ST-70 issue

I picked up a pretty clean factory wired ST-70 last spring - I kind of wanted to get to know the stock unit for a while before restoring it. I believe the unit is stock, but that it has seen some work this century. The output tubes are the Russian Mullards, and presumably there was some work done under the chassis at that time as well. I bought it from the owner's father, who was fuzzy on the details. I say presumably because I haven't been able to look inside. The chassis screws are that double dot screw that they used on some of the factory wired units, and I don't have that tool. I know I can easily buy a tool or drill the screw out but I didn't have any real reason to do so during this evaluation period. Up until now.

This evening I was listening and happened to notice that one of my output tubes was glowing much more brightly than the others. Upon closer inspection, I saw that the plates of the tube were glowing bright orange as if molten. There was no issue with the sound on that channel. I turned down the volume for a few seconds, saw that the glow did not diminish, and switched off the power. I've heard of tubes 'red plating' before, and I believe I've just witnessed the phenomenon.

My quick scan of some search results leads me to believe that this is likely caused by power supply capacitor failure - and I may well be dealing with an original quad cap. Just thought I'd get some thoughts from you guys before I figure out how to get past those pesky screws...

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It is unlikely that a power supply problem would cause ONE tube to red-plate, since the power supply feeds all four output tubes. If you do as PeterCapo says, and the problem stays with the same socket, the socket contact could still be bad, or you could have an open 1000Ω resistor to pin 5...or a leaky coupling capacitor on the printed circuit board, feeding that tube; or a bad solder connection anywhere along that part of the circuit.

I tried swapping in a tube from a backup set, and shut it down when it started lighting up funny. Looks like I gotta bust those screws and see what it looks like below decks, examining resistors and any other components on that tube socket.

Finally got past those screws. Below decks looks very clean. Some of the caps look newer than I'd think they would after 50 years - particularly the green 50 MDSs on the terminal strip and the 5.6 MFDs on tube sockets V2 and V7.

But I see some things I didn't expect:

1. The 1K resistors that should be connected between pins 5 and 6 of the output tube sockets is instead connected to the circuit board directly at points 1, 2, 22 and 23. This rings a bell - I've read about that mod somewhere. The odd thing is that the #6 lugs on the tube sockets are clean as in they've never had anything soldered there. I find this strange for a factory wired unit.

2. The sockets on the front panel that are used for bias set and PAM preamp power are not wired, with the exception of a single wire to each socket on the pin used for bias set. Maybe this is normal on later models?

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PeterCapo - I think you had the correct approach. After I looked around under the hood and found no obvious issues there, I cleaned all the tube pin sockets with alcohol and intradental brushes. Tested with the backup tubes, then went back to the matched quad. Sounds good.

So that was that socket conductivity issue that made such a strong case for solid state back in the day!

As with so many things in life, taking a technological step back usually involves more burden of effort, but greater satisfaction on performance. Maybe we can generalize that much of modern convenience is about ease at the expense of quality.

So welcome to the audio world of quality. There's a bit more bother, but you won't be sorry!

But pardon me...with 73 posts, I see now you're no noobie toobie-file.

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PeterCapo wrote:Another thought about tightening the contacts...  The contacts I've thus far seen in original Dynaco sockets look kind of like the letter "C" from the top.  If yours are also like this, then when tightening them, I’d suggest pressing on the "top" and/or "bottom" to try and close the split in the "C" a bit, if you see what I am getting at.  I would avoid pressing the one side directly opposite the split, as this might open the split more.

Si si, I see the "C".
Best thing I've found to accomplish this rather tricky task is the smallest jeweler's screwdriver you can find. It's one of those little jobs that you simply have to DO to get a feel for. Once you get the feel, it seems easy. But first time, take it slow! Good luck.

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Thanks for the input on the issue. I'm still wondering about the unexpected wiring of the front panel sockets (not so much) and the 1k resistors (in particular).

The assembly manual says the front panel socket cannot be used for bias set is it is wired for preamp power. So I figure that most buyers in the 60s would be using preamps with their own power supply so the socket was wired for easy bias set instead pf preamp power.

The wiring of the 1k resistors is still puzzling me. I remember reading something along these lines at some point - but I've been reading everything I could find on the subject for 10 years! So whether this is an official circuit revision is later models or a modification published by Van Alstine or somebody...

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Hi --- Peter, for point of information, I have several Dynaco ST70's (factory and kit forms) and the some Heathkit Mono Pre-Amps (originally used with Heathkit W5M and W4 Mono Amps) that have the same power plug wiring as the Dynaco Mono Amp Power Plugs and ST70's front panel power receptacles.

The front panel tube sockets were fully wired on my factory built ST70 and it looks like the same wiring that was used in the rest of the Amp, so maybe at some point they wired everything and at some point did not??

If I remember correctly, there were also EICO and Knight Amplifier kits that had the same tube socket power receptacle and my Heath Mono Premaps worked just fine with one of the EICO Amps.

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It makes sense that they stopped wiring them for use with the old preamps as time went on and their powered preamps became more popular. My unit has a serial number of 6638812 on a tag on the back, so it would have been produced not lone before to the one in the image. I wonder if the 66 is the year.

PeterCapo wrote:It’s beginning to seem like you might possibly have a museum quality original there.

If only that meant I could sell it for the big bucks!

Interesting - thanks, Bob. That never would have occurred to me - if you look at the products list on the old Greg Dunn Catalog History page it's the 4th product on his list. But then if you look closer at a link from his page you are correct: http://home.indy.net/~gregdunn/dynaco/components/partno.html

The interesting thing is that although it was professionally assembled, it has the look of something handmade rather than coming off of an assembly line.

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daveshel wrote:Interesting - thanks, Bob. That never would have occurred to me - if you look at the products list on the old Greg Dunn Catalog History page it's the 4th product on his list. But then if you look closer at a link from his page you are correct: http://home.indy.net/~gregdunn/dynaco/components/partno.html

The interesting thing is that although it was professionally assembled, it has the look of something handmade rather than coming off of an assembly line.

Equip of that era WAS hand assembled. Robots was way later and way larger series.