SET OTL ZNF Again

[keto]

Would that be a "C4S plate load" on the 6AS7?

[xcortes]

I'm getting 34v bias from the curves, at 100v P-K and 65mA. But an rp of 300 ohms. It's hard to read the Tung-Sol curves though.

You can run the 12AU7 with 1.2v bias or so, and at 2mA plate current it will have 50v at the plate with +/-25v peak output. Not enough to drive the above, but plenty for a 70v/45mA operating point.

You can easily have direct coupling, or a high-power operating point, or a C4S plate load; you can probably get any two of those with the Crack power supply, but not all three!

I was looking at the WE 421A tube. It thought they were equivalents. It's clear now that they aren't. I'm looking now at the 6AS7G Tung-Sol datasheet to make sure we're on the same page.

And you're right, of course, 100v p-k, 34v bias and 300 ohm rp. Good for 1.8 watts and 1.35% THD. Not bad! Not direct coupled but I could use a C4S load on the plate (yes, Tom, the plate of the 6AS7). At 65mA I'd better use big heat sinks. Would the heat sinks of the Speedball upgrade be enough? Maybe not since the speedball upgrade is for 29mA.

And at 70v/45mA I get 25v bias. With a grid voltage at 50 the cathode would be at 25v and the plate at 95v. I get about 800mW of output, 7% THD but rp goes up to about 700 ohm.

So I guess I'll cap coupled at 100v 65mA and use the original 12Au7 parameters and a big heatsink on the 6AS7G.

[xcortes]

I think I can answer my own questions on the heatsink.

From this post: http://www.bottlehead.com/smf/index.php/topic,1373.0.html

With fantastic info:

"Be sure to check the operating limitations, 300 volts and 0.67 watts dissipation for the MJE340. (You can add an Aavid 5775 heat sink and get up to 1.67 watts dissipation.) Run at least 10% of the output current in the bias string; 20% is better but not always practical."

"You can use TO-220 transistors - we use the MJE5731A in PNP alpplications - and get 1 watt bare, 2.5 watts with the clip-on we use for the Paramount soft-start upgrade, and 5.5 watts for the big extruded one used in Eros and the Speedball."

So in my case, with a B+ of 170v and a plate voltage of 134v (100 p-k + 34v bias) I'm droping 36v at 65mA or 2.34 watts. So a 5731A with the big speedball should work.

[keto]

That's great to know. Such a C4S-loaded 6AS7 would make a neat buffer for the 1K load of the Marchand XM46 PLL crossover.

[Paul Joppa]

Just a note - I was mistaken; the Speedball has a smaller heatsink than the Eros heater supply. Looks like I'll be doing some more careful calculations with various heatsinks over the next several months, might have some things showing up in teh summer (?)

[xcortes]

Thanks Paul.

I think I'll load it with a resistor. If it all works out well we can later put the C4S in. 

[xcortes]

This project died because the philip speakers arrived destroyed. Two years later another pair is coming my way. Time to resurrect it. My crack was converted to a guitar amp so I'll have to buy another one.

More coming soon.

[2wo]

Oh dear, these were the feild coils you were talking about a year or so ago?...John