Parallel Output Transformers

[gingertube]

Hi,
I have a 1958 Single ended Guitar Amp with a blown output tranny to repair. It runs a SE 6CA7 (EL34) at 250V and 100mA idle to deliver 10 to 11 Watts Output. This is a vintage Aussie Maton Amp.
6CA7 Data Sheet says that this requires and anode load of 2K. The speaker is the original 2 Ohm 1957 Aussie Rola Speaker (in good nick).

I looked at teh hammond 125 Series and found the 125E is rated for 15W and could be strapped for the 2K to 2 Ohms impedance ratio required BUT it has a max DC current spec of 80mA.

So plan B is to use 2 off 125C in parallel.

Now if I parallel the primaries and parallel the secondaries the turns ratio and hence the impdance ratio has not changed - so I need to starp each tranny for 2K to 2 Ohms before connecting them in parallel - IS THIS CORRECT or have I over simplified the problem.

Advice gratefully received,
Cheers,
Ian

 

[jerluwoo]

i think you will need to set each for 4k to 4ohm.

 

[Fuling]

I´d try only one 125ESE at first, those extra 20mAs through the OPT will probably not cause any troubles in this application.

 

[DF96]

4k to 4 ohms and 2k to 2 ohms are exactly the same ratio!

I would try one transformer, but perhaps reduce the quiescent current too. You won't notice the difference between, say, 8W and 10W.

 

[revintage]

Isn´t 125FSE or GSE available downunder?
But I agree with the others, the ESE will probably withstand a lot more than 80mA.
If parallelling you could go down to two CSE.

 

[jerluwoo]

doh you are correct sir. 4k to 2ohm would be the correct setting. the plate will see parallel impedance of the primaries.

 

[gingertube]

Hi guys,
This received from Hammond.

The good news is that the 125ESE will do OK at 100 mA.

The reason being that your primary impedance is at the low end of the range rather than high. When the DC current is raised above the rated value, the effective permeability of the core decreases and in turn lower the primary inductance and impedance. You need the higher open circuit impedance to get good low frequency response. Since this transformer was designed for a range of primary impedance, there will be more than enough inductance / impedance for the 2000 ohms you are matching to. Not well know but this transformer performs very well ( almost HiFi ) at low impedance.

So I will fit a single 125ESE.

Fuling, DF96, Revintage - good advice. Thanks

Cheers,
Ian