DIY Curve Tracer

[mogliaa]

Originally Posted by avincenty

Did you build your own?

No, not this time. I got one of those integrated filters in the socket. It's rated to 1A and has a classic pi network for common-mode filtering.

On a separate note, found an interesting impact on poorly regulated DC regulation on heaters of an DHT:


4-65a triode curves | Bartola Valves


Also found a couple of tubes with high level of distortion thanks to the curve tracer. Will upload some pictures next time so I can show the impact of it.

Finished GM tester and the THD jig. It works like a charm. For THD I follow Morgan Jones' approach but measuring valves in common cathode with CCS load and calibrating input to achieve 10Vrms output to measure THD. Useful to select good valves from a set!

[7N7]

Interesting stuff; well done.

When I had all the kit I tested quite a lot of valves for distortion; I too copied MJ's methodology - using mu-followers. because of the ex BBC test set I had I had to reduce the output of the valves and I reduced this by 20dB - using 27k and 3k resistors. Sometimes - when of course not using the mu-follower I overlooked the fact that a valve does not like driving 30k from its anode and was horrified at the vast increase in distortion that resulted from the error.

It was very instructive though and worth remembering!

Paul

[mogliaa]

Hi Paul,
I do test them just with an CCS. Distortion then is impacted by Ra in parallel with the interface input resistance (100K) so the latter is negligible.
Interesting to find that 26 and 01a have similar distortion and the 4P1L outperforms my best 46 I have! Need to look some other ones and do similar tests..

When found low gm valves, I used to curve tracer to identify the ones with misaligned electrodes or other issues. Really interesting tests, I wish I had more time to,do this....
Cheers,
Ale

[7N7]

Originally Posted by mogliaa

Hi Paul,
I do test them just with an CCS. Distortion then is impacted by Ra in parallel with the interface input resistance (100K) so the latter is negligible.
Interesting to find that 26 and 01a have similar distortion and the 4P1L outperforms my best 46 I have! Need to look some other ones and do similar tests..

When found low gm valves, I used to curve tracer to identify the ones with misaligned electrodes or other issues. Really interesting tests, I wish I had more time to,do this....
Cheers,
Ale

Yes fascinating work; I really enjoyed this sort of thing. Whilst most people are in this in pursuit of the perfect sound, I was only really in it for the valves themselves!

Do you have some distortion figures for the likes of 01A and 26 to hand? I never tested any of these antiques - I think the oldest I did was 37 which really was only any good at low levels - ask it to do anything at all and that was that!

Apart form the obvious 6SN7 derivatives, from memory I got very good results from 13D3 (6158), 6072/12AY7A and triode strapped E282F - The first section of 6DN7 and 6EW7 gave good results also. The best examples of these types gave THD around -50dB but E282F was measured at better than -60dB though figures at these levels were too close to the equipment's inherent distortions; I usually tested at +28dBu output.

Testing differential pairs was also very interesting, since of course since a diff pair doubles odd-order harmonics one could check against the SE figures and rule out equipment distortion. I once tested the test set itself and found its THD was at -65dB.

P.

[mogliaa]

Very interesting Paul. I do measure at +22.22dBu as have some limitations on my tester. I think I could get to that level, need to check though.
26 measures around 0.17%-0.19%. I have measured probably about 10-15 valves. Need to do a more rigorous test and look at deviation as well.

Planning to measure some E180f, 6e5p, 6c45, 6n6p and see whether these chaps are good ones as would expect.
This test protocol is brilliant to select best pairs for the amp, preamp, etc.
Cheers,
Ale

[7N7]

Originally Posted by mogliaa

Very interesting Paul. I do measure at +22.22dBu as have some limitations on my tester. I think I could get to that level, need to check though.
26 measures around 0.17%-0.19%. I have measured probably about 10-15 valves. Need to do a more rigorous test and look at deviation as well.

Planning to measure some E180f, 6e5p, 6c45, 6n6p and see whether these chaps are good ones as would expect.
This test protocol is brilliant to select best pairs for the amp, preamp, etc.
Cheers,
Ale

Excellent: I look forward to the results. E180F I did use as a triode; it was quite useful with its mu of 50 or so and good gm. And they used to be so cheap! CV3998 as I recall. I never tested D3A as a triode; do you have any?

P.

[mogliaa]

No D3a am afraid. I only have e180f Mullard (golden pin) which are superb. Was planning to use them in an DAC project.

So many ones good for testing, just is difficult now to source good valves

[7N7]

Yes those proper industrial valves - D3A, E180/280/282/810F usually had gold pins. A pity that the Loctals didn't have gold-plated pins - would have saved a lot of trouble!

[mogliaa]

A bit off topic here, which loctal valves do you recommend? 7n7 for sure, but which other ones are,worth looking for?

[7N7]

Originally Posted by mogliaa

A bit off topic here, which loctal valves do you recommend? 7n7 for sure, but which other ones are,worth looking for?

I haven't used many actually. Yes 7N7 of course, the Russian directly-heated 4P1L is very good I think and 7C5 is a nice cheaper alternative to 6V6. I not like 7AF7 or 7A4 (supposed to be a 6J5) because I tested these for distortion and they are very poor.

Paul