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. Could that be the one.

Anyone have an idea on this

[Enzo]

My first idea is that the amp has worked with this transformer for the last 57 years, and transformers do not change turns ration over time. So I would be inclined to think it is correct for the amp, regardless of how it looks.

Your meter reads the resistance of the speaker, but its impedance is likely 4 ohms nominal.

Triode mode? You are running two pentodes in parallel. It is a single ended amp. The book lists 5500 ohms under some conditions. You get 2700 ohms. There are two tubes in parallel. Two 5500 in parallel looks to me like 2750. Pretty close.

So what are the actual issues with the amp you are trying to correct?

[jrubin]

The test was to ensure that the transformer was ok, by matching the ratio to the specification I insured that all was good with the windings.


With a plate voltage of 320v I see a value of 8000, even half, 4000 (as read in parallel) would be high, unless I was supposed to compute the plate voltage as half when calculating too, as 160V that would bring the value down to 5000 and half of that is 2500. I feel as though Im changing the math to solve for an intended value

[Enzo]

I think the key here is that like so many other specs, it just isn't that critical. How many amps these days will run on either 6L6 or EL34 with the flip of a bias switch? yet the tubes want different impedances. The data sheets on the tubes offer design center numbers, not precise measurements.