Dynaco ST-35: Getting Ready To Rebuild

Good to hear its working! I want to build one of these so bad. You should like the Sovteks. :thumbsup:

Well I hope you get to do it. It was a lot of fun to build and of course the fact that it works and sounds good makes the build seem all that much more enjoyable. My all in cost was a good bit more than I thought it would be when I started but there is nothing like one build to make you want to do another and I do love Dynacos. Might need to go find a pair of MK III's now... Hehe, I would have to save for a good while before that happens though. I will say that I think this amp beats my ST-70 though I haven't done a switch out to compare. It is sweet and robust all at the same time. And certainly provides a lot more power than my speakers need. All in all, I am very happy.
 
Thats what I keep reading. The ST-35 sounds better than the ST-70. I have a VTA ST-70 I built and like. Now I have to build a ST-35! :thumbsup:
 
Paully -- Congrats on your successful project!! The ST-35 is such a wonderful amplifier.

If I may however, the stock design operates the output tubes at ~ 11.25 watts of plate dissipation each, as based on voltage levels provided by Dynaco. This is also assuming that the unit is operating from the rated line voltage of 120 vac as well. In a typical environment with today's line voltages however, that usually gets pushed right to the Design Center maximum plate dissipation rating for the tubes at 12 watts a piece, and sometimes a little more.

In contrast however, under EFB operation -- and at today's line voltages, the output tubes typically operate with a plate dissipation of just 8.35 watts each, which represents < 70% of the Design Center maximum rating for the tubes, and very conservative operation. Using the more modern Design Maximum rating system, the tubes operate at ~ 58% of their maximum plate dissipation rating. And, you get a bunch more power and lower distortion to boot.

Again, congrats on a job well done!

Dave
 
Paully -- Congrats on your successful project!! The ST-35 is such a wonderful amplifier.

If I may however, the stock design operates the output tubes at ~ 11.25 watts of plate dissipation each, as based on voltage levels provided by Dynaco. This is also assuming that the unit is operating from the rated line voltage of 120 vac as well. In a typical environment with today's line voltages however, that usually gets pushed right to the Design Center maximum plate dissipation rating for the tubes at 12 watts a piece, and sometimes a little more.

In contrast however, under EFB operation -- and at today's line voltages, the output tubes typically operate with a plate dissipation of just 8.35 watts each, which represents < 70% of the Design Center maximum rating for the tubes, and very conservative operation. Using the more modern Design Maximum rating system, the tubes operate at ~ 58% of their maximum plate dissipation rating. And, you get a bunch more power and lower distortion to boot.

Again, congrats on a job well done!

Dave

I read your design described as a neat trick in another thread when I was making up my mind which way to go. Given theirs and your more detailed description, more power and running the tubes well below maximum, I have to say that is pretty awesome trick indeed. You are quite the designer! So basically I can run any output tube I want to without fear of burning it out from too high plate dissipation. That opens up a world I want to explore. I like these Teslas, I just might leave them for a little while longer then try the Sovteks, then... I am really happy I bought your boards now! Awesome, and thanks for chiming in.

And thanks, your directions were pretty easy to follow for a guy who can't read a schematic and you were great answering a question I had. Overall, I would recommend your boards to anyone who asked.
 
One thing I have noticed is the power transformer buzzes and vibrates. It is not terrible and is less since I put in Dave's board but it is still quite noticeable. I was curious if it is a problem I should fix. Also, it doesn't seem to get as hot now either. Should I get a Dynakits power transformer and replace the old one? I assume the physical vibration isn't good for the sound and perhaps the old one is at the end of its usefullness.
 
Its kind of common I think. The power transformer in my Dynaco FM-3 tuner vibrates at start up and calms down once the tubes heat up. I'm not too worried about it though.
 
Its kind of common I think. The power transformer in my Dynaco FM-3 tuner vibrates at start up and calms down once the tubes heat up. I'm not too worried about it though.

