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Author Topic: Building 4-1K amp - parasitics and other problems  (Read 8258 times)

Posts: 71

« Reply #45 on: July 05, 2008, 07:44:16 AM »

I do have a 10 ohm resistor in the B- lead for measuring plate current. Can you measure grid current there also?

Posts: 1010

« Reply #46 on: July 05, 2008, 08:23:31 AM »

The plate bypass leads have nothing to do with VHF. They are on the wrong side of the plate choke.

 >>> My mistake, I was actually referring to the plate coupling capacitor and any additional inductance created by excessive lead lengths, then confused that with the bypass.

Sorry if I confused you Dennis....the 4th was a long day, but a good time.

I hope everyone else had a great day...


Posts: 9748


« Reply #47 on: July 05, 2008, 01:55:12 PM »

If the B- lead floats from the chassis, you can certainly measure grid currrent there.

The B- lead has to go all the way up to the amplifier standby relay contact.

You should CLAMP that lead to the chassis with diodes both in the power supply and in the RF deck. The cathode of the clamp diode goes towards the B- lead, the anode to the chassis.

Then the grid meter goes from the B- lead to the chassis and it will read grid current of both grids. The negative terminal is to the chassis. The positive to the B- line.

The clamp diodes have to have a few times the voltage drop across the grid meter or they will affect current reading, and naturally they need bypassed with a low impedance capacitor like a .1uf low voltage disc with very short leads.

The clamp diodes will conduct if the HV positive lead ever faults to the chassis. That would tend to lift the negative rail to full HV in a NEGATIVE polarity. With only a B- clamp resistor, the negative rail might lift to nearly full HV for a few seconds while the capacitors discharge. This can severely damage components in the amplifier. Another problem is if a B- wire is disconnected the negative rail can lift to full HV negative.

 With the diodes arranged so the cathode is to the negative rail and the anode to the chassis, they will conduct and hold the negative rail at a few volts. This will ensure operator safety and protect the grid and plate meters and other parts from a fault or open in the HV system.

There isn't any possible fault that would be the opposite polarity, but I've also seen back to back diodes used.

Never depend on a resistor to hold the B- line to chassis potential like some circuits use.

73 Tom

Posts: 71

« Reply #48 on: July 08, 2008, 07:49:22 AM »

I have many arrl handbooks and the radio handbook by Bill  Orr. In all these books I have never seen the circuit you describe used. They all use the 10 ohm or larger resistor in the b- to chassis to measure plate current. I have never seen clamping diodes used in this way.

My circuit uses this resistor as a shunt. Diodes across this resistor would prevent the meter from working.

Can you draw me a circuit or refer me to a circuit that uses this technique?

Thanks Dennis

Posts: 39

« Reply #49 on: July 09, 2008, 06:11:48 AM »

Go to this site:
If you have trouble with the URL put "triode board" in google.  Then on the menu select download the manual.  It is an education on amplifier and power supply protection techniques.  It explains the metering, safety issues, surge protection.  
You need the diodes Tom recommends.  I have first hand experience using them and still managed to destroy some components in my amp when a "believed to be good" tube shorted grid to plate.  My diode worked for awhile (a second or two ?) as the  plate supply capacitor dumped amps into the amplifier, and then a small wire connecting the board the diode was mounted on to ground opened up.  Somewhere in the docs I think you will see that the diode should be mounted directly to chassis, and that's why Toms suggestion to use two of them (one in the amp and one in the ps is a good one).  It won't upset your metering if you do like it says in the triode board discussion.  With advice from G3SEK, W8JI, and W1QJ you can't go wrong.
73, keep going,
Rick  K2XT

Posts: 71

« Reply #50 on: July 22, 2008, 01:08:57 PM »

Sorry I haven't been keeping everyone up on my progress.
My power supply took a dump.
I've been attempting to rewind a transformer for a new supply.

Steve I sure would like to get a schematic of your grid driven 4-1k amp. Or anyone else who might have a link to a schematic.

The last time I tried the amp (before the ps went) it was looking pretty stable. I was getting good power from 15m.

If anyone has attempted rewinding a large HV transformer I sure could use some hints.

Posts: 31

« Reply #51 on: August 08, 2008, 12:50:57 AM »


This thread is a really great read. I just didn't see ne1 mention ni chrome wire for the parasitic choke, or even stainless steel wire.

It seems logical the the lower the resistance of wire the better. but not if you want to swamp a RF choke or RF inductor.

Also round wire is better that flat strap conductors, round has more skin effect R, so again swamps the inductance thus lowering the Q, and it's ablity to make the amp become an osc at VHF.
 I found all this at:
AG6K's web sire

73 OM


Posts: 31

« Reply #52 on: August 08, 2008, 12:52:39 AM »

My bad they did mention ni chrome and stainless steel
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