[Amps] A Symptom of Low Bias?

Fri Mar 6 17:06:51 PST 2009

I was just thinking about that, whether or not the HV metering should 
reference chassis or B-, guess you just confirmed that one for me!  I don't 
have a HV meter in this amp, but i think the next one i build will have a 
meter with a switch to monitor a combo of things.

Definitely taking this experience forward and building another one 
eventually for another band.  Thinking about 2m SSB now, though i do have 
almost 200w there now.

73,

Scott KBONLY

----- Original Message ----- 
From: "Mike Schelly" <hinrgdj1 at comcast.net>
To: <TexasRF at aol.com>; <kb0nly at mchsi.com>
Sent: Friday, March 06, 2009 5:45 PM
Subject: RE: [Amps] A Symptom of Low Bias?

> Good call Gerald,
>
> That little bit of negative grid current used to be a mystery to me also!
> I've always monitored the Hi_V thru a suitible multiplier resistor string 
> to
> CHASSIS,
> Wrong!!!! Instead of chassis, the hv meter - should go to B-. I learned my
> lesson.
> Depending on shunts/meter movements it my be more obvious to some than
> others.
>
> TNX, Mike K4EAR
>
>
> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of TexasRF at aol.com
> Sent: Friday, March 06, 2009 3:23 PM
> To: kb0nly at mchsi.com; amps at contesting.com
> Subject: Re: [Amps] A Symptom of Low Bias?
>
> If there is a plate voltage metering circuit, there could be a sneak path
> for this current to flow through the grid meter. If it does, it would be 
> in
> the reverse direction compared to normal grid current.
>
> Assuming your bias circuit is located between cathode and ground, the B-
> should be connected to cathode. The plate voltmeter (if there is one) 
> return
> should also be connected to the cathode.
>
> 73,
> Gerald K5GW
>
>
>
>
>
> In a message dated 3/6/2009 2:05:23 P.M. Central Standard Time,
> kb0nly at mchsi.com writes:
>
> I can  borrow a scope and check that, I found a website explaining 
> basically
>
> the  same way you just described!  I have always just used a good peak
> reading meter but I don't have much for options on 6m yet, I want to build
> or buy something eventually.  I have a Heathkit HM-2102 that i used  for 
> 2M,
>
> but it can be used for 6M by changing one cap value and  recalibrating it 
> so
>
> i will do that along with the modifications to  increase its full scale
> range
> that seems to be pretty popular and known to  work on them.
>
> Can you suggest however why the grid meter would read a  negative value on
> keyup with no drive??
>
> As for overdriving it, i'm  keeping the drive to 20W as the spec sheets 
> for
> this tube shows 10-20W  max.  I also plan on building an ALC circuit for 
> the
>
> radio to keep it  at 20W or less when on 6M.  The FT-847 has a lot of info
> online  regarding this, its not really interfaced to the amp, its a 
> circuit
>
> connected to the ALC jack of the radio and then adjusted to keep the
> exciter
> at a specific level regardless of the front panel  control.
>
> 73,
>
> Scott
>
>
> ----- Original Message -----
> From: "Gary Schafer" <garyschafer at comcast.net>
> To: "'KB0NLY'"  <kb0nly at mchsi.com>; <amps at contesting.com>
> Sent: Friday, March  06, 2009 1:52 PM
> Subject: RE: [Amps] A Symptom of Low Bias?
>
>
>>  Forgot to mention the bias. I don't know what the ZSAC (idle plate
>>  current)
>> should be on that tube but it sound in the ball park. Usually  idle
> current
>> will be around 1/3 of full plate current +- depending on  the tube.
>>
>> 73
>> Gary  K4FMX
>>
>>>  -----Original Message-----
>>> From: amps-bounces at contesting.com  [mailto:amps-bounces at contesting.com]
> On
>>> Behalf Of  KB0NLY
>>> Sent: Friday, March 06, 2009 11:34 AM
>>> To:  amps at contesting.com
>>> Subject: [Amps] A Symptom of Low  Bias?
>>>
>>> Still fighting some small bugs in the new amp  this morning, does this
>>> sound like a symptom of the bias being too  low??
>>>
>>> When i key up with no drive the plate idles at  about 90-100mA, and the
>>> grid goes slightly negative, just enough  to pull the needle a hash mark
>>> width below the zero on the  scale.  As i apply drive the plate comes up
>>> to
>>>  about 400mA and the grid starts to come above zero, but i have to
>>>  literally YELL on SSB to get the grid to come up and see full output.
> I
>>> tried some CW on it last night and its making full output  there.  Also
> at
>>> the suggestion of some on here i tried  applying drive on FM and the 
>>> grid
>>> came up to 100mA and the plate  stayed around 400mA yet.
>>>
>>> So from the info i can find  online, in the handbooks, picking others
>>> brains, etc..  It  would seem i still have a bias issue here as its only
>>> amplifying  on the voice peaks on SSB.  I dug through my drawers and
>>>  didn't
>>> have the parts to do the mod for higher bias on the Triode  Board so i
>>> ordered some zeners from Mouser today and i will get  that done when 
>>> they
>>> arrive and see if i can further adjust the  bias and get the idle 
>>> current
>>> down some more as  well.
>>>
>>> What do you guys think??  Starting to  realize once again why i like
>>> building stuff, learning is  fun!
>>>
>>> 73,
>>>
>>>  Scott
>>>
>>>
>>>
>>>  _______________________________________________
>>> Amps mailing  list
>>> Amps at contesting.com
>>>  http://lists.contesting.com/mailman/listinfo/amps
>>
>
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