Bottlehead Forum
General Category => Technical topics => Topic started by: syncro on February 02, 2012, 05:46:45 PM
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I'm getting a DIYtube driver board for an ST-70 rebuild project and have posted on that forum for opinions about what to do for the power supply circuit. If anyone here wants to chime in either here or there, I would appreciate your recommendations.
The description of my project, the questions, and what I've found so far for options is at:
http://www.diytube.com/phpBB2/viewtopic.php?t=5182 (http://www.diytube.com/phpBB2/viewtopic.php?t=5182)
Thanks, David
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Last I heard Curcio sells a cap board. Paully has used them before.
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I rebuilt my old ST70 as well as a pair of Mk IVs using Joe Curcio's boards. They sound very good and aren't too hard to put together.
I posted a video a while back on you tube of the ST70 playing. It had a new oversized PT from dynakitparts, original OTs. Joe's cap board is inside and I left the can cap unconnected.
http://www.youtube.com/watch?v=yi-lmFVA2l4 (http://www.youtube.com/watch?v=yi-lmFVA2l4)
ST70
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi576.photobucket.com%2Falbums%2Fss207%2Fcorndog642%2FDynaco-Curcio%2520ST70%2Fa6464d75.jpg&hash=4711e21ce3afa8c56c69d539dd9234716bd13f45)
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi576.photobucket.com%2Falbums%2Fss207%2Fcorndog642%2FDynaco-Curcio%2520ST70%2Ff822c1a2.jpg&hash=270d5432513d35d16f088e791cbf38ab5e0d4f06)
Mk IV
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi576.photobucket.com%2Falbums%2Fss207%2Fcorndog642%2FDynaco-Curcio%2520Mk%2520IV%2F134a5cb9.jpg&hash=5345a195f3dcf5e4912ccd730b3e47a8dfe76bae)
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi576.photobucket.com%2Falbums%2Fss207%2Fcorndog642%2FDynaco-Curcio%2520Mk%2520IV%2FIMG_2423.jpg&hash=53b5c642cd28ba63b5ba7cfc7e7d1576c3985932)
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Thank you, Grainger, for reminding me of that one. I even have their documentation on my computer, as I recall thinking it was the best one some time ago. Good value at $65. I'll see what opinions I draw from the forum, and maybe Paully or others will chime in.
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I have had both power supply boards and they both worked very well. Different dynacos so no way to truly compare. I have the triode electronics in my current dynaco and am very pleased with it. You will be fine whichever way you go as far as I know.
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Hey, thanks, Paully for commenting on the Curcio power supplies. Yes, I just noticed last night they have a second offering as part of a more upscale ST-70 modification package. I think with your comment, their regular cap-board ($65 w parts) would be the way I'd go. I think I'll start with the standard 30/20/20/20 uF quad that came with this project chassis (slowly powering up with a variac) and hope for the best. I want to listen to the new driver board, compare the the other stock amp I've been using and then put the cap-board in later.
Also, thanks to Corndog71 for the encouragement and nice photos of your builds!
Here are some photos of my project as received: http://home.bway.net/bogle/ST-70_before/st-70-before.html (http://home.bway.net/bogle/ST-70_before/st-70-before.html)
You can see the corrosion from a previous quad cap meltdown. After I clean that up, I will want to get the new cap-board soon, I'm sure. If anything else there (other than the other power supply caps) looks or would be suspect of being fried by the quad-cap meltdown it had, please let me know.
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At an early VSAC meeting in Silverdale, I listened to Dave McDonald's ST-70 which was highly modded. Here is his description:
"I tried other output tubes, including 7027As, 6550s, KT88s and 8417s,
all in 'triode' mode. Since I had 3 db of feedback (local, not global)
from 16 ohm tap to output tube cathode, and all these except the 8417
biased at about 45 to 52 volts (at 50 Milliamperes), that amounts to a
little more than 70 volts peak from the drivers to get to zero volts
grid-to-cathode on the output tube. I searched for a little more
linearity at large swings on the driver. I wanted to retain a single
stage driver, but needed some gain too - so I "compromised" and picked a
6DJ8. I modified resistors and socket wiring to accommodate a 6DJ8.
