Tube Matching, Tube sellers and Used RCA Blackplates

[David Root]

As my little cache of used '60s RCA blackplates begins to head one by one for Valvehalla, I broke down and bought some "new" used ones. (New to me at any rate). They now cost way more than the $10 each that my expiring ones cost me 8 years ago, but I'm hooked on the tone, they ARE the reference standard for me.

(One of my cache stopped conducting after several minutes, which at 49 years old I guess it's entitled to) One other is slowly running away, now pulling 70 mA at 400V and -48V bias, following a third that went up to 120mA on my Maximatcher which triggered the disconnect.

I bought one matched pair of "used" and two matched pairs of "lightly used with matching transconductance". From a reputable vendor, not on fleabay.

The "used" pair were fairly well used judging by the flashing, but "matched" barely within 5 mA on my MaxiMatcher tester. The other two pairs of "lightly used" had good flashing but one pair again barely within 5 mA, the other pair was 10mA apart, on the Maxi Matcher.

The vendor had tested them, on what tester and at what plate voltage and bias I have no idea.

Is this a familiar story? I also bought three NOS RCA 12AX7As from the same vendor and they came out closer on my VTV preamp tube tester.

Both my machines are modern digital designs that a lot of you probably also use.

If I judge the two "lightly used" "matched pairs according to my MaxiMatcher I have one matched pair and two singles.

With NOS blackplates at $300/pair from this vendor, I thought I would be OK with "lightly used" for a lot less. I'm sure they have lots of life in them, but I expected better matching. All the vendor's test numbers were very close in each pair.

Is my experience just the way things are now?

[Phil_S]

David,
Seriously, what is your objection to a 5mA spread? IMO, in used tubes for lower cost this isn't much of a compromise. Have you put them in your amp(s) to see how they sound? I recently put a pair in an amp that were 10mA apart and I thought that would never do. I ended up leaving them in and the amp sounds just fine. I biased on the colder side to be sure the hotter tube wouldn't be "out there".

I think it comes down to this. Loosen up your wallet or loosen up your standards. If you were looking at the from an audiophile point of view, I might be persuaded that 5mA is too much of a spread, but not so in a guitar amp. Some even think it's desirable.

two more notes...
1) many tube sellers build their own rigs for testing and matching. Results on your maxi matcher probably won't duplicate the seller's results. Call him up and ask him about it...plate voltage, bias voltage, etc. What have you got to lose?
2) put them in your amp and see if they bias differently than the way they test on your maxi matcher.

Phil

[Andy Le Blanc]

tubes are 20% devises, 5ma is pretty good for a lowly guitar amp.

Ive been looking at a maxi matcher, does it have wings?

I feel for you, with the NOS, Its to the point where I won't bother using NOS
in a gigging amp, cant take the risk. I have precious stash of mullards, etc...

the more demand there is for new tubes the better they will get.
Especially with the crap digital audio that everyone suffers with .

I'd hoard the NOS as I find it.

[David Root]

Thank you Gents, I realize that 5mA is pretty much OK for guitar amp applications, but 10 mA is a bit beyond what I expected, especially as they were sold as a "matched pair".

Obviously he's not selling used RCA blackplates to the HiFi market, I'm just puzzled by his test numbers vs. mine. I guess it's like accounting, I once asked a CPA what the numbers were, he asked me, what do you want them to be?

I just got them today, but I will make a note to report back after I've put them in some of my amps.

Andy, no, the Maximatcher doesn't have wings but based on my experience it is a very useful tool especially with old used power tubes, which actually seem to change their operating points with time even if you don't use them much. I have watched them age and go over the hill, it's really quite sad. Like us, the older they get the faster they go.

On the plus side, I now have two of my original stash that actually drifted miraculously to the same point, 19.2/19.3 mA, 2.21/2.21 transconductance, probably for the next 3 months or so!

I compared my test results on the MaxiMatcher with a quad of NOS 7581As(Mesa 415 STRs) I recently bought and they checked out pretty exactly as a perfect quad, mA and transconductance. Different vendor in that case, but also NOS not used, to be fair.

[jjman]

I bought a pair of used RCA black plate 6v6s on ebay a few months ago. Also purported to be matched for ac (transconductance) and they are matched for both, per my scope. Only problem is my JJs sounds better.

[skyboltone]

I don't remember ever hearing about or giving a crap about matching back in my very early Hi-Fi days. Tubes that went in Dynaco ST-70s generally said 6CA7 on them and any number of brands. Sometimes 4 brands per amp. Sounded great! I was partial to the Sylvania big bulb jobs. They looked cooler.

[selloutrr]

I'm not understanding this on my phone but if you need tubes tested PM me

[skyboltone]

I bought a pair of used RCA black plate 6v6s on ebay a few months ago. Also purported to be matched for ac (transconductance) and they are matched for both, per my scope. Only problem is my JJs sounds better.

JJman. You got a PM.