replace output transfomer question

[BonesMachine]

i have a very novice question. i am replacing an output transformer on a 70's fender bassman amp. i have a new hammond 1750w replacement. it has secondary black (com) and white wires. i am not sure which wire hooks up to the positive of the speaker output jack on the amp black or white? any help appreciated thanks. the original ot that is in the amp has secondary black hooked up to speaker jack positive and a green wire hooked up to speaker output jack negative

[sluckey]

I would connect the black to the ground lug on the speaker jack and white to the tip lug of the speaker jack. If the amp squeals or howls swap the black and white wires.

https://www.hammfg.com/files/parts/pdf/1750W.pdf

[Ten Over]

i have a very novice question. i am replacing an output transformer on a 70's fender bassman amp. i have a new hammond 1750w replacement. it has secondary black (com) and white wires. i am not sure which wire hooks up to the positive of the speaker output jack on the amp black or white? any help appreciated thanks. the original ot that is in the amp has secondary black hooked up to speaker jack positive and a green wire hooked up to speaker output jack negative

If you have BRN connected to V4 + V5 and BLU connected to V6 + V7, then I would connect BLK to the speaker jack positive and WHT to the speaker jack negative.

[Stevem]

White is by the spec sheet said to be hot .

If you already have black as hot and the jack is bolted to the chassis it can take a ton of heat to swap the wires there if you find that the amp is hollowing.

In that case just swap the blue and the brown wires since to the rest of the amp circuit and the speakers it's the same.

[pdf64]

Once the signal polarity relationship between primary and secondary is known, it’s simple to get it right first time.
Every push pull valve guitar amp with an LTP and a feedback loop uses the same topology, apart from the AB165 Bassman.
It’s as set in stone as the inverting and non inverting inputs of an opamp.

[Ten Over]

White is by the spec sheet said to be hot .

If you already have black as hot and the jack is bolted to the chassis it can take a ton of heat to swap the wires there if you find that the amp is hollowing.

In that case just swap the blue and the brown wires since to the rest of the amp circuit and the speakers it's the same.

The secondary is a single winding with no intermediate taps, so it has no inherent common and hot. It does have a polarity, however, and how the primary is wired determines which way the secondary should be wired.

Bassman 100's tended to have the BLU and BRN wires reversed compared to Twin Reverbs and other big amps. The OP has already told us that his amp is wired like a typical Bassman 100 as opposed to a Twin. That means that BLK would have to be hot in order for the output signal to be in phase with the LTPI input.

[BonesMachine]

i dont feel so bad now i guess not that obvious the amp likes the ot black wire going to speaker jack tip. the other way squeals. hope that doesnt cause any damage? so the ot primary blue and brown are the same thing correct and doesnt matter which one goes to what tube socket?

[Ten Over]

so the ot primary blue and brown are the same thing correct and doesnt matter which one goes to what tube socket?

It matters. BRN goes to V4 + V5. BLU goes to V6 + V7.

[BonesMachine]

ok thanks thats how i have them hooked up. if its not too complicated to explain just wondering what happens if they are swapped?

[Ten Over]

ok thanks thats how i have them hooked up. if its not too complicated to explain just wondering what happens if they are swapped?

If you swap the BRN and BLU wires, then you get positive feedback instead of negative feedback.

Bassman 100 NFB Arrows.png

The red arrows show the phase of the audio signal. The up arrow at the grid of V3A changes to a down arrow at the plate because that stage inverts the signal. The down arrow at the grid of V4 changes to an up arrow at the plate because V4 also inverts the signal. The dots on the OT indicate that the phase on the BRN wire will be the same as the phase on the BLK wire. The up arrow on the BLK wire ends up as an up arrow on the grid of V3B. V3A and V3B form a differential amplifier, so it amplifies the difference between the two grids. Since both grids have signals of the same phase on them, the difference is less than if only one of the grids had a signal. The diminished differential signal reduces the output of V3B, so this is negative feedback.

Bassman 100 PFB Arrows.png

If you reverse the BLU and BRN wires, then The BLU wire is out-of-phase with the BLK wire. Now there is a down arrow at the grid of V3B which puts it out of phase with the grid of V3A. Since they are out of phase, the difference between the two signals is greater than if only one grid had a signal on it. This increased signal causes the output of V3 to increase which increases the differential signal at the grids which increases the output even more which .... This is positive feedback and it results in that terrible screech.

[pdf64]

Though it usually does in most Fenders, positive feedback might not cause oscillation. It can just result in a different tone, and stuff like presence controls acting weird.

[BonesMachine]

thanks for the explanation. i was just thinking power supply not signal phase