two el34s paired with a 100 Watt OT

[pdf64]

Ah ok, there were a few unlabeled pics from an email I sent them months back. Here's the OT then!

How have you ascertained that OT is a 1750R?

[Helmholtz]

The other thread I linked above suggests that original primary Zaa is 3.8K.
Hammond replacement would be 1750RA: https://www.hammfg.com/files/parts/pdf/1750RA.pdf

[pdf64]

But do we know that the model reference of the amp in question is YBA-1A? I can’t see where that’s been stated.

[Helmholtz]

I understand the amp has an original and working OT.
Question is about its Zaa.
2k doesn't make sense with 2 tubes. 3.8k would be much more likely..
Of course measuring conversion ratio would be best/safest.

[loudthud]

I understand the amp has an original and working OT.
Question is about its Zaa.
2k doesn't make sense with 2 tubes. 3.8k would be much more likely..
Of course measuring conversion ratio would be best/safest.

The Hammond site isn't working properly. I think part of the problem is that the Traynor part number changed over the years. One thing says the 1750RA was Traynor P/N A1303. In post 13, the pic says A1301. I can't access other pdf data sheets on Hammond's site.

There are at-least three eras for the YBA-1A. The earliest is the rounded cabinet style pre 1970. Then there is the Rubber Baby Bumper years like the pic Guitarist posted, and the last series with rubber bumpers and sloped front panels similar to Fender Blackface with removable cabinet top. I think the first two used a power transformer that was essentially Hammond 278CX. Same transformer as the YBA-3 (160W). AFAIK the YBA-1A and YBA-3 always had a fan.

[Guitarist]

Thanks for the info. I do believe they installed a 100 Watt OT, yes. So am trying to learn how to make the best of it. The back panel isn't labelled with Ωs load IIRC.
As Hammond is likely correct with their recommendation, then this is now equivalent to their 1750R and in '72 it was A1301. It has 2000Ω Primary.
Both amp and parts model exactly match their sheet.

Serial #: 3040897 YBA-1A Mark II 1972

I may just add a variable resistor for feedback.

I am alI suppose I could install a 4Ω reactive load prior to the output jack I have the parts. I wonder if that would get induced hum.
What would you recommend? One advantage is that the power transformer is so oversized that it can handle anything, even KT150s which put out 140 Watts a pair. But I don't want to blow up my '72 Greenbacks or other speakers either!
But headroom is great; I loved my Ampeg VT22s and later VHT 4U rack power amp.

Thank you.

[Chuck H]

if the OT were for a 100W amp and had a 2k primary impedance you could just run the amp into a 2x load impedance and your OT will reflect 4k to the pair of power tubes. Entirely acceptable for el34's and 6l6's.

The Hammond replacement OT is already appropriate for a pair or tubes rather than a quad.

[Helmholtz]

Ok, I'll try again.

A 100W / 2K OT is meant for 4 tubes, period.
No serious amp designer would specify 2k with 2 tubes.
Zaa = 2k with 2 tubes means excessive tube currents/dissipation and short tube life especially with that high B+.

You need to measure your OT to be sure!

I don't doubt it's a 100W OT. That means it can safely handle 100W.
A 100W OT is likely to have somewhat better efficiency than a 50W OT.
So it might give maybe 5% more output than a 50W OT in the same amp.

High power mainly comes from high B+.
With your voltages and a primary impedance of 3.8k the amp might put out around 85W clean with EL34s (so a 50W OT won't do here).
To be sure you need to measure power.

The impedance ratio of the OT affects the NFB.
Lowering the NFB series resistor might cause instability.

[mozz]

A lot of guitar amplifier output transformers are not "optimized" for the amplifier. That does not mean it is not possible. Maybe the designer wanted a 100w output transformer and all available at the time was 2k, so he used it and it worked. Could even be a typo in the spec sheet. If I felt enthused, I would test mine and post the reading. Possibly why he left the 16 ohm tap hidden. Most cabinets would be 8 ohm or more common 16, that would make it see 4k.

I've seen tweed fender champs 6v6SE with 16k output transformers into 4 ohms so that would be 8k. But they also used lower value that would be more like 4k, they both work.

It may be over dissipating at 8 ohms, there is a fan, how do you calculate how much above spec you can run a tube with a semi forced air cooling?

[Helmholtz]

To me all this guessing and speculating seems like a waste of time.
A simple voltage measurement would definitely tell.

As said, it makes a difference to NFB if it's 2k or 3.8k.

[Guitarist]

OK, I'll try and measure the ratio over this week. My affordable VOM has a bit of a scope built in. But I'll try a couple things: Ohms first then inserting a test tone (measuring the source AC level) then compare the Secondary AC signal...

thanks!

[Helmholtz]

OK, I'll try and measure the ratio over this week. My affordable VOM has a bit of a scope built in. But I'll try a couple things: Ohms first then inserting a test tone (measuring the source AC level) then compare the Secondary AC signal...

DCR is not related to impedance. No scope needed here, only AC voltmeter.

To find the voltage conversion ratio:

- Disconnect all OT wires from amp (make notes where each wire goes or take pics). Don't forget to disconnect the ground wire of the secondary.
Make sure no wire can touch an other one or anything else (e.g. by taping the isolated wires to a piece of wood).
- Pull power tubes.
- Connect the amp's heater voltage to the secondary and measure voltage after connecting.
- Measure voltage across the full primary (might be around 150VAC, so don't touch primary wires or CT).

Divide primary voltage by secondary voltage and square the result. This gives the impedance ratio.
Multiplying with rated output impedance (8 Ohm) gives the primary impedance.

[g1]

Traynor used the A1301 OT in two models of amp, the YBA1A and the YGL3.
The YGL3 has 4 EL34's and is labelled as 4 ohm output. (see schematic attached)
The YBA1A uses 2 EL34's and is labelled as 8 ohm output.
I think it is safe to say that for the YBA1A, the OT primary is effectively 4K primary.

The 1750R is designed for the YGL3 usage, it will work for the YBA1A but you should use the impedance tap that is half the load value.
The 1750RA is more suited to the YBA1A if you are matching impedance tap to the load.

Serial #: 3040897 YBA-1A Mark II 1972

The first 3 digits of the ser. # are the date code. That ser.# came off the assembly line the 4th month of 1973.

[Helmholtz]

Now that would perfectly explain the confusion about OT primary impedance.
So they used the same OT either as 2K/4 Ohm with 4 tubes or as 4k(3.8k)/8 Ohm with 2 tubes.