Dynaco st-70 hookup
- Thread starter canuck13
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Can we see pics of the underside?
Looks like the scribble says Rk=10 ohm. That would change the bias setting voltage, but without knowing what they did with the connections I have no idea where to poke the meter. Pics will probably tell the tale though. Curious if the 1/4" jack is for setting the bias, or if thats an input or what. If it is for setting the bias, it has to be a stereo jack to allow both channels. Either way I'd probably change that to something less annoying or eliminate it entirely but need to know what it is first.
I don't completely hate the input RCA jack mod. I could justify making myself a new plate where the old RCA jacks were that hosts a pair of tip jacks for setting the bias. Need to put in a ground jack though, probably where the old mono switch was.
Looks like the scribble says Rk=10 ohm. That would change the bias setting voltage, but without knowing what they did with the connections I have no idea where to poke the meter. Pics will probably tell the tale though. Curious if the 1/4" jack is for setting the bias, or if thats an input or what. If it is for setting the bias, it has to be a stereo jack to allow both channels. Either way I'd probably change that to something less annoying or eliminate it entirely but need to know what it is first.
I don't completely hate the input RCA jack mod. I could justify making myself a new plate where the old RCA jacks were that hosts a pair of tip jacks for setting the bias. Need to put in a ground jack though, probably where the old mono switch was.
Posting a good picture of the underside of the amp would show what's been done.
Honestly, TRS for biasing two tubes with one jack is kind of a cool idea. Not really intuitive so I wouldn't install it on someone else's amp, but for personal use with a custom cable it's a clean solution, especially if you've only got one chassis hole to work with. Better than drilling another unnecessary hole IMO.
Edit: in this case biasing two pairs of tubes, since ST-70 has a common cathode resistor
Edit: in this case biasing two pairs of tubes, since ST-70 has a common cathode resistor
Last edited:
Lavane
Lunatic Member
People like myself love tinkering with the ST-70. It truly is the Chevy small block V8 of amps!
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Lavane
Lunatic Member
Right? When I saw that jack, in my head I instantly saw two DMM's hooked up to that jack with an adaptor cable. You would even have your ground connection!
radioactive
tube gear fanatic
Looks like the jack is just a 1/4 inch jack to test and adjust bias as has been said but just a mono jack not stereo for adjusting the bias for both tubes such as Fairchild did with the 260 but there was also a balance pot as well that used the same 1/4 inch jack.
The rca jacks are moved from where the pre output power was .
Yes more pictures are nesccarry.
The rca jacks are moved from where the pre output power was .
Yes more pictures are nesccarry.
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To remove uncertanties and make this a Dynaco amp again i suggest you remove all extra stuff and rebuild it according to
the dynaco manual. That way any questions with adjustments and performance is solved. As for now it's only a munged amp that
lacks documentation.
the dynaco manual. That way any questions with adjustments and performance is solved. As for now it's only a munged amp that
lacks documentation.
analogadict
Active Member
The choke has been moved from below deck to above. Rectifier changed to ss . Speaker terminals changeed, a pic of the rear panel is necessary to determine how the impedance selection is made. Not faulting the mods but documentation is required to understand them and use the amp correctly.
Regards,
Regards,
It's a stereo phone jack, so there was some kind of special cable to measure the bias on each channel, maybe both at once? Hard to say. You either have to make such a cable or add different jacks for measuring the bias. What a pain! The chrome-plated end bells, what *might* be silver wiring replacing even the transformer leads, make me think it was a "professional" custom modification. Someone went to a lot of trouble, that's for sure. It would be a huge job to restore it anything resembling original condition.
It most certainly looks like the 1/4 jack is for bias setting. Sleeve is grounded with the nut and washer on the front. The tip and ring go to the cathodes and I can see one cathode resistor but cannot read the value.
As @gadget73 said looks like it may say 10 ohms. You need to confirm the value of the resistor. And since it has SS rectification I think it would be a good idea to know what the plate voltage is.
With the plate voltage you can calculate the plate dissipation to arrive at an appropriate voltage drop reading for bias setting.
See if you can read the value of this resistor. There is one on the other side might be a good idea to make sure it is the same value.
