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EL37 amplifiers

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Refugee
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There are few amplifiers that are built from new to use the EL37 valve.
I came by four of these as a skip rescue in the early 1980s.
They are Bell and Howell 16mm cinema amplifiers and after a little tinkering they sound excellent.
There have been many myths about the EL37 and electric guitar amplifiers recently.

What i would like to find out is if anyone has also come across amplifiers that were built from new to run with EL37s so that we can enjoy a chat about them.

 
Posted : 02/05/2012 8:46 pm
Anonymous
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I only know of EL33 and EL34
I didn't even know EL37s existed until a few months ago
I had a 19" rack with 2 x amps, each mono with 4 x EL34 ex-PA from a Belfast Mill. This sat in Living Room in 1980 connected to a time switch and a radio as an Alarm clock :)
The amps had a Vibrator pack and could run off 12V. We drove around Co. Antrim countryside "blues bros" style with 2 x home made speaker columns and BOTH of these in the car on back seat (without rack :) ) and driven with a microphone and cassette player.
Such fun :D

http://www.radiomuseum.org/tubes/tube_el37.html
FRENCH!

Translation via Google

Significant differences between the tubes EL37, 6L6 and EL34

Warning: contrary to what is given approximately in some summary characteristics, EL37 and 6L6 tubes are not at all equivalent. Heating demand of 0.9 A 6L6; that of EL37: 1.4 A. If, in some configurations mediocre (they will anyway consumption for heating and even for different anodic réultat), they can be interchanged with the limit, they must be treated in very different hifi to get them the best qualities. In addition, mounted in pseudo triode, the 6L6 has no interest while the EL37, it can deliver an interesting power.

The best configuration in terms of distortions in PPAB1 for EL37 is as follows:

Go and g2: 325V, Rk: 130k (-22.4 v) Ra-a: 4k; Rg1: 500k; Vg1-g2: 43V. Power output: 35W to 3.5% dist. (At 30w, it has only 1% dist.)

The 6L6, in this configuration can work well if you wear Rk to 190Ω. and Raa to 3.8 k. Puissane the output of 30w is then approximately 2% of dist. (But no drops at 1% until about 20w).

If we treat the EL37 to get more power, it greatly increases the distortion. While for the 6L6, the best configuration is the most powerful (much more complicated and expensive to achieve, it is true):

Will: 450V; Vg2: 400v; g1-37v; Ra-a: 5.6 k; Rg1: 100k; Vg1-g1; 70v. Output power: 55w at 1.6% distortion.

The EL34 is still different, the best stereo configuration is as follows:

Will: 375VDC; Vg2: 470Ω knew Va; g1: -32; Ra-a: 2.8 k; Rg1: 500k; Vg1-g1: 45v. Output power: 44W 5% dist. (at 35W, it has only 1.8% of dist.)

For the EL34, all other configurations offered by manufacturers significantly increase the distortions

 
Posted : 02/05/2012 8:56 pm
Refugee
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That blues brothers style amp sounds like the transistorized one i built in the early 1980s. It used to eat alternator diodes and had a crude switched mode auto-transformer in it and carried a road-side repair kit for both itself and the car.
That translated spec sounds like it has been lifted right from the Mullard application circuit for the EL37 that i found last year. The most powerful circuit i found was two EL37s with transmitter valves as cathode followers. I have never heard of a real one being built though.
Most of it is all the myths about electric guitar amplifiers producing mythical kinds of distortion when over driven with EL37s fitted. The web auction prices went through the roof for a while. They were selling for over £100 each at one time. What a pest if i want spares!!!
The skip rescue included a few spares that were NOS in water damaged boxes. The EF37As did not take to kindly to the wet and gave up there shielding as you can see in the photos.
I did also have an issue with an output transformer where during manufacture the final turn of the primary had slipped out from under the waxed paper and a bodging worker had stuffed it down the side of the start of the winding so that it arced. This got it past test and out of the door but it did not last long!!!