Later I took out the driver load resistor, coupling cap, and EL34 Grid
return resistor. These three were replaced by the Lundahl interstage
transformer. The interstage gives more linearity than a simple plate
resistor. I also bypassed the bias pot wipers with large caps to ground
(the bias is applied at the bottom end of the secondary), so I can draw
a little grid current on peaks (but by the time you get there, it does
not sound pretty, it just recovers well). Then I removed the 3 dB
feedback - the EL34 is returned through a 1 Ohm current sense resistor
to ground, instead of to the 16 Ohm tap."
The picture shows where he got to by VSAC/Vancouver in 2007(?) or so with 300B's.
His address then was: Dave McDonald <[email protected]> in case you want to compare notes with him.
Cheers and good luck! Hank in Eugene
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Dear Hank in Eugene,
Thank you for the insights in to some other possibilities with the ST-70!
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So, related to this is a question I have about A/C connection to the amp. The schematic shows a fused line and a switched line from the mains. Do these correlate with hot and neutral, or does it matter?
Intuition, and an idea about safety tells me hot line gets switched.
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Safety designs have come a long way since the S-70 was developed.
IIRC, the hot line should go to the fuse first, then the switch, then the transformer. The other end of the transformer goes to neutral. And the third wire, safety ground, goes to the chassis. There are special rules for how it is attached. The cord (if you are not using an IEC socket) should have enough slack in the neutral and ground lines that, if you yank the cord out, the hot line breaks first, then the neutral, and finally the ground - so the device is unpowered before the safety ground is broken.
There are lots of rules like these, and I don't promise that I remember it correctly - look up the UL rules on the web, there are plenty of sites.
The important point is, a rule gets added whenever enough people die or houses burn down; each rule is based on real-life tragedies, not plausible theories.
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Super info., and logical to put the fuse and switch on the same hot leg! I would rather be a "plausible theory" than a "real-life tragedy." Unfortunately, my house is a two-wire abode (1950), so the ground-to-chassis only causes noise. At least on another ST-70 I tried it, then after I heard the noise I disconnected it at the chassis. I will be able to reconnect and try again someday when I run a ground to the stereo receptacle.
Thanks for the help.
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I've read on other forums that since the dynaco amps were designed without the ground then it shouldn't be attached. That's how I've gone with all of my rebuilds and I haven't had any problems.
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Paully and I have rebuilt ST-70 amps, he has had two. The first thing we do is ground the chassis and remove the 10 ohm resistor that floats the input common from ground. With those two mods there is no noise.
We both have had grounded outlets. I guess it is a moot point if you have un-phased two prong outlets.
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Paully and I have rebuilt ST-70 amps, he has had two. The first thing we do is ground the chassis and remove the 10 ohm resistor that floats the input common from ground. With those two mods there is no noise.
We both have had grounded outlets. I guess it is a moot point if you have un-phased two prong outlets.
This will be good to know someday - when I have a ground...
Now I have another conundrum, and no responses on the ST-70 forums, such as at:
http://dynacotubeaudio.forumotion.com/t1148-st-70-project-parts-id-help-needed (http://dynacotubeaudio.forumotion.com/t1148-st-70-project-parts-id-help-needed)
My question is about a spare part inherited with the ST-70 chassis. I wonder if the black choke-looking item is useful for an ST-70 rebuild. It has the following text/labeling "L1" on top and "150-G" on the side.
I've posted photos at http://home.bway.net/bogle/ST-70%20-%20spare%20parts/ (http://home.bway.net/bogle/ST-70%20-%20spare%20parts/)
Is there a way to tell if this part is useful for an ST-70 rebuild?
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David,
Looking at the picture linked above that is a choke but not the choke used in the ST-70. The one you want is a C-354 which you can get from several sources.
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If you'd like a cheap option, Classic Valve offers a board for $19.50 - http://www.classicvalve.ca/dynaco.html#ST70P
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I have used the SDS boards several times and really liked them.