Stability issues with my original 1966 JTM 50

[Helmholtz]

This is a very early JTM 50 (model 1963 Super PA Amplifier).
It has the original JTM 45 circuit/components, uses a GZ34 rectifier and came with a pair of original GEC KT66s.
But the OT is a Drake 784-128 with a primary impedance of 3.5k, meaning an EL34 OT.
(The correct OT for KT66s would have been an 8k Drake 784-103.)
Furthermore the OT was wired totally wrong, which confirms that it must be a replacement.

I fixed the OT wiring and a few other issues, modified the bias circuit and fitted EL34s.
Now the amp is working fine and puts out around 48W at the onset of clipping.

Here's the issue:

While the amp looks stable on a speaker load, there is a ~100kHz oscillation with a resistive load at low presence settings even without signal.
(That behaviour is totally different from what I saw with the Vibrolux Reverb, where instability only showed with a speaker load.)

I tried different compensation methods and could stop the oscillation on the load resistor.
BUT anything that increases stability with the resistor seems to cause instability with the speaker load. I saw just the opposite with the Vibrolux.

Will update when I have further results.

The most sensitive method I found to detect instability is to feed a square wave of maybe 400Hz to the PI input and vary signal level, as oscillation often only shows at specific levels..

[pdf64]

Do you think it’s due to the OT, eg excessive phase shift or a resonant peak in the response?
Doesn’t that OT have a 100V secondary output option?

[Helmholtz]

Do you think it’s due to the OT, eg excessive phase shift or a resonant peak in the response?
Doesn’t that OT have a 100V secondary output option?

Well yes, but all OTs have more than one resonant peak above 30kHz and phase goes up and down.
I've measured frequency and phase response of a number of OTs but don't feel able to predict stability in the amp.
The OT does have a 100V secondary.

I think the main problem is the large feedback ratio of the amp.

[Chuck H]

Is it possible that the large feedback ratio is intended for the kt66 tubes and their associated higher primary impedance OT? Maybe the ratio just needs to be adjusted for the OT and power tubes you have in there (probably less NFB).

[Helmholtz]

Is it possible that the large feedback ratio is intended for the kt66 tubes and their associated higher primary impedance OT? Maybe the ratio just needs to be adjusted for the OT and power tubes you have in there (probably less NFB).

Good point!
Just saw that the EL34 version of the JTM 50 used a 47k NFB resistor instead of the 27k here.
Will calculate the difference in power stage gain between EL34s/3.5k and KT66s/8k tomorrow, too tired now...

[Helmholtz]

Here's the frequency response of the OT:




It is an original Drake 784-128 from the 60s, which is the first EL34 OT Marshall used. From around 1969 it was replaced by the Drake 784-139 which no longer had the 100V winding.

[Helmholtz]

Power stage gain comparison:

Power tube gain at medium output can be roughly calculated from G = gm * ra.

For EL34 gm = 11mA/V, ra = (3500/4) Ohm = 875 Ohm, so G = 9.6 .
For KT66 gm = 7mA/V, ra = (8000/4) Ohm = 2000 Ohm, so G = 14 .

The plate signal gets stepped down to the 16 Ohm secondary by the square root of ra/16, giving 7.4 for the 3.5k OT and 11.2 for the 8k OT.

So total power stage gain is 1.3 with EL34s and 3.5k OT vs 1.25 with KT66s and 8k OT.

Means that power stage gain (and thus total open loop gain) is roughly the same for both cases.

[Chuck H]

Please excuse my lack of technical education, but in case it helps I will observe that these figures are (I assume) calculated with a purely resistive load. Might things change with the difference in the OT's primary impedances and how this affects damping factor @ frequency when using an actual speaker. In other words, could there be a greater difference in phase @ frequency between the two (el34's/3.5k and kt66/8k) affecting loop stability at 100kHz ?

At any rate, you could still try decreasing NFB to levels more commonly used for 2xEL34 Marshalls. Maybe Marshall recognized the same issue you're seeing and this is why most 2x EL34 Marshalls use less feedback.?.

I guess I could also observe that since the amp is said to be stable played into a speaker this isn't a problem, per se, since you probably don't play the amp into a resistor But at your level of electronics prowess I suppose the instability into a resistive load could cause anomalies in bench test results?

[Helmholtz]

Might things change with the difference in the OT's primary impedances and how this affects damping factor @ frequency when using an actual speaker. In other words, could there be a greater difference in phase @ frequency between the two (el34's/3.5k and kt66/8k) affecting loop stability at 100kHz ?

I definitely expect the 8k OT to make a difference. Not so much because of a different damping factor but rather because of different frequency and phase response of the OT.

At any rate, you could still try decreasing NFB to levels more commonly used for 2xEL34 Marshalls.

Sure, but I don't want to change the original circuit as I'm actually after a JTM45 sound and am looking forward to test the amp with KT66s (never really cared about EL34s).
Also the amp seems to be stable with a speaker load.
In any case the oscillation stops with presence above 12 o'clock, because that lowers the NFB ratio.

Maybe Marshall recognized the same issue you're seeing and this is why most 2x EL34 Marshalls use less feedback.?

Most certainly.
Interestingly, Zollner reports a 150kHz instability with a JTM45 using KT66s and an 8k OT at low presence setting.

[pdf64]

As you’re not keen on EL34, how about 6CA7, aren’t they supposed to be a beam type pentode?

[Helmholtz]

As you’re not keen on EL34, how about 6CA7, aren’t they supposed to be a beam type pentode?

No experience with 6CA7s in a Marshall. Do you like them?
Might have some used GEC KT77s in my basement.

But I know that I love the sound of a JTM45 with original KT66s and I have a few GECs.
I think this amp deserves them.- even if I have to change the OT .

[g1]

Might have some used GEC KT77s in my basement.

How old?

[Helmholtz]

How old?

70s or early 80s. Why do you ask?

[g1]

70s or early 80s. Why do you ask?

Some of them go for high $. The one's I have from 60's with nice painted logo I am told can go for around $600/pair. Of course used ones less, but still can be valuable.

[Helmholtz]

Some of them go for high $. The one's I have from 60's with nice painted logo I am told can go for around $600/pair. Of course used ones less, but still can be valuable.

I would have thought that original KT66s are more valuable?