 
Posted : 02/05/2012 10:55 pm
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I'm sure I saw them in a PA from a factory which my cousin rescued in the early 70s.

http://www.r-type.org/exhib/aad0111.htm

It says it replaced the EL31, which appears to be a rare beast.

It was introduced in 1946 and obsoleted by the EL34 in 1954, so it wasn't front line for long and at a time just after the war when there was comparatively little demand for hi-fi and guitar amps. The cinema and PA market would have been quite small.

In some ways a bit like those B8B valves which were only made for a period of a few years just after the war, 7B7 and all those.

A valve data book I've got fron the early 50s mentions EL31 through to EL38 apart from EL34.

EL37s and guitar amps? It's probably been 'discovered' along with lots of other valves.

Pete.

 
Posted : 02/05/2012 11:49 pm
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EL31 not as rare as EBL31?
I just repaired a top cap on a EBL31 that had snapped at the glass. At first on test the Ia & mA/V needles didn't move more than a fraction of one division. I turned up heater to 6.8V and the emission slowly rose. When I handed it back it was about 35% emission and 40% gain at 6.3V. He'll maybe put and EL33 and diode inside the base and top cap on side. Or EL84 + diode on adaptor if it gets too poor though NOS are occasionally offered at the £35+

 
Posted : 03/05/2012 12:01 am
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I assume EL31s are rare because I've never come across one and the two or three on-line valve collections I looked at didn't have an exhibit as far as I could see.

I've got one of Jim Fish's price lists from maybe 6 or 7 years ago and he had EBL31 for £14, EL31 for £8 and EL37 for £23, EL41 £9 and UL41 £14. Times change though.

I wouldn't strongly argue for the relative rareness of them.

Pete.

 
Posted : 03/05/2012 12:16 am
Refugee
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Cheers for the link XTC. It is the link i was thinking of. It is the closest i can get to what i have. Mine have an EF37A driving directly into the phase splitter, a little less gain but a bit more power. It says 25 whats on the base of the output transformer. There is another link to the amplifier with the transmitter valves as cathode followers.

 
Posted : 03/05/2012 3:02 am
Refugee
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Have a look at this website and we will be able to work out if i am on a winner in a blind listening test with my amplifiers.
http://www.ekcoaudio.com/
It is a bit of fun to see what has become of an old trade name.

 
Posted : 03/05/2012 1:47 pm
Valvebloke
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What i would like to find out is if anyone has also come across amplifiers that were built from new to run with EL37s so that we can enjoy a chat about them.

I don't know of any other amps that left the factory with EL37s in but a few years back I wanted to try the same arrangement that Jeffrey mentioned i.e. to run Quad IIs with them in. That was when I found out how expensive they are. I did hunt for some among the dealers at the NVCF. None of them had any. One told me that their rarity now is at least in part down to the fact that they just weren't very well made. He said that Mullard launched them as a competitor for the all-powerful KT66 and 6L6 tetrodes which were ruling the medium-power audio roost around the time of WWII. But their cathodes were poor and very many of them failed through loss of emission. Mullard did learn from this experience but instead of using the information they'd gathered to improve the EL37 they instead built it into the design and production of a new valve - the EL34. Perhaps they thought it would be easier to sell this than to rescue the reputation of the '37 ? So the long and the short is that the '37 was only in production for a relatively short period and many of those that were made have largely given up the ghost now. I do have one amp with EL37s in it though. This beast (a 300W PA amp made by Marconi Marine it seems) uses a pair of them to drive four DA42s in Class B. Not subtle, nor easy to pick up either (50-odd kg !).

VB

AmpRegen http://www.ampregen.com

 
Posted : 26/05/2012 10:42 pm
Refugee
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Does that amp run to a single output transformer?
They appear to have been used as drivers in very big amplifiers much more than as main outputs like in the amplifiers i have.
Perhaps that is why i can't find any data on the amps.

