{Collins} Re: AM Modulators

Quoting Clio Gunsmith <cliogunsmith@xxxxxxxxxxxxxx>:

.. the 4-125A tube used as a AM modulator in Class AB1 we
> find that two tubes in push pull at 2500 volts on the plates will produce 330
> watts output and exhibit a plate to plate load of 20,300 ohms. If we connect
> four tubes in push pull parallel the output would increase to 660 watts. Will
> the plate to plate load resistance be cut to 20,300/2 or 10,150 ohms??? 


Yes, I would expect it to be half what it is with two tubes.  Matching may be a
advisable: to get each tube in the parallel pairs to carry about half the

My limited experience with modulators of that power class is that you have to
search hard for the right modulation transformer.  And if your modulator tubes
are not seeing about the right plate to plate load impedance, the thing won't
work right.

Do you have a transformer already that you are trying to build a modulator
around?  If so, you may find it easier to simply use 4-400's and run them at
higher plate current.  All else equal, lowering the modulator plate voltage and
increasing the plate current (or either alone) will reduce the needed plate to
plate impedance.

Similarly, if you un-load the RF final, to comply with amateur regulations of
1.5 kW PEP for instance, the "RF Plate Load" goes UP and your modulator may
become unhappy.

If you have a commercial modulation transformer and the P-P impedance is lower
than the needed 20,000 ohms, it may be intended for triodes such as the 833.

If you have a mod transformer with unknown turns ratio, put some volts on the
primary (12 volts, or way higher if you like) and measure the seconday
voltage.. this gives you the turns ratio. Square that for the impedance ratio.

You can operate a modulation transformer away from it's intended impedance
levels (assuming the impedance ratio is right) if: 
 A) The differnce from intended impedances is modest
 B) You derate the thing. (A rule of thumb is to not excede either voltage or
current levels expected under normal impecance levels.)  

So, at twice the impedance levels, the (audio) voltages must not be more than
7/10 normal. This will reduce the power handled by half. (Twice or half the
normal impedance level is likely way to much change.)

1)Anyone with a mod transformer for a Desk Kilowatt or KW-1, let me know..
*might* need one here!

2) 600 watts of audio will 100% modulate a 1.2 KW carrier - that is nearly 5 KW

3) Class AB1 has no grid current, and so theoretically zero power is required.
(No, a 12AU7 driving the 4-125's may not work well.)  If you drive the grids
into condution, the driver needs to deliver power into a varying load. (Yes, a
pair of 6L6's triode connected driving the 4-125's with a hefty driver
transformer, will work fine.)  Swamping the driver may be needed to keep
everything happy.  If you are homebrewing such an arrangement, do test
carefully for RF oscillations in you modulator section - it can happen.

4) (I know, I digress somewhat):  The final RF stage must be operating at
suffient grid drive to support output at four times the standing carrier power

5) Having a scope to see the transmitter output can be invaluable.. either
simply on the carrier or arranged as a modulation monitor with trapazoid

6) As archaic as it may seem, a spark gap at the modulation transformer primay
may save your transformer a trip to the re-winder.

Happy Modulating

Roy Morgan
7130 Panorama Dr.
Derwood MD 20855
Sponsored by the Collins Collectors Association http://www.collinsradio.org
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