Unknow O.T. Check my maths.

[chocopower]

Hi,

i have an old chasis with two 6V6, one 5Y3 and one 12AX7.
No marked speaker impedance, so..

I aplied 36,2 vAC to primary/plate taps.
I get 0,65 vAC in secondary.

so:

36,2/0,65 = 55,6923

55,6923 square = 3101 ohms

If a pair of 6V6 at 285vdc need to "see" 8k, i have a dilema:
With a 2 ohm load i get 6k2
With a 4 ohm load i get 12k4

the closer value i get is with the 2 ohm load (maths say 2,56ohms).
2 ohm is not usual and i would like to use some 4 ohms speakers i already have.

[Phil_S]

Yes, it's pretty much guess work from this point. My gut is telling me you've got 10K : 3.2R.

If you have them, a pair of 4 ohm speakers wired parallel is 2 ohms. I think I'd go with that.

[VacuumVoodoo]

Repeat the measurement with 4R load on secondary side.
If you're using a variac to excite the primary winding than by all means go to 200V or higher range. This will give results closer to the conditions in normal use.
A second test to do is to determine ratio from each half of the primary to secondary. This will tell you if both halves of primary really have same number of turns. Ok, +- one turn.

[tubelectron]

Repeat the measurement with 4R load on secondary side.
If you're using a variac to excite the primary winding than by all means go to 200V or higher range. This will give results closer to the conditions in normal use.
A second test to do is to determine ratio from each half of the primary to secondary. This will tell you if both halves of primary really have same number of turns. Ok, +- one turn.

Exactly. And if you can use an audio generator at 400Hz sinus instead of a variac, it's better, but in any case, don't forget the secondary side load !

A+!

[Cliff Schecht]

Since most OT's impedance is rated at 1 kHz, I test everything at 1 kHz.

[chocopower]

Repeat the measurement with 4R load on secondary side.
If you're using a variac to excite the primary winding than by all means go to 200V or higher range. This will give results closer to the conditions in normal use.
A second test to do is to determine ratio from each half of the primary to secondary. This will tell you if both halves of primary really have same number of turns. Ok, +- one turn.

So, i just put a big 4 ohm resitor in the seconraries and measure throw it, isn´t?
What´s the reason? simulate a real situation?

I have no Variac, but i can put the 230vAC main suministre, via a ligh bulb, just for safe.
I have done in the past without problem. Even, i use it to test for shorts in trafos.
I take note about check the primary center tap.

[chocopower]

New measurements.

Vin: 220vac
Vout: 3,98

220/3,98 = 55,274

so: 55,274 sqared = 3055 ohm
Voltages between primary extreme and CT: 119v - 101v

Some pics:
https://picasaweb.google.com/davidageit ... gvb5gvOzag

[rdjones]

New measurements.

Vin: 220vac
Vout: 3,98

220/3,98 = 55,274

so: 55,274 sqared = 3055 ohm
Voltages between primary extreme and CT: 119v - 101v

Some pics:
https://picasaweb.google.com/davidageit ... gvb5gvOzag

So the new measurements are different by just barely 1.5%.
There's more difference between the 2 sides.

Also note that with that high of a test voltage and the low line frequency you may be getting some core saturation.

rd

[chocopower]

Power Trafo is marked:

12518-5
138639

Any idea where it cames?

[chocopower]

i found my self..

138639

138 = Stancor (Chicago-Standard)

39 week / 1956 (could be 1966, but those caps seems older)

[tubelectron]

Also note that with that high of a test voltage and the low line frequency you may be getting some core saturation.

Exactly. You should have found a greater difference than 1.5% between loaded and unloaded... Additionnally, 50 or 60Hz are probably too low frequencies for guitar audio output transformer, IMHO... So there may be a problem of core saturation.

A+!