Tube data vs output impedance -- an example case

[xtian]

Trying to get a grip on OT primary impedance vs. output tube set for PP amps.

Example case: Marshall 1987 with a pair of EL34 power tubes. 3.7k ohm primary.

Now a peek at EL34 tube data: http://www.duncanamps.com/tdsl/show.php?des=el34

First question. Why, in the Application Data table, for AB1 P/P amps running at around 400 volts on the plates (Va), is the grid2 voltage shown as HIGHER than the plate voltage? There is only one, 450/375, where the plate is higher than the screen.

For plates running at 400 volts, then, the Zout is 3.4k. This must be for a pair of tubes, right? SO that matches, say, the Classic Tone OT, with it's 3.6k impedance.

Next, if you add two EL34s, for a 2203 Marshall e.g., do you halve the impedance to 1.7k ohms?

[martin manning]

I don't know why the screen voltage is shown higher than the plate. Check out one of the manufacturer data sheets for similar data.

For two EL34 at 400V plate voltage 3k4 a-a is a typical load, and if you add two more the load is halved.

[Reeltarded]

Yes on the 100 from 50. 1/2 the primary.

Umm. Shhh magic starts around 2k2 and goes to 2k5. (X2 for pair instead of quad.. so 4.4-5k.

[VacuumVoodoo]

Occam's razor: obviously somebody swapped Va and Vg2 in when putting together thyat table. BTW, the Va in the table are voltages on the plate but there's no data on B+ supply voltage to the CT on the transformer.
Rely only on original TFK/PHILIPS/Mullard data sheets, not on tables of unknown origin.

[xtian]

You mean not everything we read on the internutz is true?

Thanks, guys. One more long-standing head-scratcher scratched.

[Colossal]

The example where the screen voltage is higher than the plate voltage, data were taken from a Fender

[Roe]

most plexis have lower screen voltages than plate voltage both at idle and when running. the obvious except would be JTMs that run fender style 470R screen resistor and no flying resistor

[sepulchre]

For plates running at 400 volts, then, the Zout is 3.4k. This must be for a pair of tubes, right? SO that matches, say, the Classic Tone OT, with it's 3.6k impedance.

Ah. That must be why, after getting all my voltages, etc., Alden At Heyboer told me I should have my OT wound at 3K4, which I of course did.

[Lindz]

Hey Reel

who makes a higher impedance OT that you like similar to what you describe? Most I see listed are like the Classic tone are 3.6 ~3.7k for a 50 watt, or ~ 1.8k for a 100? Do you get custom winds?

What is it you like vs the more standard impedance?

Even better, do you have examples of clips where one can hear the difference or examples of what you like in Marshall style builds?

I am always intrigued by your posts where it seems you have really listened to and tweaked that circuit in a lot of ways - multiple masters, resonance, feedback, coupling caps, etc. and always seem to have interesting ideas when people post..

And your posts always crack me up - total wisecracks out of nowhere that make it a pleasure to read this forum sometimes...

[Reeltarded]

Over on Metro there were a couple tests from stock 1k7 to 3k.

This one is close. Bassman head replacement. 4k2 and cheap.

http://www.classictone.net/40-18001.pdf

In some original 100w Marshalls that sound Thee Best have 2k2 and 2k5 OT. It's between an Express and stock. Less linear, but not quite so tilted. The lowend is never overbearing. The grit is fantastic and the whole thing gets angrier but smoother. It's actually flatter response in my ears.

[Lindz]

Thanks for the tip - I will see if I can find the thread at Metro

I actually have one of those classic tone trannies here for a 5f6a build. Maybe I will throw it in one of my Plexis to have a listen

[katopan]

The grit is fantastic and the whole thing gets angrier but smoother.

Extra screen compression... mmmm, yum.