How is transformer impedance selected here:

[Y2KEDDIE]

I've seen a speaker (4 ohms) connected to a 4: 25,000 ohm input transformer . The 25K side is in series with a 500K pot which the center tap feeds the grid of a 6AV6 tube/valve.

This is for an intercom circuit. I'm wondering how the transformer ratio and pot values were determined. Should I be analyzing this based on impedance of the tube grid circuit, or is the transformer just to step up the signal voltage from the speaker used as a microphone? (or both).

 

[mofy]

The transformer needs to be able to drive the secondary impedance, in this case the 500k pot, which feeds the tube grid. How is the primary of the 25k transformer wired? What is its ratio.?

 

[Y2KEDDIE]

My question is; How/why are the values determined?

As I originally stated: the primary Z is 4 ohms, the secondary is 25K ohms, The 25K ohm secondary is in series with a 500K ohm resistance to ground, The tuns ratio calculates to be 79:1. (square root of 25k divided by 4), It would seem a proper load would be a 25K pot not 500K. I'm trying to understand why the designers choose the values they did.

The tap on the pot connects to a .0022ufd cap to the grid of a 6AV6

The speaker is being used as a microphone

 

[hevans1944]

They probably were not trying to match impedance so much (you only need to do this for maximum power transfer) as they were trying to provide a usable level of audio for amplification by the 6AV6. In other words, the transformer is being used to step up the voltage from the speaker used as a microphone, not to match impedances. A one megohm pot would probably work just as well.

 

[Y2KEDDIE]

They probably were not trying to match impedance so much (you only need to do this for maximum power transfer) as they were trying to provide a usable level of audio for amplification by the 6AV6. In other words, the transformer is being used to step up the voltage from the speaker used as a microphone, not to match impedances. A one megohm pot would probably work just as well.

I agree with you. The circuit in from a 1966 RCA tube manual (pg 544-545). The Circuit description says: "...Transformer T1 matches the voice coil impedance of the 4-ohm pm speaker....to the 2500 ohm input impedance of the 6AV6 amplifier stage....."
It just seems by adding a 500k resistance in series ,it would effect the impedance matching they are trying to achieve. On the other hand; if they want a 1:79 voltage step up , followed by a high impedance voltage divider it makes some sense.

These kind of things keep me awake at night when I should be sleeping, LOL.

 

[mofy]

It does seem strange to use the 500k pot for a 2500 ohm input impedance. Perhaps the transformer was designed for something else and just adapted for this application. That is quite common, as making unique components can get quite expensive.

 

[Y2KEDDIE]

The 6AV6 feeds a 6EH8 followed by a 3000 ohm to 4 ohm output transformer. It would seem the designers would of used the same transformers for input and output if possible, to save cost of spec'ing two different transformers. I guess the Output transformer wouldn't produce enough voltage for the 6AV6.

The input transformer is 4: 25K, the output is 3K : 4.

 

[mofy]

But what are the power ratings of the 2 transformers? I bet the output transformer is a lot larger. But it is only a possibility, I wouldn't get too caught up in it. It is also possible that the input transformer provides a wide range of adjustment for component variations and or varying applications. But again, it's just a possibility.