Modifying a Pye 75 Output Valve

[Radio Tech]

Has any member ever done a modification where the EL41 o/p valve in the Pye P75 has been modified to accommodate the EL84 valve in the o/p stage.

The reason I as is that the P75 I have has a hum that I thought at first was the smoothing but checking all the smoothing and HT out did not solve the problem, it then appears that the EL41 seems to be the culprit but I haven't got a known good EL41 only EL84's so the question remains apart from changing the valve base has anyone done the mod to replace the EL41 with a EL84.

Ken

[kalee20]

Mullard did list operating conditions for EL84 to emulate the EL41.

It would be a case of alter the cathode resistor to make the anode current the same as what it was with the EL41. If that's correct, then the load will be right.

You can find what the current would have been from the service info, the cathode voltage and the cathode resistor value (this'll give the cathode current, but cross your fingers and hope that the EL84 and EL41 have the same partition between anode and screen-grid - it should be near enough).

The benefit is that there's more EL84's floating around than EL41 - and you'll be operating it at under 75% of its power dissipation rating anyway!

[cathoderay57]

Hi Ken, if you haven't butchered the set yet, hang on a couple of days. I have some NOS EL41 valves and can send you one for cost of postage if you want. You have a PM. Jerry

[Robert Gribnau]

It seems to me that the best solution by far is to take the offer of cathoderay57.

But about the EL84 as a replacement:

The EL41 in the Pye 75 is running very modestly at Va = 205 V / Ia = 23 mA, Vg2 = 194 V / Ig2 = 3.2 mA, and Vk = 5.7 V. So the anode dissipation is only P = I x V = 0.023 x (205 - 5.7) = 4.6 Watt, so practically half of its maximum plate dissipation of 9 Watt (while the maximum plate dissiaption of the EL84 is 12 Watt).

I find it hard to make up from the available data in the datasheets on the EL41 and the EL84 what the value of the cathode resistor would have to be if you would change the EL41 into an EL84. But I think it is safe (enough) to just try it without changing the cathode resistor (being 220 Ohm). If the current becomes too high (so if the cathode voltage gets substantially higher than 5.7 V) you just use a cathode resistor with a higher value (like 270 Ohm). Or if you want to play it safe, start by changing the cathode resistor to 270 Ohm. You can than always lower that value if the cathode voltage is substantially lower than 5.7 V.

[PYE 405]

You could perhaps get away with using an EL42, certainly for testing purposes, but of course if you don't have one, then Jerry's offer is the better option.

[paulsherwin]

Back in the 60s, service techs would routinely replace obsolescent B8A valves with B9A near-equivalents, simply drilling out the valveholder rivets and fitting a B9A holder with screws.

This simple replacement is generally frowned on now, but it's possible to fit a B9A socket above the original one using long bolts and spacers (maybe scavenged from an old wavechange switch). Wires can be run down the central spigot hole of the B8A socket and soldered to the pins, leaving the original wiring unchanged. This mod can be easily reversed by a future restorer.

If you have access to a replacement EL41, that's obviously a cleaner solution though.

[G6Tanuki]

I've done a UL41-to-UL84-conversion in an Eddystone 840A; just fitted a B9A base, rewired, and tweaked the cathode bias resistor to set the anode current to 30mA [a bit less than the original UL41 - you don't need much volume for communications-receivers] and it has worked fine for the last ten years or so.

[Rich Woods]

Originally Posted by paulsherwin

Back in the 60s, service techs would routinely replace obsolescent B8A valves with B9A near-equivalents, simply drilling out the valveholder rivets and fitting a B9A holder with screws.

This simple replacement is generally frowned on now, but it's possible to fit a B9A socket above the original one using long bolts and spacers (maybe scavenged from an old wavechange switch). Wires can be run down the central spigot hole of the B8A socket and soldered to the pins, leaving the original wiring unchanged. This mod can be easily reversed by a future restorer.

If you have access to a replacement EL41, that's obviously a cleaner solution though.

Hi Paul, why is it generally frowned upon now. It seems an eminently sensible way forward? Is it the “keep it original at all costs” approach that a lot of folks favour? Personally, often working on a budget over the years, I would rather repair the set with more reliable & easy to source valves as per using an octal base & GZ34 in my Murphy. I’ve never really been into component stuffing either as to me the functionality and performance of the set is more important than creating an illusion of originality inside. I get why others do what they do but I think that their motivation while being just as valid is perhaps a little different rather than more favoured than mine & perhaps some others on the forum.

[cathoderay57]

The OP is of course at liberty to do whatever he wants since it is his set. My thoughts are that there isn't usually much to choose between the cost of an EL84 and an EL41; the former being available in larger numbers nowadays. My personal preference is to keep a set original where this is reasonably feasible and affordable but, sometimes, needs must. Each unto his or her own. Jerry