Dynaco ST-70 with Lovan modifications

[Bob@FM]

What's up with those heatsinks (?) around the output tubes? Unless I'm mistaken, don't tubes want to radiate their heat into the surroundings rather than have it absorbed and reflected back by pieces of metal stuck to the outside? It's not like the glass envelope needs to be cooled, and those certainly aren't cooling the plates, so...

I think the tube cooler idea is you are increasing the size of the heat radiating area, and adding convection cooling. This combination likely lowers the internal tube plate temperature.
The small internal tube plate get very hot, and so radiates very effectively into a black surface totally surrounding it. Then that black surface gives off heat into the surroundings.

Like the way you use a finned heat sink to draw heat from a smaller transistor in a power supply or SS amp. Heat always flow from hotter object to cooler one, so no heat would go back to the tube, it is likely many hundreds of degrees hotter than the cooler wrapped around it.

I would imagine if you design / adjust your tubes to run at sensible idle currents, you probably will do more to extend tube life, than adding these tube coolers.

 

[Bob@FM]

...will do some checking on the two transistors and the zener diode in the circuit. As you can see by the photo, it will take some maneuvering to get one of the boards loose so the components can be removed. I'll keep you posted and thanks again.

Since you have a variac, I would remove all tubes, (mark where they were) and run it at very low voltage (maybe 20 VAC out) and carefully take some readings (if possible) on that lower PS board. Everything solid state is suspect. Not sure if a bad output tube would also cause this, but that is another thing to check - test output tubes if possible. If the resistor gets hot with no tubes, then it's likely not the tubes.

 

[Pio1980]

That does look like it was made using the Curcio amp kit featured in Volume 1/1989 of Glass Audio magazine.
It takes an old guy to remember it.
The Glass Audio article has compete information including circuit diagrams.
I built one back then and it still resides in a dark corner of my basement.
The complete kit included a complicated regulated power supply contained on the lower circuit board housed inside the chassis.
There's no room inside for the topside board.

The tube coolers and window dressing on top were not part of the kit.
To my ears, the circuitry was way more complicated than needed, to achieve the end result.

As would seem others.

 

[gadget73]

would be interesting to see how it actually performs vs some much less complex way of getting it done. I can appreciate this for the engineering perspective but I don't really enjoy complexity for the sake of complexity. Just seems to be overly fussy and unreliable in a lot of cases and who needs that?

I'd still have to fix it for the challenge of it all, and if it sucks once it works, well at least I know it sucks.

 

[nj pheonix]

I was reading the copy for the curcio boards.
I can't comment on the finished product.
That said, most of us have heard an st70. Also most of us have read Dave G's baseline testing article. I have a lot of respect for dynaco iron ( having owned mk3s and still owning a 70)
I tend to doubt its as good as as good as mcintosh or marantz 8b iron as implied in the ad.
Part of the brilliance of the 70 was the excellence of the original engineering with the simplicity of the design.
It would be nice to have hard #s for the modded version. While #s don't necessarily tell the whole story, I wonder if anything useful is gained
A number of aftermarket boards have been available for a 70 but no hard data supporting why they're better