Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on June 08, 2014, 03:55:59 am

Title: Doubt about calculating impedance of output transformer
Post by: kagliostro on June 08, 2014, 03:55:59 am

A friend has restored an old italian combo amp, the Corland HiFi
Here some photo
http://imgur.com/7BE8YSY&C6gvhx0&BEGXeEK&dNuvUFt&lSvYymL&ZBJdBbA&19BeY9z&K37I3lT&b3QDmLm&fuxHWJv&OyaN59l&HnvdORG&5zTHEje&DvwrmhJ&hDvZx0c&827WXwn&rFQHGVl#0 (http://imgur.com/7BE8YSY&C6gvhx0&BEGXeEK&dNuvUFt&lSvYymL&ZBJdBbA&19BeY9z&K37I3lT&b3QDmLm&fuxHWJv&OyaN59l&HnvdORG&5zTHEje&DvwrmhJ&hDvZx0c&827WXwn&rFQHGVl#0)

He has draw a copy of the layout and I've draw the schematic

I'm in doubt about the impedance of the two OT

the amp is a PP but as told by the manufacturer (an 80 yo guy) at the time the amp was build it was difficult to find OT so the PP amp was buind using two SE OT with secondaries in parallel

Trying to establish the impedance of this transformers, knowing that the Speaker is a 4ohm, we ended to consider it as a pair of 5600ohm/8ohm speakers

but I'm not sure about and I'm confused

Does someone is able to establish which is the right impedance of this two transformers ?

the transformation ratio is near 26.5:1 (one is 26.8:1 and the other is 26.25:1)

and they are connected the way you can see in the attached image


Thanks

K

Title: Re: Doubt about calculating impedance of output transformer
Post by: PRR on June 08, 2014, 11:18:25 pm

EL84, cathode biased, 340V supply, use 8K or 10K loading.

If I compute the original turns-ratio correctly, I get 9,333 Ohms CT to 4 Ohms.

3500:6 is 583:1 impedance ratio and 24:1 voltage ratio.

Two windings in series makes 48:1 (48.3046) ratio.

48^2 * 8 Ohms is 18,432 Ohms CT, which is awful high.

48^2 * 4 Ohms is about 9.3KOhms CT, which is a fine load. 

> transformation ratio is near 26.5:1 (one is 26.8:1 and the other is 26.25:1)

Missed that.

26.8 plus 26.25 is 53.

53^2 * 4 Ohms is 11,257 Ohms CT, maybe not bench-perfect, but excellent for odd-parts design.

Title: Re: Doubt about calculating impedance of output transformer
Post by: kagliostro on June 09, 2014, 02:23:52 am

MANY THANKS PRR


So 11,257 ohms CT, and my calculation for each transformer of 5600ohm/8ohm (5618ohm/8ohm) is to be considered correct


pointing that the two secondaries are connected in parallel and the primaries in series


thing that when the OT are connected on the circuit halves to 4ohm the secondary impedance


Thanks again, that thing was confusing me


Franco