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taj 31st December 2011 10:42 PM

Sanity check please? ST-70(-ish) power supply
1 Attachment(s)
Advice and suggestions requested:
I am refitting my Dynakit ST-70 with Gregg V's (Geek's) driver board, and while I am at it, converting the tube rectification to solid state. Naturally that requires a re-think of the voltage dropping scheme to make the driver board happy.

The driver board expects 400v for the phase inverter (LTP using 6CG7), and 250v for the 5751 voltage amplifier. I'm guessing about the current draw from the LTP as I am not familiar with Gregg's unusual implementation. (info here)

The output section is stock ST-70 (P-P EL34 UL) and I'm using all the stock Dynaco transformers (the cloth wire ones).

I modelled this with PSUD2, but I am new to that, so I don't know if it's done well. (see attached pic). I will use a CL150 ntc thermistor after the diodes. The first current tap in the model is for the EL34s, next is for the LTP, and last for the voltage amplifier.


Attachment 257810

taj 31st December 2011 10:48 PM

One question I have: The secondary resistance of the power transformer I entered into PSUD is measured from center tap to one end, naturally the total will be double that (approx.) Should I be using the 59 ohm total. for that value in PSUD2?


indianajo 31st December 2011 10:57 PM

I'm a little vague about the use of the CL-150. If this is a GE sensing part, it is 5 ohms cold, .15 ohms hot. It is small enough high voltage will short right across it when the dust gets thick enough or the humidity is high. I'm using a 47 ohm 5 watt resistor between the transfomer and the rectifier tube to protect my transformer from shorted rectifier tubes. It doesn't drop a lot of voltage at these currents. Using the CL-150 on the primary of the power transformer makes more sense.

taj 31st December 2011 11:24 PM

I just copied the CL-150 usage from the Mullard KT88 project power supply. From that conversation I understood that it lessened the impact of the HT being available before the heaters get the cathodes up to temp (or maybe to ensure the bias voltage is fully operational before the HT is). Not required with a tube rectifier of course.

I've heard of folks using a different NTC thermistor (CL-90?) on the primary side. I'm too much of a non-EE to know much beyond following the example of others here, and trying to understand their reasons.


Burnedfingers 1st January 2012 10:52 AM

Generally from what I understand when going from tube to SS rectification a resistor of somewhere from 10 ohms or so is put in series from pin 8 of the tube socket. Some are actually building their own SS rectifiers inside a octal base and fitting the resistor inside the base with the diodes. Making your own SS rectifier instead of buying it is cost effective and allows for an instant swap back to the tube rectifier when you want to go back.

My personal usage on my 70...I have installed permanent diodes from pins 3 to 4 and pins 5 to 6 if my memory is correct. The diodes allow the strain to be taken off the 5AR4 but still allow for the slow B+ ramp. When I want to go to the SS mode I pull out the 5AR4 and pop in the home made rectifier with the dropping resistor and the voltage remains the same but you still are able to hear what I consider to be the difference between the tube rectifier sound and the SS sound. Hope this helps

Rundmaus 1st January 2012 10:53 AM

Keep an eye to the allowable peak reverse voltage of the 1N4007 diode, which is 1000V. One half of your transformer secondary gives 360V, so each diode sees 720V for parts of the mains cycle. Also, these are RMS values, so you're getting quite close to the allowable reverse voltage in the peaks - too close for my taste...

Also, mind that L-filters need a minimum current for the L to behave as an inductance. I don't remember the formula right now, but the smaller L is, the higher is the minimum current. For low values as 2.5H the minimum current might be larger than the 200mA you're drawing from it. Maybe it's ok, but should be checked.

Greetings, and have fun!

Burnedfingers 1st January 2012 11:53 AM

I have two diodes in circuit. Two under on socket and two inside the tube socket. Doesn't that make it a little better?

Rundmaus 1st January 2012 01:40 PM

@Original Burnedfingers:

My comment concerning the diode ratings was referring to the original post! :)


indianajo 1st January 2012 01:49 PM

Yes, 1n4007 is physically too small to hold off 720 V at high humidity or in the presence of dirt. I salvaged from a Wyse terminal board a diode with plastic rings about 1/2 " along the leads with leads 1" long holding the diode up in the air. That ought to hold off high voltage. Air is good for about 1000 v/inch, dirt much less.

boywonder 1st January 2012 02:08 PM

Taj: Your PSUD model looks fine to me.......assuming that you are close to the needed current on each stage.

I believe you are correct in using the resistance for 1/2 the secondary, ie one end to center tap.

As far as the NTC/thermister goes, look at the data sheet for a CL-140, CL-90, etc. They give a cold resistance and a hot resistance at the rated current. Problem is, with typical tube PS designs, we are not pulling very much current, especially on the secondary side, so they never get very hot. I believe that's why it's quite common to see a CL-90 on the mains side of the transformer, as you are drawing more than a couple hundred ma, and can get the device warmed up.

Is 450V the typical voltage for the ST-70 outputs? If the power transformer can handle it (probably not if it's a stock ST-70 transformer), you can reduce the 450V and increase the idle current to have a larger percentage of the available power as class A....just a thought

A larger choke (More H) would help reduce the ripple voltage and allow you to reduce B+ depending on it's resistance.

Indianajo: PSUD doesn't have lots of choices for diodes; so the 1N4007 is usually the one picked for a typical FWCT model. I'm assuming Taj will be using 1200V Fairchild FREDs or 1200V Schottky's or something similar if the budget allows.

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