DIY Curve Tracer

[euro21]

C3g (triode strapped): 11mA CCS, 230R+100uF in cathode.
About four dozen used tubes.

5.9-7.4V RMS out, 0.031-0.068% THD
52-66V RMS out, 0.14-0.55% THD.

[euro21]

C3m (triode strapped): 160V, -6V, 100R in cathode, 16mA anode current.
Two dozen used tubes.

70Vpp out, 0.11-0.61% THD
160Vpp out, 0.28-0.99% THD.
240Vpp out, 0.50-0.96% THD (six tubes already clipping).

Excellent driver tubes, but requires selection.

[7N7]

Originally Posted by euro21

C3g (triode strapped): 11mA CCS, 230R+100uF in cathode.
About four dozen used tubes.

5.9-7.4V RMS out, 0.031-0.068% THD
52-66V RMS out, 0.14-0.55% THD.

Excellent numbers; the C3's reputation is obviously well deserved.

[mogliaa]

had to bring my ferrograph RTS2 test set back to life to test THD somehow. And indeeed I screwed up Pete Millett's interface. Measured THD on 26 at Ia=6mA, Va=160V and Vg=-10V on a RCA ST valve and THD was 0.05% @ 10Vrms

Now, need to repair Pete Millett's interface!

[7N7]

Originally Posted by mogliaa

had to bring my ferrograph RTS2 test set back to life to test THD somehow. And indeeed I screwed up Pete Millett's interface. Measured THD on 26 at Ia=6mA, Va=160V and Vg=-10V on a RCA ST valve and THD was 0.05% @ 10Vrms

Now, need to repair Pete Millett's interface!

Yes a good figure 0.05% - about -65dB I think. Trouble is that at these levels one really has to trust one's test gear

P.

[euro21]

Trust in God, and keep your powder dry! :-)))

My basic test equipment:
Heathkit IG-18 (strongly modified, only sine wave) 0.001-0.002 % THD;
Leader LFG 1300S 0.1-0.15% THD sine/square;
Kenwood VT-176 dual channel AC voltmeter (300uV-100V), dual output to sound card (max. 1V);
1:10 scope probes;
2 ch passive preamp (for low value THD measuring) -protecting sound card input-;
M-Audio Transit USB sound card;
Visual Analyzer program (running on laptop).

[andyjevans]

They say "You can't beat Wales at Rugby - you can only score more points than them"

I say "You can't beat the 26 as an input tube - you can only get better figures".

andy

[mogliaa]

Originally Posted by andyjevans

They say "You can't beat Wales at Rugby - you can only score more points than them"

I say "You can't beat the 26 as an input tube - you can only get better figures".

andy

Andy,
The analysis was only around THD and specially the tube's linearity at large voltage swings. I do like 26 and 01a a lot, no doubt about that.
Both tubes have THD around 0.02%-0.06% at low signal levels...

Trouble is here that I screwed up my Pete Millett's audio interface whilst playing with these valves!
cheers,
Ale

[7N7]

Originally Posted by euro21

Trust in God, and keep your powder dry! :-)))

[...]

Yes. When I was doing this, the equipment I used was a BBC EP14 distortion test set, Marconi wave analyser, Marconi HF voltmeter, Tek 60MHz 'scope.

I also had a signal generator and a function generator, but the distortion from these was risible.

The BBC set's THD was around -65dB anyway, so it was better that I tested at higher levels. Even though I no longer have the equipment I do still have the data in my book.

As a matter of interest I did a lot of testing on type 955 "acorn" triodes, because I built a line stage with them. As single valves with 50k resistive anode load, about 140V on the anode, driving my usual 30k pad, THD at +28dB (19v rms) was consistently about -37dB (about 1.5%) with 3rd harmonic about 25dB below that. At +20dB (7.75v rms) the figure was -45dB. However, configured as differential pairs which I have used in the line stage, still with resistive loads, at +28dB, THD was -63dB (0.07%) with 3rd at -70dB (0.03%), which is perfectly acceptable to me!

Paul

[mogliaa]

Originally Posted by euro21

Ale, I think you have a little weak bunch of #26s. :-)

Each of all -in my collection four ST type Silvertone, and a pair Cunningham CX326- distortion spectrum below -60dB (2nd), and the best pair THD only 0.0526% (7V RMS out).

B+ 220V (Salas SSHV2), heater bias (10R/50W, Coleman regulator), 6mA (DN2540) cascode CCS anode load, 220nF FT3 teflon coupler, 100K Shinkoh resistor as load.

Sylvania 26 ST - randomly picked from the good batch of ST 26 I have:
THD=0.03% @ Vo=7Vrms - to compare against your measure above
THD=0.052% @ Vo=10Vrms - to compare against my damaged audio interface!

Both were tested at Ia=6mA, Va=158V, Vg=-10V using my curve tracer circuit in transconductance mode. Anode load is DN2540 cascoded CCS. Fixed bias. Input and output caps are russian PIO....

Good thing is that my tester is working, bad thing is not only that I damaged my audio interface but that I need to retake all THD measured I did yesterday