Loud squeal (oscillation?) after replacing output transformer

[slidincharlie (Carlo P)]

My '59 Gibson GA18-T amp (2 x 6V6) has got a tiny output transformer. Not that it sounds bad, but I'd like to get some more volume and balls out of it.
So I put a tweed Deluxe-style Hammond output transformer in it.
Problem:
the amp squeals like crazy with the new tranny, even with the volume at zero.
Could it be an oscillation issue due to bad wire layout? I tried to replicate the old layout... I tried to move around the new wires with the amp on (and with aching ears) to no avail.
I attach two photos. The chassis photo shows where the central tap and the primary wires are laid.
Is there a way to chase oscillations? Or could the new tranny be no good for the amp?

[martin manning]

I'm going to guess that you have positive feedback going on. Try swapping the connections (just the wires from the transformer) at the outer primary wires or the secondary wires to change the phase of the output by 180 degrees.

MPM

[Daver]

Try swapping the OT leads to the power tubes. You may have created postive feedback when replacing the OT. I think the GA-18T uses negative feedback from the power amp. The new OT's start and finish of the windings are probably different from the original one.
Dave

[jrfrond]

You do not have to go near the tube sockets. Reverse the secondary leads to the speaker. Yes, you Do have negative feedback going on.

[Enzo]

As long as the NFB wire stays where it is now.

[drewl]

You know you may have positive feedback
Switch the plate leads......I've had to do this more times than I can count especially on builds.

[slidincharlie (Carlo P)]

Fixed!

Fixed!
I reverted the plate wires from the OT and the squeal went away.
Thanks everybody.

[jrfrond]

You do not have to go near the tube sockets. Reverse the secondary leads to the speaker. Yes, you Do have POSITIVE feedback going on.

Oops! Brain fart.

[Daver]

Glad it worked Carlo. Now, how does it sound with the new OT?
Dave

[slidincharlie (Carlo P)]

It sounds GREAT!
Ranges from mellow pussycat tones to a growling roar...
And louder than before...

[Enzo]

Can't think about the ones that got away, John, we have to think about the ones we brought home.

[TD_Madden]

It sounds GREAT!
Ranges from mellow pussycat tones to a growling roar...
And louder than before...

Charlie, was that updated transformer a direct-fit, or did you have to drill? I'm almost afraid to give my Explorer any increased power since it has the original P10R still in it...guess I could pack that speaker away and replace it as well!

[slidincharlie (Carlo P)]

TD,
I had to drill one hole to accomodate one of the screws. There's more than enough room to host the Deluxe tranny.
My amp came with a replacement mid-60s CTS alnico speaker, probably one of those that went into the Super Reverb.
I guess that this amp would sound gorgeous with a good 12" alnico speaker and the upgraded OT...

[bloom]

You know you may have positive feedback
Switch the plate leads......I've had to do this more times than I can count especially on builds.

I had the same squealing problem and I just reversed the plate lead and it is working great. Wonderlful! It's been couple weeks I was working on my amp and I was really panicked when I heard that noise. Thanks a lot guys!