Derwent radio valve lineup question. Any valve gurus?

[Gabe001]

Just need to pick your brains on a little dilemma with a set I'm working on.

I've just started work on a lovely little Derwent I got from Retrotech. It's a 1940s set which is based on the Raymond electric F17 chassis. I'm quite familiar with them as I worked on both an F17 and a JG Graves in the past, all effectively clones of each other with some very minor differences, I suppose dependant on what they could get hold of post war. There's no schematic for the Derwent per se (that I can find) but the F17 one is generally accurate, as it was for the JG Graves. The differences are generally down to type of capacitors used and value of the electrolytic cans

My question relates to the valve line up in this set

V5: 5Z4 which is correct - RM says AZ31 but I've always seen these with a 5V filament (U50 or 5Z4). In any case, easy enough to verify

V4: 6F6. I've always seen these with an EL33. Also easily checked from the cathode resistor - the EL33 requiring a much lower value. I believe the 6F6 is a "get it going" drop in but I'll probably keep it and correct the bias arrangements to suit.

The problem lies with the rest of the valves

V3: 6Q7GT - should be EBC33
V2: 6K7 - should be EF39
V1: 6K8G - should be ECH35

Whist I do have replacement E-series valves, is there any way I can check from the circuit whether the set RF/IF stages of this particular radio were originally intended to work with 6 series valves or the E series valves? Other threads and previous experience suggests the correct line up is the E-series valves, but I find it a bit odd that every single valve was replaced.

PS: I do know that the 6K8G and ECH35 top caps are tied to different grids, but apparently this doesn't seem to matter in practice and they seem to be broadly interchangeable. The rest seem to have the same pin layout and operating conditions apart from ra (plate resistance)

Thanks for you're thoughts, Gabriel.

[Sideband]

Apart from the EL33, I think V1, V2 and V3 are basically interchangeable. I've certainly used them interchangeably over the years and not noticed any difference. There might be a problem with smaller sets where I think the 6 series are taller than the E series. Whilst originality might be desirable, I doubt that all the E valves will have been changed for 6 series unless the set was in the hands of a valve changer......! More than likely the 6 series were cheaper than the E series when the set was built so they won the day. Personally, I wouldn't be too concerned but it's worth dealing with the output valve of course.

[Gabe001]

Thank you

There seems to be an additional 280 ohm cathode bias capacitor added to the original 170 ohms. This is correct for the 6f6. It looks like either a factory modification or done a long time ago.

[cathoderay57]

Hi Gabriel, the 6F6 is slightly kinder to the mains transformer heater winding, drawing 0.7A as opposed to the EL33 0.9A. Not that it makes that much difference.... Cheers, Jerry

[Herald1360]

Given that two of the E series valves have 0.2A heaters and all of the 6 series ones (apart from the 6F6) have 0.3A heaters, the 6F6 can't hurt. If you want to reduce the heater load further, a 6V6 would be even better.

[Gabe001]

Thank you. I'll stick with the 6F6 for now and the 6 series lineup and see how it performs. I think the 6F6 has a "slope" that's closer to the original el33 than the 6v6 anyway, so may give better volume, but I could be wrong.

I like these post war bakelite sets, they're a bit of a hotchpotch electrically but they're charming.

[Herald1360]

EL33 gm = 9

6F6 gm = 2.55
6V6 gm = 3.75


Neither particularly close to EL33 but 6V6 a bit livelier than 6F6.

[Gabe001]

They're not the loudest of sets even with el33 so this may be problematic. I'll report back.

[Gabe001]

6f6 sounds fine

[mister valve]

You mentioned "6Q7GT should be EBC33"

Actually if the valve specified by the equipment manufacturer is 'EBC33', then the Anode load resistor will be in the region of 47k ohms which is really too low for the high impedance triode in a '6Q7' which should have an anode load resistor of about 220k ohms.
A nearer equivalent to the 'EBC33' would be a '6R7' and 'DL63'.

It seems that the '6Q7' were more plentiful and easier to get, so were often used. They will work, but not work efficiently, as a substitute for an 'EBC33'.

Regards substituting output valves, be careful 'EL33' is a high slope output pentode and only requires a small negative grid bias (6V) for class A operation. Both '6F6' & '6V6' require much more, and if used as direct substitutes without altering the the bias they will run much 'harder' with increased Anode / screen currents, especially in fixed bias circuits. Auto biased circuits will attempt to compensate, but even they are limited. Both 6F6 & 6V6 are pretty rugged and would probably tolerate the increased current, but output transformers, mains transformers, rectifiers, & chokes (where fitted) may not be quite so tolerant, so will cook or go O/C.

[Gabe001]

Thank you very much, that's really helpful.

Regarding the 6f6, the set originally had a 170ohm cathode bias resistor. This was snipped and an additional resistor was added in series to a combined value (from memory) of about 470ohms, which is correct for the 6f6. This looked like a very old modification, so I suspect it may have been like that either from the factory, or very early on in its life. The set sounds good with the 6f6, with sufficient volume. I think the el33 would have been louder but it doesn't matter.

I'll check the anode resistor for the ebc33, that's really helpful.

There's no schematic for this set so working from that of the Raymond electric f17, my experience, and information online. Interestingly, there is a Derwent similar to mine on eBay at the moment also with a 6f6g (but with an ebc33).

[mister valve]

Some manufacturers of immediately post war sets, would use what ever valves they could get hold of at the time. Often the same model would use different valve line-up's and the circuit would be suitably modified to cater for the different ranges.

Looking through some circuits of sets using the International Octal bases, sets designed for high impedance triodes such as '6Q7' & 'DH63' seem to have anode load resistors ranging from 220k - 470k ohms (personally I think 470k is a little too high, but it must have worked)! whereas sets designed for low & medium impedance triodes such as 'EBC33', '6R7' & 'DL63' tended to set their anode load resistors between 47k - 100k (47k & 56k being the most popular). Sometimes the resistance in the anode circuit, was made from two resistors in series and decoupled by a large value capacitor, The resistor values above, only refer to the anode load resistor between the anode pin and the large value decoupling cap. The other resistor being part of a conventional H.T. decoupling R.C. network. (any anode stopper resistor fitted, is excluded).