Stability issues with my original 1966 JTM 50

[Helmholtz]

The term 'beam tetrode' didn't seem to get used much by valve manufacturers in the 40s-60s. GEC then revived it in the 70s, perhaps to differentiate their KT series from Philips competiton.

No matter how you call the different constructions, I tend to think that there are characteristic differences between comparable tubes of both types

Comparing EL34 and KT77 data, I see a less pronounced knee, less kink, higher screen current and considerably lower ra with the EL34. There may be other differences.

I couldn't find the data for a beam tetrode 6CA7.

Both the RCA-30 and the 1973 GE databooks list the 6CA7 as a power pentode.

[pdf64]

I’ll try to get around to making some comparative open loop output impedance measurements for the amp with EL34 and 6CA7, to see if the apparently differing designs result in a differing output impedances.

[Helmholtz]

Here's what I see with a speaker load and a 33p cap across the 27k NFB series resistor, using a 400Hz square wave test signal:



Zooming in shows that the oscillation frequency is just above 440kHz (!):

[Helmholtz]

Did some more testing with speaker load and different square wave signal levels and found instability also without compensation. So that's new.

But - EUREKA - I finally found a simple solution for both load types.

I had tried a number of different cap values from 33p to 250p across the 27k NFB resistor, but none of them helped.
Until I finally found a 22p cap and that indeed stops the sustained ~110kHz oscillation with resistive load as well as the strange 440kHz oscillation with speaker load.
A 15p cap didn't, though.
There's still considerable square wave ringing with resistive load but that can be greatly reduced by the series RC filter between the PI plates I mentioned earlier.
Not sure if I will use it, as it's not required with speakers.

Interesting to note, the 110kHz show as a peak in the CL frequency response, while the 440kHz with speaker load don't.
I looks as if the 440kHz resonance only develops with a moving speaker voice coil, which presumably changes/modulates speaker HF impedance.
The delayed oscillation in the scope pics seems to confirm this theory.

[pdf64]

I've eventually got around to checking and comparing output impedance of a '50W' type amp, 3k2 primary OT, operated open loop, with my pairs of Mullard xf4 EL34 and GE 6CA7.

EL34 6CA7
1W 80 50ohms
20W 28 33
max sin 32 28
max sqr 14 23

I hope the format on that comes out ok
Edit - nope, see table
The 6CA7 seem a bit weaker than the EL34, output power is lower, I suppose they may be nearer to the end of their useful life. That may account for the discrepancy in output impedance when fully overdriven .
Otherwise, they seem pretty similar.
It seems impressive to me that GE managed to make a valve with such similar performance using beam plate, rather than suppressor grid, technology.

I used Dave H's method to calculate output impedance, as per post #9 of https://music-electronics-forum.com/...tput-impedance At lower power levels, the method becomes very sensitive to the tiniest variation of measured voltage. So in reality, the 1W results are probably pretty much identical.

power output	EL34	6CA7
1W	80 ohms	50 ohms
20W	28	33
Max sine	32	28
Max square	14	23

[Helmholtz]

I've eventually got around to checking and comparing output impedance of a '50W' type amp, 3k2 primary OT, operated open loop, with my pairs of Mullard xf4 EL34 and GE 6CA7.

I'm surprised by the low output impedance values.
With an OT impedance conversion factor of 100, 30 Ohm output impedance would mean an effective tube ra of only 3k.
That's assuming you measured at the 8 Ohm output.
What measuring frequency did you use?

Did you see the output impedance plots in Zollner's book?

[pdf64]

With an OT impedance conversion factor of 100

Can you explain that? I don't understand where 100 has come from.
The OT voltage ratio is 20.1 for the 8ohm output and load used, so I thought that impedance ratio would be x404, so 12k (= 30 x 404) at the anodes?
The results are similar to what I've got previously, in the thread linked.

What measuring frequency did you use?

IkHz

Did you see the output impedance plots in Zollner's book?

Yes, but a while ago, can't recall the details.
Without an index, it's so hard to find stuff just from the table of contents.
What section are the plots?

[Helmholtz]

Can you explain that? I don't understand where 100 has come from.
The OT voltage ratio is 20.1 for the 8ohm output and load used, so I thought that impedance ratio would be x404, so 12k (= 30 x 404) at the anodes?
The results are similar to what I've got previously, in the thread linked.

A 3.2k/8 OT has an impedance conversion ratio of 400 at the full primary or 100 at each half-primary.
A voltage ratio of 20:1 for full primary means 10:1 for each half-primary (as voltage transforms proportional to turns number).
Impedance ratio is voltage (or turns) ratio squared.

[Helmholtz]

Yes, but a while ago, can't recall the details.
Without an index, it's so hard to find stuff just from the table of contents.
What section are the plots?

https://www.gitec-forum-eng.de/wp-co...-impedance.pdf