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Re: [Amps] 572B/T160L tube class C

To: Carl <>,
Subject: Re: [Amps] 572B/T160L tube class C
From: Alek Petkovic <>
Date: Sun, 22 Aug 2021 08:23:04 +0800
List-post: <>
Yeah, The AC mains voltages are all over the place here. Even more so these days with everybodies (including mine) solar arrays cramming extra power down the lines.

I use common 5W wire wound resistors in the 6.3V filament line. Combinations of 0.1 and 0.22 Ohm units, series/parallel to get the required resistance at a power rating that will not create too much heat in the resistors.

Typically, a FL-2100 runs at 6.9V at the filament pins, with 245V mains, so I aim to destroy about 0.6V to 0.7V.

Two tubes in parallel run 8A at 6.3V, so the resistor needs to drop 0.6V to  0.7V at 8A. Therefore the resistor needs to be between 0.075 and 0.0875 Ohms. The power dissipated in the resistors is 5 to 6 Watts, so I aim to have at least 20 to 30 Watts capability in the resistor combo. This means that they run very cool.

In the above example, 4 x 0.22 Ohms in parallell will give me 0.055 Ohms and 4 x 0.1 Ohms will give me 0.025 Ohms. Put those two units in series and you have your 0.08 Ohms at 40Watts.

The above example is perfect in theory but in reality, you have to solder them into place and usually add a short piece of thick wire into the line. This adds resistance, so in my experience. 3 x 0.1 Ohms in parallel and and the same again in series, gives you 0.066 Ohms  at 30 Watts and that is usually perfect to give the nominal 6.3V at the filament pins.

The series/parallel combination of 5W resistors can be made compact enough to fit under the chassis of the 2100 amplifiers.

The Ameritron 811 amplifiers are not as easy to physically modify but it can be done. These amps would be better candidates for resistance wire in series, due to they way they are built. Those bloody mother boards with everything on them are a pain.

I've also done it with a couple of Dentron Clipperton L amplifiers. 16A of filament current that the 4 tubes draw, presents extra power dissipation challenges for the resistors and the resulting bundle gets big and untidy but there is room inside the cabinet, down one side from memory. Those jobs were a long time ago.

Anyway, the result is that the owners of the amps that have been modified in this way have never been repeat customers for buying 572B tubes, other than to buy spares.

Probably another benefit of adding this resistance is the reduction of around 15% of filament inrush current. The Chinese Shuguang 572Bs are prone to filament breakage and this may go part of the way to minimising that problem.

The Shuguang tubes are sometimes hard to get due to irregular production runs. When they are in short supply, the factory rejects seem to find their way into the market.🙁

One time, I had a whole box of 10 tubes turn out to be rubbish. Luckily, my supplier in China is a personal and family friend, so I got my money back on them, in the form of a credit for a new lot of 10 when the next production run came in.

I do have a true RMS meter but only got it recently. I have never compared the readings against my other meters. I will do it for my own curiosity. Thanks.

73, Alek, VK6APK

On 22/08/2021 12:43 am, Carl wrote:
Interesting Alek and thanks for the input. Have you come up with a specific resistance for the various 572B amps rhat would make it easier for others?

Around here the 120/240V lines can vary from about 118-126/236-252 at times due to population increase and frequent storm rebuilds along with rerouting distribution.

I have Liberty Utilities fairly well trained by now to keep the pole pig tap across the street at 236-242. Increases lighting life also.

At one time there were at least 3 Chinese 572B producers. Except for Shuguang the others didnt want the headaches of dealing with hams and concentrated on the Audiophool market where profits were much higher.

I picked up a nice variety of resistance wire from a surplus electronics shop about 40 years ago to deal with Dentron and others. A Clipperton L, MLA-2500, and DTR-2000L get rotated thru the vintage TX benches, mostly on AM and a little CW and SSB.

Be very sure to use a TRUE RMS meter for measuring right at the tube sockets. Most of the cheap imports dont qualify. I currrently use a military surplus Fluke FM76 from when they dumped a load on the market many years ago.

