Many years ago I got a box of 3 dozen pulls from a music store in Montreal for a buck a piece. There were 6 Tungsol 5881 in there, the rest were GE and Phillips 6l6s, all looked visibly excellent. They should have been bad tubes pulled by the tech but with these large music stores you never know who's doing what. I took the lot for like $25 US. The 5881s proved to be a matched quad and pair drawing 35mA in my 5F6A clone at -45V An other bunch proved good too drawing around 40mA. 2 Draw 50-60mA, the last dozen 70-90mA.
I don't have a tube tester and I've been sitting on these last ones for 20 years, can I assume that they are ready for the dust bin, or are they usable if I just give them a lot of -V to bring the current into reason? In other words is what I'm doing testing good/bad or am I just rating good tubes from 1 to 10 like groove tubes does?
Last edited by rp on Thu Aug 26, 2010 7:27 pm, edited 2 times in total.
That's right. You have a bunch of great tubes! How come I can't get deals like that?
Case in point, I just bought a matched pair of NOS RCA 6L6 GC blackplates from Gregg Levy at Hi-Test Guitars. He asked me how hard or soft I wanted them on a 1-10 scale, I said they're for a '70s ODS build so let's try 8, so these are pretty hard & clean.
Gregg said to bias them at maximum 32 mA Ip, 30 if I liked the tone.
I put them on my MaxiMatcher and they came in right around 40mA Ip at 400 Vp and -48V bias, so I figure these are going to need -52V or better bias in my amp. Gm was close too, 2.4 & 2.5.
David Root wrote:That's right. You have a bunch of great tubes! How come I can't get deals like that?
Forget it, you can't any more, w/ the internet everyone knows the value of everything everywhere. Heck you can check ebay on your iphone in a blink. I spent from 1980-1995 finding Easter eggs in pawn shops, thrift shops, yard sales. Montreal in that time was pawn shop / little music store heaven, though whatever you took home sure stank of cigarette smoke. Lot's of killer Brit stuff for crazy cheap. I wish I still had everything I found up there. Including the beat but intact '56 Gibson ES125 I got in a pawnshop for $125! It was fun while it lasted.
Internet has it's uses like forums, but I miss the Easter egg hunting. Did just pick up a beat but fine G&L SC1 on craig's for $500 guess that's good for 2010 but $125 would have gotten a bigger smile.
I put them on my MaxiMatcher and they came in right around 40mA Ip at 400 Vp and -48V bias, so I figure these are going to need -52V or better bias in my amp. Gm was close too, 2.4 & 2.5.
That's a 16W bias point....69.5% for the 23 watt TS 5881. A -52Vdc bias voltage will bias them colder, and you'll likely see a slight rise in your Vp..is that what you want ?
Dumble circuits don't run anywhere near 70% of Pa, nearer 50% sounds better and the tubes last much longer. Since these are NOS RCA blackplates, that's just as well!
These are 6L6GCs not 5881s, so normally I'm looking for about 15W or so quiescent dissipation, ie about 35mA, however since they are so strong the seller advised absolute max. of 32 mA Ip and 30 if it sounds OK.
I would expect the Vp to rise with a lower dissipation, yes, but probably onle a couple of volts or so, so no tonal changes expected because of that. So I would think it might go from 443Vp to 445, maybe. Will be checking them out in the amp today.
Update. Had to remove the AC bias dropping resistor (1K2) to get to bias these hard blackplates to 32 mA cathode (~31.5 Ip). Bias voltage necessary is -61.1V! Plates/screens went up to 445V, no significant change on any other voltages except when I switched V1 from smooth long plate Telefunken to Mazda silver anodes, plate voltages dropped significantly, from 191/192 to 178/179. Still sounds very good, this is a '70s ODS so ~180V in the clean is OK. Wall voltage was 120.1 VAC.
An other bunch proved good too drawing around 40mA. 2 Draw 50-60mA, the last dozen 70-90mA. 
