Cossor 210SPT

[rockinmark]

Hi can I use a Mullard VP2 in place of a Cossor 210SPT?

[cathoderay57]

Probably wouldn't work well because the 210SPT is a straight pentode whereas the VP2 is variable Mu type. The signal characteristics are therefore different. An SP2 would be alright. Cheers, Jerry

[cathoderay57]

Hello folks. I've belatedly had a brief look at the Cossor 483 battery set circuit, for which Mark is looking for a 210SPT. This is a regenerative receiver that uses the 210SPT for what is described in the receiver service hints as a regenerative pentode detector. In the Cossor 483 the 210SPT is running with 60v on the anode, supplied via a 70k load resistor, with regen feedback via the reaction capacitor to its own grid 1 via the secondaries of the previous IFT. The 210SPT grid 2 is running at only 22v via a 500k load resistor. HT+ is 120v. Clearly it seems best if he obtains and uses the correct valve. In post #2 above I said a Mullard SP2 could perhaps be substituted but in this application I'm not sure. Since the SP2 requires 135-150v on both the anode and grid 2 I suspect it would be unlikely to work well and fiddling about with its anode and grid 2 resistors is probably a bad idea. Any thought please? Cheers, Jerry

[cathoderay57]

Update. I did some tests on an SP2 using the VCM163 valve characteristics meter. All tests done with zero bias on grid 1.

anode: 60v; grid2: 22v; (as per Cossor 483 circuit) Ia=0.1mA gm= 0.3mA/v
The SP2 is unlikely to work well under these conditions

anode: 60v; grid 2: 60v; Ia=0.7mA gm=0.6mA/v Ig2=0.3mA
The SP2, at least under DC conditions, is behaving close to the circuit requirement but to increase the voltage on grid2 from 22 to 60v would mean changing R7, the grid2 load resistor, from 500k to around 180k.

If stuck for a 7-pin 210SPT this might be worth a try. Comments welcome, particularly anybody who can explain how a regenerative pentode detector works. I cannot get my head around how near-self oscillation would filter out the HF carrier and result in rectified modulation signal. Cheers, Jerry

[PJL]

It's a conventional leaky grid detector that rectifies the RF on the grid. The regeneration is fed from the anode through a regeneration coil wound on the IF transformer.

The test meter used in the service sheet was 0-150V @ 1K/V so 150K. That would kill the grid2 volts fed from 500K and significantly reduce the anode volts.

The SP2 is listed as equivalent to the 210SPT and is described as suitable for leaky grid detection.

[cathoderay57]

Thanks, that makes sense. Sounds like it would be worth a plug and play. Cheers, Jerry

[TowerRadio]

The Mullard book suggests the SP2 is a near equivalent and the screen volts would have to be increased to the same as the volts on the anode, so some component adjustments would be needed. Les

[PJL]

With the low g2 volts due to the meter, the screen current will be substantially reduced (along with the anode current) so difficult to say what the screen voltage would be in circuit.

[TowerRadio]

As the 210SPT seems to be unavailable but the SP2 is I guess its up to rockinmark to decide if he wants to try the SP2 or not (it seems he has a lot to do on this set in any case)Les