1959 Gibsonette Audio transformer impedance matching

[JRUBIN]

Hey everyone,

Im working on a 1959 gibsonette and I have tested the audio output transformer which matches the exact ratio as listed in the technical literature which is 27.5:1

The finals for this AMP are dual 6V6 running parallel single ended into the transformer in class A
The speaker reads 3.5 OHMS on the meter.

From this, my math says that:
Z= 756.25 OHMS (27.5 ^2)
P= 2647 OHMS (Z * 3.5)

According to my book a 6V6 expects P to be 8000 ohms PLATE to PLATE not 2647. But i think that is for a PUSH PULL configuration. Triode mode mentions 1960 OHMS.

I understand that single ended in parallel cuts the impedance in half, that would still be 4000
Could that be the one.

Anyone have an idea on this

 

[thorpej]

Just looking at the schematic you can see it's not triode-connected.

Let's look at your math again. 27.5:1 winding radio would be 3025 ohm for a 4 ohm load (you can't translate direct resistance readings to impedance, but it's probably a fair bet that's a 4 ohm speaker). Your parallel-single-ended output stage is going to be using the Class A characteristics for the 6V6. 5K load is common, although I believe Fender used around 7K in the Champ. Anyway, since it's parallel-single-ended, when you're doing the load line analysis you want to double the primary impedance of your transformer primary and do the calculations as for one tube. So, that would be approximately 6K load, which is in the ballpark.

So, what I'm saying is: I think you're all good!

BTW, cool amp!