One day I was shopping for EL38s on Ebay. The EL38 is a Mullard designed pentode used for television line output stages. It features a 25W plate along with a beefy 400V 8W screen. The downside for many users is the top plate connection. With plate (anode) caps high voltage is outside the chassis and for safety reasons this tube may not appeal to everyone. However this may not be such a concern if you're making a DIY tube amplifier. Specifications for the EL38 can be found here.
I came across a very strange looking EL38 that I had never seen before. It looked exactly like a KT77 with a top cap. There was only one, but I was curious enough to bid and ultimately win.
Upon receiving the vacuum tube, I confirmed that the plate, getter rings and mica conformed exactly to the GEC KT77. I knew there were two versions of the KT88 with top caps - the TT21 and TT22. Was this a top cap version of the KT77? If so, I may have discovered a cheap way to get the famed KT77 sound at reasonable prices. So I started to use Google and find out everything I could about this version of the tube.
On this site I found references to the GEC EL38/CV450 and saw that the GEC version was called the E3375 - the 'E' standing for Experimental. I was curious enough to buy some, so I found a supplier in England called Langrex. At the current exchange rate, I was able to order a quad of E3375s for roughly $40ea. A little steep for an unknown tube, but perhaps I was letting my emotions get the better of me.
While I waited for the tubes to arrive, I bought a set of 1/4" top cap connectors on Ebay. Langrex shipped quickly and I popped open the first box to find this:
The only push-pull tube amplifier I had on hand was my pair of trusty Dynaco Mark IIIs. I added the plate cap leads to pin 3 of the tube socket, plugged in the tubes and set the bias adjustment at the lowest point. And got no sound. Hmm... this is strange. Fearing something was wrong, I shut the power off and suddenly, as the amplifier powered down, music started to come out of the speaker. Here was a clue. The bias voltage dropped as the amplifier power supply was shut down, allowing the tube to conduct. So the E3375 is in fact the GEC version of the EL38. Like the EL38, it requires much lower bias than the stock 6550/KT88 used in the Mark IIIs. See the specifications here for the operating points for the EL38.
I quickly modified the bias circuit, replacing the 10K pots with 25k units. The larger range of adjustment allowed me to finally bias these tubes in. In the end the sound was pretty good, but not at all what I was expecting from Genalex. Perhaps the Dynaco Mark III output transformer was not the optimal load. From my further experimentation with a single ended amplifier, the entire EL38 family of tube seems to 'work' better biased hard into class B.
As to why the E3375 was even made, I can only surmise that GEC wanted to compete with the Mullard EL38 for TV tube sales. Or perhaps the British military required a batch of new EL38s for some older equipment and GEC took on the contract. They already had the KT77 on hand and just modified the screens to conform to the EL38 specifications.
So if you're into rolling EL38s and don't mind spending a little dough, the E3375 is worth checking out. But be aware that using them in a stock amplifier may require some modification since they will require a lower bias point.