[Heath] DX-60B Update

[Glen Zook]

Several things:

First of all is that the DX-60B meter is not actually reading plate current!  It is reading cathode current which includes the screen current.  As such, you are not reading the true power input.  The ratings "assume" 600-volts of plate voltage where a meter reading of 150 mA results in an input power of 90-watts.  However, the 6146 typically has about 12 mA of screen current.

>From the RCA Transmitting Tube Manual, typical Class "C" has a screen voltage of 250 VDC and a screen power input of 3-watts.  Using the formula P = IV, then P/V = I.  As such, 3/250 = 12 or a screen current of 12 mA.  This screen current has to be subtracted from the cathode current rating to get the actual plate current.  Therefore, 150 mA - 12 mA = 138 mA.  When multiplied by the plate voltage of 600-volts, one gets the actual plate input power of 82.8 watts, NOT the "rated" 90-watts!

Heath did this with the DX-20, DX-35, DX-40, DX-60- series, DX-100, and DX-100B as well as several other tube-type transmitters.  This was done to keep the average amateur radio operator, especially the Novice Class operator, from abusing the final amplifier tube(s).  When the Novice Class operator was running 120 mA (the current stated, in the manual, for "75-watts input") the perceived power would actually be 72-watts.  However, subtracting the 12 mA screen current, the true power input would be 64.8-watts.  Since the Novice Class power input rating, when the DX-60- series was first introduced, was 75-watts, having a true power input of around 65-watts kept the operator perfectly legal even though he / she was actually running somewhat lower power.  Frankly, this difference in power would not be detectable on the other end of the QSO.

The average efficiency, taking into consideration circuit losses, etc., of a Class "C" stage is typically between 50% and 60% on the bands up to, and including, 20-meters.  However, this usually "drops off" as the frequency increases, especially with the 6146 and 40% is not uncommon at 10-meters and, depending on the actual tube, can be even lower.

If one is getting an output power of 45-watts, and the actual input power is 82.8-watts, then the efficiency would be 45 / 82.8 = 54.3% which falls well within what the expected efficiency would be.

Glen, K9STH 

Website:  http://k9sth.net



________________________________
From: Doug & Kathy Davies <dkdavies3 at gmail.com>
To: Heath at puck.nether.net 
Sent: Monday, April 9, 2018 9:24 PM
Subject: [Heath] DX-60B Update



The replacement parts for the DX-60B arrived today, R35 the 270 ohm, 2W resistor, R34, the 100 ohm 7W resistor and C19 a 0.001uf, 1.4kv disc ceramic cap, all of which are in the power supply circuit.  I installed them all and proceeded to the test phase.  Everything went according to the manual as far as the tune-up was concerned.

Measuring the output, the rig is putting out about 45 watts to a dummy load..  The spec says the input power is 90 watts so I would think the actual rf output should be around 70-75 watts.  Not sure why this is happening.  I'm getting the proper grid and plate current to the 6146 which tests good with a tube tester.  Any ideas here?

Anyway, it's good to have the old girl running again and I really appreciate the help I received.