EL37 amplifiers

I was under the impression the EL37 was an EL38 without the top cap, and was meant for use as a line output valve.
I was also under the impression that Bell and Howell were American. Chances are it originally used 6L6's, and have susequently been replaced.
Can you give us any more information please?
A photo of the underside and the components used will help figure out where it was made.
Also things like value of cathode resistor, ht voltage, output transformer ratio.
Cheers, Rob.

It looks to me like the EL37 was just a very late big envelope valve. They can be over-driven a lot more than an EL34 because the glass envelope is formed so that the glass is further away from the glass in the location where the anode would get hottest. I am amused at the Roswell story getting in and will add it to the long list of bass guitar owner myths that come up when i look EL37 up on the web.
I once tried EL34s in one of my amps however they were too tall for me to get the case top on so they came out again never to revisit these amps again however they did play all right.

I was under the impression the EL37 was an EL38 without the top cap, and was meant for use as a line output valve.

I don't think the EL37 was ever advertised as a line output valve was it ? This ad http://www.r-type.org/adverts/adv495.htm definitely pushed it as an audio output valve. The EL38, on the other hand, was very clearly meant as a line output valve http://www.r-type.org/adverts/adv366.htm . In addition to the top cap it also had internal ceramic insulators to give the necessary HV withstand. And the characteristic curves of the two valves are pretty different (the EL38 cuts off at substantially less negative Vg for example).

VB

Ok, thanks VB.
Maybe it was the other way round then, the EL38 was developed from the EL37. Like you say TC anode plus ceramic insulators. The limiting values and electrode structures are similar.
Maybe the TV guys can help, what came before the EL38 as a line o/p valve?
Rob.

I have now seen it the other way round with line output valves being used for audio.
What fun!
I am as of yet still looking for another owner of an amplifier that uses the EL37 as an audio output that was made from new to do this and popping them into a Fender amp does not count does it
We can have loads of fun finding out just how rare real EL37 amps are.

Maybe the TV guys can help, what came before the EL38 as a line o/p valve?

I hope so. I'm way out of my depth here. All I know, from the National Valve Museum website, is that the EL38 was introduced in 1946 as was the EL37. That was immediately after the war and before the war quite a lot of tellys had mains-derived EHT. So maybe there wasn't much history of Mullard line output valves before the EL38 ? Where's Trevor when you need him ? (Answer: in bed if he's got any sense.)

VB

I'd still like to know where the amplifiers were built.
If they are American then they would not have originally used EL37's.
BTW even with mains EHT, you still need a line output valve.
Rob.

BTW even with mains EHT, you still need a line output valve.

This is where I'm showing my TV ignorance. There were valves like the Mazda Pen46 as early as 1940 apparently. I guess I'd expect Mullard to offer a similar product.

VB

The HMV/Marconi pre-war TV's used KT63 (Osram) as line-output valve I believe. Certainly listed for the 904/905 & 706/707 respectively.

Also KT44 was used on Pre-war TV as line-output for the HMV 1800/1802/1850 Marconi 710/713

The HMV/Marconi pre-war TV's used KT63 (Osram) as line-output valve I believe. Certainly listed for the 904/905 & 706/707 respectively.

Also KT44 was used on Pre-war TV as line-output for the HMV 1800/1802/1850 Marconi 710/713

Was that a typo for KT36 by any chance ? My 1951 Osram data book lists the '36 explicitly as a line output valve but it doesn't appear in my 1938-39 edition nor in any of my short-form Osram guides (I've got them up to 1940). I've got no Osram books or guides between 1940 and 1951 though so I don't know when it was first listed. The KT44 isn't in the 38-39 data book either but the National Valve Museum says it's a 1939 valve so maybe that's not surprising. The pin connections for the '44 are shown in my 1939 and 1940 short-form guides (but not in the 1938 one) but there seems to be no other data given at all.

VB

Was that a typo for KT36 by any chance ?
VB

Hi VB,

Not a typo I have the 1939 service manual in front of me and is a direct quote from that.

Regards
Chris

That's impressive. The KT63 has no top cap and an anode dissipation limit of just 8.5W ! The 1938-9 Osram book says Type KT63 is particularly suitable for use in the output stage of AC receivers or car radio and suggests that a single tetrode-connected one will give an output of 3W. I guess the demands of those early tellys were pretty gentle.

VB

The HMV radio I grew up with used a KT63 for output.

KT63 is very similar to 6F6.

Don't forget that WWII intervened and the TV service was suspended during the war. Valves designed for new developments in TV just before the war would have been put on hold, as there was no market and probably no military use. it would have taken time to get the show back on the road, even after broadcasting resumed.

Pete.

KT63 is very similar to 6F6.

Yes. When I was about 13, I replaced the low emission KT63 with a 6F6. If the KT63 was the original it would have been about 17 years old - the set was new in 1940.

However, when I was much younger I can remember my mum asking the engineer who fixed it if it was "the power valve gone again?"

Obviously, a 'power' valve was the wireless equivalent of the 'picture' valve ...!

(As discussed here recently ...)

Other contemporary line o/p valves were 6P25 and 6P28.
Later on came 6CD6, EL36/360/509/519
Rob.

It seems to me that pre-1950 high powered audio valves weren't a very big market, there were strong beam tetrode competitors in the KT66 and 6L6 (Mullard were pentode and originated tha patents, although there were all sorts of cross licensing and sharing agrements). Mullard/Philips just didn't take it particularly seriously, and when they did, it took a lot of R&D to develop the EL34. So the EL36 and EL37 were a half-hearted effort and their salesmen sold what they had to sell as well as they could.

Pete.

I guess you're right Pete.
For the domestic market, 10 Watts or so were plenty, easily achievable with a couple of 6V6 / EL33 / KT36 etc.
Higher power applications were generally sound re-inforcement / PA, zero bias triodes in class B were dish of the day.
Rob.

The EL37 was used by Pamphonic immediately post-war (1946-7) as output valves for PA amplifiers. In push-pull the output was quoted as 25W. A good workhorse, even if as mentioned it was often hard-run. The key difference was a much higher gm than the KT66. So when I restored a Pamphonic 601B PA amp recently I couldn't understand why it needed so much drive with the KT66s I had to hand until Valvebloke put me right on the gm issue vs EL37s. The EL37s have 11mA/V gm vs 6.3mA/V for the KT66.

Paul Taylor (Tech Dir of Pamphonic) started off the 601A PA amps in '46 with EL35s (and SP41s) then quickly moved to the newer EL37s (and EF37s) for the 601B version in '47.

This old brochure http://www.pamphonic.co.uk/023_Pam_sound_equipment_-_marked_up.PDF is an interesting marked-up copy of the original 601A amp, ready for the release of the 601B, in Paul Taylors own hand, showing the valve (and sensitivity) changes.

I tested a set of the EL37s that had been in Valvebloke's monster amp and all were virtually dead. I assume that 24/7 PA service over very many years did for them. From memory, the DA42s were really in quite good nick still.

- Jeremy

Those PAMs look more like my cinema amplifiers and were indeed made to work with EL37s as push pull outputs. Did you see that link to the amps on that dreaded auction site?
Silly money.