Looking at the VA amp first - probably my fav config here.
Its is an EH 12AY7 with one half as a grounded cathode stage with 68K on the plate and +2.0V bias - direct coupled to the other half set up as a cathode follower. The output of the CF is cap coupled to a tone stack.
B+ here is around +250V, Current in each half is around the same at 1.7mA, with the voltage drop across the GCA plate resistor about the same as across the CF cathode resistor (82K).
It drives a tone stack thru a big 0.22uF orange drop cap.
Gain is x27.7 or 28.9dB, so for an input of 3Vpp max, gives an output at the GCA of 83Vpp. The CG drops this a little, to 79Vpp.
....
So, with 1KHz sine applied to the amp front panel main input set for a 3Vpp signal to the grid :
REW signal generator, -12.9dBFS level output from Motu balanced analog out
GCA stage
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AY7 (1) plate has THD 2.46%, THD+N 2.46% at 76Vpp output
H2 -> 1.64%, H3 -> 1.68%, H4,5 -> 0.5% each, higher approaching 0
1% THD 35.4Vpp, 3% THD 79.1Vpp
response at 76Vpp output is 20Hz -0.4dB, 20KHz -1.3dB
CF stage
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AY7(2) cathode has
THD 2.50%, THD+N 2.50% at
75.3Vpp output
H2 -> 1.54%, H3 -> 1.81%, H4,5 -> 0.5% each, higher approaching 0
1% THD 33.1Vpp, 3% THD 78.8Vpp
response at 75.3Vpp output is 20Hz -0.4dB, 20KHz -1.4dB
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So the AY7 here, in a moderate gain setup, has a reasonably high THD at 2.5%, and about equal H2, H3. components.
There is still significant H4, H5 dropping towards 0 for higher orders.
Not much noise, the THD being about the same as THD+N; so supply decoupling is good despite the lowish CMRR of this stage.
(class A with 68K plate, no nfb)
The CF adds very little to that - around 0.05% and virtually the same freq response.
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That's what I would expect from this stage, more or less. It's tending towards the high side of my expected range.
Low THD by my reckoning would be around 0.7%, high around 3.5%
while I'm here, I'll do a sub of other EH AY7 I have here and also maybe an AV7 in that position for comparison.
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Testing EH AY7 nbr 2 tube, for the GCA+CF combo shows THD+N 3.34% at 72.7Vpp.
Thats quite a bit less output and quite a bit higher THD! Good thing that it is currently in a 6L6 deluxe type amp, as the PI.
There it tamed the whole amp down from the dangerously strident levels of the Groove Tubes AX7 it replaced - not as much gain, not as much 'grrrrr' but .. sweeter at the chimes and more responsive
But still able to give more with the turn of the Master vol
So, out with that one!
nbr 3 tube shows very similar as last THD+N 3.6% and 74Vpp output.
That one is used in rack preamp mostly the same as this SE amp. It goes back!
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So for a sample size of 3 ehAY7, I get a range of 2.45% to 3.7% - to get the lowest would be a 1/3 chance. I do have some more of these in-use, which I'll measure another time.
How about some AV7 - here I have a sleave of unused nos rca (70s) av7s. I tested the first one in the el34pp amps earlier and it gave something like 2.5% in that circuit, which is just a GCA, no CF.
This circuit isn't optimised for an av7, but anyway, as a relative measure and to check the spread of thd, it is a useful test.
These aren't burned in and I'm not letting much time to settle - just quick in and out.
av7 nbr 1 3.37% at 66.8Vpp output
av7 nbr 2 3.34% at 66.8Vpp
av7 nbr 3 5.48% at 64.6Vpp
av7 nbr 4 3.31% at 51.6Vpp
av7 nbr 5 6.37% at 60.8Vpp
av7 nbr 6 8.20% at 60.9Vpp
and nbr 1 again for a reality check! 3.32% at 67.2Vpp
THD rises with warming ... then stabilises after 3mins or so.
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SO quite a spread there on the AV7s in this 'quick-test' 3.34% .. 8.2%, with nbrs 1,2 a good match, 5,6 match but with more than double the distortion and 10% down on the plate volts achieved. Nbr 4 is a freak outrider
, destined to sit in my little Champ (esque) 6V6 'character piece'. nbr 5 is pretty ordinary and again, suitable for less demanding tasks
But first! I await the arrival of a set of 4x 6829 equivalents. If any of them match up with lowest THD, I'll use them instead.
A better test obviously would be to measure the halves seperately. That would characterise the tubes more accurately but not so the current configuration.
I'm not going rewire to take advantage of uber-matching possibilities here, because it's *this amp* I'm tuning, not a 'tube-tester'.
The first two nbrs 1,2 - lowest THD, would be the best candidates in the el34pp hifi amps - which is where they will go.
I'm pretty sure they were lower THD in that single triode GCA setup, at 2.5% or so.
However, the golden ear types do say that that's only a starting point! The 'sound' is the main acceptance criteria.
I am not in that league - I can tell the diff between a AY7 and a AX7 though - and that's taken a long many years of guitars, amps and building!
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Next up!
EH AT7 - I use this in a limiter build, the d-oac. 6.23% at 95.9Vpp. Pretty high, but sample size = 1 !
EH AU7 - I use this all over the place for line stages, CFs and things that need 7.5mA or so. THD+N 2.21% at 38.6Vpp
I have a bunch of nos 5814A and 5963 on-route - these au7 equivalents will be really interesting to check against the au7 types I have used up to date - mostly the eh variety and a nice bunch of Baldwin organ pulls from many years back.
And an AX7 phew 19.1% THD at 72.6Vpp
Note this isn't a fair test - the 12AX7 is not set up here, But there you are.
Dropping it's grid amplitude doesn't help much - about 12% is the minimum before it all goes bad. This is a really old one pulled from an mid-60s japan EL84 pp hifi amp.
AX7 nbr 2, a Sovtek of the cheapest variety, shows 15.8% for 75.6Vpp dropping down to 7.5% or so with much reduced applied signal.
More on AX7s another time.
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Back to the AY7 nbr 1 - this is the original tube in this amp.
Re-measure, THD+N 2.64% at 76.7Vpp (which is about the same as I found before)
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SO what do I conclude here from this simple first set of tests ?
- firstly, the eh AY7 is generally a reasonably low distortion tube, and selecting from 1 in 3 gives one 'lowest' THD tube
So far it's my best performer
- secondly there is a lot of spread in nos rca av7s
I can get one good matched pair and 2 good singles, with one best suited in a guitar amp.
and , to get more accurate results :
- the circuit should be biased properly for the toob-under-test
- should characterise each triode seperately for twin triode bottles
and finally,
- y m m v
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Even given the rough and uneducated way which I am profiling my toobs ....
It's great to be able to make even these characterisations.
Room EQ Wizard's THD measurement capabilities are wonderfully easy to use for these repetitious tests.
Tedious to perform but wonderful to peruse the executive brief!
I've only done this for the GR tubes in my limiters before. Now all future builds will be tested in this way and then have tubes selected to suit the purpose.
A great new day in my builds has begun
