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Reviews Categories | Amplifiers: RF Power - HF & HF+6M | Hallicrafters HT-33A Help


Reviews Summary for Hallicrafters HT-33A
Hallicrafters HT-33A Reviews: 1 Average rating: 5.0/5 MSRP: $$795.00
Description: 1000Watts Input.
Product is not in production.
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KZ4B Rating: 5/5 Oct 14, 2009 20:20 Send this review to a friend
kz4b@hotmail.com  Time owned: more than 12 months
The 50+ year old amp puts out 600 watts continuously and 800 watts PEP + using (what I believe to be) the original Penta Labs PL172A tube with well less than 100 watts drive from my matching Hallicrafters HT 32A Sideband/AM Exciter. It puts out approximately 200 watts of AM carrier with about 25 watts of drive carrier from the same HT 32A exciter. Audio reports on both AM & SSB with this combination are excellent using the HT 32A/HT 33A combination!

The amp weighs 120 lbs. and is built "like a tank" including a massive choke input high voltage (HV) power supply that uses an oil-filled (approx. 2,300 "working" V.D.C.) HV filter capacitor. The input to the amp is un-tuned--but a very steady nominal 50 ohms due to a "swamping resistor" across the grid & cathode of the PL 172A. This means it would be an ideal linear amp for use with an exciter having load impedance-sensitive solid-state finals!

A couple of easily overcome cautionary notes are in order. The PL172A amplifier tube has a tendency to get gassy from a lack of use over many years and is NOT commonly available. My tube was gassy (evidenced by excessive plate current draw under all conditions)-but was rejuvinated by leaving the filament of the tube on for up to a week with the plate voltage TURNED OFF. This allows the "getter" compound inside of the tube to react with the offending gasses inside the tube with the help of the heat from the tube filament. EIMAC made a substitute tube (with a ceramic/metal instead of glass/metal enclosure) designated 8295 which is said to be less likely to become gassy. Fortunately--each brand of tube is often available from larger "military surplus" / NOS tube suppliers for around $350.00. Shop around!!! The original HT 33A amps used tube-type (3B28, etc) HV rectifiers--but most people (including myself) install solid state substitutes which slightly increase plate voltage/output power and greatly reduce heating internal to the amplifier. The HT 33A is wired for 110/115 VAC only due to it's age of manufacture. If you have 240 VAC available--you can always find a 1 kVA or larger "buck & boost" (best value) or conventional two-winding 120/240 VAC step-down transformer in order to minimize power supply voltage drop. Get a reproduction or original HT 33A Operation Manual from the Internet and READ IT. As best I can tell--the only differences between the HT 33A and the later HT 33B were the addition of a separate plate current meter (two meters total--instead of one common multimeter)and a higher screen grid voltage (which made no significant operational difference from my experiments). NOTE that a small metal heat sink/metal pad goes in one pin of the PL172A/8295 tube. If it is missing--fabricate and install the metal pad before operating the amplifier if you value the tube. And lastly--there is a manually-resetable plate current overload circuit breaker located on the front of the HT 33A that often has a tendency to trip at ever lower plate current values with much use and aging. The make and model number was located on the back of my circuit breaker--so I was able to "Google" the (still-in-business) manufacturer to obtain a new replacement allowing full-power operation.
 


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