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

[Helmholtz]

https://www.hammfg.com/files/parts/pdf/1750RA.pdf (100W/3.8k)
https://www.hammfg.com/files/parts/pdf/1750TY.pdf (50W/3.8k)

And it's a Gibson OT?

How this?

[mozz]

Yba-1a. The one he listed is there. 1750 RA. It's a 100w transformer.

[Coop]

I removed the OT. Removed its end bells. It looks pretty cooked. Took some some resistance readings, which is practically the same as before. I peeled back the tape, but didn’t see any breaks that’s fixable. Looks like it burnt somewhere in the middle of the windings.

mozz, I’d consider selling it, but I wouldn’t want to ship it.

[g1]

It's interesting that Hammond call both the 1750R and 1750RA replacements for the original Traynor A1301.
The R is 2K primary and the RA is 3K8 primary. I wonder which is closer to the original.

[Helmholtz]

It's interesting that Hammond call both the 1750R and 1750RA replacements for the original Traynor A1301.
The R is 2K primary and the RA is 3K8 primary. I wonder which is closer to the original.

An Raa of 2k is not appropriate for 2x EL34/6CA7. Tubes would overdissipate at medium output.
But fine with 4 tubes. And this is a 100W OT, so likely for a quartet.

Generally a 100W OT will not noticeably increase output of a 50W amp.
It might improve bass response, though.

[mozz]

You've got output taps on the first one. I know my YBA-1A only has 1 output, and i remember looking all over and nowhere does it tell you what impedance, so i take it 8 ohms. I did have a output transformer monster here from a 8417 amp years ago, gave it away to a friend who wanted to build a amp. He never built it so i asked for it back if he wasn't going to use it, now he says he can't find it.

I would guess just about any 100w output trans would work.

[g1]

An Raa of 2k is not appropriate for 2x EL34/6CA7. Tubes would overdissipate at medium output.
But fine with 4 tubes. And this is a 100W OT, so likely for a quartet.

Agree it is not appropriate. Just wondering what actually was original. Sometimes they did things that were 'wrong' because they had parts for something else, or maybe assumed most users would be running 2 cabs.
As far as I know, the R was sold as the replacement before the RA came along. And the RA may well have been introduced when someone told them 2K was 'not right' for the application.
Measurement of an original would show.

[Helmholtz]

or maybe assumed most users would be running 2 cabs.

That would make things even worse with an Raa of 2k.

[g1]

That would make things even worse with an Raa of 2k.

Yes, I was thinking about that backwards.

[Chuck H]

Ok, well... These are images of actual transformers in the YBA-1 amps (which clearly differed from year to year). It's pretty clear that the current Hammond offerings are short of the effort and wouldn't be direct replacements. As noted in the current Hammond description they have fudged a bit and only matched mounting where it seemed practical/(read profitable) Look at the iron stacks!!! Holy cow!

[loudthud]

The pic Chuck H posted on the right is the original YBA-1A with the "slide in" chassis from about 1969, although it's rare to see them with a choke. When they changed over to the "flip top" chassis about '72, the OT got smaller like the 1750RA. 3.8K primary at 100W is correct for the YBA-1A. Note: If you can find the big OT like the one on the right, there is a 16 Ohm secondary with the wire cut short. You can remove the end bell and splice a wire to the 16 Ohm tap.

You can see where the power transformer has been replaced in the right pic. The original power transformer was identical to the Hammond 278CX. They are so heavy, they can rip loose when they are hanging upside down. The same power transformer was used in the YBA-3 (160W) which had four EL34s and a 2K OT.

Don't confuse the YBA-1 and the YBA-1A. The YBA-1 (Bass Master) is 45W with 430V B+, the YBA-1A (sometimes called Bass Master Mk II or just Mk II) is 90W with 560V B+ and a fan.

[Helmholtz]

Looks like different tubes as well.
90W with two EL34s and an Raa of 3.8k is not possible (without frying the tubes).
No problem with 6550s.

[mozz]

I've measured 85 watts clean with mine and about 120w clipped. 1kHZ. Pretty sure mine is the 3.8k version. 6CA7's. Plate voltage is about 580v. I don't know what they were biased at and do not know how long they would last.

Agree the second picture shows KT 77/88 or something different. These are rich man's Marshalls.

[Helmholtz]

I've measured 85 watts clean with mine and about 120w clipped. 1kHZ. Pretty sure mine is the 3.8k version. 6CA7's.

I should have said "not possible with a solid design".
I know that two EL34s/6CA7s can deliver in excess of 90W.
But that requires an Raa of 11k and screen voltage not above 400V (with a B+ of 800V)
Under these conditions and an idle current of 25mA max the tubes will not overdissipate.

But an Raa of 3.8k with high B+ and screen voltage (in combination with little PT sag) means severe overdissipation with signal and thus short tube life, no matter how cold the bias.
I wouldn't call this a solid design.

Typically lower Raa at high B+ means more power, but at the cost of high plate signal current and dissipation.
Max. safe B+ with an Raa of 3.8k is 500V, giving up to 70W clean output.
Higher B+ necessitates a higher plate load to tame plate currents.

Agree the second picture shows KT 77/88 or something different.

A KT77 looks like an EL34 and has similar specs.
KT88 or 6550 would be a good match for this amp.

[g1]

90W with two EL34s and an Raa of 3.8k is not possible (without frying the tubes).

That is the common response. I see that you clarified to say not reliable (for tubes). But they were not prone to eating tubes and Traynor warranty has always been extremely generous.
I don't know exactly how they got away with it, but they did. And they were no harder on tubes than other high power amps.
This seems to be one of those situations where the theoretical and practical are far apart.