• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Looking for Dynaco ST-70 Power Supply Info/Specs

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Travis Clarke

Travis Clarke

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Joined 2006
#1
Hi guys, I've decided to work on a car tube amp project and I'd like to use the Dynaco ST-70 as a 1st revision design. Considering how rare and expensive REAL commercial tube car amps are, I think this would be a great project and a good starting design.

The problem is, as I need to come up with my own power supplies, is that I don't have the voltage requirements listed anywhere, at least as far as I know. Also I don't have an amp to serve as a reference yet.

The ST70 manual has voltages listed at sockets and various other points in the amp, but not at the points shown in the attached pic:

  • The negative voltage for the bias supply, (RD/BL wire) going to the bias circuit
  • Voltages at numbered/circled points 1-4 in the power supply pic

If anyone knows what those are I'd really appreciate it.

Thanks.
 

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#2
MartyM said:
The ST70 manual has voltages listed at sockets and various other points in the amp,
but not at the points shown in the attached pic:
Click to expand...

These will vary somewhat with the unit, tubes, and the line voltage.
Bias supply: -70VDC (after a silicon rectifier, replacing the selenium one), only 3mA needed.
HV supply (2, 1, 4, 3): +435/415/375/305.

Remember the AC filament supplies. Sounds like you won't need a 5VAC winding or rectifier tube.
 
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#10
20to20 said:
Go down to WM and get a 250W inverter.
Click to expand...

Hi, I played around with the "average" AC inverter, and they're prone to shutting off when attempting to power a large transformer.as I discovered.

I could use a true sine wave inverter, but using that big, heavy, and expensive power transformer is something I'd prefer to avoid (and true sine wave inverters are not cheap either). Milbert also uses switching supplies in their very nice BAM 23ab car tube amp.
 
#11
MartyM said:
Hi, I played around with the "average" AC inverter, and they're prone to shutting off when attempting to power a large transformer.as I discovered.

I could use a true sine wave inverter, but using that big, heavy, and expensive power transformer is something I'd prefer to avoid (and true sine wave inverters are not cheap either). Milbert also uses switching supplies in their very nice BAM 23ab car tube amp.
Click to expand...

Did it trip out from high surge, or low voltage? Where did you make the connection, cigarette lighter, or direct battery connection with the engine running, or stand alone battery?
 
#13
MartyM said:
I'm pretty sure it was from the initial inrush current. That was powered directly from a fully charged battery at the time I it.
Click to expand...

Ok, well it wouldn't be the transformer causing an inrush that trips the high current protection, it would be because of solid state rectification into a high capacity filter, plus the heater load. The ST-70 draws 190W or less running, but until the 5AR4 starts up there is only heater current being drawn. So if your inverter has enough capacity watt-wise it ought to not trip off from the initial turn on of a tube rectified ST-70. It might take a 350W inverter or one rated with enough peak load. The amp uses a 3W fuse. Good luck with your project.
 
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