12AX7 exercise session

[Stevem]

I had the time to do some plaing around with a Vibrochamp that I fixed for I guy and since its a simple average circuit I stuck 14 different tubes into the V1 spot and took measure of the max signal output getting pumped out to the 6V6.
My test out was done by pumping .150 volts @ 1k into the little bugger and all of the 14 tubes that I popped in it where tested and then rated on my freshly calibrated Hikcock 600.

The first line stack you see here is the tube type and on types that have different size plate structures that is noted for instance sp, stands for small plate version, or long plate version of which on this test there where no long plate versions.

The next two lines are how the tube tested out in regards to its A section and its B section.a grade of 100 would equal the tube being perfect per its 1954 design spec!

The last line is the signal out put voltage that both the A and B section where able muster in the series string of the preamp section/ first gain stage, tone stack, gain stage/ output driver.




GE/Fender red lable
7025, V1a , V1B , signal output voltage.
88 92 45

SINO/ Mesa
7025
82 90 48.5

GE 12ax7/ECC83
white lable,
81 84 45

Sino/ Groovetubes
12ax7
90 96 46.5

Sovtek 12ax7wxt
88 92 49

Sino 12ax7B
114 116 46.7

RCA 12ax7A, SP
74 76 42.2

Ruby/Sino
12ax7a
104 104 48

Ruby/Sino
12ax7a
84 80 46.3

EI/Groovetubes
7025
112 100 50.3

1969 RCA
12ax7a,
80 86 40.8

Sino 12ax7B
132 142 46

Sino/ Peavey 12ax7
super 7s
92 80 46

Sovtek 12ax7wa
88 72 39

Note that for instance there is a near 30% difference in the drive signal output between the EI tube and the Sovtek 12ax7wa and your gonna hear that and feel that big time!

enjoy!

[billyz]

Interesting that the much maligned Sovtek's test closer to the vintage USA tubes than the rest. I always found the sino tubes to be a little harsh and nasty sounding to my ears, even if they have higher test numbers.

[matt h]

(deleted)

[martin manning]

Three numbers are given for each tube, of which the first two are section 1 and section 2 test results. Are these test results indicating plate current (emission) or transconductance (Gm)?

[Stevem]

That first gain stage and the recovery gain stage are A and B respectively and the number for each are gm.

A lot of the New Sino tubes I get my hands on are well over spec on gm, some to the tune of testing out at 148!

I have found the with these tubes the more over the level of 98 they test at, the more like brittle crap they sound in use especially if they are being over drivin!

Also as commented on many of the tubes be they N O S or not where not new, my posting on this was to purely show the voltage output difference in relation to how they test out in a tube tester that test them in a meaningful way!