Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JB on November 29, 2014, 03:26:44 pm
-
I'm building a JTM 45. Nearly done, had it running with a pair of 6L6 in, now for some KT66's.
I've got 7 NOS GEC 66's. I've run them all through my valve tester - AVO CT-160. AVO's test spec for them is 250V Va, 250V Vg2, -15V Vg1 and Ia should measure 62mA with Gm at 6.3.
I've got the following, Ia ma/Gm
71/5
67.5/6
62/7
58/5
57/5
56/5
49/5
So in terms of emission the nearest to spec are the 67.5/6 and 62/7. But I've got separate bias trimmers so can match the standing current. Gm very slightly different but this is a guitar amp so probably negligable?
Or I could go for lower emission pair with matching Gm. I don't think it'll make a difference but would be interested to see what people think about differences in Gm in an output pair. If they were further apart, like Gm=5 on one and Gm=9 on another would that make any audible/measurable difference to the output?
-
I've got 7 NOS GEC 66's. I've run them all through my valve tester - AVO CT-160. ... But I've got separate bias trimmers so can match the standing current. Gm very slightly different but this is a guitar amp so probably negligable?
Or I could go for lower emission pair with matching Gm. I don't think it'll make a difference but would be interested to see what people think about differences in Gm in an output pair.
I'm mildly envious of your selection of KT66's and your tube tester. :icon_biggrin:
According to RDH4, the proper test for output tubes is a power output test, which makes sense when you're matching tubes for the type of service they'll perform. I imagine it's a bit like making sure the 0v grid-line on a set of plate curves is at the same spot, rather than just the idle point. While this is a do-able test a number of ways, let's assume we can't readily perform a power output test for each of your tubes.
The next most-important number is probably Gm, as that's mA/V or plate-current-change per unit of grid-voltage-change. Gm measurements are the most-valid when the plate current of all measured tubes was the same at the time of Gm measurement, because all tubes exhibit higher Gm when their plate current is increased. However, we do hope that the Gm measurement will indicate something of how the tube will perform dynamically more than the simple idle current measurement.
So in your case, if matching was the upmost priority I'd pick tubes with matching (or very close) Gm. As you mentioned, you have bias trimmers to make the idle currents exactly equal.
Will it matter much? No, probably not. The only change that could happen is you get the maximum clean power output with perfectly-matched tubes (and circuitry), with somewhat earlier distortion for anything else. RDH4 did an experiment with power triodes and purposely picked 2 triodes of different types where one had half the Gm of the other. The result was 5% higher THD than if the tubes had been perfectly matched. Those were triodes and these are beam power tubes, but it ought to tell you matching is not the be-all-end-all.
-
GM of 9:5 is a barely audible rise of 2nd harmonic distortion. Which is not always bad for music.
I'd save the 49 and 71 for a bad day, use any two of the others.
-
Thank you for replies both technical and pragmatic! I think I'll try a pair in the 50's mA range and then those two in the 60's and see if there is any noticable difference. I doubt there will be.
I bought the CT160 about 25 years ago from a test equipment surplus store for £20! I then phoned AVO and a rather surprised man went and found their last ever valve data book (tells you all the test conditions and thumbwheel settings). They hadn't sold one in quite a while....
The 66's were partly from a job lot my dad bought at a ham radio bring and buy sale 20 years ago. He saw a man on his way to the stall with a box of about a dozen of them, plus rectifiers and a Quad II pre-amp - took £15 for the lot!
-
Yep, there used to be screaming deals in strange places 15-20 years ago. Around that time I bought 20 Telefunken ECC83's for $1 each at a hardware store in Nashville... The previous owner had been into old radios and had moving boxes full of loose tubes. I wish I knew more when I went through them, as I probably would have found many more valuable tubes for next to nothing.
-
An amateur radio operator who is my friend, I think 15-20 years ago, has sold a stock of tubes, where many ECC83, got a piece of bread for this and even now, when he remember, he bites his hands
Franco