6P3S-E output transformer primary impedance

[maxdissipation]

Hello guys
Long time lurker here, and from today a member also.
Grabbed a stash of these beauties and now I have an urge to build something with them
Some guys use them in place of 6l6gc, others in 5881 place, also tried looking at various russian forums and datasheets, but foumd nothing. So I will have to ask the same old question since I found ranges vary from 4k to 8k
What difference should I expect soundwise? And whats the ideal primary impedance for anode voltage in the 400-450 range

[DeepBias]

You can find the primary impedance yourself by simply injecting an AC voltage into the secondary and calculating the turns ratio. For example, if you take a filament transformer and inject 6.3 V into the secondary, note the AC voltage measured at the primary (plate-to-plate for PP). Divide that primary voltage by 6.3 to get the turns ratio. So if for example you measure 189V AC at the primary, 189 / 6.3 = 30 (30:1 ratio). Then, if the tap you injected the 6.3 V into is the 8Ω tap, the primary impedance is 30^2 * 8 = 7200 Ω (7.2 kΩ) primary impedance. You can also inject an AC voltage into the primary, but you’ll need a fairly large AC source to get a readable value on your DMM at the secondary, remember an output transformer is step-down.
 

[maxdissipation]

Thanks for quick response
I was wondering what would ideal trasformer winding ratio for 6p3s-e in push pull arrangement be, but to not complicate things I asked for primary impedance, as its much easier to say 4k, 5k or 7k that relaying on turns ratio betwen primary and secondary windings
I measured various OTs, using the method you just described
I just attach 1V 1kHz signal from my audio generator to the secondary winding, and read the voltage on the primary side with my multimeter

[mountainhick]

VTADIY load line calculator lists this tube. Typically, the default in the calculator is a design center for the tube. Scroll down the tube selection list. It will show 3K for SE and 6K for PP:

https://www.vtadiy.com/loadline-calculators/loadline-calculator/

You can use a range. Going with lower impedance increases output current (steeper load line), and raising impedance decreases current (flatter load line). If you want to get deeper learn to run the load lines.

Increasing current too much doesn't help much for guitar frequencies, might increase "fullness", but much more happens prior to the power tube. Decreasing current very far reduces (coincidentally) max dissippation.

[DeepBias]

Yep what mountainhick said ^^ The ideal plate-to-plate primary impedance for a push-pull pair of 6L6GC or 5881 tubes operating at 400V to 450V plate voltage is typically 4 kΩ to 6.6 kΩ, with 5.6 kΩ seems to be a very common sweet-spot.

From what I found,

6P3S-E (6П3С-Е) : (Max) Va: 450V Vs: 400V Pd: 20-23W
6L6GC : (Max) Va: 500V Vs: 450V Pd: 30W

The 6P3S-E is the military / ruggedized type and can safely handle around 420–450 V on the plates, but not quite as tough as a real 6L6GC at 500 V+. The ideal plate-to-plate load is the same ballpark as a 6L6GC.

So yes you can use the same transformer impedance as for a 6L6GC, but keep plate and screen voltages a bit lower (ideally ≤ 430 V plate, ≤ 400 V screen).

 

[tubeswell]

The 6P3Se is very robust and was designed for military use. Pmax = 20.5W Run them like 5881s - they can easily handle 450-460 plate voltage.


(But are a completely different tube from the 6P3S - which you should avoid like the plague)

[tubeswell]

Datasheet info attached

[HotBluePlates]

I was wondering what would ideal trasformer winding ratio for 6p3s-e in push pull arrangement be

You decide the "ideal" primary impedance for your amp.  To get there, you need to know the intended supply voltage, the intended screen voltage, and the desired power output.

   - The supply voltage sets a limit on the Peak Plate Voltage swing available.
   - The screen voltage sets a limit on the Peak Plate Current.
   - Output Power is Plate Volts x Plate Current, and will be Peak Plate Volts x Peak Plate Current / 2

   - For Class AB:  OT Primary Impedance = Peak Plate Volts / Peak Plate Current x 4
   - For Class A:    OT Primary Impedance = Peak Plate Volts / Peak Plate Current x 2  (but only an option if certain limits are not exceeded)

That's the reason you see many different value for OT Primary Impedance.  There's no such thing as a "right value", unless discussed in connection with available voltages and currents.  Change voltage, or change current, and the "ideal" impedance changes.


* And the amp will still work well and sound good if you don't pick "ideal" impedance.  Either power output will be less, or distortion will be higher, or both.

