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Hallicrafters HT-33B linear amp throws internal breaker

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Aug 6, 2014
If a single-pentode linear like a Hallicrafters HT-33B has a gassy tube like a PL-172, will flipping the High Voltage switch on cause the Cathode Current to spike and throw the breaker a.k.a. the High Voltage switch back off? Or does that sound like something else? I tried leaving the filament on for 5 days.

I was excited to acquire a matched transmitter(exciter) Hallicrafters HT-32A and HT-33B linear amp (which could double for a deadman for a sea wall, and might). At some point years ago the former owner cleaned them up, replaced wiring, electrolytics caps and, as is common, swapped out a couple of the 33B's big, toasty rectifiers with ceramics. It then faithfully served as a phone patch to Antarctica with a beam atop a tall building.

Now, I partially disassembled them, dusted & reasonably cleaned everything, and found the 32A exciter to work CW wonderfully on 40m and to sound fantastic received on 10m via cross-country web sdr on SSB & AM.

The linear, however, immediately tried to peg (at least upwards of, if not all the way) the meter set to cathode current before throwing the internal breaker which flips the high voltage switch back off.

Mind you, the previous owner and a review online both cautioned me the glass tube PL-172 typically gets gassy "as evidenced by excessive plate current under all conditions" but is commonly getter-rejuvenated by keeping the filament switch on for up to a week before use. I did this for 5 days before testing (assuming the filament was really on just because the switch was up and the fan came on, and it doesn't have a bad filament transformer or something). I'm not really confident 48 more hours will do the trick.

Now, I do have THREE more PL-172's to try, so I guess I'm more fortunate to have them than unfortunate to have to leave each one filament-on, getter-basting for a week each before trying them one by one (the poor old fan).

If this cathode current meter-peg & breaker pop is symptomatic of a hopelessly gassy pentode, I'm stoked because one of the remaining 3 spares may rejuvenate. But if you can tell me it sounds more like a shorted transformer or something I'm less stoked.

Any insight on this is super appreciated!
Thank you
Aug 6, 2014
is the bias reversed ??

Is my control grid bias wired backwards (thus positive?) :?:

A supposedly working AB1 linear amp (Hallicrafters HT-33B)
has appeared to have instantly fried two PL-172 pentodes in a row :oops: upon turning High Voltage On (filament was on a long time, zero excitation yet, bias control in full counterclockwise begin tuneup position)

maybe they were just faulty, but does this seem right --
(with the tube removed and filament on) :

Checking control grid bias:
with DC voltmeter's black negative probe in amp socket hole 2 (holes 2 and 6 correspond to control grid pins, more on that below)
and red positive probe on the amp's anode/plate clip
I'm reading 156VDC (not -156VDC)
to read negative, or -156VDC I would have to reverse the probes

Did someone accidentally reverse the wiring and I've got positive power to bias? That would explain the electrocution noises, pegged current drain, throwing internal breaker, and seemingly creating two dead tubes that were not prior...I know a jarred, shorted tube could, too, but please advise the probe positions and polarity indicated thereby, above.

Thank you very much

P.S. – additional info.:
Flipping High Voltage On without the pentode in is fine, including with the ceramic rectifiers in.
Funny, too – both the tube data sheet and schematic both say pin holes 2 & 6 both go to control grid but 6 does nothing. Hole 2 gives the above readings against anode contact.

The heater (holes 3 & 5) is reading 7 VAC, supposed to be no more than 6.3 (5% high) per the data sheet and definitely no more than 10% high a.k.a. 6.6VAC... all capacitors were supposed to be reasonably new but now I can't get a straight answer, they're of course not the old wax paper, but maybe they're getting out of date again, the electrolytics...
Inordinately excessive current drain supposedly indicates gassy tube common of glass PentaLabs PL-172 and is supposedly, commonly rejuvenated by leaving filament on up to 1 week (multiple sources). I don't think gas is my sole problem/drama here, but I had tried one of the tubes 5 days and another 3 (both sounded like they blew up the world, pegging current meter and the first threw the switch back off). The 3 day one didn't trip the switch but now does nothing with high voltage on like it's not in the socket (after again with the arcing sounds and current meter jumping over 400mA), the High Voltage on indicator bulb now burns with that tube but no reaction to bias knob, plate tuning or loading. Like the tube blew up. I am not trying to “warm up tubes for days”; it was a degassing rejuvenation resort technique advised by both a PL-172/HT-33B enthusiast on eHam and the seller if current drain is excessive under all conditions, like beginning tuneup. I realize this is not appropriate for all tubes in absence of plate current.

Thank you very much in advance. Is my bias accidentally re-wired to being positive instead of negative?? (probe polarities/positions listed above) This would be great to find out as Yes because I could fix it myself and try one of my last two, irreplaceable pentodes!
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