[Boatanchors] Clamping control question for DX-100

[Bob Young]

Hi Garey,

Yes it's a 6AQ5, good memory, The problem I'm now having is that in the 
directions it says after applying plate power and checking that everything 
is OK (It is) put the VFO-crystal knob selector in a position without a 
crystal and then turn the clamp control from it's fully counterclockwise 
position until the meters just reads zero implying (to me anyway) that it 
should be reading below zero right now. It's already reading zero now in 
it's fully counterclockwise position and turning it doesn't move the meter 
at all. I've turned it back and forth and don't think it it's shorted 
although I suppose I should check that also. The meter works and will go 
below zero. It was about 3/4 clockwise when I first got it. I'm sending this 
to the lists also as there is another guy in the same boat as I am right 
now,

Bob Young
Millbury, Ma
KB1OKL


>From: Garey Barrell <k4oah at mindspring.com>
>Reply-To: k4oah at mindspring.com
>To: Bob Young <youngbob53 at msn.com>
>Subject: Re: [Boatanchors] one more DX-100 question
>Date: Sun, 04 Feb 2007 17:30:25 -0500
>
>Bob-
>
>I haven't seen an answer to this yet.
>
>The clamp control is a 6AQ5 (I think, going from memory) that has it's 
>plate connected to the Final screen dropping resistor.
>
>With no signal, the 6AQ5 and by dragging down the screen voltage to the 
>finals, it cuts them off.  Otherwise, a Class C stage with no drive will 
>conduct heavily, drawing as much current as the supply or final tubes will 
>stand until the fuse blows!  Sound familiar?  :-)
>
>When drive is applied to the final stage by keying the drivers, a small 
>sample of the drive power is rectified and applied to the grid of the 6AQ5, 
>cutting it OFF, and allowing the final screen voltage to rise to the 
>operating point.  When the drive is unkeyed, the 6AQ5 conducts again, 
>dragging down the final screen voltage and cutting off the final stage.
>
>The adjustment is made to set the bias of the 6AQ5 stage to draw "just 
>enough" current to drag the final screen voltage low enough to cut off the 
>finals.
>
>73, Garey - K4OAH
>Atlanta
>
>Drake 2-B, 4-B & C-Line Service Supplement CDs
><www.k4oah.com>
>
>
>
>Bob Young wrote:
>>What is the clamp control? Is it the bias for the 6146's? I can't find it 
>>on the schematic. The reason I ask is that I just had a scare. I got the 
>>plate voltage going fine on 116Vac line voltage and the next step said to 
>>adjust the clamp until the meter just touches zero, well evidently I 
>>turned it too far because lights out! It took me a half an hour of 
>>checking transformers' primaries, secondaries etc, to remember that my 
>>variac is fused @ 2 amps, that solved that problem thank God. But now how 
>>much do I turn the clamp and what is it? The clamp was already at zero on 
>>the meter, I should have brought it back to fully counterclockwise as it 
>>says in the manual (or left it alone) and then set it, when in doubt 
>>follow directions I guess,
>>
>>Bob Young
>>KB1OKL
>>
>>