PYE CR/AC converting from PX4 to pentode help required
 

I have a Pye CR/AC however I don't have a decent PX4 what I do have are several MKT4's now I'm aware that this isn't a million miles away from being able to do the job however I need to know what needs to be tied to what for it to work properly and what is likely.to need adjusting on the chassis and to what.

Re: PYE CR/AC converting from px4 to pentode help required
 

Consider connecting the MKT4 is a triode, easy if you have the five pin version with the side terminal for the screen grid.

DFWB.

Re: PYE CR/AC converting from px4 to pentode help required
 

Yep that's what I'm asking how to do lol

Re: PYE CR/AC converting from px4 to pentode help required
 

Screen grid to the anode, suppressor grid to the cathode.

Re: PYE CR/AC converting from px4 to pentode help required
 

As long as the valveholder is of the five pin type then it's a simple valve substitution job. You'll need a 390 ohm bias resistor and a 25mfd bypass capacitor both of which will be connected to the centre pin of the socket. A wire link from the anode pin to the side terminal of the valve completes the conversion.
Got no information for a triode connected MKT4 but as the valve will be connected as a triode it will have a much lower anode impedance compared with normal pentode operation.
From the Valve Museum: http://www.r-type.org/exhib/aaa1006.htm

DFWB.

Re: PYE CR/AC converting from px4 to pentode help required
 

First fit a B5 socket. The HT is a bit high for the MKT4 so probably best to rearrange the biassing to take the bottom of the coupling transformer grid winding to ground rather than the bias chain tapping and use a bypassed cathode resistor which will reduce the Vak a bit. You'll need to experiment a bit with the resistor to get the cathode current to about 30mA. Maybe start with about 1k?



Might be easier to use the MKT4 as intended, as a tetrode. Similar attention to biassing would be needed.....

Re: PYE CR/AC converting from px4 to pentode help required
 

Using a beam tetrode or pentode in triode wired raises an interesting issue:

The MKT4 screen is specified as maximum of 225V DC. The circuit can be reworked to lower the quiescent voltage BUT if you have it connected in triode mode the maximum voltage on the screen will be considerably higher than this as the audio signal will be added to this.

Re: PYE CR/AC converting from px4 to pentode help required
 

Screen voltage rating is for pentode/tetrode mode where the anode can go below screen voltage and leave the screen dissipating significantly.

In triode mode with screen strapped to anode, the anode isn't going to go below screen potential and the screen dissipation is limited.

What the voltage capability is in this mode, who knows if the data sheet doesn't include it. But it's very probably different to normal usage.

David

Re: PYE CR/AC converting from px4 to pentode help required
 

Vao isn't given in the data sheet but its usually around twice Va, I suspect Vg2o will be around twice Vg2.

If you have several MKT4's try one suitably configured, not a lot to lose.

Lawrence.

Re: PYE CR/AC converting from px4 to pentode help required
 

I don't know the particular model of PYE radio you have, but if it is worth it, there is a company that is making PX4 valves. They are not cheap but they are well made - better than the Marconi / Osram originals. They make two versions of this valve, one on the original 'British 4 pin' (B4) base and the other on an American 'UX4' base so you have to specify which you want. The valve envelope shape is 'Balloon', the same as the original first production type. The manufacturers name is "KR audio products" and they have a dealership in the UK to order from.

About 2 years ago I had to purchase a matched pair of "PX25" valves for the top of the range, (for the mid 1930's), "HMV 800" radio-gramophone. The price for the two valves plus postage was nearly £600 ! But they are well made and will almost certainly outlast me (lol). I think the PX4 valve is in the region of £250 - £300 + P&P each but I can't find a price list on their site now.

Re: PYE CR/AC converting from px4 to pentode help required
 

Just checked the anode voltage of the PX4 in the Ferranti Arcadia, it's 243V. I'd guess it'll be much the same for the Pye CR/AC.

DFWB.

Re: PYE CR/AC converting from px4 to pentode help required
 

I think the point would be I'm not spending a couple.of hundred quid on a valve for a radio all be it a very nice one that cost me a tenner that couple of hundred quid would buy erm more radios lol

Re: PYE CR/AC converting from px4 to pentode help required
 

Am I right in thinking the early px4 had an anode voltage of 250 and it was raised to 300 when the valve was redesigned ?

Re: PYE CR/AC converting from px4 to pentode help required
 

"Am I right in thinking the early px4 had an anode voltage of 250 and it was raised to 300 when the valve was redesigned ? "
That's right, as a successor to the LS6A the PX4 was introduced in 1929 for use in battery and mains receivers and was designed to operate on a maximum anode voltage 250. The valve was uprated to 300volts in 1933.

DFWB.

Re: PYE CR/AC converting from px4 to pentode help required
 

Yes I see your point ! In my case I felt that my radio-gram (HMV 800), with its advanced features and fifteen valves (13 + 2 rectifiers in parallel) and original EMI K3 auto-changing record deck, it was perhaps worth replacing the two 'dilapidated' PX25 output valves in order to keep this top of the range instrument as originally designed.

I also have two radios that use 'PX4' O/P valves, an "HMV 443" & a "Bush PB63" although the Bush set uses an AC044 valve.... Mullard's version of the PX4. So when one or other fails I will be in the same boat as you (so to speak)

Re: PYE CR/AC converting from PX4 to pentode help required
 

The Mullard PM24M is a directly heated output pentode. This valve connected as a triode could be considered as a replacement for the PX4.
From the Radiomuseum: https://www.radiomuseum.org/tubes/tube_pm24m.html

DFWB.

Re: PYE CR/AC converting from PX4 to pentode help required
 

Ok PM24M I have what do I strap together or do to run it as a triode please people

Re: PYE CR/AC converting from PX4 to pentode help required
 

To turn a pentode into a triode, you need to join the second grid (which would normally be connected to near full HT) to the anode. Then the initial acceleration will be provided by the anode voltage.

If the third grid is brought out separately, you could join it to the anode, but if it is internally connected to the cathode this will not do any harm.

Re: PYE CR/AC converting from PX4 to pentode help required
 

What needs done in the way of anode load ??

Re: PYE CR/AC converting from PX4 to pentode help required
 

The original PX4 was a triode and loaded as such. Whatever that load was it'll do to try your triode strapped valve.