[AMRadio] Ranger Audio... the numbers


Larry Taft ltaft at charter.net
Fri Nov 17 12:36:16 EST 2006


Gary has, I think, hit the tube on the plate cap.

I went looking for a set of curves for the 6L6 in my pile of old books 
and data but no find yet.

Just from "rule of thumb" for audio tube amps you should get a couple 
hundred volts audio swing (not related to BIRD WATTS) at each plate to 
ground.  50 v is only a couple of watts audio.  Soft tubes or/and 
shorted xfmr.

Try disconnecting  the RF section from the power supply and put a 5K 
power resistor across the output winding of the mod xfmr.   Now you can 
test the audio chain and figure out where it is failing.  This removes 
the possibility of RF back into the audio chain and power supply 
variations from the load of the 6146.  Actually, just pull the 6146 AND 
hook the 5k resistor across the mod iron output leads.

Now, Redemption!  I earlier said to reverse the OUTPUT leads on the mod 
iron.  Reversing the plate leads on the 6L6s would definitely make the 
ampalator into an oscillafier.  It isn't often that I'm really wrong.  
Just ask any of my ex-wives.

73, Larry  K2LT

Gary Schafer wrote:
> It's beginning to sound like a partially shorted mod transformer. With 20
> volts signal on the grids of the mod tubes I would think that you should see
> at least 200 volts on the plates??
>
> Could be bad 6L6's.
>
> The reason that you see a slightly distorted audio pattern on the audio
> amplifiers when in tx and not in standby could be the result of the negative
> feedback loop trying to correct distortion from the mod transformer.
>
> Can you easily get high modulator plate current with audio applied? Will the
> mod plate meter peg or nearly so if you hit it hard? If it will that is an
> indication that the tubes are good and the transformer may be shorted.
>
> 73
> Gary  K4FMX
>
>   
>> -----Original Message-----
>> From: amradio-bounces at mailman.qth.net [mailto:amradio-
>> bounces at mailman.qth.net] On Behalf Of Jack Schmidling
>> Sent: Friday, November 17, 2006 9:45 AM
>> To: Discussion of AM Radio in the Amateur Service
>> Subject: Re: [AMRadio] Ranger Audio... the numbers
>>
>> Gary Schafer wrote:
>>
>>     
>>> Check at the center tap of the mod driver transformer with your scope
>>>       
>> and
>>     
>>> see if you see anything there. Could be a bad or missing bypass cap
>>>       
>> there.
>>
>> It's brand new but I didn't have a 15mf so I used a 47mf.. ditto for C59A.
>>
>>     
>>> Check the jumpers on the plug in the back to be sure that the proper
>>>       
>> taps on
>>     
>>> the mod transformer are selected.
>>>       
>> All ok.
>>
>>     
>>> Check the clamp tube circuit. Could be holding the screen slightly low
>>>       
>> on
>>     
>>> the 6146 and clipping the modulation peaks to the screen.
>>>       
>> Screen voltage is 150 and the nom is 190.
>>
>> However, R35, the Modulator Screen Voltage Adjustment resistor seems to
>> go to the clamp circuit and it is not at all clear what is going on
>> here.  It seems to me that diddling with it changes the screen voltage
>> on the 6146.
>>
>> So does diddling with the modulator current effect the 6146 screen
>> voltage or do I have a screen voltage problem?
>>
>>     
>>> Check modulator screen voltage.
>>>       
>> It is the nominal 250V... but again, why is it nominal if that resistor
>> is there to diddle with the current?
>>
>> js
>>
>>
>> --
>> PHOTO OF THE WEEK: http://schmidling.com/pow.htm
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>>
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>
>
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