PM24M Puzzle

['LIVEWIRE?']

Earlier today, I re-checked all the resistors involved, including the variable resistor, R1, and the connections between them. All measure close to the stated values. I had already replaced R9 (640k) with a 560k - the nearest value I had to hand - the only reason R9 failed is that one of it's end tags broke away (like all the original components the resistor is about 84 years old after all!) I haven't yet measured the o/p valve's filament current, but that I can easily do. From the glow visible I would say it's O.K.

[ms660]

It's the filament's DC current that should be checked, the fact that the filament glows doesn't mean there's a DC path to HT -ve.

Lawrence.

['LIVEWIRE?']

Fair enough, Lawrence, but I did earlier check that there was continuity through the heater wiring, and that the centre tap of said winding was/is indeed earthed.

[ms660]

No problem, sounds like the valve then.

Lawrence.

[Station X]

Has the heater current been measured?

I recently had a problem with a 12V lamp which wouldn't light. There was 12V at the lamp holder, the filament was intact and had the expected cold resistance. It turned out that a connection in the wiring from the switch to the lamp holder was high resistance.

[Sideband]

If I get a chance this weekend, I'll get my 274 up on the bench...it could do with a polish and scrub up anyway so I may as well vacuum out any dust in the cabinet. While it's being given a birthday (it's just had its 84th birthday) I'll see about checking some voltages. Might help by giving you a guide. I'm not sure just how good the PM24M is although there is plenty of volume. A PM24A will work but I don't know what effect it has on the bias for the other valves.

[PJL]

You have measured the anode, filament, grid and screen grid voltages at the pins of the valve socket?

You mentioned you rewired the screen grid as it had a series resistor? You should have HT on the screen grid. The symptoms are consistent with the screen grid being earthed,

The trader says it should be a PM24A so I suspect the series resistor was required for the PM24M to reduce the anode current but it should have gone to HT not earth.

[ukcol]

Originally Posted by 'LIVEWIRE?'

When I first checked the radio I found that someone had wired a 170 ohm resistor between pin 5 of the 24M and chassis, effectively earthing the screen grid.

I may have misinterpreted what you said but I took it from your first post that you had access to the circuit and that you had put right the error mentioned in the quote above.

Peter is right of course, the screen grid must be connect to HT+ for the valve to pull anode current.

[Sideband]

Originally Posted by PJL

The trader says it should be a PM24A so I suspect the series resistor was required for the PM24M to reduce the anode current but it should have gone to HT not earth.

The Trader sheet does indeed say that it should be a PM24A. However there is a valve chart inside the set that clearly says PM24M. The 'A' does work but with reduced output. The 'M' actually packs quite a punch.

['LIVEWIRE?']

As I'm fairly certain I stated earlier, the voltages on the anode and screen grid are both around 300v, when measured with my DMM. at the valve socket, and on both sides of the o/p transformer primary, which measures a little over 600 ohms. I replaced a resistor in the control grid circuit, besides replacing all the waxed paper caps. I downloaded the Manufacturers service data from this site quite a while ago, and have been working to/with that manual. An original label inside the back cover of the radio clearly states that the o/p valve is a PM24M, though a 24A would probably work in the circuit, albeit with lower volume as Sideband says. I have no idea who put the 170 ohm resistor between pin 5 & ground, and in any case I removed it ages ago, long before I powered up the radio. IMHO no resistor is needed between G2 (pin 5) and HT+, and none is shown in the diagrams, so I've wired g2 directly to HT+ as per Colin's circuit excerpt.

[PJL]

It's definitely the valve then.

Looking at the datasheets it looks like the PM24M would result in less anode current than the PM24A. I have the 834A which uses the same chassis and I am fairly sure that has a PM24A. More gain but less power.

[Sideband]

Hi 'Livewire'.

I put my 274 up on the bench and have some voltages for you. The HT on mine is down by only 6 volts at 214V so shouldn't make any difference. I'm using the Trader sheet references (which refers to the 834 radio....same chassis).

V4 GB potential divider R15/R16 junction, -13.9V. From this junction, R10 connects to R11 and R12 in series with the control grid of V4. At the junction of these two resistors there is -11.9V. The other end of R12 connects to the control grid of V4. At the grid we have -11.4V.

I hope this is clear.

While mine is having a clean and vacuum out I checked all the resistors on the tagboard. I was amazed that all of them are still well within tolerance considering they are 84 years old. I last checked them 12 years ago when I restored the set. It's a great pity that the 994V detector valve is totally unobtainable as a new one of these I think would greatly improve performance. I'm told that an AC/HL is a reasonable sub so I might try one of those sometime.

['LIVEWIRE?']

Thanks for the info, Sideband. Sorry not to have replied earlier, but I've ben away for a few days. Barrie (Bazz4CQJ) has offered to test the PM24M for me, so I'll get that off to him ASAP. I'll check again the voltages on the 274, specifically those on the V4 GB potential divider chain. I, too, was amazed that all the resistors on the tagstrip were within tolerance after 84 years, though I don't know for how many of those years the 274 was in regular use.