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In Reply to: PCC 189 a replacement for PCC 88? posted by uwe on July 30, 2006 at 09:30:19:
The ECC88 = 6DJ8. The PCC88 = 7DJ8. The ECC189 = 6ES8, which is a variable mu (gain) type. The PCC189 = 7ES8 and it is NOT suitable for audio service.AI preamps are notorious tube eaters, as your experience indicates. The "best" tube for service in your unit is NOS Russian 6n23p-ev. The Russian NOS is TOUGH and has decent sonics.
Eli D.
Follow Ups:
Howcum, Eli?Are they running them outside (or on the edge of) their ratings?
Poinz
can work wonders. If nothing else just a change in cathode resistors.The preamp is very simple. A 6922 with plates in parallel, a common load resistor and output cap, with individual cathode bias resistors. Beats me why guys put money into burning up tubes rather than some changes.
Russ
They don't know to make the changes. It's funny, though; I mentioned to ARC below that I've tested all the tubes I use for small signal use in-circuit sonically for operating point. There's always (so far) a current necessary to get the device 'on the cam', and in every case, this is nowhere near max. Pump the tube with lots of current and voltage you may get better tech specs, but the sound doesn't improve.I would think that a manufacturer of a boutique piece would test more rigorously since operating point and load are the prime determinants of the sound you get. Gofigga.
Aloha,
Poinz
Several causes; you hit the primary cause right on the head; the tubes are run at or near max voltages, but more importantly they are run SO FAR up on their plate curve in the AIs and many others that they just roast. Unnescessarily. Preamps like the Audio Reasearch SP10 are much easier on tubes (comparatively) The second major cause for tube eater phenomenon is that often the 7308s (or whatever version 6DJ8 type) being used are not CURRENT MATCHED per triode half; combine that with low or no negative feedback that would normally go a long way towards correcting this and other issues and the tubes die fast. Essentially the high current side becomes a "current hog" and radically overdissipates-- this of course reduces the high current section to junk quite quickly, but it also impacts negatively the low current half. Also after the higher currrent section wears way down, now the OTHER half becomes a current hog and thus the entire tube, which should (and does when PROPERLY selected and used in a proper design) last OVER 10,000 hours can toast in as little as a few hundred hours! OUCH for those who tube up with Telefunken CCa and other big $ tubes! Transconductance matching simply doesn't cut it in certain demanding designs. Personally, I cannot justify such designs as there are many ways to reach Valhalla without popping off 12 $350 tubes every 6 months!
Through the years I recall hearing of a couple other manufacturers who ran tubes (and other pahts) very hard, some to the extent that it became uncomfortable to work the controls on the piece, due to heat.A funny thing; when you look at the book for small signal tubes, the specs are always quoted at max plate watts and near max current. Maybe some manufacturers just believe the book. :^)
I have found that there is a certain amount of current required to get any small signal tube (that I have tried) 'on the cam', and that this current is, in every case so far, nowhere near max. The 5965, f'rinstance, is rated for 16mA, but after about 3mA the sound doesn't change (up to about 5mA, where I stopped testing). I run them at about 3.2mA just to make sure. Okay, I'm not getting the wonderful technical specs that I would if I ran them at 200V and 10mA, but the band don't care.
Oddness,
Poinz
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