Idea: singleton power tube trading forum?

[elipsey]

How many of you have a sock drawer full of orphaned power tubes? I do, and I often wish I could could get a mate to some oddball tube that is hard to buy just one of. If the monetary stakes were very small, I wouldn't mind if it was used or mismatched.

So for example I might post something like:

Seeking/trading from North America:
sovtek 6L6wxt+ (have 1)
sovtek 5881 1990's (have 1)
JAN/philips 6L6wgb '85,week8 (have 1)

Does anyone else find this idea appealing, or do you think it's just a lot of trouble to get wheezy old power tubes?

(I have even considered building a single ended amp just for my poor orphaned octal power tubes, but it's hard to imagine one circuit to work well with all of them....)

[DRH1958]

Wouldn't this come under "Flea Market"? I'm somewhat new but this seems to fit there.

I have a cathode bias, 6L6 SE, 10-12 watt, amp that takes almost any tube(6L6) I throw in there. The bias might not be optimum but it seems to work fine. The cathode bias method seems to mostly self-regulate. This should help those lonely tubes feel wanted again.

[elipsey]

Wouldn't this come under "Flea Market"? I'm somewhat new but this seems to fit there.

Oops, I agree, but I can't figure out how to move a thread...

I have a cathode bias, 6L6 SE, 10-12 watt, amp that takes almost any tube(6L6) I throw in there. The bias might not be optimum but it seems to work fine. The cathode bias method seems to mostly self-regulate. This should help those lonely tubes feel wanted again.

Sounds good, tell me more. Like a 5F2a kind of thing? I have built a couple of those for other people, but never kept one. Did you use transformers with a higher B+ and lower primary impedance for the 6L6?

I was thinking a single ended princeton reverb would be cool, but I'm a little fuzzy on how to make the tremolo work with an SE power stage. Maybe an adjustable fixed bias single ended amp would be interesting to try...

[DRH1958]

Oops, I agree, but I can't figure out how to move a thread...



Sounds good, tell me more. Like a 5F2a kind of thing? I have built a couple of those for other people, but never kept one. Did you use transformers with a higher B+ and lower primary impedance for the 6L6?

I was thinking a single ended princeton reverb would be cool, but I'm a little fuzzy on how to make the tremolo work with an SE power stage. Maybe an adjustable fixed bias single ended amp would be interesting to try...

You could just let this one die and post about the tubes over in flea market. It's a subdivision of Hang Out/Parking Lot.

As for the amp, it's an old PA amp, a Fannon FMA-11. It's been a couple of years since it re-did it but I'll try to recall details. It is a single channel amp that I rebuilt the front end on because it had the old grid leak bias system. I changed it to the standard cathode bias and also tone caps to .047 Black Beauties. It has one input, vol and tone. Simple, low insertion loss, great tone. Two 12AX7 stages, input and tone makeup/driver. So just one 12AX7. I kept the power section just as it was because there was nothing to fix, I liked how it sounded. It's just a cathode bias SE 6L6 output with B+ around 320v and screens at 300v if I recall. It has a 25uF cathode bypass cap. I found a good deal on three 6L6s that didn't match, orphans, two GCs and one GB, and all test at different Gm values, but all sound great but have somewhat different sounds. Nice amp to tube roll with.

Here's a Vibro Champ schematic for SE tremolo. Looks like the oscillator output is taken off the cathode through the intensity pot and modulates the bias of the second half of the 12AX7.

[Justin Thomas]

Hopefully that worked... SE PR, sort of. Gretsch Compact Recording or Reverb, not sure. Might be a starting point. Sounds pretty dang good, too. Not much crunch, but probably easily fixed...

Justin

[Enzo]

Or don't worry about matching. Take some old amp that works, and just stick a random pair of 6L6 (or whatever) into it. How does it sound? When Fender made all those fine old revered amps, they never matched tubes.

[Chuck H]

There's a forum here called:

Trade Tent

My silly idea and tboy drank the KoolAid. Then he went and made me moderator there. Overall the idea hit the floor and didn't even bounce. I got to looking at all the speakers I was amassing and thought it would be great if we could all swap speakers instead of buying every model you want to try. Not unlike this tube swap idea.

[DRH1958]

Or don't worry about matching. Take some old amp that works, and just stick a random pair of 6L6 (or whatever) into it. How does it sound? When Fender made all those fine old revered amps, they never matched tubes.