 
Posted : 27/05/2012 1:09 am
Valvebloke
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Yes, it's push-pull with each side running two DA42s in parallel. The output stage is also driven through a 'bridged transformer' arrangement. The circuit is very close to one in the 1957 GEC book 'An Approach to Audio Frequency Amplifier Design'. They describe a 175W Class B amp using a pair of DA42s and this is pretty well the same thing but with doubled-up output valves and no front end (I guess there would have been a pre-amp on a separate chassis). The GEC book specifies KT66s as the drivers of course.

I've just measured the lam stack on the output transformer and it's 40 x 100 x 115 mm, so large but not excessively so for 300W+. However PA engineers know that speech intelligibility is actually improved by restricting the bass response. So I doubt this thing will go very low, and it's deep bass that really drives the need for great big output trannies.

VB

AmpRegen http://www.ampregen.com

 
Posted : 27/05/2012 9:52 am
Refugee
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Being an old cinema amplifier the output transformer is almost that size in order to get the much needed bass response. I can measure the lam stack if you like by checking out the next one before i power it as it is likely there was a bad worker at the time of manufacture.

 
Posted : 27/05/2012 10:24 am
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I recall Morgan Jones; in his book; making some rather negatively biased (groan!!!!) comments about the EL84 valve; despite it's undoubted usefulness. Along the lines of 'too much valve in too small a bottle'
I'm not really a bottle man so am not best placed to comment.
I haven't come across many duff Mullard EL84s. I've got some that look like they've been to hell and back, but still work.

 
Posted : 27/05/2012 11:10 am
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It wasn't me y'r honour. I never condemned the EL84. It certainly gets very hot. I've heard people say the ECL86 isn't "up to the job". I have no idea of the truth of any of these assertions. Certainly the Magnoval base and to an extent the loctal base give opportunity for "bigger bottles" with "modern" tube design (button base). But I was fascinated to realise recently that actually 6146 and other octals are NOT traditional "light bulb" pinch base in Octal holder but similar to Loctal/Magnoval button base with thin wafer and outer tubes for Octal socket. Such tubes are obvious by narrow metal ring on the base rather than tube glass vanishing into a tall Octal base cup hiding inline leadout wires and bottom of pinch.

 
Posted : 27/05/2012 11:22 am
Valvebloke
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There's a well doumented case of 'trouble with EL84s' from Leak. When they designed the TL/12+ (the replacement for the KT66-based TL/12.1, which was bulky, relatively expensive to build and getting long in the tooth) they decided to use EL84s. They also wanted to preserve the output power - a generous 12W - and the low distortion (<0.1%). This wasn't easy given the EL84's specified operating envelope. So they breezily designed the amp to run the valves beyond Mullard's specs. The worst happened. The valves lasted long enough to get through prototyping and product launch and the introduction of the very similar Stereo 20. Then, and only then, did they start to fail, going into thermal runaway and destroying themselves and other amp components. The output stages had to be redesigned. Fortunately it was (just) possible to do this by changing only the UL tapping on the output transformers. But it was a salutary lesson. Apparently Leak took to referring to EL84s as 'valves' and KT66s as 'proper valves'.

VB

AmpRegen http://www.ampregen.com

 
Posted : 27/05/2012 11:49 am
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Yes,
Don't ignore data sheets even if it seems to work :)

 
Posted : 27/05/2012 12:10 pm
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Judging by how few amplifiers were designed to work with the EL37 as a main output valve it would be difficult for it to get a reputation either way. The main failing of the amplifiers i have is looking strongly like bad workmanship during manufacture and not the individual components as you can see from my photos. From what i have found so far it looks like the EL37 was mostly used in hard driven PA amplifiers as a driver and not as an output valve it its own right as it is in the case of my amplifiers. The only problem i have had is one that went gassy after driving into the shorted winding for a while.