Ham since 1955

----- Original Message ----- From: "Alek Petkovic" <>
To: <>
Sent: Friday, August 20, 2021 10:08 PM
Subject: Re: [Amps] 572B/T160L tube class C

Over the years, I've imported around 200 572Bs from China.

They are definitely not as good as the US ones but they have proven to be very serviceable, when treated right.

The single biggest proviso that I drummed into everybody that I ever sold them to is to make absolutely certain that the filament voltage stays at 6.3V. The Chinese tubes fail very quickly with filament voltages of around 7V+, which are the rule in both Yaesu, Ameritron and Dentron amplifiers when connected to Australian 240V+ mains voltages.

It is a very simple thing to add appropriate resistance to the filament circuit to bring the voltage back to 6.3V. I've modified very many amps in this way for many hams. The result has been that all those amps are running happily on Chinese tubes, which are lasting many years.

73, Alek, VK6APK.

On 21/08/2021 8:00 am, Stan Gammons via Amps wrote:
I agree. I've had so so luck with the Chinese ones. They are nowhere
near the quality of the American made tubes. When America made lots of
tubes; I think we made the best tubes in the world. But, I'm a bit biased :)



On 8/20/21 6:52 PM, Carl wrote:
The Chinesw 572B and 811 (it is NOT a real 811A, not even close) are mostly all junk looking for a trash can. They sometimes let a good one sneak thru.

Best bet is to pay the freight for a real NOS Cetron or United Electronics,
the only USA real manufacturer who private labeled them for others.

----- Original Message -----
From: "Jeff Blaine" <>
To: <>
Sent: Thursday, August 19, 2021 10:33 PM
Subject: Re: [Amps] 572B/T160L tube class C

How much service do you expect from a new set of tubes? It's one thing to
know "it can do it, if I want" - but then not actually need the
capability. It's another thing if you are saying "I want to run a 572b
24/7 at full output."

About 10 years ago I was working on a SB200 which was initially setup to run RTTY. I did quite a lot of profiling of those using the Chinese tubes of the era. Watching very carefully to respect the Pd max of the tubes, I
found a typical 25% drop in Po over about 100 hours of testing. The
testing was typically 3-15 minutes key down 100% carrier per interval.
Plus on-band rag chew, contesting & DX (this was pre FT8).

Carl is about a million times more experienced than I am but I think maybe the Pd spec vs. actual was a bit optimistic for the Chinese tubes I had because this drop off seemed excessive. However my abuse of those tubes, compared to typical ham use, was really bad. Also the SB200 positions the
tube horizontally which may be a factor as is the generally poor
circulation even with my augmented cooling.

In the end, I abandoned the 572b and went with the GI7T which was an
excellent performer by comparison although it required quite a lot of
changes to the SB200. Unless there is a specific reason to use the 572b, if I were building a high duty cycle amp in the future, I would probably
want to use a metal/ceramic type tube instead of a glass one.

Good luck!


On 8/19/21 8:50 PM, Carl wrote:
That tube was designed for AM BCB service by Taylor but didnt catch on
there or for AM hams as the T-160L in the late 50's.

It was later bought by and designated the 572/T-160L for Unitrd
Electronics alone.
They couldnt keep up with demand and sold production rights to Cetron who
had a much larger production facility.

UE then became the 572A and Cetron the 572B. The step top (shouldered)
glass was Cetron and the round top was UE.

At some point the T-160L was dropped for both.

You may also find a 572B with the round top and the Cetron name, those
were built by UE as the demand was even too much for Cetron alone at
times....such as the SB-200 and the Clipperton L.

I have examples of both (no Taylor T-160L) as well as other versions
including OEM named such as Dentron, Waters, plus Raytheon, GE, and
several others who did not build their own. The top shape ID's the

I cant remember ever seeing an actual T-160L spect sheet or the tube.

Ham since 1955

----- Original Message ----- From: "Fuqua, William L." <>
To: <>
Sent: Thursday, August 19, 2021 8:56 PM
Subject: [Amps] 572B/T160L tube class C

I am looking for full power data for the 572B/T160L operating ICAS
class-c CW and AM.
The only thing I have found thus far is in an ARRL handbook, but it is
obviously the 811A specs.
Not the full 160W plate dissipation specs.

Bill wa4lav

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