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Old 1st January 2012, 04:41 PM   #11
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Quote:
Originally Posted by boywonder View Post
Taj:
As far as the NTC/thermister goes, look at the data sheet for a CL-140, CL-90, etc. They give a cold resistance and a hot resistance at the rated current. Problem is, with typical tube PS designs, we are not pulling very much current, especially on the secondary side, so they never get very hot. I believe that's why it's quite common to see a CL-90 on the mains side of the transformer, as you are drawing more than a couple hundred ma, and can get the device warmed up.

Is 450V the typical voltage for the ST-70 outputs? If the power transformer can handle it (probably not if it's a stock ST-70 transformer), you can reduce the 450V and increase the idle current to have a larger percentage of the available power as class A....just a thought


Indianajo: PSUD doesn't have lots of choices for diodes; so the 1N4007 is usually the one picked for a typical FWCT model. I'm assuming Taj will be using 1200V Fairchild FREDs or 1200V Schottky's or something similar if the budget allows.
1N4007 is great theoretically and in prototyping, but I cleaned a lot of dirt off my 50 year old ST70 last year. Better longer arc path diodes are probably available salvage in a CRT display or somewhere.
CL anything is too physically small to not be bridged by current at high voltage assuming dirt or humid air. Calculations are great, but 1000v/in/.3 in=300 volt standoff. I'm using a russian 5AR4 but putting a silicon diode in series couldn't hurt. CL 90 on transformer primary might solve the big "whang" noise when you turn it on.
ST70s came originally with a 525 VDC rated B+ capacitor. I removed mine in 1971 when I bought the ST70 and found it had low watts. This is probably a surge voltage rating, as modern caps are rated, instead of a steady state rating. I have used 450 VDC rated panasonic & nichicon radial lead caps in my 1961 dynakit ST70 with fabric transformer leads, so far so good. The wall voltage has increased from 110 VAC nominal in 1961 to 125 VAC nominal in 2011, with lots of people on organforum reporting 130+ V on their receptacle, So designing for high voltage with original transformers is a good plan.
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Last edited by indianajo; 1st January 2012 at 04:46 PM.
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Old 1st January 2012, 07:55 PM   #12
taj is offline taj
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All good points. Thanks. Some filler information...

Yes, The only reason it shows 1N4007 is because of PSUD2's choices. As boywonder says, I will use much higher rated FREDs or whatever. Maybe Vishay SF1600, as used in a popular DIY guitar amp power supply.

Regarding line voltage, the Dynaco power transformer is spec'd at 117VAC, and my line voltage is 120-122VAC typically. So not as bad as it could be, in general. On the original ST-70, the voltage at the reservoir cap is listed as 430V with the GZ34, so it will be higher with the SS diodes and higher line voltage, but not as big a difference as other rectifier tubes, or other locations, I will consider higher line voltages though.

I won't really know what I have to contend with in terms of current load and voltage supply until I get it hooked up, so I was just hoping to get a ballpark idea beforehand, hence the PSUD2 sanity check request. Thanks for the reply boywonder, I respect your answer.

Rundmaus: The new load on the old power supply parts, from the output section and driver boards will be pretty close to what it was in the original ST-70. A little more, but not much. So using the existing parts should be fine.

indianajoe: Would insulating the leads of the CL-150 reduce the chance of dusty/humid-part arcing?

..todd

Last edited by taj; 1st January 2012 at 07:57 PM.
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Old 1st January 2012, 08:20 PM   #13
rmyauck is offline rmyauck  Canada
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Here's that diode mode. I first read about it on this form from Kevin Kennedy.

tube rectifier diode mod

UF4007 may be quieter.

More info on a really good replacement can cap if you want to keep that.

http://dynacotubeaudio.forumotion.co...s-layout-photo

Lot's of ST-70 info at diytube.com

Hope everybody had a great Christmas and a Happy New Year to all!

Randy

Last edited by rmyauck; 1st January 2012 at 08:32 PM.
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Old 2nd January 2012, 01:36 AM   #14
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Greetings Taj,

May I ask where you got those current tap values simulating the driver and PI?

