Pye Empire. Output stage conversion to tetrodes?

[FERNSEH]

A triode connected PEN45 has similar characteristics as the Mazda 6L18 triode which means it's not particularly powerful.

Correction to my previous post. Shouldn't the anode resistance be written as ra?

External resistance is written as Ra.

DFWB.

[Argus25]

Originally Posted by lloydwells

I'll have a bash at the 6N7 just ordered two cheap as chips the set is unrestored and works but one of the AC044s fitted is way way down on test now I could spend X pounds replacing them or pinch the good one for my Pye CR/AC and use the saved cash on the cabinet of the empire which seems a lot more sensible

Good idea, I have also suggested the very capable 6N7 before. Parallel the two triodes in each bottle (connect grid & plates) and use two 6N7's to replace the two PX4's and set up the heater voltage and I think you will need to add some cathode to ground resistors, two, or just one will do, bypassed with a 25uF electro to set the operating point. With zero grid voltage (wrt the cathode) these have a plate current around 17mA and you possibly might want to reduce that with cathode resistors.

[PJL]

The 6N7 is not a starter unless I am missing something. It is designed for class B operation and requires a positive grid voltage that will draw grid current to get anything from it.

The datasheet is here http://www.r-type.org/pdfs/6n7.pdf.

[PJL]

Triode wired PM24M's might work and that will be almost a drop in replacement.

http://g3ynh.info/valves/old/Data-Bo...G/MVG-p034.pdf

[FERNSEH]

"Triode wired PM24M's might work and that will be almost a drop in replacement."
Actually I was considering the PM24M as a replacement for the PX4s in the Beau Decca TV.
But watch the anode voltage. Isn't the PM24M VA max 250V?

DFWB.

[Radio Wrangler]

Originally Posted by FERNSEH

6V6 Ra. According to the RCA manual the plate resistance of a triode connected 6V6 is 1,960 ohms. Isn't that higher than the PX4?
DFWB.

This set has the PX4s fitted with neutralising networks, effectively positive feedback trying to cancel the negative feedback implicit in Cag of the triode. This will act to increase ra of the valve (well, to reduce the amount ofthe reduction caused by the negative feedback).

So the output impedance of the PX4s will be modified from the datasheet values.

David

[lloydwells]

I really quite fancy the 6p3p mainly due to them being pence to buy and looking roughly the right size if someone could scribble on the diagram a way of sorting that I'd be elated obviously ignoring the heater requirement .

[Argus25]

Originally Posted by PJL

The 6N7 is not a starter unless I am missing something. It is designed for class B operation and requires a positive grid voltage that will draw grid current to get anything from it.

The datasheet is here http://www.r-type.org/pdfs/6n7.pdf.

Yes that is the way they are characterized for class B operation and the drive power is substantial in that mode, however you will notice they run in class A too with about half the bias level of a typical small pentode, this is because unlike some class B triodes that are nearly cut off at a low grid voltage, they have a substantial anode current still, at zero grid volts.

So biasing each triode to a low anode current of around a few mA you can still get a good anode current swing out of them in push pull before there is any substantial grid current.

It has been some years since I used one as a class A output amplifier, but it produced good output with minimal drive requirements, I would have to set one up again to determine the exact power and also try them in push pull and the ideal bias setting. But I agree that to get anything like the power out that PX4's could deliver the driver transformer would have to be changed most likely to a step down type to support the grid currents.