Upgrading ST70 PS w/ SS question

[zigzagflux]

I will be building a PS for my ST-70. Use HEXFREDS to the following filter:

47 uF cap
ST70 choke
70 uF cap
ST70 choke
100 uF cap

no load output is about 510V.

Question is, do I want to put a resistor in the xfmr center tap return to drop voltage (figure somewhere near 100 ohm) or should I bias my EL34's a little lower?

This has got to be a question many have asked, but I couldn't find a thread on it.

Thanks.

[Eli Duttman]

Put a pair of CL150 inrush current limiters between the ends of the power trafo's rectifier winding and the FREDs. The CL150s will slow the B+ rise down a little. That slight delay is enough time for the bias supply to rise and protect the EL34s against cathode stripping.

Connect the CT of the rectifier winding directly to ground. Take advantage of the extra B+ Volts. Use combination bias on the finals. 2X shared RC bias networks connect the cathodes of each pair of "finals" to ground. Size the bias resistor to drop the extra B+ Volts at the idle current you like. In addition to "burning" the extra B+ Volts, the RC bias networks compensate for minor matching errors and allow a single bias set pot. to work well with less than perfectly matched pairs.

[zigzagflux]

Maybe it does not matter here, but I have fixed bias (4 independent) for each output tube.

I like the specs on the CL150. Does it get fairly hot and reduce R or stay somewhat near the 5 ohm level?

Also, my controls consist of two independent timers driving relays, so I can delay turn on of B+ as long as I like, and control inrush with the second timer. So practically speaking, an NTC is not all that necessary here.

At the same time, you lost me on the combination bias. I expect to drop the 510V down to around 475V with the two chokes in series. So I basically need another 50-60V drop. Straight resistors concern me because now I just make regulation suffer. Series zeners sounds downright stupid.

What are you referring to?

[Eli Duttman]

CL150s will not fully heat up in this situation. That's not a problem here, as you have more Volts than you need.

CL150s have 2 advantages over timers and relays. CL150s are inexpensive/simple and they keep the power trafo from going BOING at turn on. The downside of NTC thermistors is that the protection they provide is gone, once they heat up. Don't turn the amp off and back on immediately.

Combination bias uses both cathode resistors of a fair size and an adjustable C- supply. Dropping some of the extra B+ Volts in cathode resistors of appreciable value does not impact B+ regulation and it stabilizes the operating point. Since you already have 4 bias pots. on hand, use a dedicated RC network between each EL34 cathode and ground.

BTW, if you keep the 1st filter cap. small, the power trafo will run cooler. Also, you can tweak the value of a small 1st filter cap. to hold the final B+ rail voltage down. Pile the energy storage up after the inductance.

Think about a small cap. after the FREDs. Follow with a LC section for each channel. You get better channel separation that way.

[zigzagflux]

Now I think I follow you. 1K resistor between cathode and B- equals 50V drop, effectively lowering my B+ supply. Cap in parallel to bypass audio.

That's gotta be a big cap, I would think, to be on the output stage. Isn't that why the original design kept a low v-drop across the cathode resistor, effectively eliminating the need for the cap?

[zigzagflux]

One last thought: I'm not using any of my filament windings on the pwr tranny. What about bucking the secondary?

[Eli Duttman]

I think you mean wiring 1 or more of the filament windings in the bucking configuration. You wire the filament winding(s) in series with the PRIMARY.

You have considerable flexibility in adjusting the B+ rail voltage. Resort to combination bias is probably not going to be needed.

[zigzagflux]

I guess it's a matter of nomenclature.
In my industry, the primary and secondary do not determine high and low voltage, but rather direction of positive power flow.
Therefore the primary of ST70 is 120V, secondary is 720VCT.

In the end, I could either buck the 720V 'secondary' with one or more filament windings (just reverse polarity), or I could add additional turns to the 120V 'primary' (keep polarity in phase).

Come to think of it, I kind of like the idea of adding more turns to the primary; that way, the 720V winding simply puts out less voltage.

Tranny runs at lower VA for a given current, instead of wasting watts by dropping B+ with resistors. Fewer concerns about regulation.

Not bad at all.

<|:^)

[bear]

My suggestion is to reduce the cathode resistors in the output tubes to as close to nil as you can manage!

Lower = cleaner bass each and every time it has been tried by anyone I know in a ST-70.

The purpose of those cathode resistors is/was to make it easy to bias the tubes with an old time Simpson or VTVM, and to act as fuses in case the tubes runaway or short.

Size the new lowZ resistors appropriately to maintain the "fuse" capability if you want that...

One could consider a choke input filter, given the higher secondary voltage??

The problem with import EL34s is that they will not handle the screen current that NOS tubes would have, and will flame out faster if pushed harder, which is what will happen with a higher B+. Until they fry they will sound ok fine...

One can consider means to decrease the screen current, but then you get less max output! (dog chases tail?)

Then too, I think they sound best rigged as triodes, not as UL pentodes or pure pentodes... so the power drops some in that case too.

After having worked with them for a long time, and having at least one audio friend who went seriously bonkers into them (now quite a bit more than a decade back...) the overall conclusion was that it is better to have them sound good than to put out more power... since you can't seem to get both.

Oh btw, the more C you can put in the spot that feed the outputs, the better... which then leads to wanting to put in a heftier power transformer... (dog chases bone?)

As always, ymmv.

_-_-bear

[zigzagflux]

That's very good advice, bear. I was on fence about adding more cathode resistance.

In the end, I'll add turns to the primary. Did a quick test last night, and output was around 460 at the first cap bank. I can deal with that, especially with two chokes in series.

And I will take your advice about some additional, local capacitance at the output stage. Maybe an ASC PP or something. I don't think I'll bother with PIO caps until I do my SET project.

Thanks, everyone !