[Phil_S]

Greetings. I received a suggestion that I might find help here at RAT
answering questions about winding an output transformer.

I have a pair of small SE OT's. I don't know the origin. The came to me on
a derelict chassis from eBay. I harvested them and set the aside. The
other night, I thought I run some low voltage through to discover the turns
ratio. While doing that, there was a spark at one of the leads and I
promptly shut down the test. My instinct was to Ohm the windings to see if
they were OK or if I let the smoke out. Unfortunately, the meter read 265K,
so I concluded the tranny was toast, which lead me to the impulse to open it
up. Later, I discovered the meter was toast and the tranny was probably OK.
Anyhow, I had already unwound the primary (100 or 101 turns) and opened the
tape on the secondary, damaging and then removing about 60 turns in the
process.

I am hoping I can rewind the secondary for a SE 4.5W or 5W guitar amp --
your typical 6V6 Fender Champ and the like. I've done quite a bit of
reading and I'm thinking this can be done by hand if the primary is
suitable. I've gathered up all sorts of information, made calculations, and
whatnot, but feel that I would benefit from running this by someone who has
actually done this and has a better understanding than I do. Remember, this
is a for a guitar amp, with the relevant frequency range of about 82 - 5000
Hz.

Since I know the secondary turns, assuming 100, I ran the low voltage test
on the twin of the OT I took apart. I think the twin (still intact) has
3423 turns. Subtracting 60, the victimized OT has 3363 primary turns left.

I've been trying to determine the wire gauge on the primary without a proper
tool to do it. Using the scrap, I am getting about 3.6z on 63" of wire.
I've got 642 ohms left on the primary winding, suggesting 11,235". At about
4" per turn, that's about 2800 turns. I go with the higher number from the
electrical test, but I think this is reasonable confirmation. I also found
a chart stating 54.97 ohms per 1000" for 38 AWG. If I have 12,000" then 12
* 55 = 660 ohms, and I have 642 ohms. So, I believe the wire is 38 AWG. Is
this heavy enough for a 5W OT? Please read on for the description of the
lams.

I'd like to wind a 4-8-16 ohm secondary. Assuming 3363 primary turns, and
5K: 4-8-16, I'm coming up with turns ratios of 35.4-25-17.7 and secondary
turns of 95-135-190, respectively. Is this the correct calculation?

I've got another chart that shows current capacity at 600 cm/Amp and 700
cm/Amp. I figure your typical Champ amp, where Va=350 and Ia=0.045 The
current ratio is the primary multiplied by the turns ratio? IOW, 45mA *
35.4 = 1.6A? The chart says 19 or 20 AWG secondary wire for this measure.
It seems on the heavy side to me, but I don't really know much about this.
What is the correct sizing of the secondary wire gauge?

The lams, the core area is 18mm x 19mm = 3.42cm2 = .52 sq in. The I's
measure 54mm x 9mm. The E's measure 54mm x 36mm. The spaces between the
E's are about 9mm x 27mm -- thats the space where the bobbin fits. The
stack is 19mm high, but you already know that from the core area measure.
Everything seems to fit nicely in multiple of 9mm, so I guess this is a
European tranny, though I am in the US. That is why I'm using metric
measures -- it seems to fit. I got rather confused trying to figure the
number of primary turns, but have concluded that 3400 turns is plenty. Any
objection/other thoughts?

I don't know what sort of lams these are. Any way to tell? Eyeballing it,
the 19mm stack is about 60 lams. I am not wanting to take them apart and
make a mess of them.

Lastly, the air gap. It's there undisturbed. I don't have any way of
measuring it. Im inclined to leave it. I my estimation, it is not as
thick as a piece of copier paper so maybe 0.05mm?

I'd be very grateful if someone will help me out here and get me pointed in
the right direction. If you think what I've got isn't appropriate for a 5W
amp, what then? Please suggest another output tube.

Thanks.

Phil