[maxdissipation]

Thank you all, I find your answers very helpful
What is so confusing to me is that for same exact tube (5881-6p3s-e) fender used 4k2 primary impedance, marshall used 6k6-8k, friedman in dirty shirley used plexi 50 watt transformer 3k4 more or less in the same voltage range

[maxdissipation]

I was wondering what would ideal trasformer winding ratio for 6p3s-e in push pull arrangement be

You decide the "ideal" primary impedance for your amp.  To get there, you need to know the intended supply voltage, the intended screen voltage, and the desired power output.

   - The supply voltage sets a limit on the Peak Plate Voltage swing available.
   - The screen voltage sets a limit on the Peak Plate Current.
   - Output Power is Plate Volts x Plate Current, and will be Peak Plate Volts x Peak Plate Current / 2

   - For Class AB:  OT Primary Impedance = Peak Plate Volts / Peak Plate Current x 4
   - For Class A:    OT Primary Impedance = Peak Plate Volts / Peak Plate Current x 2  (but only an option if certain limits are not exceeded)

That's the reason you see many different value for OT Primary Impedance.  There's no such thing as a "right value", unless discussed in connection with available voltages and currents.  Change voltage, or change current, and the "ideal" impedance changes.


* And the amp will still work well and sound good if you don't pick "ideal" impedance.  Either power output will be less, or distortion will be higher, or both.

I would like to know more about that
How can I predict or calculate peak plate voltage and current for class AB?

And how different primary impedances change sound?

[pdf64]

https://www.vtadiy.com/loadline-calculators/loadline-calculator/

[maxdissipation]

Great, I can see how parameters (grid voltage, plate current and plate voltage) all interact to create same output power
But I would like to know tonewise what different output transformer widing ratio does
Lets say we have same plate voltage for 6p3s-e and compare 50 watt plexi OT (3k4) with jtm45 OT (6k6-8k)
I would like to test it myself, but cant afford that in the moment

[tubeswell]

Great, I can see how parameters (grid voltage, plate current and plate voltage) all interact to create same output power
But I would like to know tonewise what different output transformer widing ratio does
Lets say we have same plate voltage for 6p3s-e and compare 50 watt plexi OT (3k4) with jtm45 OT (6k6-8k)
I would like to test it myself, but cant afford that in the moment

You see how it sounds, you hook up an OT with multiple secondary winding taps and you switch the speaker between the different taps. (It’s that simple)

[HotBluePlates]

And how different primary impedances change sound?

Mostly, they do not change the sound.  (That's just a silly guitarist-assumption)

Except!  The load impedance used can change how the output tubes distort (if you even drive them hard enough to distort), which then affects the sound.

The graph at the bottom was published by RCA for a single-ended 6L6.  RCA published the associated condition in their 6L6 data sheet (Page 2, under "Fixed-Bias Operation", 2nd column from the left).  Here, the 6L6 has 250v on the plate and 250v on the screen, and a bias of -14v.

   - RCA's data sheet condition lists a load impedance of 2.5kΩ and an output power of 6.5 watts.
   - Notice on the graph that power is not maximized, 2nd harmonic is 10%, and 3rd harmonic is almost 3%

   - The graph shows that if the load impedance is increased to 4kΩ (a 60% increase), power output rises to 7.3 watts.  That's "more ideal," right?

   - Notice also for "4kΩ load" that Total Distortion has reduced.  Except... 2nd harmonic is <1% but 3rd harmonic is >7% and 4th harmonic is >2%.

Large amounts of 2nd harmonic distortion may be barely noticeable, but all odd-order harmonics, as well as higher even-order harmonics are more readily heard.  The 4kΩ loading will sound like "more distorted" even though THD is less, and the big increase of 3rd harmonic puts a bright edge on the sound.

RCA chose the "less ideal 2.5kΩ load" because even though distortion is measurably higher, the types of distortion are less-audible, and the 2.5kΩ sounds like it has more clean output power.

So changing load impedance can change distort-types/amount, which can change the amp's sound.  Also "odd-order harmonic distortion" is not necessarily bad, as it is the predominant kind of distortion you've heard from distorted push-pull amps you think you like.

[pdf64]

My take is that to guitarists, distortion means clipping type overdrive.
EE and hifi references to distortion aren't the distortion guitarists would recognise as being distortion, more like a warmer / richer / chimeyer / grittier etc tonal quality.