New to me - Dynaco ST-35

Hey guys!

New to this forum, longtime AudioKarma and AudioHeritage Member.

I purchased my ST-35 on eBay about 6 Months ago. It has been running flawlessly - and continues to do so.

I was told the amp was worked on by Matt Kamna from the Portland area - he seems to have done a great job with the amp, as it sounds and performs great!

I'm wondering if you guys would be able to help me understand, based on these photos, what has been done to the amp.

The previous owner mentioned:

• Power Transformer
  
• Caps & Resistors
  
• Power Switch & LED
  
• Input & Output Terminals
  

http://home.indy.net/~gregdunn/dynaco/components/ST35/  

https://s-media-cache-ak0.pinimg.com/736x/da/71/69/da71697f69e0020c9d1833861d8a6c94.jpg  

http://www.thehistoryofrecording.com/Manuals/DynaCo/Dynakit_ST35.pdf  

A great deal!

a) The filter caps are now separate, not a multi-cap.
b) OEM had no on-off switch at all (or LED).
c) OEM outputs had two taps.
d) Looks like separate bias supplies for each channel.
e) The power-transformer is new.
f) The boards are original, but appear to be recapped with boutique caps, and possibly boutique resistors.

I have linked the original assembly manual and a few OEM pictures as well.

Welcome to the monkeyhouse, cj. Hours o' fun await you here.
That looks like a great little amp now. Someone took great care in restoring it. I can't think of anything else that could have been done. Probably last years!

Thanks for the insight, guys!

Peter - that reading was very informative.

Can you elaborate on the amp now having separate bias supplies? Was this done because the filter caps are now separate instead of being combined in a multicap? I have read that most EL84 amps do not include bias adjustment - what does having separate bias supply accomplish?

Is this the original bias supply:

Note the interpolations - and that I am guessing from what I can see. Not stating with absolute certainty.

cjwebber wrote:Thanks for the insight, guys!

Peter - that reading was very informative.

Can you elaborate on the amp now having separate bias supplies? Was this done because the filter caps are now separate instead of being combined in a multicap? I have read that most EL84 amps do not include bias adjustment - what does having separate bias supply accomplish?

Not so much that there are separate caps above the chassis, but notice the four electrolytics and what looks like precision resistors below the chassis. Separate bias supplies reduce problems from output tube failures or from the tubes aging at different rates from each other and from the other channel. Which, in turn, greatly stabilizes the amp. Hafler was notorious for running his tubes at the bleeding edge of their limits and was very nearly as cheap as Mad Man Muntz in his circuit designs. So this apparent change is 'all good'!

Is this the original bias supply:

Yes, it is. "Nice and Simple". Until things go bad. However, there is virtue in simplicity and few things do go bad. But, something clearly did with that example - and that necessitated a new power transformer. So, *you* are way ahead of that bus.

Coming in late :-)
There is no thing as a "bias supply" in a st-35. It's a cathode biased where all 4 tubes share one 100ohm
resistor. And there is a very good reason for that.

I would recommend to get a build manual for st-35 ( it's available at this site ) and start to
revert to it's factory configuration.
Apart from D. Gillespies EFB mod there is nothing done that improves the st-35 ( in my opinion),
reverting it to factory will only improve it.
That said, you might want to leave the B+ supply with it's separate caps as-is, it seems to
be a functional equivalent ( i have not looked that deep, but with the manual you can examine
yourself.

The one thing added in this amp seems to be the power switch , an item left out by some
reason during design. Keep it if you like, i prefer ( again a personal preference) to remote-control
poweramps with a solid-state relay. YMMW.

The st-35 is generally considered as the crown on dynacos work, it the last tubeamp designed and
it has the best sounding transformers, but and that's the down side, it need sensitive speakers.
It's also one dynaco that uses current production tubes, and it run's the tubes very conservative, there is no hitted dangers, and any brand of correct type will do well.

EFB is worthwhile to look at , see tronola.com http://tronola.com/html/ps_mods_for_st-35.html
for a discussion of the st-35 and what possible with modern components.

Congrats to a very fine amp!