I have been doing some reading and it does seem pretty common. I remember going through this rigmarole with the power transformer on my ST-70 which I eventually replaced since no solution offered resolved it. Of course replacing it fixed the buzzing, but with that one it was so bad it was audible from the listening position. The ST-35 is less, but it doesn't settle down any like yours and I can feel the output transformers vibrating from the power transformer. If it doesn't mean it is going to fail, I suppose I shouldn't worry about it if it is unlikely to be affecting the sound too much through imparting vibration into the chassis.
 
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Somewhere yrs. ago I read about putting Diodes (for sure) and maybe caps with them to block any possible DC on the line voltage before the PS trans so probably if one's lucky the same effect as the soln's Peter posted in #35 for little $. Low cost to try anyway!
 
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A new power transformer might or might not help. Have a look at post #35 in this thread. I mentioned some alternatives short of replacing the power transformer.

Thanks, I am going back to reread that post. I had forgotten about it. So thanks for the links, they look like some good things to try.
 

Fantastic. I am going to try to isolate the transformer first. The buzz isn't so bad that if I can just isolate it a little and keep the vibrations out of the chassis a little better it might be a good enough situation. I put the amplifier on my PS Audio P500 regenerated power but it didn't drop the buzzing. I would have thought there would be absolutely no DC coming out of the PS Audio? But I can still pursue this option if all else fails, good to have a back-up plan.
 
Question for those who know, if I am going to put a grounded power cord in the amplifier I assume green/white goes to ground. Between the black and the white wires, which one goes to the fuse (if I am remembering how things were connected). I assume the white wire (neutral) goes to the fuse and the black wire (hot) goes to wherever else the original power cord is connected. Thanks!
 
Well, the last modifications are done. I put the Hammond choke in the power supply. A word of advice, if you decide you are probably going to do a choke go ahead and put it in. Wiring it in later is a bit of a pain! Also, put in a grounded power cord. Last was putting in some plastic stand-offs on each of the middle tube sockets and cutting them so they just touch the bottom of the chassis. Seems like they do a good job of supporting the middle of the board and keeping it from flexing too much when changing output tubes.

I am going to live with the buzzing power transformer for now. If it gets worse I will just have to pursue options that were pointed out to me in the thread.

So hopefully nothing more to do any time soon. It is playing and playing wonderfully!

Thanks to everyone who offered help, I really appreciate it!

 
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Just an update for fun, put the amp back in rotation and it is playing now. Still using Sovtek EL84M tubes though a quad of Ei is on the way from Upscale Audio. Also playing around with some Ei 12DW7 at the moment. The amp is still sweet sounding and a pleasure to listen to. Well worth what I put into it.

Not the prettiest picture, but it is how I have it set up at the moment.

 
Looks great! Can you show where/how you mounted the Hammond choke? That shallow chassis gets awful busy with the new boards that you used.
 
You bet. Unfortunately the left channel has developed some hum and buzz that I need to hunt down but it still sounds pretty nice! But here is the picture. I didn't even try to mount the choke on the chassis. It is mounted on a board that the Dynaco sits on also and then ran the wires underneath.

 
Thanks Paully, I have a ST 35 kit, a set of Dave G's boards and been planning on incorporating a choke, mounting the choke on or under the chassis.
 
Thanks Paully, I have a ST 35 kit, a set of Dave G's boards and been planning on incorporating a choke, mounting the choke on or under the chassis.

I used a Hammond 193H. I would have to put the amp on stilts to mount it underneath. It has a 5H rating which is significant upgrade from the 1.75H of the Dynaco choke and both are 62 ohms resistance though here we are replacing a 50 ohm resistor on the board (if memory serves and I am looking at the right choke for a Dynaco that most people use). If you mount it underneath you have to completely disconnect it to get at the inside of the amplifier when trouble shooting and having to go back and forth could be a real pain. I have enough slack in the line the way I mounted the choke that I don't have to disconnect the wires to get inside the amplifier nor is the choke near output transformers or forcing the amp to be on legs 6 inches high. Just a thought.
 
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