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Weird World of Tubes : EL38 - the other pentode output tube

Posted by kstagger on December 27, 2007 at 07:29:19

note pic is from my stash of tubes - cut out a non-EL38 from the pic

The EL38/6CN6 is a Philips/Mullard design for use in the television line output stage. They are 25W tubes, much like the EL34 and also have an 8W / 400V G2. This makes them better suited for ultralinear usage than other common sweep tubes. EL38s are extremely well constructed with ceramic insulators and a large cathode area. They look quite similar to the famous EL37 - but of course have the additional plate cap and the specs are different.

Why aren't they popular to use?

First of all, they require a plate cap which seems to automatically discount their use with many people. They also require much lower bias voltage than an EL34 - which means you have to modify your amplifier bias supply - either by increasing the available range or using a dropping resistor. They also don't seem terribly suited for hard Class A use unless you are willing to give up some power - red hot spots on the plate have been reported if you go over 20Ws of idling.

There are four known variations of this tube - the early original Mullard version with a larger bulb, grey glass and black plates. Reportedly have lower plate dissipation and G2 voltage. (second one in pic)

The more common Blackburn made Black base and brown base - they appear to have the same internal structure. I've found these branded Amperex, Mullard, Rogers, Valvo, RCA, etc. (first tube in pic)

Later production are French made and have gray glass with a metalized envelope covering the bottom third of the tube. (third one in pic) I've also seen an Ebay auction of an EL38/EL39 that appears to be a Russian tube!

If you want to check out this tube, you will need the plate caps - 6mm or 1/4" size and run the wire to pin three of your output tube octal socket. Also the amplifier must be able generate low enough bias voltage - with a stock Dynaco 70 bias power supply I could only get 20mA per tube. I've used this output tube on an updated and modified Dynaco 70 (Triode input board and SDS Powersupply board) and was surprised how good the EL38 sounds. Classic big tube type of sound and I much preferred it over the Matsushita EL34s I had on hand.

Check out the specs and some operating points here : Retrovox

Going Market prices on these are as low as $10-12, with an average of ~$25. Again, this is more of an European tube and there are more sellers of this tube type across the pond.

Edits: 12/27/07 12/27/07

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