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Author Topic: Amplifier loading questions  (Read 1419 times)

Posts: 282

« on: February 26, 2006, 02:47:09 PM »

Hello all,

I was hoping to catch Tom, W8JI and have him address some amplifier loading questions.


I have read numerous forum postings in which you address amplifier tuning and loading questions, for which I have some additional questions.

The following is a summary of the equipment that I am using;

Exciter - FT-847
Amplifier - AL-80B converted for 6M monoband service, using a 2-20pF vacuum variable plate tune cap, the original plate air variable cap was moved to the tune position.  The plate blocking cap is a 1000pF doorknob cap and the bypass cap at the base of the HV choke is a 500pF doorknob type, both 7.5KV rated.  The HV choke is 24 turns of #22 ga enamel wire.  The tank coil is 3 1/2 turns of 1/4" copper tubing with 1/4" spacing between turns.

The questions that I have are the following;

1.  When I tune the amplifier, I am not able to use the full output of the exciter.  At 87 watts output the IG is 105ma. and the IP is 625ma. and amplifier output is 1030 watts.  This is well above the stated IVS rating.  By doing this, everything you have written, I am not able to tune the amp properly.  Are the IG and IP readings what you would expect on 50MHz?  When talking about exciter overshoot, how do I handle the tuning issue when I cannot get to full rated exciter output?

2.  I have the power transformer jumpered for 245VAC with the incoming power measured at the outlet at 244VAC, with no load.  I have not measured it under load.  The HV meter shows 2800VDC no load and 2600VDC fully loaded.  Prior to suffering a HV short to gound, the HV read 3100V. This brings more questions;  a.  Does the lower HV reading have anything to do with a low IG reading and a high IP reading?  b.  Should I, or would changing the transformer tap bring the HV back up to the 3000V range where it is supposed to operate?  c. Would a HV short to ground fail a component and cause a lower HV output or fail something in the meter reading circuit for HV?  d.  I have a Fluke VOM and a HV probe, would this give me an acceptable HV reading to compare to the onboard HV meter?

3.  What is the safest way ( for personal protection and for component protection) to measure the filament voltage?

4.  As you had previously posted, after the HV short to ground, I did have a shorted diode at D117.  After replacing this, the IG and IP meters are working properly.  I also replaced the Q301 and Q302 on the meter board.  This corrected the peak reading problem that I had.  The onboard wattmeters are reading about 200watts high.  I imagine the meters are in error as they were designed for HF service and not for 50MHz?

I apologize to all for the long posting.  I appreciate everyone's input and do not mean to limit this to one person's reply, but had referenced Tom as he has written quite a bit about amplifier tuning and loading.

Good day!

Doug - N9XTF


Posts: 10248


« Reply #1 on: February 28, 2006, 06:19:26 AM »

Tom's e-mail address is listed in his profile. Give him time to answer, as he's a busy guy!

Alan, KØBG

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