Comments for page: PS Mods for ST-35

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Posted by Bill February 05, 2015 - 07:36 pm
Hey Art, I use a Hammond choke in my ST-35 which replaced the 50R. The specs are similar to the one you cite. I have a few of them and wondered if adding a resistor in series with the choke might be a solution to nudge the overall resistance after the first cap closer to 100R? I could try different values to get the desired B+ reading.

Posted by Art Grannell February 05, 2015 - 03:10 pm
Ohm’s law shows that for the voltage to remain the same if the current is halved, then the resistance must be doubled. The decision to use either a 50 or 100 ohm series resistor in a power supply filter, is a trade-off between output voltage and the amount of ripple in the output. Higher 120Hz ripple may not have a noticeable effect on the push-pull output stage since some of it will cancel. The voltage amplifier and phase inverter (7247) however, is more sensitive to ripple noise. If you feel that the slightly higher voltage is desirable, then you could try the 50 ohm resistor. Hook up a speaker to the output, short the input, turn it on and listen closely to the speaker for 120Hz “buzz”. If you find it objectionable, change the resistor to 100 ohms, and see if makes a difference.

Although I don’t use them, chokes can work quite well in a power supply. Their electrical action is quite a bit different from a resistor, so you don’t want to “match” the 100 ohms. Trying to do so could result in a choke that is too small for the ST-35 current requirements. I’ve heard of builders using the ST-70 choke, the C-354, in an ST-35 with good results. Like all electrical devices, chokes have their advantages and drawbacks. For absolute best choke performance, all the components in the power supply need to be considered, and the whole supply optimized as an entire circuit.

This is easier than ever today, thanks to computers. PSU DESIGNER II will allow you to model your power supply, change values as you wish, and immediately see the effect of changing components and values:

My personal objective in an amplifier now under construction is to keep the B+ voltage to the output stage in the same range as the original Dyna circuit while raising the supply voltage for the 7247 to 365 VDC by employing a separate dedicated rectifier, filter and voltage regulator circuit.

---Art Grannell

Posted by Bill February 04, 2015 - 05:08 pm
Getting back to the mono amp queries I had a couple of pages back. I found the site ( that talked about this, and it was agreed to double the 6.8K resistor, but there was disagreement about the 50ohm resistor. One guy said to double it as well (as you do), but Shannon Parks said to leave it at 50ohm or there would be too much voltage lost. He recommended a choke instead of the resistor as well. I'm now thinking about doing the mono block thing and wonder about this issue. It's hard to source a 100 ohm choke.

Posted by Bill January 11, 2015 - 01:53 pm
Thanks Art

Posted by Art Grannell January 10, 2015 - 09:21 pm
I’m guessing that the capacitor you have in mind is about 100 mfd. I’m not aware of any reasons why a low output impedance power supply cap should harm other components in a tube amplifier. However, I do question the usefulness of an extremely low ESR cap in this application, since the resistance of an eight inch long primary lead to a Dynaco Z-565 output transformer is at least double the resistance you mentioned. If the large poly cap(s) needs to be mounted “outboard”, as would probably be necessary in an original ST-35 chassis, then the resistance of additional wiring would be added to that figure.
It’s also interesting to note that modern electrolytic caps, as compared to poly caps, store more than ten times the energy, or power, in the same volume of space.

Whatever you decide, remember that large inrush currents are potentially damaging, and that with large power supply filter caps, some sort of control is necessary.

---Art Grannell

Posted by Bill January 09, 2015 - 01:58 pm
Can power supply caps have too low an ESR value? I have some ClarityCap poly caps (which are really big), but I can use them in a ST-35 build. They have an ESR value of 5.7 mOhms which I guess is substantially lower then the original can cap spec. Someone told me that replacing an original power cap with one that has significant lower ESR value can harm downstream components. Can't see it myself, but is there any truth to this?


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