Ameritron AL82 stresses tubes

[K2OWR]

The Smaller AL080B has cutoff bias on the tube at all times until RF drive is applied, so it’s idling current literally is “zero.”  When you apply RF drive, that’s detected and then the amp is “biased up” to normal Class AB2 bias.  It’s newer design not implemented in the AL-82 for some reason, although they are still 3-500ZGs.

Has anyone ever made a mod to the AL82 that would incorporate this method? As it is, the tubes draw a lot of current and glow red needlessly when there is no rf drive.

[W6LAR]

What is "A lot of current?"

[W6LAR]

Ok, I should add to asking "What is a lot of current?" with why would you key the amp and not apply RF drive at in some timely manner? Yes the AL-82 does apply bias without RF being present. As per the manual its set at 75 Ma for CW and 150 Ma for SSB. Now I know that my tubes do not glow red when biased at these levels in my AL-82. So if someone has an AL-82 and their tubes are glowing red at the prescribed bias levels I would suspect there is more wrong than improper bias. Yes it would be nice to incorporate the "no bias until RF is detected" and maybe someone can fill us in on that.

Have a great day....

[K2OWR]

:::: My particular amp, when keyed with no drive, will draw about 250ma in
the SSB position, which is more than the manual says it should. The tubes
are brand new, and the ones I took out behaved the same way. Ameritron
basically said "so what" when I posed the question. Both Zeners test ok.
I just felt that the method used in their AL80B models made a lot more sense
and I wanted to retrofit my amp to that configuration and wondered if anyone
had ever done it. It would likely not be difficult to simply copy the method
used in the AL80B, with the appropriate mods to accommodate the extra
tube.....no?

As to keying and not applying drive, on SSB that means that the tubes are
seeing the high plate current all the time which makes them glow red a lot
more then they would if they were cut off in between words....like the newer
circuit.

BILL K2OWR

[K6AER]

You mentioned the tubes glow red under a non RF drive condition. Normal current in the amplifier keyed mode is about 100-150 mA depending on the bias zener diode and manufactures design. There is a possibility that the tube bias zener is shorted. At 110 mA (normal idle current) at 3300 volts the tube dissipation should be about 360 watts or about 180 watts per tube. With normal tube cooling that is well under the 500 watts dissipation per tube. Normal anodes on 3-500’s will not glow until driven beyond idle current.

If you are comfortable checking the bias zener on the AL-82 that is where I would go first. It is if I remember correctly it is a 7.5 Volt 10 watt stud mounted zener 1N2917A

You did not mention what the idle current is when keyed. The amount of idle dissipation using EBS is a small part of the total tube dissipation when the amplifier is being driven to 1500 watts out. This is why many older 3-500Z amplifiers don’t bother with EBS circuits. When the EBS circuits are not adjust properly they can add significant amount of low level  distortion.

You can add the EBS-1 bias switch board for about $50.00 if you feel the need.

Good luck.

[WB2WIK]

Hi Mike,

Bill actually did mention what the ZSAC was, it's 250mA, which is 100mA higher than the manual says, and about 100mA higher than it needs to be.   Thus, his tubes start to turn red (after a while, I imagine) even without drive when the amp is keyed.

I think this is non-destructive and probably won't even age the tubes any faster, but it certainly does waste power and just makes a big shack heater.  In the winter, probably good, in the summer, probably bad!

The circuit used in the AL-80B works well and I'm surprised they didn't implement it in the AL-82 and the "bigger" amps over time.

I suspect Bill's 7.5V zener may have drifted to a lower voltage value and did recommend he check it; if it's "good," he can add a few 1N5402s or something in series (opposing polarity) to boost it up another few volts and knock the idling current down a bit.

[W8JI]

The Smaller AL080B has cutoff bias on the tube at all times until RF drive is applied, so it’s idling current literally is “zero.”  When you apply RF drive, that’s detected and then the amp is “biased up” to normal Class AB2 bias.  It’s newer design not implemented in the AL-82 for some reason, although they are still 3-500ZGs.

Has anyone ever made a mod to the AL82 that would incorporate this method? As it is, the tubes draw a lot of current and glow red needlessly when there is no rf drive.

I don't know why people worry about the red color in the anode. It does not wear the tube out.

