predictable, though some EL84s (like JJs) might be harder to get in range.
Shannon


I am using a matched quad of new ElectroHarmonix EL-84's. Running a little hot, adjustment range at the test points is
about .67 to .47 ... can't get any lower.

What do you think ?

... tom
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Re: ST35 Rev E!

Postby Shannon Parks » Fri Sep 13, 2013 6:23 am

TomMcNally wrote:I am using a matched quad of new ElectroHarmonix EL-84's. Running a little hot, adjustment range at the test points is
about .67 to .47 ... can't get any lower.

What do you think ?

... tom


Holy guacamole, those are some roarin' tubes! They don't seem to match the classic datasheet curves at all.

10V zener + 10 ohm current sense (times 67mA) + variable pot @ 0 ohms = 10.67V (+/- 200mV)
10V zener + 10 ohm current sense (times 47mA) + variable pot @ 100 ohms (times 47mA) = 15.17V (+/- 200mV)

So you have an adjustment range of 10.67V to 15.17V on the selfbias adjustment. Verify that your 10V zener is actually 10V and that my computed voltages match yours at the cathode, pin 3. Maybe just verify this at the 15.17V setting, as we don't want to burn those babies up. If it does turn out to be 'hot' tubes, get a few sets of four of these, 12V, 13V & 15V zeners (all in stock):
78-BZX85B12-TAP
78-BZX85B13-TAP
78-BZX85B15-TAP
You can use the first class mail shipping option on this small order and it should be very cheap.

If you really have 15V at the cathodes at 47mA, I would go straight to the 15V zener. The 12V and 13V may not be enough:
(12V + 10 (times 35mA) + 100 ohms (times 35mA) = only 15.85V

Let me know, Tom. If these EL84s are going to be all over the place, then I might just change the 100 ohm variable pot to 200 ohm to save headaches. You could also do a quick fix by changing your 10 ohm resistors to 100 ohm. I originally had the 12V zener in mine, but changed to 10V zeners due to low current (less than 35mA on all my Reflector 6pi14pi tubes).

Shannon
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Re: ST35 Rev E!

Postby TomMcNally » Fri Sep 13, 2013 7:42 am

I'll experiment with it. I can try other sets of tubes also. I can raid my Rev B and my
Dynaco ST-35 and my Harmon Kardon amp and see how they play.

You might want to document the mod in the manual, and also show where the test points are on the schematic.

... tom
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Re: ST35 Rev E!

Postby Shannon Parks » Fri Sep 13, 2013 10:40 am

TomMcNally wrote:I'll experiment with it. I can try other sets of tubes also. I can raid my Rev B and my
Dynaco ST-35 and my Harmon Kardon amp and see how they play.

You might want to document the mod in the manual, and also show where the test points are on the schematic.

... tom


Will do, Tom. Looking over my own curve data for fifty-seven different EL84s, I think it's just a matter of strong EL84s.

Tentatively, I'll probably change the parts to:
Code: Select all
Zener Mod:
Omit R17, R20, R23 & R26. Sub the following:
   4   R18,R21,R24,R27   12V zener 2%     78-BZX85B12-TAP      0.10
   4   R19,R22,R25,R28   200 ohm Pot      72-T93YB-200         1.58


This will broaden the biasing range, though some weaker tubes (like my Reflectors) may not get to 35mA.

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Re: ST35 Rev E!

Postby Shannon Parks » Sat Sep 14, 2013 11:18 am

Here's a more detailed answer to the MIL-SPEC rating versus commercial rating. My parts list gives the Industrial rating @ 125C. Each resistor actually has four ratings, which can be confusing:

Example for a CMF60/RN60 resistor:
MIL-SPEC up to 70C: 1/4W
MIL-SPEC up to 125C: 1/8W
Industrial up to 70C: 1W
Industrial up to 125C: 1/2W

Except for marking the CMF & RN resistors are identical.

http://www.vishay.com/docs/31027/cmfmil.pdf
http://www.vishay.com/docs/31018/cmfind.pdf

Hope this clears up any confusion.

