Tray or YBA-1A Very Low Output - Please Help

[GoldtoneWorkshop]

Sounds like they've got good B+ on all tube plates after reconnecting the 100k plate resistor.

[Helmholtz]

Anyway a shorted OT primary section won't allow for significant output.

[g1]

Those EL34s should have over 600v on them.

B+ is around 540V, so I guess you meant over 500V ?

[g1]

As Helmholtz suggested, I checked resistance between the OT CT, and the plates of each tube socket
...
V4 - EL34 - Pin 3= 0.9 ohms

Disconnect that OT wire from V4 pin3. Re-check resistance from that wire to OT CT wire.

[mozz]

B+ is around 540V, so I guess you meant over 500V ?

Searching my posts, mine is at 580v, i thought it was 625v. I know it was really up there. Another person reports 570v.

[The Dude]

......Low resistance at V4 plate means a shorted OT primary.

^^^^^^^That!

[loudthud]

In the first post it says the amp was POWERED UP after the fuse and the output tubes were replaced. How is this possible with one side of the OT shorted to ground ?

Something is not making sense here.

Are you sure it was pin 3 and not pin 2 that had a low resistance to ground ? What is the resistance from B+ to each output tube pin 3 ?

Was this amp originally equipped with a circuit breaker (push to reset) instead of a fuse ?

[Helmholtz]

In the first post it says the amp was POWERED UP after the fuse and the output tubes were replaced. How is this possible with one side of the OT shorted to ground ?

No indication that the OT has as short to ground.
The low resistance is between power tube plate and OT CT, so all makes sense.

[Coop]

Disconnect that OT wire from V4 pin3. Re-check resistance from that wire to OT CT wire.

Helmholtz. I done the resistance check as you suggested above. Disconnected OT wire from pin 3 on V4, and checked resistance between that wire, and OT Center Tap (Red wire) = I get 1.1 ohms. So I assume that means the CT of the output transformer is bad?

[Helmholtz]

. So I assume that means the CT of the output transformer is bad?

It means that one half of the OT primary has a short.

That kills most of the output.

[Coop]

It means that one half of the OT primary has a short.

That kills most of the output.

Well, at least we know what the problem is now. Looks like a good replacement is the Hammond 1750RA. which will cost around $150 USD. I don't mind spending that on it, but the amp isn't nice cosmetically. I'd probably keep it ... It would be great if I could locate a used OT. It may need a new fan too. It's making some bad noises. But, I may be able to lube it, and it be ok. Anyway, thank you for your help, guidance, and knowledge. And, Thanks to everyone else that chimed in on this one. You all are greatly appreciated!!!
Coop

[mozz]

Both transformers on those amps are monsters. That looks to be a exact replacement, gives the primary ohm as ~ 48 ohms each side. If you have it totally disconnected and aren't getting very close to that it's bad, as was said. I doubt you will ever find a factory part or even a parts amp. Best off to splurge and get the correct one. Or sell the broken beast to me.

[Helmholtz]

Can you see anything suspicious when removing the endbells?

[Chuck H]

Hmmm..? The Hammond site only has one OT offered for the Yorkville/Traynor models. And it's a Gibson OT? Seems strange since as far as I know Hammond made their bones as guitar amp transformers with the Traynor amps. There may not be a high demand for a YBA-1 OT but I would have thought they'd have offered it anyway as a nod to Hammond company history.

My own YBA-1 was a later model with the inline inputs and the weird tone stack rather than the better known Marshall/Fender clone-ish earlier version. It still sounded impressive though. I don't remember the OT being impressively large though. A little smaller than my 50W Marshall JMP amps I had at the same time. Maybe they changed the OT for the YBA-1 when they changed the circuit.

That said... I'll wager the original YBA-1 OT was probably very similar to one of Hammonds off the shelf 16XX series transformers. Does anyone know what the primary impedance was for the older YBA-1?

[Chuck H]

Okay. I just did some looking around and Hammond DOES offer a YBA-1 OT. Why it's not listed on their guitar amp transformers page I don't know. The model is 1750TY. It looks very much like the transformer that was in my amp. So, smaller and with a different footprint than the earlier YBA-1.

This, FWIW.