Being a proponent of single ended topologies, I am constantly on the lookout for singleton power tubes. Since I’m working on designs with all single ended signal chains, things like matching and bias points as less important to me than tubes which test strong and are reliable. One source containing lots of singleton “New Old Stock” and “Used” power tubes is Ebay. Often I can get a single power tube for far less than buying from one of the more established vacuum tube business on the web. But I have begun to wonder about whether these are really such a great deal after all.

Increasingly I have been noticing an interesting (and annoying) phenomenon associated with these tubes. They just don’t sound very good. I don’t mean to imply that they are not as advertised or that I was intentionally “swindled” or cheated. When I test these tubes, for the most part, they do indeed “test strong”. Usually amplification factor (µ) and transconductance (Gm) seem to be well within specifications, regardless of the test method. But when placed in an amp they are “muddy” or “fuzzy” or simply lack clarity. And when tested specifically for distortion they far exceed the levels that would be deemed acceptable from a new or NOS tube. But I have formed a theory.

My theory is that these tubes are guitar amplifier culls. This issue seems to be prevalent in singleton power tubes common in push pull guitar amplifiers; 6V6s, 6L6s, EL34s, etc. Most of these larger guitar amplifiers tend to push the power tube at the edge of, or exceeding, their maximum ratings. And this is intentional. It’s at these high voltages and high currents where the tubes produce the interesting and complex tonal qualities which artists covet.

However the consequence of this is that tubes don’t last. On those big duals and quads the tubes are abused. The Fender Twin Reverb with its 6L6 quad run at max dissipation (and screens over max) or the Baseman 100 with its 450V plate voltages and over maximum dissipation both are prime examples of abusive tube operating environments. And when tubes fail, it’s generally just one not both (or 3 or 4). Which brings us to the core issue. Bias matching.

As anyone who has one of the big amps knows, when you lose a power tube, if you just get one replacement, getting the output stage bias to balance is a nightmare. Even if you get the two (or four) tubes to balance, it doesn’t last. Soon one tube is sucking all the current with all the tonal problems that entails. So the answer, is to go get a new matched pair (or quad), fine tune the balance, and then all is well. But what of that “still good” but orphaned tube? And this is where the problems start.

No one wants to simply trash that “good” tube. It cost a fair amount of money the first time and “it’s still good”. So people with these big amps tend to collect a handful of these singletons in the back of the amp, or on a shelf somewhere. Until at some point they’re in a guitar shop talking to someone that says “You could sell them on eBay” or “I’ll sell them on eBay”. So they reliably test the tubes and list the “good” ones (i.e. those for which the tube tester needle points to “Good”) for some nominal fee.

But are they really “good”? My theory is no. I think that after hours and hours of running beyond maximum in guitar amplifiers, the electrode coatings have begun to outgas and breakdown. The overall µ and Gm may still be in specification, but the distortions (both harmonic and intermodular) are seriously degraded. They just aren’t the tubes they started out to be.

Now in all fairness, these singletons may be suitable for use in small single ended amplifiers where someone is looking for a unique tone. Used in a little Fender Champ or a Gibson Epiphone Jr. these may be suitable tubes. But in a high fidelity music amplifier, probably not.

So has anyone else had any experience with the “not up to snuff” singleton tubes? I’d like to hear.