JCM900 Frequency Doubling Weirdness

[Ted]

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

Please triple check what nickb says.

The centre tap IS the centre tap. 44/42 ohms to plates 86 ohms plate to plate.

I'm beginning to suspect that I'm doing something stupid. WOuldn't be the first time. But I need to work out what.
( I should say that I did try playing a guitar through the amp and it did sound terrible).

[nickb]

The centre tap IS the centre tap. 44/42 ohms to plates 86 ohms plate to plate.

I'm beginning to suspect that I'm doing something stupid. WOuldn't be the first time. But I need to work out what.
( I should say that I did try playing a guitar through the amp and it did sound terrible).

Wild suggestion: Does the power full/half switch have any effect on the problem?

[Helmholtz]

Can you see signal voltage at the OT primary CT? Maybe a bad filter cap (connection) causes some multiplication effect of the half-cycles?

[alexradium]

did you try disconnecting feedback?

[Ted]

Changed the filter caps. No effect.

[Ted]

Feedback disconnected.
No Effect

[Ted]

The effect is still there in half power mode

[Chuck H]

Have you tried different input frequencies with the same result? Have you tried another pair of tubes?

[Ted]

OK. Got it. I'd already changed output transformers before I started poking around with the scope - I'd injected 2VAC into the transformer secondary and the voltage on the primaries looked a little unbalanced, so I swapped it out with a brand new Marshall part.

Foolishly, I assumed that the new, straight-from-Marshall transformer would be OK...
When I measured the CT-plate and plate-plate resistances everything looked sensible.
However I tried the 2VAC on the secondary thing again. Was getting sensible voltages from CT-Plate on each side.

However plate-plate was showing almost nothing. A quick look at the plates with the scope...
Both sides of the primary are in phase.
I said a bad word. Several times. Tried an old Laney 50W transformer that I had on the shelf, and all is rainbows and unicorns.

I'm going to have a word with the parts department at Marshall in the morning.

Thanks for your help everyone. Thought you'd like to know how this one turned out.

[nickb]

OK. Got it. I'd already changed output transformers before I started poking around with the scope - I'd injected 2VAC into the transformer secondary and the voltage on the primaries looked a little unbalanced, so I swapped it out with a brand new Marshall part.

Foolishly, I assumed that the new, straight-from-Marshall transformer would be OK...
When I measured the CT-plate and plate-plate resistances everything looked sensible.
However I tried the 2VAC on the secondary thing again. Was getting sensible voltages from CT-Plate on each side.

However plate-plate was showing almost nothing. A quick look at the plates with the scope...
Both sides of the primary are in phase.
I said a bad word. Several times. Tried an old Laney 50W transformer that I had on the shelf, and all is rainbows and unicorns.

I'm going to have a word with the parts department at Marshall in the morning.

Thanks for your help everyone. Thought you'd like to know how this one turned out.

Yikes, again!

So glad you found it. That was subtle.

So why the same problem with the original transformer?

[Ted]

Yikes, again!

So glad you found it. That was subtle.

So why the same problem with the original transformer?

It wasn't the same problem - the problem was that it sounded terrible - weak and distorted. I hadn't looked at the original transformer with the scope. It was a couple of days between trying it out with each transformer. So it probably sounded terrible but in a different way. The human brains memory for audio is around 15 seconds apparently.

[Helmholtz]

OK. Got it. I'd already changed output transformers before I started poking around with the scope - I'd injected 2VAC into the transformer secondary and the voltage on the primaries looked a little unbalanced, so I swapped it out with a brand new Marshall part.

Foolishly, I assumed that the new, straight-from-Marshall transformer would be OK...
When I measured the CT-plate and plate-plate resistances everything looked sensible.
However I tried the 2VAC on the secondary thing again. Was getting sensible voltages from CT-Plate on each side.

However plate-plate was showing almost nothing. A quick look at the plates with the scope...
Both sides of the primary are in phase.
I said a bad word. Several times. Tried an old Laney 50W transformer that I had on the shelf, and all is rainbows and unicorns.

I'm going to have a word with the parts department at Marshall in the morning.

Thanks for your help everyone. Thought you'd like to know how this one turned out.

Wow, who would/could have thought of something like this? It means that one of the 2 primary halves was connected at the wrong end to the CT. In result the signal voltages of the 2 power tubes are not added as supposed but substracted instead.

Good job, though!

[nickb]

Wow, who would/could have thought of something like this! It means that one of the 2 primary halves was connected at the wrong end to the CT. In result the signal voltages of the 2 power tubes are not added as supposed but substracted instead.

Doesn't say much for the transformer manufacturer's final QA process, does it?

[J M Fahey]

Just curious what brand was that transformer?
By your words, not a Marshall original part (meaning Drake or Dagnall)

I will laugh all the way to China and back if it turns out to be an expensive MM though

[Ted]

Just curious what brand was that transformer?
By your words, not a Marshall original part (meaning Drake or Dagnall)

I will laugh all the way to China and back if it turns out to be an expensive MM though

It was a Dagnall, bought direct from Marshall.