check my math: transformers

[dobsont]

Trying to budget-build a 5e3, using vintage/found transformers, just like every other idiot out there.

PT 350-0-350 on the HV, measured unloaded, out of circuit.
it has taps for both 5v and 6.3v heaters. It also has some other low voltage 2ndry taps that originally connected to a pot labeled "hum balance". Not sure on these.

Voltage a bit high, but suitable with a NOS 5y3, no?

OT - has 3 leads on the primary side: blue, red, brown. Originally came from a phono amp circuit with 2 6v6s in PP arangement.

Hooked my variac to both blue and brown leads on the OT primary, dmm to the secondary side 'common' and 8ohm tap.
applied AC until DMM gave reading of 1v.
Measured output from variac at 45v.

so turns ratio is 45:1

now I get a little lost.
to get the Impedance ratio I square the turns ratio:
45x45=2025
and then to find the primary impedance I multiply by 8, no?
2025x8 =16200
so this thing has a 16k primary? not useable?

but what if I figure it this way:
ELR of 2 6v6 pp ab1 ab2 is 8000
8000/2025 = 3.9ohm

So can I run this tranny with a 4 ohm load on the 8 ohm tap and be happy?

Judging purely by dimension, the OT is probably capable of handling output in the 12-15 watt range -- it is very similar in dimension to the OT on a Blues Junior. Not all that scientific, I know.

I think the PT will be usable, the OT, not sure. Where have I gone wrong here?

[Bruce / Mission Amps]

That looked OK to me... just use two 8 ohm speakers in parallel for 4 ohms and it should be fine.
Not sure about the hum balance.
I don't know if the lower voltage winding is separate from the rest of the windings and if there is or is not a center tap winding but if they are separate, you could build a low voltage DC supply with it, a half wave rectifier, full wave or FW bridge SS diode rectifier, and a big 1000uF 100v cap then apply the low voltage DC through a couple 100 ohm resistors across the filament supply for better hum reduction.
Using a 1 watt 250 ohm POT instead of the two 100 ohm resistors, you could put the outside leads of the pot across the filament supply (lugs 2 and 7 of the 6V6s or 9 and 4+5 of the 9 pin sockets, and apply the DC voltage to the center lug through a 1/2 watt 47 to 100 ohm resistor... thereby changing how much DC voltage is applied to either side of the filament supply leads.

[dobsont]

Thanks Bruce! I'm blown away. You're helping me despite the fact that I haven't bought your kit. That is class. (and when I blow up my OT, guess where I'll go for another...)

I'm sorry, I'm a little daft. Bear with me, please.


If I understand correctly, the bottom line is that, if I run a 4 ohm load off what is labeled as an 8 ohm tap I get
2025 x 4 = 8100, and I'm perfectly in spec.


The hum balance stuff is probably gold -- I've got to sit with it a bit and let it sink in. I'm pretty new. Can you tell?