A bunch of extra capacitance in there. It appears the original quad cap might only be used for the driver board supplies.
As @gadget73 said looks like it may say 10 ohms. You need to confirm the value of the resistor. And since it has SS rectification I think it would be a good idea to know what the plate voltage is.
With the plate voltage you can calculate the plate dissipation to arrive at an appropriate voltage drop reading for bias setting.
See if you can read the value of this resistor. There is one on the other side might be a good idea to make sure it is the same value.
A bunch of extra capacitance in there. It appears the original quad cap might only be used for the driver board supplies.
Wow, thats a mess underneath.
lets see if we can work through it a bit here.
Tube rectifier is gone, and it looks like a cap lives in that space. Don't hate this, the 5AR4 is a trouble spot. The choke lives on top, don't love that but functionally it doesn't matter a ton.
The grey caps bottom side on the top left corner live where the choke originally lived, so thats why that is up top now.
The old rectifier has been replaced with a single section cap. I can't quite tell if the quad cap is still in use. It looks like at least one section is open. I see resistors on it but not sure if any wiring actually goes there. I'm sort of curious if it was abandoned in place and subbed with the cap in the rectifier spot, the two grey ones in the top left, and the light blue cap.
orange caps upper right corner is the bias supply stuff. This appears basically stock, just with new caps. Likely no problem here.
I'm assuming the bias rectifier has been replaced but I don't see it. I do not see the old selenium though. No issue there either.
The 1/4" jack on the front appears to be for checking the bias. Functionally this works, I'm not a fan though. As I mentioned earlier I'd change this out for pin jacks and probably add a grounded pin jack somewhere to make the whole process cleaner and easier.
RCA jacks are the RCA jacks.
Looks like all of the transformer leads have been clipped back and spliced to new wire. I don't hate the idea but its all white wires which makes tracing a lot more difficult. If you wanted to build a bomb nobody can disarm this is a great tactic, if you want something thats not a pain in the ass to work on, this isn't the way to get there.
If this were mine, I'd leave it solid state rectified with an appropriate resistor to drop the voltage and revert the power supply to an otherwise stock config. New quad cap, wire all that up as it was meant to be. Put the choke back downstairs in the original location. I'd probably put the rectifier socket back, and do the diodes and resistor there as a plug-in, but allow the option of running a tube rectifier if anyone ever wanted to.
lets see if we can work through it a bit here.
Tube rectifier is gone, and it looks like a cap lives in that space. Don't hate this, the 5AR4 is a trouble spot. The choke lives on top, don't love that but functionally it doesn't matter a ton.
The grey caps bottom side on the top left corner live where the choke originally lived, so thats why that is up top now.
The old rectifier has been replaced with a single section cap. I can't quite tell if the quad cap is still in use. It looks like at least one section is open. I see resistors on it but not sure if any wiring actually goes there. I'm sort of curious if it was abandoned in place and subbed with the cap in the rectifier spot, the two grey ones in the top left, and the light blue cap.
orange caps upper right corner is the bias supply stuff. This appears basically stock, just with new caps. Likely no problem here.
I'm assuming the bias rectifier has been replaced but I don't see it. I do not see the old selenium though. No issue there either.
The 1/4" jack on the front appears to be for checking the bias. Functionally this works, I'm not a fan though. As I mentioned earlier I'd change this out for pin jacks and probably add a grounded pin jack somewhere to make the whole process cleaner and easier.
RCA jacks are the RCA jacks.
Looks like all of the transformer leads have been clipped back and spliced to new wire. I don't hate the idea but its all white wires which makes tracing a lot more difficult. If you wanted to build a bomb nobody can disarm this is a great tactic, if you want something thats not a pain in the ass to work on, this isn't the way to get there.
If this were mine, I'd leave it solid state rectified with an appropriate resistor to drop the voltage and revert the power supply to an otherwise stock config. New quad cap, wire all that up as it was meant to be. Put the choke back downstairs in the original location. I'd probably put the rectifier socket back, and do the diodes and resistor there as a plug-in, but allow the option of running a tube rectifier if anyone ever wanted to.
I blew up the image and the quad cap is supplying the driver board A and B voltages to 19 & 20. One section is unused and I cannot tell if it is doing anything else.
