Henry RF AMPLIFIER

High Voltage Arching/Popping

 NE7X - 11/28/2011

 

If your Henry vacuum tube RF amplifier is experiencing intermitting HV (High Voltage) arcing or popping, this is most likely due to:

1. Higher then normal HV

2. No HV regulation

When the HV is running higher then the circuit is designed for, the excess HV will jump/discharge to a ground point. In some cases when the HV arc/pop occurs, while watching the HV meter you will see the HV spike >4KV and then come back to normal HV values (~3KV) after the pop. Another sign is when keying the amplifier, the HV will drop to near zero.

 

Things To Check:

• An open equalizing resistor across one of the HV filter electrolytic capacitors. Normally there are eight filter capacitors in series with an equalizing resistor in parallel across each capacitor (8 resistors total). You will need to remove one end of each resistor from the circuit to test its ohm value. Reference the schematic for resistor values. When testing, all resistors should be +/- 10% of each other.

• An open or shorted HV electrolytic capacitor. Henry used high quality computer grade electrolytic capacitors. You will need to remove the capacitor by the two mounting screws.  You will see a rubber gasket in-between the two terminal screws. If the capacitor shorts, a plastic tit will push through the rubber gasket and protrude outward.  This is an internal "fuse" which opens the capacitor when the capacitor shorts. Another indication of a bad capacitor is it will be covered with a wet sticky liquid. This is the internal electrolytic compound which gives the electrolytic its properties. This indicates the capacitor is no longer the value written on the case, most likely its a much smaller value. For Henry RF amplifiers which use a single oil filled capacitor, check for oil leakage.

• Shorted ALC zener diode. This diode is located across the electrolytic capacitor closet to the B- side of the eight series HV capacitor chain. When the diode shorts, the last electrolytic capacitor is electrically removed/by-passed from the series HV capacitor chain, resulting in only seven capacitors in the filter circuit.

• Open B- HV negative ground return resistor.  This resistor connects from the negative HV side of the HV series capacitor chain to chassis ground. It is used for HV current imitating in case of a HV over-surge (kind of like a fuse). When it opens the HV plate voltage will drop to zero when the amplifier is keyed. I have more information about this resistor on the following webpage: Henry_B_minus_resistor.htm

• Cracked HV wire insulation. Over time the HV B+ wire(s) will become dry, hard  and brittle. Any movement or vibrations of the wire could cause it to crack, exposing the bare wire to no insulation. Its recommended to replace all dry/brittle HV wire, especially on the B+ side. When doing so, consider HV wire lead dressing. Do not place or run the HV wires near low voltage or signal wires. There could be some interaction. Replace the HV wire(s) with new soft flexible 30KV wire. You can find some on http://ebay.com for less then $10.

 

Preventive Measurement Recommendation/Enhancement

For about $20 worth of insurance, added four 10 Ohm / 10 Watt pulse resisters in series with the HV B+ line feeding the RF deck. If there is any short duration HV spike on the HV B+ line, the pulse resistors will absorb the HV spike, kind-of like a HV shock absorber. I learned this enhancement from my childhood friend Tom W8JI, who designed RF amplifiers for Ameritron.

Henry 2K Classic

Add four 10 ohm (40 ohms total) HV Pulse Current Limit Resistors in series with the HV B+ line feeding the RF deck. One 10 ohm resistor for every 1000V of the HV supply. They provide HV pulse current limiting if a voltage surge occurred on the B+ HV line due to a tube shorting or filter cap going bad.

 RCD Part Number: 175P-10R0-FBW

Ameritron Part number: 110-1100-1

Description: RESISTOR, 10 Ohm 10 Watt, 175P, 10%

P = Pulse Current Limit Option

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GOOD LUCK in troubleshooting your issue. If you find a different cause then the above items listed, please email me with what the symptom and fix was. I will add that information to this webpage so others may benefit from that knowledge.