New Member- First Tube Amplifier Acquired Dynco ST70 Custom Questions on Mods

[Jordansky]

Hello,
I am new to audio in general and tubes as of one week.
The unit I got was made by Jim Fosgate and it works great.

[I am running Klipsch quartets with redone insides and titanium tweeter, I have black ice dac coming soon (this weekend!) because it seemed fitting to get something else to match the amp- in the meantime I am reduced to some Bluetooth streaming through an old Russ sound device or phono via Nad series 20 as preamp which isn't sounding as great as the bluetooth, annoying me... I will get a good phono pre amp eventually as well.]

**My questions are about the unit I have in particular and what is going on with it as far as modifications and capabilities- wattage range etc?
It has two switchable inputs, a volume knob and a loudness switch.
On top of the unit are two potentiometers as well as two red banded metal areas which look like terminals of some sort.
I was hoping and would be extremely thankful for information about those in particular as well as any extra information anyone may see or spot on from the photos.
My enthusiasm for this hobby is just a new glowing love in my life and I couldn't be having more fun and emotions enjoying it.

 

[cademan]

It's already been modified. I would leave it and enjoy it as it is.

If you look at standard ST-70's, they don't have volume or loudness controls on them. They are just basic standard power amplifiers which put out 35 watts per channel.

EDIT: Unless you know what you are doing and know how to adjust the bias, I would not touch the two potentiometers on the top of the chassis.

This one also uses a solid state rectifier where the original rectifier was a 5AR4 tube.

 

[Lavane]

And you really, really need to cover the ends of those two wires coming out of the output transformer that are just hanging there. The ones at the bottom right in the picture. The two on on the left have already been insolated. You don't want them shorting out to the chassis, that would be an expensive mistake.

 

[jayclick]

With the two inputs (never saw THAT on a ST70 before!) and potentiometer I wonder if it is modded to also serve as a preamp. You might have an 'integrated' amp there...

 

[mrwood200]

Welcome new member!
And that is a good looking amp! It does appear to have some similarities with the ST70. I wonder if it has that 'odyssey' circuit & may be more similar to some of the black ice amplifiers.
I would anticipate that the pots & terminals are for setting bias. Do you have a digital multimeter?
I am concerned that there are 2x pots missing from the panel. I see 4x terminals, 2x pots & 2x open holes. Usually there is a pot & some test point for each output tube. Maybe someone modified the amp so 1x pot sets bias for 2x tubes?

 

[Jordansky]

With the two inputs (never saw THAT on a ST70 before!) and potentiometer I wonder if it is modded to also serve as a preamp. You might have an 'integrated' amp there...

Yes, It seems to work just fine without a preamp. Interesting-

 

[Jordansky]

Welcome new member!
And that is a good looking amp! It does appear to have some similarities with the ST70. I wonder if it has that 'odyssey' circuit & may be more similar to some of the black ice amplifiers.
I would anticipate that the pots & terminals are for setting bias. Do you have a digital multimeter?
I am concerned that there are 2x pots missing from the panel. I see 4x terminals, 2x pots & 2x open holes. Usually there is a pot & some test point for each output tube. Maybe someone modified the amp so 1x pot sets bias for 2x tubes?

Hello, thank you. I will buy a multimeter. So the top I am glad I did not touch! Bias seems to be the answer.

 

[Jordansky]

And you really, really need to cover the ends of those two wires coming out of the output transformer that are just hanging there. The ones at the bottom right in the picture. The two on on the left have already been insolated. You don't want them shorting out to the chassis, that would be an expensive mistake.

Gotcha- those photos are from the ebay listing I bought it from- however I will certainly be warry of this thank you.

 

[Jordansky]

It's already been modified. I would leave it and enjoy it as it is.

If you look at standard ST-70's, they don't have volume or loudness controls on them. They are just basic standard power amplifiers which put out 35 watts per channel.

EDIT: Unless you know what you are doing and know how to adjust the bias, I would not touch the two potentiometers on the top of the chassis.

This one also uses a solid state rectifier where the original rectifier was a 5AR4 tube.

Got it, will not touch

 

[FB night]

The seller stated he was in charge of selling Jim Fosgate audio collection, I would reach out to him and see if he can find any documentation on the custom amps that were built, there might be some papers somewhere. Other custom Fosgate amps have been built "for friends", who needed to know what to do as well.

 

[RS Steve]

So I assume it is set up to run 8 ohm speakers only, and if you want to run 4 ohm or 16 ohm you'd need to solder one of the other leads coming out of the output transformer in place of what is currently soldered there?

 

[Dswankey]

Based on the amount you paid for it, just leave as is and maybe follow up on what @FB night mentioned as the seller seems legit and focus's on selling vintage audio you never know he might have some background info on the original design.

 

[Jordansky]

The seller stated he was in charge of selling Jim Fosgate audio collection, I would reach out to him and see if he can find any documentation on the custom amps that were built, there might be some papers somewhere. Other custom Fosgate amps have been built "for friends", who needed to know what to do as well.

Thank you- I reached out and he has responded openly.
I recommend https://www.ebay.com/str/werksmagic very much if anyone is interesting in these other custom Jim Fosgate built units he can be trusted.
Thank you everyone! I