Interstage transformer primary question

[Groover]

In the Musicmaster Bass amp the primary of IST is wired in parallel with a 15k plate load resistor, but I don't know the impedance of the primary.

How do I figure out what the resistor value should be with a known primary impedance?

I assume these values would change for different B+ voltages, so some guidance on that would be appreciated too.

[Steve Conner]

Hi Groover

The purpose of the resistor is to smooth the frequency response of the interstage transformer, by damping resonances in it and swamping its magnetizing inductance.

In an amp with no overall NFB, the resistor value just affects the tone, so you can adjust it to your taste. A triode driver tube will damp the transformer somewhat with its plate resistance, but a pentode will hardly damp it at all.

In something like a Fender 400PS, that wraps a NFB loop around both output and interstage transformers, the phase shifts caused by the magnetizing inductance and self-resonance can send the feedback loop unstable, so they need to be swamped. The 300/400PS used quite a low-value resistor here, and connected the driver tube as a triode for even more damping.

In a 400PS, you probably only get one shot at getting it right, too.

PS: Making the resistor equal to the specified primary impedance for the interstage transformer would be a good starting point.

[octal]

I recently built an amp with a 10K:90K CT interstage/PI transformer driven by a 12AU7. Adding a 15K resistor paralleled with the primary of the transformer caused a surprising reduction in gain, but really cleaned up the squarewave response of the amp. It sounds much smoother and cleaner too, but not still not overly sterile.

You could install a 50K(?) variable resistor and adjust it to taste, or for the best squarewave response without compromising gain too much, then install a fixed resistor of the nearest standard value. Use great caution if you go the variable resistor route, as there will be around 250 V on both legs of it!

Cheers,

Nathan