HB Transmitter RF Deck

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W7SOE:
Finally got around to sketching a rough schematic of the RF deck. (PDF Attached)  I would appreciate any help in understanding what is going on .

To my inexperienced eyes, I see:

Two 813s, parallel, grid-driven.

What appears to be a clamper circuit (the 6y6 and 0d3).  This circuit is confusing with both the grid and screen currents involved.

The band switch is a turret style coil switch (B&W), combined with the variable cap at "E" I think this is the input tuning.  (Shown here as a box on the lower left)

The caps at the "B" and "C" inputs are ganged.  One, I believe is "grid tuning" (whatever that is) and the other goes to two porcelain insulators located at the bases of the 813s.  Perhaps these are for neutralization plates that were not needed?

The Plate and Tank circuits look straight forward.  The antenna loading and capacity adjustments are new to me....

I hope the schematic is mostly correct if not complete on the input side.

Thanks

Rich

W3RSW:
Ok Rich, in reference to your email, here's the explanation.
If you can obtain an old "Radio Handbook" or any "ARRL handbook" from the 60's or earlier you'll see you have a standard circuit of the era.

What you have is a traditional dual 813 rf deck with pi net output, extra and switchable loading capacitors in the output of the pi and if you've drawn that switch correctly, it progressively loads up the additional loading capacitors in parallel for at least two positions.  The first coil off the plate is probably for 10 meters, the rest of the taps and add'l loading caps. good for bands to 80 meters.

Your screens are 6YG clamp tube protected, that is grid bias is normally established by an input signal of about 15 to 20 watts which causes current to flow through a grid dropping resistor, hence making the grid about minus 160 or -170 volts with respect to the cathode (filaments) of the 813's.  If there is no grid drive then the clamp tube keeps the 813's more or less cut off.  Without the clamp tube, during loss of drive in AM or in CW service when the drive is cut off, the 813's would have no negative bias on the grids and hence conduct way too much plate current, "run away and melt down."  The OD3 cuts the screen voltage to zero when grid excitation is removed, not just a somewhat but still a safe current if it's not in the circuit.  You want to be sure the OD3 goes dark, no current, when grid excitation is removed.

I need to see a little more of your turret circuit to make other than a conjecture of your grid input circuit.  What are the approximate values of the capacitors shown? This will help identify the lash-up.  Usually a neut. cap. is very small, few plates or only two, wide spacing.  The grid tuning will have many plates and is usually around 100 pf.  The turret probably has link coupling, a few turns on the 'ground' side of each coil from the input jack and many turns on the grid side.  The grid side of the, say 100 pf, cap in parallel with each coil will be above ground. There will be around 300 to 800 pf from that lifted ground to real ground.  The neut. cap. will feed from the plate back to this lifted ground in order to make the neut. "bridge" complete.  You feed rf into the amp. without any HV and tune for minimum output as you adjust the neut. cap.  It may already be set up and close so don't fool with it at first. You'll need to get some instruction and practice here. 

Also, you'll get better linearity if you feed the screen dropping resistor from the HV on the non-modulated side of the mod. transformer.  Your schematic shows the screen dropping resistor coming from the modulated side, same line as feeding the plate choke. You'll need another HV jack and wire run to the HV power supply to do this. What you already have will work but has a little more distortion than the two lead HV I've described.

Anyway you have a classic; it'll shine when you get it on the air.

 

WB2RJR:
Rich,

That Clamp tube circuit is a dead ringer for the one shown on pg. 154 of my 1963 ARRL handbook under protecting screen grid tubes.

The amp appears to have grid leak bias (no seperate grid bias supply) so the bias is developed  by grid current flow when the amp is being driven. If you lose drive the grids will no longer have negative bias and the 813s will conduct like a mother.

So bias from the PA grids is applied to the 6y6 grid and this biases the tube off, no conduction. If drive is lost, (as in keying an earlier stage) the grid on the 6y6 goes to zero it conducts dropping the screen voltage and turning the 813s off. The VR tube is used to completely remove any screen voltage from the 813s.

I hope that makes sense. Remember I'm a petroleum geologist not an EE.

Marty WB2RJR

W7SOE:
Thanks Rick and Marty,
    I know the risks ;) of exposing my inexperience here but I have some basic questions:

Where does the negative grid bias come from?  It looks to be provided by the RF exciter, but is this not a sine wave at RF frequencies?  How does this become a negative DC offset?

The clamper:
I see this as the input RF biases the 6Y6 grid to a negative voltage cutting off the tube.  In this state the 0D3 regulates a positive voltage to the screens of the 813s.  When RF is lost the 6Y6 is turned on, the 6Y6 plate voltage drops, and the 0D3 is off.  Is this even close to being correct?  Where would the 813 negative screen voltage come from?  I assume it would have to be negative to cut off the tube.

Rich
 

The Slab Bacon:
Rich,
       the way that the clamp tube is supposed to work is: When rf drive is applied to the final tubes some of that rf is rectified in the grid circuit creating the negative operating bias accross the grid leak resistor. This cuts off the clamp tube and the amplifier acts as normal. When the incoming rf drive falls off the bias goes away  the clamp tube starts to conduct. The conduction of the clamp tube "clamps" down (shunts) the screen voltage to the final amp to a level that limits the run away plate current of the final tubes to a level that wont cause them to melt down. Without any negative biass at all it is sometimes difficult to get a tetrode to cut off by just clamping down the screen, but it will keep it at a safe level if adjusted properly.

Grid leak biass was the way to go for many old timers. I prefer to have some fixed protective biass as well and eliminate the clamp tube. But that is my preference and it requires an additional power supply for the bias.

Johnson Rangers have only grid leak bias on the final, and a clamp tube. You actually have to drive the final into class C operation. You can actually watch the output come up and the plate current drop back as you increase the grid drive to the final. Its kinda neat to watch.

                                              The Slab Bacon

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