New Output transformer - Need to Change Tube Bias ?

[Dave H]

As another test, I connected an alligator lead from the speaker terminal with the green wire attached, directly to the chassis ground and it doesn't change a thing in the gain or sound.

So I no longer believe the mysterious green wire is part of a negative feedback loop, but just a chassis ground for the speaker.

You could have saved time by first checking with a DMM to see if the mystery wire was grounded

For it to have negative feedback there would need to be two extra wires, one to ground one side of the speaker and a second from the other side of the speaker back to the amp's NFB input.

[Helmholtz]

Question : Are speakers generally grounded through the chassis ? May sound stupid considering I've worked on tube amps for a few years now, but I always thought the speaker voltage was just carried through the low volt side of the transformer after the excitation of the ouput transformer in a loop, and that the voltage was insulated from ever interacting with the chassis ground !

In most tube amps one side of the OT secondary is connected to the amp ground/chassis. (This way also one end of the speakers voice coil is grounded.)
While this is mandatory for amps with global NFB, it is not necessary without NFB but may help to avoid oscillation and guitar PU feedback issues. It is even better to also ground the speaker frame (especially in combo amps) as this provides some shielding of the voice coils.

[Helmholtz]

You can see in your photos that the secondary of the OT is wired directly to a speaker, and then two wires put the other speaker in parallel.

Yes, the speakers appear to be wired in parallel - but out-of-phase.

I thought they should be wired in series for 8 Ohm with the new OT (post #1)?

[Chuck H]

Yes, the speakers appear to be wired in parallel - but out-of-phase.

I thought they should be wired in series for 8 Ohm with the new OT (post #1)?

That isn't Harold's amp. It's a photo I found on line and posted to help the discussion of the mystery wire.

[Helmholtz]

That isn't Harold's amp. It's a photo I found on line and posted to help the discussion of the mystery wire.

Oh, I see. Thanks.

One more remark: Some speakers like the ones in the photo have their minus terminal riveted to the speaker chassis. This allows only one of speaker chassis to be grounded in a series arrangement. Otherwise one of the speakers would be shorted.

[Chuck H]

I remember a build where I needed to ground the speaker frames to eliminate a reverb oscillation. I thought it was strange at the time. I suppose there may have been interaction between the reverb tank transducers and the speaker coils being fed back into the amp. The extra lead is a cumbersome implementation and if I get the amp back on my bench I may try reversing the reverb tank phase.

[g1]

One more remark: Some speakers like the ones in the photo have their minus terminal riveted to the speaker chassis. This allows only one of speaker chassis to be grounded in a series arrangement. Otherwise one of the speakers would be shorted.

Also causes problem when used with a bridged type amp and the frame is grounded. Fender has cautioned about this for one of their Mustang amps as they run a ground wire to the frame for shielding. ("There are speakers on the market that have the negative terminal common with the frame of the speaker – these speakers CANNOT be used in a Mustang amplifier, as this will result in damage to the power amp.")