The AM Forum

Hi All,

I don't remember where, but I've read somewhere that the impedance ratio of a modulation transformer has influence on the audio response.

I.e, I have build class b 2x811a modulator. I'm planning to build a plate modulated single 813 class c amplifier.

The modulator runs (currently) on 1100v. According to the tubedata, it would have a plate to plate impedance of around 11k
The 813 will run on approx 1300v and will have an impedance of around 9k
The optimum transformer ratio has to be 1:1.2

From what I've read, is that a ratio close to 1:1 will affect the audio response.
Am I wrong?
When it is true, would it be better to use a higher ratio transformer and live with the mismatch?

PA4WM

The transformer ratio will affect the maximum % of modulation available, not the frequency response.

Don't worry too much about impedance mismatches. A good rule is to use all of the secondary winding of the transformer, "all of the copper", don't use the secondary taps unless there are extreme circumstances.

Bill

Using  mod trans with the DC part of the final HV going thru the secondary will magnetize the core to some extent, and thereby limit the freq. response on the bottom end, mostly. 

A good way to eliminate this is by using a "Heising" hookup, with a choke blocking the ac current off the mod trans secondary, and a cap passing the same.  

If not engineered properly, those parts can be resonant, giving audio rolloff or bumps.  

The mod trans primary inductance can effect non-linear response by ringing on squareish wave audio, but that is unusual with quality iron run at or below ratings.

Class "B" iron always is picky, and some designers swamped the secondary of driver and mod transformers with resistors to achieve a more linear response across widely varying impedances working into reactive loads

73DG

Its better to have the RF load impedance higher than the modulator than the other way around but for only a few K its probably nitpicking.

Carl

I run two 811A to a single 813 at 1450V on both with 4.2V bias on the 811's.  Using a Stancor mod xfmr,  9K ohms on the primary and 5K ohms on sec.  Works fine but I'm also using a 50hy reactor with the mod xfmr.

Fred

I was probably mistaken then. I thought a ratio of 1:1 was effecting the audio response.

I have a mod transormer on hand with a ratio of 1.5:1, not ideal for the 811a - 813 (1.2:1) setup, but that's what I have to live with.

Understood about the modified heising approach. It's not easy to find large inductors over here, but I keep my eyes open.

The mod transformer has a gap, so intended to run DC through it.
Would it still be possible to use a reactor and capacitor with it?

WM.

That should be no problem.  You might try and re-set the laminations to close the gap if you do it that way, to get a bit better response out of it. ;)

73DG