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Dynaco Stereo 35 replacement output transformers

Slightly off-topic, but does anyone have a schematic for the "Enhanced Fixed Bias" circuit used here:

http://tronola.com/html/efb_in_a_dynaco_st-35.html

And would the Dynakit Parts ST-35 kit benefit from this modification? Dynakit Parts sells an add-on board with four individual bias pots, but this mod looks a lot simpler.

I don't want the huge "EFB power supply" add-on board that stretches across the whole amplifier.
 
The Enhanced fixed bias board is very nicely done. And has DG's trademarked EFB circuit. It is well worth it. I had already bought the bias kit Dynakit sells and had it sitting on a shelf. So I recently installed it along with removing the 50 ohm resistor and installing a 62 ohm choke DynaKit sells. Either way is a nice upgrade, but the Enhanced board is probably the better choice.
Here is a picture of the bias kit and choke I installed.
 

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Lavane said:
The Enhanced fixed bias board is very nicely done.
Click to expand...

Agreed, but I don't want it for various reasons.

Lavane said:
Here is a picture of the bias kit and choke I installed.
Click to expand...

I'm thinking about that option, definitely the choke, but I like the simplicity of the mod posted in the link that I gave instead.

At a certain point, we lose the vintage character of the little amp. IMHO. :)
 
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Yeah, you can put me in the camp to not overthink/overmodify the ST-35. Great small vintage amp and easy to work on/restore.

I have a restored one with dynakitparts boards. Thinking about building another one with the choke being the only thing I would add.
 
personally I'm not sentimental. Cathode bias sucks in an AB amp and EFB is a lovely replacement that performs better and makes life on the tubes easier.
 
RadioOn said:
Admitting to some sentimentality in my opinion.
Click to expand...

Ditto. Millions have loved the ST-35 over the decades with no mods at all or minimal mods. Making it into something it wasn't = build something else that better suits your preferences and opinions. Me, I want something reasonably close to the original. Maybe add the choke.
 
Lavane said:
Here is a picture of the bias kit and choke I installed.
Click to expand...
Do you have a schematic for the bias kit from Dynakit? A friend of mine is building a ST-35 clone using mostly original parts. We've both been trying to determine what that kit actually does.

Jack
 
JustMe2023 said:
Ditto. Millions have loved the ST-35 over the decades with no mods at all or minimal mods. Making it into something it wasn't = build something else that better suits your preferences and opinions. Me, I want something reasonably close to the original. Maybe add the choke.
Click to expand...

You asked if the Dynakit ST35 would benefit from EFB (tm). The answer is YES. Don’t do it if you don’t want it. But you will never know what you missed :D

You asked for the schematic. Check Dave’s Store on www.tranola.com. The assembly manual provides the schematic.

Edit: Dave’s store is at: http://www.tronola.com/html/daves_store.html Thanks for spotting the typo @nerdorama
 
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it provides a small amount of positive voltage from to the grid in order to adjust the tube currents individually. The main EFB regulator sets overall current flow, the trimmers are just to balance things out. This is from Daves SCA-35 article but same idea on the ST-35. I'm assuming the resistor numbers are different.

upload_2023-4-2_10-30-23.png
 
Lavane said:
The Enhanced fixed bias board is very nicely done. And has DG's trademarked EFB circuit. It is well worth it. I had already bought the bias kit Dynakit sells and had it sitting on a shelf. So I recently installed it along with removing the 50 ohm resistor and installing a 62 ohm choke DynaKit sells. Either way is a nice upgrade, but the Enhanced board is probably the better choice.
Here is a picture of the bias kit and choke I installed.
Click to expand...
The Dynakit board just lets you bias each output tube. This helps to keep them all in balance. DG's board allows for biasing them as a pair. The original ST-35 just uses a resistor to auto bias them all. So a matched quad is important to start with. But the tubes in the quad may not age the same. So one or more could get weaker than the others. I like having a separate bias control per tube.
That bias kit doesn't effect the signal path. My ST-70 has separated pots for each tube, and my M-125 mono blocks have one per pair. DG's board takes it all to another level.
 
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TriodeLuvr said:
Was this in answer to my question? Are you referring to the Dynakit "Bias Control Upgrade"? There's no LM337 or heatsink shown in their photos.

https://www.dynakitparts.com/shop/st35-bias-control-upgrade-kit/
Click to expand...

It appears as if the Dynakit board is still cathode bias, but separates the single cathode resistor into 4 along with individual bias adjustments. In this case, the performance of the ST-35 will actually be REDUCED in favor of the convenience of indiviual bias adjustment. The reason for this is explained in Dave's initial EFB introduction article related to the SCA-35 but applies equally to the ST-35:

http://tronola.com/html/a_new_look.html

This should be considered mandatory reading for anyone considering mofication of the the SCA or ST 35 as well as for general enlightenment on the operation of the ouput stage. It should be noted that the performance increase provided by the EFB circuit is niether trivial or the subject of only subjective impression. It is substantial and easily measured.

EFB can be implimented without the PCB, although I'm sure Dave would appreciate it and, for the ST-35 is described in this article in the Dave's Lab section:

http://tronola.com/html/efb_in_a_dynaco_st-35.html
 
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RadioOn said:
Yeah, you can put me in the camp to not overthink/overmodify the ST-35. Great small vintage amp and easy to work on/restore.

I have a restored one with dynakitparts boards. Thinking about building another one with the choke being the only thing I would add.
Click to expand...
I felt the same way at first. I ran the amp for over a couple years with just the choke mod. Shortly after building the amp, I bought the bias kit from another AK'r in bartertown. It sat on a shelf until recently. The tubes should last longer keeping them in balance. The small reduction in performance I haven't noticed. I couldn't hear a difference.
<edit> I run this amp with the cage on. I just don't like how it looks without it. So I can't just glance over and see if a tube is getting the snot beat out of it and red plating, like what I saw recently in another amp I have. Being able to bias each tube gives me some peace of mind.
 
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Thanks for the schematic Lavane!
Lavane said:
I run this amp with the cage on. I just don't like how it looks without it.
Click to expand...
I agree completely, and that goes for the ST-70 too. Dynaco amps were designed in an era when people didn't want to see the tubes. Audio gear was generally considered incomplete if it didn't have a cover or cage. I don't understand why all the aftermarket companies continue to sell kits and upgrades using PCBs. I'm thinking about making aluminum plates for my friend with standard tube sockets and all the driver components hand-wired underneath.

Here's an advertising photo showing the chassis he bought for his project. I think it came from Triode Electronics or somewhere similar, but apparently no longer available.

Dynaco ST-35 redux.jpg

Jack
 
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