Source of 10 Watt ceramic resistors?

[Yorkie]

I'm making good progress restoring a Cossor Melody Maker 500 reported under restoration stories, but may need to replace a couple of high wattage resistors. They're the old 'body - tip - dot' carbon rod types o ne I estimate to be probably 5 Watts, the other, hidden away under a tag-strip, I estimate to be 10 Watts. The smaller of the two should be 12k but has drifted up to 15K (+25%), the larger (hidden) one should be 2k7 but has drifted up to 4k2 (+55%). Until I power the set up, I won't know to what extent this will pull the HT down on the anodes and grids of the valves, but ordinarily I'd change them anyway, being so far out of tolerance.

I say 'ordinarily' because I've search places such as Maplin, ESR, Farnell, Rapid etc, and was surprised to note that none of them stock 10Watt ceramic resistors in such high values - they only seem to stock a few, mostly up to 1K. As 10Watt ceramic resistors are compact anyway, (50mm x 10mm x 10mm) I'd just replace both with 10 Watt ones, rather than say 7 Watt.

I've tried googling 10 Watt ceramic resistors but nothing on the horizon. Neither are their any metal-clad ones, but in any event, the wattage of metal-clad resistors must be de-rated by at least 50% if not mounted on heatsinks.

Maybe I'm not looking in the right place - any ideas anyone please?

I've attached an annotated pic - as I said, the one in view I'd estimated to be 5 Watts - the one hidden beneath the tagstrip, which is quite chunky, I'd say is 10 Watts.

[Radio Fixer]

The one in view I would guess a rating of 2W, 3 at most. See chart below...

   

Why not lash a couple of 2W in series or parallel and get it working and measure power... Then get some proper ones. Vey unusual for sets back then to have 5 and 10W resistors except in mains droppers or smoothing in lieu of a choke.

Of course you can often calculate it to a good guestimate from a study of the circuit.

Gary

[Radio Fixer]

Just had a quick look and the 2.7K seems to be R15. No O/P valve anode current, so screen + other valves only, so guess 15mA = 40V and 600mW.

The 47K looks like R5 on the layout and not 47K but 12K? If so the voltage on those screens is given as 80V so if HT = 200V say then 120V across it. About 10mA then and 1.2W.

I take it you have the Trader Sheet, if not then just ask

Done quick so please check as got to dash.

Gary

[Yorkie]

Thanks for reading my post and for responding so quickly Gary - it's so long since I used to buy carbon rod resistors back in the 50s that I've lost touch with their wattage ratings and I was just guessing at the likely wattage. You're right of course about working out the requirements from the circuit. I'll fit a couple of parallel resistors temporarily to bring the set onto spec, check the voltage drop and hence the current being drawn, then calculate the wattage from that. The 2K7 one is for HT smoothing and carries the full DC HT. The smaller of the two is R5 on the Trader Sheet 1133 and is in the screen grid potential divider to V1 & V2. I might find when I power the set up, that the screen voltage at that point isn't far off the required 80V - 15
% high isn't too far off.

The set did work before doing anything at it, though volume was low. I haven't yet tested the valves so I won't know what state they're in. The resistor in anode feed to V3 (R11 on the Trader Sheet) should be 680K and has drifted up to 1M1 (62% high), so that, coupled with the high HT smoothing resistor won't have helped matters. (I've replace R11). As might be expected, the cathode by-pass cap (25uF 25V has gone to heaven, but the smoother/reservoir caps are fine. I don't normally stuff caps, but modern 15uF 25V caps are so tiny that they look a bit silly and it's only a few minutes job to stuff the can of the original cap so I often do that.

As I'll be passing this set on, I don't want to do so unless it's safe and working to spec. (I'm passing it on - not palming it off!).

[AlanBeckett]

Welwyn W22 would be OK for the 12k and W23 for the 2k7.
Farnell have them. I expect others do.

Alan

[Radio Fixer]

Can stick you 3W types in the post if you want them

G

[Yorkie]

Thanks Alan and Gary.

I'll see how things shape up and may take you up on your offer Gary.