Enzo, I know you aren't a big fan of the whole matching/testing thing. What would be your acceptable limits for mismatch of bias mA for a pair of 6L6GC? Somewhere along the line, the bias will be too different and be bad for the OT as far as heating goes. One tube will be doing almost all the work. I mostly agree with you, in fact I purposely mismatch tubes some because I like how this sounds better than closer matched tubes. Better second order harmonics from mismatch.

[Chuck H]

It's been noted here before, and this harkens back to a time before mine, that "back in the day" when you bought a pair of tubes the guy behind the counter just grabbed two and slid them your way. Maybe so, but I'll bet it wasn't uncommon to at least try to get a pair from the same batch or shipment. And I've also read that in that same era a given batch or shipment of tubes would be close enough to a set performance standard and tolerances so that further matching was unnecessary. You absolutely couldn't, and shouldn't try this with modern tubes and I wouldn't do it with random NOS either. I don't often disagree with the E man, but I think his notion dates to a prior epoch When tubes were made with ideals that apply to the manufacture of semiconductors instead of bulk factory output like clothes pins.

[Enzo]

I buy matched sets of tubes, I am not religiously opposed to matching. What I AM opposed to is the mantra that somehow matching is ultranecessary and oh so important. CHuck's grab two off the shelf is indeed how it was done. And when Fender made amps, big boxes of tubes sat there and pairs were grabbed for amp chassis as they moved past the station. The last thing I am concerned about is whether a few milliamps difference in the two sides of an output primary will hurt it. It is enameled wire wrapped around iron. If it gets a few degrees warmer, it won't suffer. It will take a hell of a large mismatch to saturate the core from currents. The primary deficit of unbalanced tubes is potential for hum in the stage. That is why Fender even went to a bias balance control in some models, rather than a bias level control. One can make a case for and against as to any sonic effects. My point in this thread was that left with a lonesome tube, instead of wailing oh my god I have no mate, we might try just grabbing another homeless tube and seeing how they sound together.

I have no standard as to how large a mismatch might be. If I found a large difference between push and pull idle currents, I'd find out why. If the circuit is doing it, I can correct that easy enough. If the tubes are doing it, then I can either ignore it, or change the tubes, whichever seems warranted in the particular case.

[elipsey]

Yeah, the virbro champ is cool, but i kinda want reverb. That Gretch schematic is crazy and I'd be lying if I said I understood it. It seems like it has the wrong number of transformers!

I don't worry very much about tube matching per se, but I do like to have the same type of tubes at least. The point of trading was supposed to be finding an unmatched mate of the same brand and type for orphaned tubes. Although I did try playing my 60 Watt Fender with one 6L6GC and one 5881 for awhile, which I must admit worked way better than I expected.

Anyway, I was thinking a single ended princeton reverb would be cool. Do you think it would work to use an adjustable fixed bias SE power stage so I could just use the bias modulating reverb it already has?

It seems unlikely that the Class A bias point for a 6L6 would be in the right spot for the trem to work "out of the box", but maybe with some fiddling...?

[Chuck H]

Yeah, the virbro champ is cool, but i kinda want reverb. That Gretch schematic is crazy and I'd be lying if I said I understood it. It seems like it has the wrong number of transformers!

I don't worry very much about tube matching per se, but I do like to have the same type of tubes at least. The point of trading was supposed to be finding an unmatched mate of the same brand and type for orphaned tubes. Although I did try playing my 60 Watt Fender with one 6L6GC and one 5881 for awhile, which I must admit worked way better than I expected.

Anyway, I was thinking a single ended princeton reverb would be cool. Do you think it would work to use an adjustable fixed bias SE power stage so I could just use the bias modulating reverb it already has?

It seems unlikely that the Class A bias point for a 6L6 would be in the right spot for the trem to work "out of the box", but maybe with some fiddling...?

Hmmm.?. Well... No AB so no crossover distortion garble. That's a plus. OTOH the tube would be driven into a disproportionate cutoff region relative to dissipation. I imagine it would be somewhat like it is in higher voltage amps that employ bias vary trem. That is, best at lower drive levels and not very good at high drive levels. You'd basically need to bias and drive the power tube so that it would conduct as class A but NOT be driven too dangerously high on the positive bias swing of the trem. It seems plausible to me. It would take some tweaking, but should be do-able. Though not ideal for either the bias vary trem or class A bias. Drive level and any amount of class A overdrive hoped for are going to be the lynch pin in the idea.