Another Dynaco ST-70 thread

Give it a shot and see what happens. Don't do anything to that transformer until you determine what the issue is. Frannie i've read the same, JJ's fry, the dynaco mullards seem to last.
 
Ok, so I went down to the shop and could not find the old can cap (yet) but I found the second set of new parts I purchased in case I bought another ST-70. So I have another new CP 20/20/20/20 @ 475 volt cap to install and see what happens. Please check back tomorrow and see what happened with that (I will do it early tomorrow (like 7:00 EST) then post the results. Doc
 
I have sovtek 7199, Electro-harmonix EL34's and a Chinese 5AR4 in it now (not a NOS after all.) I went through a couple of JJ 5AR4's quickly and I have a couple of Ruby 5AR4's new in the box as back up's. I also have a perfect second ST-70 with all original tubes and components running the speakers now so I could do some tube swapping but I will be really pissed at myself if I screw up the good amp trying to fix the problem child. Again many thanks, Doc
 
Hiya,

JJ's are all over the place and I don't recommend them at all. The Rubys are pretty good actually as they seem to screen out bad ones.

I am guessing that Chinese tube may be a Shuguang which I have heard are ok but not enough to form an opinion on.

As far as overall reliability I am guessing that other ST-70 has the factory Mullard in it and that should be all you need to know about the gold standard.

Frannie
 
I've heard rumors that the CE cans can suffer from early crib death - I have one in my McIntosh that I think has failed after sitting in storage. I'll check and make a post about it eventually. I'm pretty wary of them now.

Maybe one alternative is to restuff a can with some of those pencil-type caps, and put two in series (with a dividing network) for the 525V section?
 
Seems to be a few bad new can caps found on the forms. Probably a bit leaky Must be a shelf life issue. Anyway it's hard on the transformer and rect. tube. Be nice to have one of those ESR cap testers!

I think trans. will only be failing (overheating) if you put too much of a load on it in the form of a leaky cap etc. Should be okay once fixed.

The diode mod with UF4007 ( as quieter ) will help rect. tubes last.


http://www.audiokarma.org/forums/attachment.php?attachmentid=327473&d=1328400937

Randy
 
I don't think SS rectification is quieter than valve rectification, and I wouldn't recommend the diode replacement on the ST70 because the B+ will be even higher.
 
Pretty sure that mod (which I did and didn't raise the B+) doesn't replace the rectifier, rather adds a layer of protection to the tube.
 
Sorry - I must have misunderstood. I thought you were removing the tube and substituting diodes, which will definitely raise the B+. As you pointed out, if they are supplemental, it won't.
 
You guys are the best. The can cap had failed. After installing the "other" new can cap the hum disappeared and the bias was set back to 1.2 dcv. The power transformer lives to fight another day! New readings:

Can Cap:

Square 394 dcv (was 328)
triangle 448 (was 371)
half circle 306 (was 256)
plain 460 (was 375)

5AR4:

8 & 2 460 dcv (was 375)
3 & 6 384 acv (was 386)

Diode .77 dcv (was .78)
54 acv (was 58)

Now the questions are.... Since I bought both the CP 20/20/20/20 @ 475 cans at the same time will this one fail as well? Did the can I just took out cause all the marginal JJ tubes to fail or is something taking out both the cans and the marginal tubes? Can I buy a better Can Cap somewhere other than AES (as in a higher voltage rated 20/20/20/20?) Let me also explain that I do not want to change the amp other than to improve the reliability and lifespan (not looking for hot-rod mods here.) I will do the diode mod at the 5AR4 if it will help with the reliability of this unit. I have read discussions about stuffing electrolytic caps into an empty can but that is beyond my ability to engineer and the DC board mod is way beyond me. My abilities are to discover / determine bad components and replace them with new / good stuff to keep my system working (sorry about that but honesty is the best policy.) Thanks again, Doc
 
Are you running it cage on or off? If I were to hazard a guess, those rectifiers may have messed up the cap can. Get a mullard/dynaco rectifier, do the diode mod to the socket as was posted and run it with the cage off if you don't already, heat is not good for caps. You can also get a higher rated can, thats what i would be doing if a dc board is too adventurous.
 
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Hiya,

I hope the OP does not mind this query. So what happens if you have to leave this cage on. I have small little things with paws that I want to keep out of electronics.

Is there anything one can do short of the advice here to mitigate heat build up other than cutting a ugly hole and running a loud fan ???

Frannie
 
Hi Frannie, I too have paws, little kids, dogs, you name it. I decided to run mine with cage on and found the heat to be unbearable, upwards of 160F. So I set out to build a cheap cooling system, an AC adapter I had laying around, a second hand fan rated at 24V .4A that I got from work and a velleman thermostat kit for $13. No damage was done to the cage, stand off's hold the board to the cage and the fan is held on using mechanics wire. It cycles every 5 minutes or so and runs for about 5 and keeps it cool. For added cooling I put a Dell heatsink on the power transformer. I'd like to find a quieter fan, for now I just listen to the music a little louder lol. I have ordered another velleman kit, I plan on running a smaller fan to cool down the cap board on my FM-3, same idea.



http://www.audiokarma.org/forums/showthread.php?t=422005
 
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I made a quiet cooling fan once (not for the Stereo 70 though). I bought a 120VAC pancake fan, advertised as being really quiet. It wasn't quiet enough. I ran it on a variac to see how much voltage I had to drop to slow it down to where it was quiet enough. Then I built a little voltage divider with 5 watt resistors to match the variac results. Works fine. I may put it back in service with my Stereo 70, just to try to help out the original PT a bit.

I also had a suspect can cap that I purchased new about 2 years ago. The can ran a little warm, right from new. The amp always had a touch more hum than I expected it should, though not extreme. Voltages were all reasonably in line with the schematic. But a NOS Mullard 5AR4 only lasted 2 years. :tears: I've since recapped it with essentially stock values stuck under the deck. The new caps stay cool, and the amp subjectively has about half as much hum as before.
 
I have a cheapo round 110 V fan from Wally-World (the type with the spring loaded clip) that blows across both of the ST-70's. I did notice the cage had become slightly dusty though so it may have run hot. Good point I will keep an eye on it and I will check the operating temps of the cans with a digital pyrometer I own. Leaving the cages off both amps is also not a good idea since I have a couple of dogs.
 
Hiya,

Well once the driver board is hidden I am guessing its pretty safe to run without the cage.

Hard to imagine with all these ST-70's in the world that someone has not designed a better case or cover ???

But that fan idea does the trick and the mod is done right.

I could live with it but I would always know its back there.

Frannie
 
Ah, so it's what I thought, cages on. I'm guessing the heat kills these caps, maybe the new caps aren't built as well. The cap on mine was swollen which is an indicator that they are on their way out. I decided to go with a cap board to eliminate the heat since it sits under the chassis, that was always my logic. If you look at high end tube amps like a McIntosh 240 the cap cans are under the chassis as well.

MC240bef.jpg


There must have been some logic in doing so. Here's mine with the cap board installed, it wasn't difficult at all. I replaced the 2 bolts going through the transformer with longer ones and cut to fit.

underwired.jpg
 
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