Sowter 9705 Preamp Transformer

[lt_texan]

Ages ago I built a preamp and posted here. That old thread is closed. But during a move, I lost my documentation and am trying to figure out exactly what I did.

The Sowter documentation for their preamp transformer it is recommended to put 600 ohms across the output to reflect 9K6 (4:1, so 600x4^2) back to the valve, but I see I did not solder in a resistor across the output of the transformer?

My amplifier (WAD KAT6550) has a measured 100K input resistance.

The specific question is, do I need to solder in a resistor, maybe 300 ohms, to reflect a load of around 5K to the 12B4A?

Did I mess this up years ago?

 

[rayma]

You'll likely get some ringing without reasonable loading.
If you have a scope, try a 1V at 1kHz square wave and experiment some.
Start with whatever Sowter recommends for it.

 

[50AE]

Loading the secondary depends on the parameter you're looking to optimize.
-If the transformer has a peaking ringing, loading will flatten it (reduce Q factor of the ringing)
-If the transformer has a dip ringing, loading can increase its Q-factor.
-Loading will extend low-frequency response
-Loading can extend or decrease high-frequency response, depending on how the transformer is built. If capacitive roll-off dominates, loading will extend the frequency response. If leakage inductance roll-off dominates, it will worsen it.
-Loading will cause more distortion for a triode stage, as it results in lower reflected primary impedance and steeper loadline.

 

[lt_texan]

No scpoe, but I have some 510R resistors laying around. So I put those across the O/T outputs and listened for a while.

I think it sounded really good before, and it still sounds really good with the 510 resistor.

So I soldered them in and will listen for a long while.

Thanks!