[AMRadio] Re: AMRadio Digest, Vol 37, Issue 45

Jim Candela jcandela at prodigy.net
Mon Feb 26 13:20:52 EST 2007

 Steve, I'm sure your getting  frustrated.  Hang in there  since the solution is close now. When it is all over and we all look back on it, it needs to make sense.  Now onto the issue as I see it.

I brought up the need for a AC compensated 1000x HV scope probe early in this thread. Nobody until John has mentioned this since. If you have a scope, then I can lend you a suitable probe. Let me know off-list.

You say the 200ma and and 400 ma currents were combined modulator current. Are the tubes sharing the load? One crude way is by color of the tube (shades of orange), or another way is to add a #47 pilot lamp in series with each 833 plate. These will glow normal brightness at 150ma through the bulb, so you should be able to run the modulator up to 300ma, and if balanced, the brightness between the bulbs will be the same. The bulbs won't know they are at HV; they will just drop the B+ 6.3 volts at 150 ma.

You said that you got 200V X 5 across those resistors (RMS). What kind of meter do you use? Watts = E^2/R = 1000^2 /4000 = 250 watts RMS, or 50 watts heat in each resistor. Those resistors should get hot!! If they do not, then the measurement is somehow in error.

 Another option without a HV probe would be to take the 833 filament CT to ground wire, and  interrupt it with a 1 ohm 1 watt resistor. Then look across this with your scope using a 1X probe. This will show the combined cathode current where I (amps) = E/R = E. If the modulation transformer is shot, expect to see the  audio waveform distort badly as you bring up the drive. In fact, the distortion may be mainly on one tube (every other pulse of Class B modulator current). Since this is likely Class B, it would be normal for each tube to only conduct 180 degrees, but more likely a little more than that.

Hope this helps,

----- Original Message ----
From: John Lyles <jtml at losalamos.com>
To: amradio at mailman.qth.net
Sent: Monday, February 26, 2007 11:28:26 AM
Subject: [AMRadio] Re: AMRadio Digest, Vol 37, Issue 45

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). 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. Although, I don't know what the standard readings are for modulator current for each 833, you are getting 200 mA per tube metered DC current. 

Also, are you certain that the levels through the 807 stage are correct for high level tone? 

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 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. 


> Message: 10
> Date: Sun, 25 Feb 2007 22:42:56 EST
> From: SBJohnston at aol.com
> Subject: Re: [AMRadio] Low modulation Gates BC-1T
> To: amradio at mailman.qth.net
> Deep in the bowels of my junk I found some resistors for a test of the whole 
> modulation section of the Gates BC-1T.  I put five large-ish (each like a roll 
> of quarters, maybe 50 watts?) 800-ohm wirewounds in series across the 
> secondary of the modulation transformer.  Thaty would seem a good load: 4000 ohms.  I 
> put an RMS voltmeter across one of the resistors in the stack.
> Applied tone to the input... enough drive to produce 200 mA of combined 
> modulator current produced about 100 volts across the one resistor.  400 mA got me 
> about 200 volts.  And the tube plates were medium-glowing at 0.2 A (what I'd 
> consider normal for 833 modulators) and quite brightly glowing (more than I'd 
> like to see on an 833) at 0.4 A.
> Didn't run it too long, but after I shut it down the resistors were hardly 
> warm at all.  
> Steve WD8DAS

> Hate to say it, Steve, but I think your modulation transformer is fried.
> Had a similar thing happen with a Ranger I mod xfrmr... plenty of plate 
> current on the 1614's... excessive, in fact.  Just no, or very low 
> modulation.  Turned out be a blown mod tranny.
> Good Luck!
> --
> 73
> -W5OMR (home for 2 days)

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