Pye Empire. Output stage conversion to tetrodes?

[lloydwells]

Ive acquired a Pye empire with push pull px4 output onrbof thems on its way out I dont want to buy replacements so I'd like one of you clever chaps to work out if its possible and what would need done to change it to something along the lines of 6l6 or kt66 push pull dont worry about the heater voltage I can sort that. Pic of output stage attached .

[G6Tanuki]

There are various ways to triode-connect beam-tetrodes or pentodes for use in push-pull circuits like yours in place of the obsolete/expensive triodes.

The big problem you have when it comes to substitution is that the PX4 has a 4-volt heater, and I can't remember any more-modern/cheaper 4-volt beam-tetrodes/pentodes with the sort of anode-dissipation you will need here.

[turretslug]

Lucky you! There can't be many of these sets still around, I hope that things like the mains/output/driver transformers are still in good order. Rather than 6L6s or KT66s, something like a triode-connected 6V6G/6F6G/KT63 pair would still give plenty of volume and keep HT current down out of deference to the mains transformer without having to be biased too far back like a bigger valve. (I assume that you still want to keep the look of classic large-base/bulb types).

[G6Tanuki]

If you can supply the right heater-voltage, a simple rewire of one of the valve-bases would allow you to use a 6N7 power-double-triode

http://www.r-type.org/exhib/aaq0194.htm

one of which will get you 10 Watts of audio. Two, with the pairs of triodes in each valve paralleled, will give you quite a bit more.

[FERNSEH]

From the Radiomuseum, info about the 6N7: https://www.radiomuseum.org/tubes/tube_6n7.html

Or the even older 6A6: https://www.radiomuseum.org/tubes/tube_6a6.html

DFWB.

[PJL]

The 6N7 is designed for class B operation and would require driving into grid current so not suitable for this application. If you are going for an indirectly heated 6L6/KT66/EL34 type wired in triode mode then I would suggest a separate LT transformer and add cathode resistors and decoupling for auto-bias. You might need to fiddle with the values of the miller effect compensation.

[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

[Radio Wrangler]

Yes, that circuit has neutralisation applied to both output bottles. This will have the effect of raising their output impedance up to beam tetrode levels.

So you might as well remove the neutralisation networks and use beam tetrodes as beam tetrodes. How about a pair of 6L6G to get not too modern envelope shapes. add droppers and decouplers for the screens, add cathode resistors and decouplers. Not minding adding a 6.3v heater transformer opens up a lot of options.

David

[turretslug]

I like the reasoning there- if tetrodes are gong to be fitted, it's nice to make use of their tetrode-ness. Hopefully, a big ol' console like this will accommodate an extra heater transformer on the PSU/output chassis and a little bit of load-shedding from the mains transformer will do it no harm.

RS currently have a good price on a 6.3V 20VA transformer that would happily deal with any of the discussed options plus supply scale bulbs if originals also prove tricky, the primary connected across the existing transformer's 230V tap.

Good luck with the set, there's something appealing about a radio whose heater current aggregates some 11A!

[lloydwells]

One thing I came on here for advice in English basically what to connect to what and what needs to be changed or removed in English not I've done this for 50 years speak just change this remove this if anyone can oblige with that I'd be exceptionally chuffed but thanks for all your inputs I can add the values of nearby components if that helps

PS as mentioned in the first post I put on I'm fine for supplying the heater voltage that isn't a concern I have umpteen filament transformers

[lloydwells]

How do we feel about the 6p3p ??

[Megatron]

Chinese version of 6L6G
what about 5V6GT 5 volt version of 6V6. would probably operate on 4 volts reasonably well.

[Ed_Dinning]

Hi lloyd, if you want to hide the heater transformer, then use an auto transformer from 4v up to 6.3v. Evan at the currents concerned it will be physically very small and easily hidden under the chassis.

Ed

[mark_in_manc]

Originally Posted by lloydwells

How do we feel about the 6p3p ??

I use 6p3s as a Soviet 6l6-equivalent, in a pair of Quad iis for which I'm too tight to buy KT66. They came cheap from Ukraine via ebay. Seems to work fine in that application - others will know much more about advising what to do here. A large-ish octal valve in a glass envelope, but not such a 'nice' shape as a KT66 or some 6L6.

Looks like 6p3p might be a Chinese version with a prettier-shape envelope, but not used them!

[PJL]

I would recommend the separate LT transformer to avoid stressing the mains transformer on a 1930's set. This set has a field coil speaker, so to keep the HT voltage right you will need to make sure the HT current remains about the same and the triodes were taking 29mA each.

[Radio Wrangler]

29mA each would put it into 6V6 territory.

for 30mA at around 300v on the anode, a 6V6 would want -17v g-k so 17v at 30 mA means a 560 Ohm cathode to ground resistor for each valve. 25 uF was the traditional decoupler value. The cathode impedance is 1/gm which is 4mA/V which becomes 250 Ohms so 25uF would roll off by 3dB at 40Hz, so a 22 or 33uF capacitor across each cathode resistor should be good.

The grid (1) circuit needs no changes, grid (2) needs a dropper resistor of about 10k Ohms and a decoupler capacitor to ground of about 0.1 uF. The beam forming plates (== grid 3) get connected to the cathode.

A heater transformer for 6.3v is needed.

The thing about 6v6 and 6l6 is that there are plenty around and they seem immortal.

Replacing a PX4 doesn't call for anything high gain and fancy. A softish curve is fine.

David

[Radio Wrangler]

560 Ohms at 30mA means half a watt. So use at least a 1 watt resistor here. 2W would be nice.

David

[murphymad]

Mazda PEN45's Triode connected should be adequate and have 4 volt filaments.
If a bit more power is required then use Mazda PEN44's. You should have plenty of HT to make them sing.

Mike...

[FERNSEH]

I converted my RGD 1046G radiogram from PX4s to a pair of Chinese made KT66s which are connected as triodes. The Ra of a triode connected 6L6 or KT66 is almost the same as the PX4 so there is no need to replace the output transformer.
The same goes for many HMV radiograms which use a pair of PX4s, use the triode connected 6L6 beam tetrodes as replacements. Remember that the famous Williamson amplifier uses two KT66s as triodes.

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.

[electrogram]

i have recently acquired a Cossor set c.1939 which originally would have had a Cossor 4XP output valve. It has been fiitted with a Mazda Pen45, many moons ago by the look of it, dont know if it is triode or pentode connected as yet.