Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shaun on June 10, 2021, 12:15:53 am
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Hi all. Got say this is the best forum ever, thanks to the big brains and generous spirits involved.
Lucky me - a guy actually GAVE me this amp today, saying he had "blown the output transformers" by failing to connect speaker dummy loads while running if for 15 minutes or so while he was on the phone. Then all he could get was humming. I didn't know what to say as I was rather busy at the time, but I opened it up later to behold Frankenstein's first attempt at the monster.
Apart from a very bad solder joint one of the 5AR4 plates (which may have stopped the thing from running at all), I couldn't believe the bizarre bazaar of added extras in there. It seems that, while a bunch of new power filter caps have been added, the old cap-can has been left in the circuit - not good. Hot glue everywhere.
As the ST70 was, and still is, so popular, I was wondering if someone with ST70 experience could advise me on how to proceed. I'm tempted to just pull everything out and rebuild according to the schematic, perhaps rehabilitating the 7199 circuit (I know the 7199 is crazy expensive, but apparently, the 6U8 replacement adapter works okay). The sutherland circuit seems to be considered an upgrade by some but not all, so I'm unsure how to view it.
I guess I'd better check the OTs properly, but they seem to be reading okay on my DCR meter. Mind you, they're still in circuit - I'm loathe to take the soldering iron to it without a clear plan of action because I can get carried away with that thing sometimes. And with my wire clippers.
Anyway, I'm sure there's a smart way and a not-so-smart way to approach this, and by seeking help, I'm trying to take the smart way:). So, if you happen to know the amp, what would you do to restore this thing to its former glory?
Thank you!
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eewww! frankenamp for sure! nice score though.
if it hums through with speakers connected, then OT's are probably good. what fries the OT without a load is fly-back (arc) generated by signal applied attempting to drive an infinite load.
measure the resistance of OT primary from CT to each plate lead and CT to UL taps, then measure for primary to secondary shorts. measure the secondary windings as well.
you have other options for the driver board that support 6922/6DJ8/12AU7 etc. search triode electronics site.
ditch the GZ34 and use a yellow jacket or some such. there are also aftermarket PCB for the power supply cap bank.
http://www.triodeelectronics.com/st70capupgrade.html
http://www.triodeelectronics.com/driverboards.html
--pete
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This is also a good read at the Triode site - the trouble shooting guide on this page (5th item) http://www.triodeelectronics.com/dynacoupgrades.html
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Great info - thanks Dummyload and mresistor. It seems my course is clear - spend a bunch of money on it :). Still, I got it free, so for a couple of hundred bucks, it's a great deal. I think I'll go ahead and replace the entire board and the driver board also. Guess I'll have to take my heat gun to it to get those old parts out.
I've been busy lately, so it may take a while, but I'll do my best to post progress pics as they arise.
Cheers.
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If you want to restore to original, that cap can is available...
https://www.tubesandmore.com/products/capacitor-ce-mfg-525v-30202020-f
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Thanks Steve. Much appreciated.
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Assembly manual...
http://diyaudioprojects.com/Schematics/pdf/Dynaco-ST70-Instructions-Manual.pdf
Upon closer examination, I'm not sure what you have. That circuit board has solid state driver/PIs, not 7199s! This is what a real ST70 looks like...
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Yes, mine has a Sutherland solid state "upgrade." Some people put them in many years back. I'll have to reinstall the original tube board for tubes.
Oh, and thanks for the manual!