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Re: ST35 Rev E!

Postby TomMcNally » Sun Sep 15, 2013 2:00 pm

Update ...

The dim bulb that put the board together left in the 47 ohm resistors with the Zener mod.
Taking them out helped a little, I still can't get lower than .37 volts though. I'll order up some
parts from Mouser and get it going.
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Re: ST35 Rev E!

Postby Shannon Parks » Mon Sep 16, 2013 3:33 pm

Cool! So the 12V zeners should work well for you. I think I'll just end up sticking with the 10V zeners and changing the 100 ohm pot to a 200 ohm pot on the parts list.

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Re: ST35 Rev E!

Postby EWBrown » Fri Sep 20, 2013 8:15 pm

During some of my tests with the Rev D boards, I found that most J/Js ran pretty close to 35 mA with 14V DC on the cathodes (pre-set cathode resistance network to 400 ohms), while the "Radio Lampa" and "Raketa" Russian 6P14P and 6P14P-EV were all over the map, some as low as 10V on the cathode for 35V, and a high of approx. 16VDC for 35 mA. I later built up a simple dedicated EL84 tester, using, for each tube, an LM317 cathode current regulator set to 35 mA, on which I could test / match two tubes at a time. This made it much easier to match and grade all my various and sundry EL84s.

Plan B is to add two more sockets, wired for 6GK6s. Tis could also be "expanded" to include 6V6GTs, 6AQ5s, et, with an appropriately lower B+ voltage

The B+ voltage in my tester is the same as in the ST-35, I set that for 365 VDC, which was what I refer to as the "Standard" ST35 B+ voltage, though in actual practice, this can be anywhere between 350 and 380VDC, depending on the power transformer and AC powerline voltage..

/ed B
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Re: ST35 Rev E!

Postby LMAN » Sat Oct 05, 2013 8:07 am

I'm planning on doing the zener mod but need some clarification. When you say:

"Omit R17, R20, R23 & R26"

do you mean place jumpers where the resistors would have been or leave then open.

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Re: ST35 Rev E!

Postby Shannon Parks » Sat Oct 05, 2013 5:09 pm

LMAN wrote:I'm planning on doing the zener mod but need some clarification. When you say:

"Omit R17, R20, R23 & R26"

do you mean place jumpers where the resistors would have been or leave then open.

Lou


Leave them open. Crank the 100 ohm pots to max resistance before first power up, too.
Good luck and let me know how it biases up for you!

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Re: ST35 Rev E!

Postby LMAN » Mon Oct 07, 2013 10:33 am

I put in my sacrificial tubes, JJ EL84's, fired it up and had no problem biasing them using 200 ohm pots with the 10 volt zeners.

Now I need to finish up the base, mount the FPE top plate to it and install the listening tubes.
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Re: ST35 Rev E!

Postby TomMcNally » Wed Oct 09, 2013 3:22 pm

I finally grabbed some 10 cent 12 volt Zeners at Radio Shack for $ 1.00 each and replaced the 10 volt ... the tubes biased up nicely.
I'll take some pictures soon.
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Re: ST35 Rev E!

Postby Shannon Parks » Fri Oct 11, 2013 12:50 pm

TomMcNally wrote:I finally grabbed some 10 cent 12 volt Zeners at Radio Shack for $ 1.00 each and replaced the 10 volt ... the tubes biased up nicely.
I'll take some pictures soon.


Radio Shack is still pretty handy! I even noticed a nice wall of Arduino stuff there last month when I needed some freeze spray in a pinch.

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Re: ST35 Rev E!

Postby chrx » Fri May 08, 2020 12:36 am

I ended up switching to the 200 ohm pots from the 100s first, and then, after still needing more elbow room, also went to 13-volt Zeners, from the 10s I stuffed initially. That combination finally got my Electro Harmonix EL84s to .350 in the middle of the range of the pots.
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