[AMRadio] B&W 5100B Modulation Update


Gary Schafer garyschafer at comcast.net
Wed Oct 13 15:24:28 EDT 2004


Tom,

Let us know what that modulator plate current actually is at about 50% 
modulation and at 100% modulation.

Awhile back I played with a pair of 6146's modulating a pare of 1625's 
with a multitap mod transformer. I found that using too high an 
impedance tap on the secondary, to get a lower ratio, would make the 
modulators draw lots of current. I could easily draw 400 ma of mod 
current or more.
A partially shorted mod transformer could give the same results.

Your idle current on the mod tubes should be around 50 ma?
Maximum current with a whistle should go to around 250 to 300 ma??

The impedance ratio I would think (off hand) should be around 5000 or 
4000 ohms plate to plate on the mod side and 2000 ohms on the final side.
Assuming running 180 watts input (300 ma) on the final.

73
Gary  K4FMX



Tom Elmore wrote:
> Yes there is a plate current position and it shows a definite rise in
> current with modulation. There are dual 5U4's in the supply feeding the
> finals and modulator and I swapped them both out with the same results. I am
> leaning towards Gary suggestion that there is an impedance mismatch causing
> excessive current draw. Does anyone have a 5100B or know what the impedance
> of the primary and secondary should be. I will try another set of caps this
> evening when I get home there are 2 , 40uf in series on the B+ line.
> ----- Original Message ----- 
> From: "Gary Schafer" <garyschafer at comcast.net>
> To: "Discussion of AM Radio" <amradio at mailman.qth.net>
> Sent: Wednesday, October 13, 2004 10:29 AM
> Subject: Re: [AMRadio] B&W 5100B Modulation Update
> 
> 
> 
>>Tom,
>>
>>Try a new rectifier tube if you have one and see what you get.
>>Easy way to check capacitors is to tack a new one in parallel with the
>>hv filter cap and see if you get any improvement.
>>
>>Does the 5100 have a meter position for modulator plate current? If so
>>what is it doing.
>>If you have the wrong impedance taps on the mod transformer, the
>>modulators may be drawing much more current than they should. Too high
>>an impedance tap into a low impedance final load will make the
>>modulators draw very heavy current.
>>
>>73
>>Gary  K4FMX
>>
>>
>>Tom Elmore wrote:
>>
>>>The caps are the first thing I should have checked and there is a good
>>>possibility they are dried up except I don't hear any hum in the audio
>>
> when
> 
>>>monitoring on the receiver. Also I can make at least 125 watts carrier
>>
> and
> 
>>>don't see much of a voltage drop on B+ when I run carrier only.
>>>
>>>-Tom
>>>----- Original Message ----- 
>>>From: "George Pritchard" <gpritchard at comtechpst.com>
>>>To: "'Discussion of AM Radio'" <amradio at mailman.qth.net>
>>>Sent: Wednesday, October 13, 2004 8:46 AM
>>>Subject: RE: [AMRadio] B&W 5100B Modulation Update
>>>
>>>
>>>Tom,
>>>It seems to me that the capacitor bank for the 600 volts is not working.
>>>Look at the B+ with a scope, and you may see the B+ drop at the audio
>>
> rate.
> 
>>>A "stiff" capacitor bank will show no audio waveform on the B+. If I'm
>>>right, the caps are died out. Is there any trace of Hum on the "dead
>>>carrier"?
>>>George AB2KC
>>>
>>>-----Original Message-----
>>>From: amradio-bounces at mailman.qth.net
>>>[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Tom Elmore
>>>Sent: Wednesday, October 13, 2004 12:35 PM
>>>To: amradio at mailman.qth.net
>>>Subject: [AMRadio] B&W 5100B Modulation Update
>>>
>>>
>>>Now that I have had some time to troubleshoot the B&W 5100B modulation
>>
> here
> 
>>>is what I found. The B+ is around 600 volts with no modulation and I as
>>>start to modulate and watch the scope when it reaches about 50%
>>
> modulation
> 
>>>the B+ drops from 600 to around 400 volts.  I changed the final and
>>>modulator 6146 tubes and see the same results. The original modulation
>>>transformer has been replaced with a Chicago Standard Transformer
>>>Corporation # A-3891.   When I remove B+ to the final's and terminate
>>
> the
> 
>>>modulation secondary into a resistive load of about 6k ohms I still see
>>
> a
> 
>>>significant drop in  B+ as I start to bring modulation up. I don't have
>>
> the
> 
>>>original mod transformer and the manual I have doesn't say what the
>>>impedance should be.  I have tried various other tap settings on the
>>>transformer secondary and don't see much of an improvement in reducing
>>
> the
> 
>>>amount of voltage drop. I wonder if it is possible that this mod
>>
> transformer
> 
>>>just will not work with this unit. Anyone have experience with mod
>>>transformers using 6146's as finals and modulators?
>>>
>>>Thank You
>>>Tom Elmore KA1NVZ
>>>Anchorage  Alaska
>>>
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>>
>>
>>
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> 
> 
> 






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