JCM900 Frequency Doubling Weirdness

[Ted]

50W JCM900 Combo.

I'm injecting a 1000Hz signal. I'm getting a 2000Hz signal at the speaker. The scope says the signal on each EL34 grid is 1000Hz.
All grounds have been cleaned, different sets of valves tried, signal is being injected on 1st PI grid. (The effect was visible when running through the pre-amp, I just wanted to rule it out). Valves are biased to around 12W, DC grid voltages ~= -40V. Tried a spare output transformer. No difference..

If I pull either one of the EL34s the signal at the speaker looks as you'd expect - lopsided, but 1000Hz. With both output valves in circuit the voltage at the speaker approximately halves and becomes 2000Hz. Odd cycles slightly lower than even ones.

Suggestions?

[nosaj]

50W JCM900 Combo.

I'm injecting a 1000Hz signal. I'm getting a 2000Hz signal at the speaker. The scope says the signal on each EL34 grid is 1000Hz.
All grounds have been cleaned, different sets of valves tried, signal is being injected on 1st PI grid. (The effect was visible when running through the pre-amp, I just wanted to rule it out). Valves are biased to around 12W, DC grid voltages ~= -40V. Tried a spare output transformer. No difference..

If I pull either one of the EL34s the signal at the speaker looks as you'd expect - lopsided, but 1000Hz. With both output valves in circuit the voltage at the speaker approximately halves and becomes 2000Hz. Odd cycles slightly lower than even ones.

Suggestions?

Suggestions? To me it sounds like a capacitor timing issue introducing a slight delay effect. Do you have a scope?
Please post a schematic your working from.
nosaj

[nickb]

Can you scope the EL34 grids simultaneously using two probes?
What are the screen grids doing?
What is the OPT centre tap doing?

Please post the output and the two grid one waveforms with vertical and timebase info.

[xtian]

Can you get intermodulation artifacts with a single frequency input?

[nosaj]

Can you get intermodulation artifacts with a single frequency input?

Well the phase invertor splits the signal in 2 ......

nosaj

[Jazz P Bass]

I would swap in a different OPT.

[nickb]

I would swap in a different OPT.

He already did ...

[g1]

Sorry to ask this, but it sounds like it could be a scope setting or glitch to me (or source gen.).
Have you scoped the input and seen it to measure 1K ?

(edit: nevermind, I see you are measuring 1K at the grids)

[J M Fahey]

50W JCM900 Combo.

I'm injecting a 1000Hz signal. I'm getting a 2000Hz signal at the speaker. The scope says the signal on each EL34 grid is 1000Hz.

Please inject 1kHz signal into input.
Set scope so you have , say, 4 full sinewaves on screen, so 4 positive peaks and 4 negative peaks.

Without touching horizontal sweep controls at all, , just adjusting vertical scale and sensitivity to match different levels along the path, follow signal from input jack to speaker terminals, until you find one stage where you have eight full sinewaves on screen, 8 positive and 8 negative peaks, only reliable indication that you have 2 kHz there.

Just do not scope power tube plates, deadly voltages there and in any case completely out of range for your scope, I don´t need them if I can see waveforms at speaker terminals.

Please upload 3 screen captures: signal entering power amp. signal a PI plates or power tube grids, and speaker out waveforms.

All with same timebase settings.



All grounds have been cleaned, different sets of valves tried, signal is being injected on 1st PI grid. (The effect was visible when running through the pre-amp, I just wanted to rule it out). Valves are biased to around 12W, DC grid voltages ~= -40V. Tried a spare output transformer. No difference..

If I pull either one of the EL34s the signal at the speaker looks as you'd expect - lopsided, but 1000Hz. With both output valves in circuit the voltage at the speaker approximately halves and becomes 2000Hz. Odd cycles slightly lower than even ones.

Suggestions?

[Ted]

Good advice here. Thanks.

Just waiting for a 2nd scope probe to be delivered....

[Ted]

Both Grids:


One Grid and Speaker:

[nickb]

Both Grids:
[ATTACH=CONFIG]51907[/ATTACH]

One Grid and Speaker:
[ATTACH=CONFIG]51908[/ATTACH]

Yikes!

Are you 100% certain the center tap of the OPT is going to HT+. Check the DC resistances of the OPT to verify the CT.

[J M Fahey]

Yikes 2 !!!
True *sinewave* frequency doubling , never saw that in a power amp.

Please triple check what nickb says.

[g1]

I've never seen that either. On the flip side, you may be on the verge of a breakthrough in 'doubler' effects!

[nosaj]

I've never seen that either. On the flip side, you may be on the verge of a breakthrough in 'doubler' effects!

That Kalamazoo one I built would do something like that when you hit the string a little harder than normal sounded just like a tremelo never did get it sussed out but I suspect the output was somehow coupling with a grid.

nosaj