Cheers!
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Old 2nd January 2012, 01:39 AM   #15
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Quote:
Originally Posted by taj View Post
indianajoe: Would insulating the leads of the CL-150 reduce the chance of dusty/humid-part arcing?
Spreading the legs of the CL-150 and a good blob of some sort of voltage specified epoxy (not black) on each lead to get the arc path out to 2 cm or 3/4" couldn't hurt. I don't think heat shrink tubing has a high enough voltage rating to help. I had replaced tubes in the preamp over the years because of popping, but when I replaced the capacitors & carbon comp resistors last year, found that I had blue arcs (in the dark) at places on the PCB I hadn't worked at (on the bottom). Simultaneous with popping sounds. My theory is the original 1961 builder (a minister) had left solder rosin on the PCB, that only acted up in high humidity. 450 VDC is rather unforgiving. Wash, wash wash wash. And nifty prototypes discussed here probably don't get used for 50 years like my dynakit equipment, or 43 years like my organs. Dead skin flakes (household dirt) make a great arc conductor.
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Old 2nd January 2012, 06:19 AM   #16
taj is offline taj
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Quote:
Originally Posted by Geek View Post
Greetings Taj,

May I ask where you got those current tap values simulating the driver and PI?

Cheers!
Hi Gregg,

Glad to see you're hanging around here. I thought you left a while back.

Those were just approximate starting values to learn PSUD2. My question was mainly about whether I was using PSUD2 correctly. Next step was going to be figuring out the correct currents (or ask you), so I could land at approx. the right supply voltage values. The 5751 amplifier is a pretty straightforward calculation once I pull up the datasheet, but the tail on that PI is beyond my comprehension. Looks like a CCS with feedback from the anodes (or something..)

What are the current draws, if you don't mind my asking?

My plan was to use solid state rectifier (I'm not a tube rectifier fan) without resistively dropping the B+ voltage down to GZ34 level , let the EL34's run at whatever B+ resulted from the power supply (and adjust their operating point accordingly), then drop the next stage voltages in the power supply so that they land at 400V for the 6CG7 supply and 250V for the 5751 supply (as your board expects.)

I love the driver board PCB. I was raised on single-sided roll-your-own PCBs, so those plated through holes are such a pleasure to work with.

..Todd

Last edited by taj; 2nd January 2012 at 06:27 AM.
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Old 2nd January 2012, 06:44 AM   #17
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Heya Todd,

Actually, I did.... a buddy directed me to this thread.

The current draw for the 5751 is ~2.2mA (both channels) and the 6CG/FQ7 you had about right - draws 15-18mA for both channels.

The PI is less forgiving... 400-415V it's OK with and the pre is more forgiving - 230-270V. The important thing for best CMFB performance is keeping 100-110V across the anode resistors on the PI

On my test bed, I've used SS and all sorts of tube rects. I've had the B+ up to 525V with a pair of MUR4100 in series on each leg or as low as 410V with a 5R4. It works.

And thanks on the board! My partner does excellent PCB work.

Cheers!
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Old 2nd January 2012, 06:59 PM   #18
taj is offline taj
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Thanks Gregg,

If I had taken 30 seconds to look at the 5751 datasheet (or even thought about it for 30 seconds), I would have gotten much closer, and may have even nailed it without a calculator. But the 6CG7 current was fluke.

Does one need sharing resistors when using a pair of diodes in series?

..Todd

Last edited by taj; 2nd January 2012 at 07:02 PM.
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Old 2nd January 2012, 07:10 PM   #19
taj is offline taj
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Quote:
Originally Posted by indianajo View Post
And nifty prototypes discussed here probably don't get used for 50 years like my dynakit equipment, or 43 years like my organs.
Thanks indianajoe,

This won't be a prototype really, it'll end up looking good in the living room, and being bequeathed to successors eventually. So it will make sense to consider its long-term health (and safety). But I won't be filling it full of expensive boutique parts unless they make sense technically.

..Todd
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Old 3rd January 2012, 12:54 AM   #20
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Hi Todd,

With guesses like that, maybe I'll have you choose my Keno numbers

Quote:
Originally Posted by taj View Post
Does one need sharing resistors when using a pair of diodes in series?
I ended up doing this. Not for the diodes, but for the longetivity of the tubes:

Click the image to open in full size.

Cheers!
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