 
Posted : 27/05/2012 12:20 pm
valvekits
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Hello Valvebloke thanks for the interesting information. Until recently I had always considered the EL37 to be one of those valves that was credited with a bit more kudos than it actually merited.
But when I started following OTTs Mullard E7600 valve tester thread I began to think it might have some guts after all because despite it being hailed as an audio valve it is used as a (triode) series stabiliser in that.

Of course there are still some who believe that the EL37 was pre Roswell and the EL34 post Roswell. :=D

Cheers

Eddie

 
Posted : 27/05/2012 12:59 pm
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Hilarious
Roswell, New Mexico, June or July 1947

Transistor Yes working models are after Roswell, but based on pre-WWII ideas and the case of the French they were using Germanium diodes recovered from unexploded WWII ordnance. WWII Germanium diodes used in valve based Proximity fuses (one nightly RAF raid would use 250,000 valves). Germanium & Silicon used in WWII microwave mixer diodes.

B7G miniature glass button tubes and near pocket size radio sets 1940.
http://www.radiomuseum.org/forum/rca_bp ... cle.html#3

http://www.radiomuseum.org/forum/common ... alves.html
http://www.radiomuseum.org/forum/from_p ... utton.html

"modern" Noval tubes very much based on Sylvania Loctal, RCA B7G and Philips/Mullard EF50, all pre-Pearl Harbor (before WWII for USA). The Modern Tubes mostly evolved between 1933 and 1943. Then many refinements into the late 1950s.

The Russian Rod Pentodes (1950s to 1960s and produced till 1991) were Military only and no Western version are the last development in Tube Technology apart from vacuum triode Planar VFD panels used as displays (1960s to now, still developing).
The 25mA filament Philips development of Dx9x series (DF97 was last ever developed) battery tubes, PIL colour CRTs and and Magnoval Line drivers are all refinements of pre WWII research.
Silicon and ICs was a big jump (Germanium other than power consumption are inferior to tubes), but this was nearly 10 years after Roswell and based on theories and Mathematics even from 1930s!

The Russians had tubes as advanced or more so than the West, yet used only transistors for power & Audio for many years and still using Germanium when Silicon well established in 1970s.

The whole "Roswell" thing is people that can't follow research. Digital Computers and programming before Roswell.
Semiconductors (even for amplifiers, not just detectors) was the most promising line of research till 1907 and then it went on the "back burner" till the 1930s due to success of valves. There are pre-1939 Military Germanium signal diodes and Domestic radios with Metal Rectifiers pre 1939 too.

 
Posted : 27/05/2012 1:28 pm
Anonymous
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I recall Morgan Jones; in his book; making some rather negatively biased (groan!!!!) comments about the EL84 valve; despite it's undoubted usefulness. Along the lines of 'too much valve in too small a bottle'
I'm not really a bottle man so am not best placed to comment.
I haven't come across many duff Mullard EL84s. I've got some that look like they've been to hell and back, but still work.

The story I heard was that in the late 50s/early 60s, EL84 was so popular they couldn't make 'em fast enough.

There were some duff Mullard valves such as the UL41.

A lot of B9A/B7G valves were electrically similar to IO and B7 designs.

6AQ5 and 6BW6 were miniature 6V6s with restricted specifications. 6AQ5, I don't know about, but 6BW6 doesn't have a great reputation. I think it was a Brimar thing. The way valve performance scales with size was explained to me once. It just seems intuitive that if you want to dissipate any power a small envelope creates problems you don't have with a large one.

As far as I know, in the West, the last gasp of mainstream valve development was compactrons and nuvistors. I've heard talk of research into externally heated ultra-miniature valves. What the Russians were doing for military purposes I couldn't say, but valves certainly had advantages over semiconductors in military applications such as resisting EMP from nuclear explosions, which was a reason why we kept RA17s in service for so long.

Pete.

 
Posted : 01/06/2012 11:49 pm
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