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Three great 6DJ8 testers: ARC SP10, SP8, D115

Posted by Bambi B (A) on July 17, 2006 at 23:10:19

In Reply to: Re: The 6DJ8 family posted by Doc B. on July 17, 2006 at 20:37:52:

Doc B.,

Yes, certainly the context of judging tubes is in respect of the euqipment and systems it's used in. On the topic of judging 6DJ8s, I think the Audio Research SP10 is one of the two or three best choices to judge the 6DJ8 as preamp gain tube- in my view the SP10 was the 6DJ8's shining moment- no other preamp made it work for a living like that.

The SP10 uses 12 of them and these, run from a separate chassis, tube regulated power supply. The phono section uses 4- of the 6DJ8s followed by an 8 tube line stage to create 74dB of gain. This gain is enough for medium-low MC cartridges without a step up transformer- but at a cost: it accomplishes this apparently by pushing the gain tubes to the limits- and beyond. The phono tubes have to be just perfect- no detectable noise, non-microphonic, and capable of taking a fairly high current.

Then, the sound of the SP10 is so very revealing- the level of detail and natural timbre istill amazes me- but it also tells you how poor the recordings and your turntable arm are- or o how noisy the tubes are. In short, the SP10 is about as close to a testing device for 6DJ8s as can be- it' pushes them to their highest specification performance and also reveals with intense clarity, whether the tubes are noisy or microphonic, or not very good sounding. I think there is a disconnect between specification and good sound- in my experience, the toughest ones usually do not sound as good as the lower rated.

In judging 6DJ8s, I'm lucky that the SP10 is the performance master in terms of use as gain tubes, then the SP8 uses two as cathode followers, and the D115 power amplifer uses three in the way power amplifers use them. As these three pieces of gear use the 6DJ8 in most ways they're used in audio, since 1985, I've had a kind of 17-6DJ8 test lab !

I once bought a big pile of unbranded Russian 6DJ8s- half were the ordinary rating equal to a 6DJ8, and the other 12 or so were the "-EB" suffix- the equivalent of the 6922- more rugged and rated for 5,000 hours instead of 2,500. These were I think around $4-5 each. I had just gone through a lot of tube fuss with the SP10- the Vice-President of Gold-Aero came to my house and went through 80- that's eight zero- tubes before he was satisfied with the two leading phono Amperex 7308s. Buying the cheap Russian ones was my way to see just what the range of this difficult tube might be. And, in the SP10, the difference from tube to tube was impossibly wide, there was one tube of the 24 or so that was as good as the 2 selected from 80 already selected 7308's, there was another 2 tubes that were acceptable but needed serious damping rings, there were 6 tubes that were so microphonic I could shout at them and my voice would be reproduced in the speakers- literally microphones, and the rest were just far too noisy. Sound quality was pretty terrible, even if the tubes was behaving in other ways.*** I learned again- the 6DJ8 family is just a crazy one -unpredictable, inconsistent, and generally chaos on wheels.

*** -I'm going to put these on Ebay someday for $65 each labelled "NOS/NIB ultra-rare Bambi B 'Trumpet Lad' 7308SL / ECC188SS".

The frame grid tubes must be difficult to make, the frame grid is almost microscopically fine and a floppy thing prone to reasonance. I've heard it said they were not an audio tube, they were actually intended for high frequency stuff- radar and test equipment like oscilloscopes. Today, there's lots of NOS and used 6DJ8s from old oscilloscopes or marked for use in test equipment.

Because of my troubles with the 6DJ8, I was forced to study this big subject, and just when I thought I figured out something about them, the audio world gave up on new production 6DJ8s and is turning to the 6H30. Who knows if new euipment is not using the 6DJ8 will that end efforts to improve new production? I notice the frenzied prices now for the premium ones- and it may be that people intending to keep their 6DJ8 stuff want a lifetime supply at hand of the ones they like. There's millions around but getting the 12 good ones is a long road paved by you throwing gold on the ground in front of you.

Cheers,

Bambi B

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