[AMRadio] Low modulation Gates BC-1T


SBJohnston at aol.com SBJohnston at aol.com
Mon Feb 26 23:07:07 EST 2007


jtml at losalamos.com writes:

> So what you are saying is that the total voltage across the 4 K resistor 
> load was 5 x 200 or 1 kV rms audio? I assume you drove it midband (1 Khz or 400 
> Hz etc). 

That's correct.

> If you have a good HV scope probe, you should look at the plates of the 
> modulator 833s and see that both have substantial swing, before you blame that 
> iron. 

I don't have a high-voltage scope probe, just a HV multiplier for a 
multimeter Maybe that could be made to work.  

> You've probably been there already, but don't these rigs have a switchable 
> attenuator in the input somewhere, for when they go to nighttime 250 watt 
> power level? Is it possible that this is in the wrong mode?  

I've tried it both ways.  And in any case, I can drive the rig with as much 
audio as it needs,even with full pad on the input, and the plates of the 833s 
are glowing out of control before I get more than 20% modulation.

> I had an RCA transformer that i gave away two years ago. Let me know if you 
> need something like that, because I might find it again from the guy who 
> took it away. 

Thank you.  Today I borrowed a mod transfomer to try as soon as it has had a 
chance to dry out a bit after coming in from the cold.  I think it is from a 
Gates BC-1F.  It is for push-pull 833s into a 5000 ohm PA load.   It doesn't 
have the driver mod winding, but I'm glad to get something to try.   If I recall 
correctly, in the RCA line a transformer from an RCA BTA-500MX or -1MX would 
be a good choice for my rig.



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