Comments for page: PS Mods for ST-35

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Posted by Bill May 24, 2014 - 11:26 am
The mono amp thing could get expensive and maybe overkill. Maybe some day. Art, with the extra capacitance, if you weren't using a slow ramp up component, would it help by putting a pair of resistors is series between the diodes and the first cap? I read somewhere 1-22R values at 2-3 watts would help as a shock absorber at start-up. Another guy said 1R at 10W is what he uses. Any merit to this and what would be good values to use?

Posted by Bill May 22, 2014 - 06:15 pm
I was thinking about doubling the R values of those particular resistors, but never thought about splitting up the filaments. Thanks for some great advise.

Posted by Art May 22, 2014 - 05:00 pm
Should you decide to build monoblocks with some version of the 774 transformer, the change to the power supply would be to double the resistance of R11 from 6.8 Kohms to 13.6 Kohms, (12Kohms or 13Kohms would be fine) and R12 from 50 ohms to 100 ohms. This change is necessary to maintain the power supply filter efficiency. Since the current draw would be halved, the resistance should be doubled to provide the same or better ripple reduction.

Due to certain variables such as AC line voltage and the specific transformer used, this may not provide the exact voltages specified by Dynaco, but it will get you close. Some adjustment to these resistance values could be necessary.

No changes are needed for the heater windings, but since two separate heater windings are available in the 774 transformer, Iíd connect one set of leads to each 6BQ5 heater to balance the load on the transformer. The 7247 heater should probably be connected across one set of windings only. Since the 7247 heater draws much less current than the output tubes, this isnít much of an issue.

Building a ďhome brewedĒ amplifier allows changes not practical in a factory built amp, such as replacing the 7247 with two separate tubes. A 6AV6 can electrically replace the 12AX7 section, and a 6C4 can replace the 12AU7 phase inverter. Other possibilities exist with Octal or Locktal based tubes. Each tube heater could be attached to an individual transformer winding. This, again, isnít very important, but there could be other advantages to using tubes other than the 7247.

---Art Grannell

Posted by Bill May 21, 2014 - 06:47 pm
Well, not mods to the PT per se, but changes to the PS and filament windings probably.

Posted by Bill May 21, 2014 - 05:24 pm
I didn't realize it was Dave who answered my first query. Thanks Dave!

I recall reading somewhere about someone thinking of going the mono route and there were responses suggesting that he would need do do some "modification" to the PA-774. I wish I could find it.

Thanks for the heads up about the Triode model. I have a larger Hammond chassis for my next assault, so I will be able to achieve better cooling.

Posted by Art May 21, 2014 - 04:03 pm
Iím glad that Daveís explanation helped you to understand the bias.

The Power Transformer that I will be using in my next ST-35 build is the Triode Electronics PA774. It is advertised as being wound with larger wire sizes than the original for lower heat generation and better regulation. My resistance measurement of each type does confirm this. Iím not familiar with Edcor transformers.
The Dyna 774 does run warm, and a factor is ventilation, since in the original chassis, itís placed quite close to the output transformers. Iíve seen a couple failures, but both were in SCA-35ís where ventilation is even more of a problem than with the ST-35.

A larger chassis would help with better cooling. Were I to build a ďpaired monoblockĒ ST-35, I would probably use a pair of the Triode Electronics PA774ís, as I donít believe Iíve ever heard of an issue caused by a transformer having too much current capability!
It could result in a slightly higher than planned B+ voltage, but normally that shouldnít be of concern, rather it may even be beneficial, as would be the additional current.

---Art Grannell

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