trying to make this OT work

[Luthierwnc]

Hi All,

I'm working on a 2nd Gen 50-watter. Years ago I got what I believe was a NOS output transformer originally built for ADA. It was supposed to be good for 80 watts and judging by the weight I believe it.

My concern is that in the wall-socket test, the impedances are right if you use the old-school assumption that a pair of 6L6's running A/B1 at 450 VDC (which is what it will be) has a Z-Out of 5.6k ohms. The more modern assumption of around 4.5k ohms puts me off a bit.

Here's the math: 122 VAC wall voltage into the outside primary windings gives me the following volts on the secondaries. The third column divides those into 122v for the turns-ratio. The 4th column squares that for the impedance-ratio and the last two columns divide assumed primary impedances by the impedance-ratio.

At 5k6, Green&yellow is pretty good for 8R and Yellow works for 4R (those are the two I need). At 4k5 (or less), I'm about 75% of ideal. I'm not sure that's enough to go buy another OT but if anyone has compelling reasons to consider, I'd appreciate your thoughts.

Thanks, sh

[Structo]

I would use the OT you have.

No sense replacing it before you have played the amp for a bit.

[Luthierwnc]

Hi Tom,

It won't explode -- that's important.

I'm also wondering about the grounding on this. My plan was to use a multi-section cap for B+2 through B+5. The shared ground was going to go to the base of the buss. Node B+1 would be grounded at the chassis ground.

In looking over some layout diagrams on 2nd gen and others, a lot of people separate individual filter caps with B+5 and B+4 near the input jack and B+3 and B+2 where I mentioned. The preamp cathodes also go to those places respectively. In other cases, all secondary nodes go together.

The questions are; whether I need to separate the filter and cathode grounds of 2/3 and 4/5 or if there is a place they can all be happy together? If the former, I'll need to rethink the cap board before I start drilling holes.

Cheers, sh