[Boatanchors] A couple more questions

[Vic K2VCO]

Chris Howard wrote:

> 1)  The PA uses two 250TH tubes in push-pull.  There are two filament
> transformers, one is a potted type and one is not.  The potted one is
> leaking gunk.  I believe I have another unpotted one I could swap in
> there.  The transformers are both connected to the same AC supply input
> and their outputs are also joined.  If I replace the gunky one, do I
> have to be careful not to get the new one wired up in an out-of-phase
> situation?

Yes, if you hook them up backwards you will in effect short them.  You 
can check by connecting two of the secondary leads together and 
measuring the voltage across the unconnected ones.  If you get 10V, you 
have connected the wrong ones!  When you get close to zero volts you can 
connect the free wires together.

> 2) The Bias supply produces -100 volts DC.  Also on the bias supply
> there is a relay which turns on a little panel light.  Even though I am
> getting the -100 volts that I think I should, the relay is not lighting
> the light.  Schematic in pdf format is available here:
> http://users.frii.com/yipyap/twiki/bin/view/Main/BiasSupply
> The relay coil seems to be good, as tested with a 6 volt battery.
> I've attempted to clean the contacts.  Otherwise, I can't figure out
> what is wrong with it.

Is the relay closing when the bias comes on?  If yes, the problem is in 
the circuit supplying voltage to the coil.  If yes, then it's in the 
bulb circuit.  Or the bulb is burned out.

> 3) When I am able to get the exciter excited, it produces about 20 watts
> of output.  Routing that into the PA, my grid current indicator still
> says nil.  The PA is a grid driven design.  Maybe I've got my grid
> current meter wired backwards or something?  I will have to watch real
> closely and see if it is doing the pin-to-the-left thing.  It seems to
> me that having grid bias and having RF input I should see some grid
> current, assuming I get the grid input tuned right. No?
> PA schematic is available here:
> http://users.frii.com/yipyap/twiki/bin/view/Main/PowerAmplifier

The filaments must be lit and the filament transformer center tap path 
to ground continuous in order for you to see grid current (I hope the 
plate supply is off!).

> 4) One of the PA controls, a grid inductance bandswitch,  uses a Millen
> right-angle drive.  Unfortunately for me, I've stripped most of the
> teeth off of one of the bevel gears in that thing.  Any good ideas on
> where to get another gear or another right angle drive?  Shaft size
> looks like 1/4 inch.  If not,  I may
> think about rerouting that function to a panel mounted wafer switch.
> If grid input is only a couple hundred volts and 20 watts of RF, I don't
> think it will require anything massive.  

If you move the grid bandswitch the additional lead length may make the 
amplifier more likely to oscillate at VHF.  Certainly it will affect the 
neutralization at the operating frequency.  I wouldn't do this unless 
absolutely necessary.

-- 
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco