Plate to Plate Load Impedance

[hesamadman]

Im looking around for an OT for a 100 what marshall build. I was looking at a replacement for a marshall jcm800 100 watt. According to the rating, it has a plate to plate of 1,700ohms. After looking up the specs of el34, it shows the plate to plate being 6500. The 100 watt will have two extra tubes in parallel, so my assumption was to half the 6500 number which is 3250 ohms. 3250 is well out of the 50% range of 1700. Can anyone shed some light on this?


I was wondering if the 1700 was suppose to be from primary #1 to CT and primary #2 to CT. Both legs would be 3400?

[Paul1453]

This is from the Mullard EL34 Data Sheet.

At 450V using cathode bias the EL34 wants 6.5K load.
So about the same as 6L6.

Go down to 375V and the EL34 wants a 3.5K load.
Using 4 tubes = 1/2 the load so 1.75K

I might be wrong, but it seems to me you can use the 1.7K load OT if you use the right voltage.

[hesamadman]

Ah I see. I was looking under wrong voltage.

[Paul1453]

Looks like using fixed bias the load is good with a higher B+. 

[hesamadman]

This might be a good place to dive into some more power amp related questions. As I have not done any power amp modifications before, I am curious what the affects/differences would be to run the el34's with 400 vs 375 volts? 

[Paul1453]

Look at the chart.

2 tubes fixed bias 375V has 48 W output with higher distortion.

400V has 54 W with less distortion.

Voltage effects current too, so that is where the PT considerations weigh in.   

I'm still learning this stuff so maybe one of the more experienced members will chime in.

[PRR]

> what the effects/differences .... 400 vs 375 volts?

7%??

Not much.

My wall-voltage varies that much when the well-pump kicks-in. If guitar amps made majorly different sounds with a mere 7% change of voltage, we'd be in trouble.

As for this load "rule": impedance is voltage divided by current. Power is voltage times current. You can usually get the same power into different impedances by changing voltage and current. As can be seen in Paul's post #1, which shows very similar powers at very different impedances, and different voltage and current.

Marshall apparently glommed the 3.5K 375V conditions and then poured more voltage (and current) on it. IIRC these amps run somewhere above 400V (but not to the 450V limit of standard e-caps!) and thus make far more than 35W.