If the anode never runs red the tube will often fail prematurely. This is because the gettering agent is coated on the anode. It activates when hot.

http://www.w8ji.com/vacuum_tubes_and_vaccum_tube_failures.htm

In the 80's Jim Aurand from Eimac Salt Lake (where the 3-500Z was designed and manufactured) visited me at Prime Instruments. Jim told me how RCA had a BC transmitter with premature tube failures. RCA used a 4-400A as a driver and ran the tube so cold it never showed color. This caused tube to not degass, the getter was too cold to work. RCA fixed the problem by changing to a lower dissipation tube and running the anode red 24/7.

Certainly the extra dissipation makes heat and wastes electricity, but it doesn't cause a tube life issue at all so far as I've ever heard. Eimac certainly never thought it was a problem. To the contrary, they were always clear about the tube needing some color, at least on occasion, to prevent failures.

73 Tom

 

[WB2WIK]

Purely in the interest of saving electric power which now costs more than it ever did, Tom, why not implement the circuit as used in the AL-80B to reduce ZSAC to zero with no drive?

73

Steve WB2WIK/6

[K6AER]

Steve,

I am going to be in LA on Wed through Sunday. Lunch?

Mike

[VR2AX]

The 3-500zg tubes in my L-4 always glow red at idle in the ssb position, if left in that state for a while, with no drive applied (200mA idle current total, at 2.5kv, or 250 watts plate dissipation per tube).

I doubt whether a tube "knows" whether bias is being applied or not, when considering the amount of plate dissipation, if the plate current is 125mA per tube then that amounts to quite a lot of dissipation, regardless of bias or no bias.

VR2AX

[W8JI]

Purely in the interest of saving electric power which now costs more than it ever did, Tom, why not implement the circuit as used in the AL-80B to reduce ZSAC to zero with no drive?

73

Steve WB2WIK/6

I have nothing to do with what they do now. It's available as an option, but IMO it is no big deal either way. I suppose if you sat around with the relay closed for long periods with low duty cycle it would matter, but I don't operate that way.

[WB2WIK]

>RE: Ameritron AL82 stresses tubes       Reply
by W8JI on February 9, 2010    Mail this to a friend!
Quote from: WB [2010-02-08 20:47]

Purely in the interest of saving electric power which now costs more than it ever did, Tom, why not implement the circuit as used in the AL-80B to reduce ZSAC to zero with no drive?

73

Steve WB2WIK/6


I have nothing to do with what they do now. It's available as an option, but IMO it is no big deal either way. I suppose if you sat around with the relay closed for long periods with low duty cycle it would matter, but I don't operate that way.<

::But my dog does, occasionally.  I'll get up from the station and go to the kitchen or something, and return to find the dog laying down right on my foot switch.

I didn't really think it mattered, either.

73

Steve

[WB2WIK]

>RE: Ameritron AL82 stresses tubes       Reply
by K6AER on February 8, 2010    Mail this to a friend!
Steve,

I am going to be in LA on Wed through Sunday. Lunch?

Mike<

Hi Mike,

Sure that would be great, especially Weds-Thurs.  Friday might be a bit busy and I don't know about the weekend yet, but they seem to be filled with "honeydews" every week for some reason...

Where will you be?  In the Valley at all??  I'm roaming the Valley and the Hollywood area, anywhere in there would be great.

73

Steve

[W8RJL]

If you are willing to go into the Al-82 and add one resistor and buy the Array Solutions QSK Master you will have NO idle current on the 3-500 tubes. The QSK Master is designed for CW QSK but a added bonus is it also allows for shorting the added bias resistor in either CW or SSB transmit mode. Works perfect  73, Ron W8RJL

[VE7RF]

Ok, I should add to asking "What is a lot of current?" with why would you key the amp and not apply RF drive at in some timely manner? Yes the AL-82 does apply bias without RF being present. As per the manual its set at 75 Ma for CW and 150 Ma for SSB. Now I know that my tubes do not glow red when biased at these levels in my AL-82. So if someone has an AL-82 and their tubes are glowing red at the prescribed bias levels I would suspect there is more wrong than improper bias. Yes it would be nice to incorporate the "no bias until RF is detected" and maybe someone can fill us in on that.

Have a great day....

##  The AL-82 uses TWO  zeners in series.  ONE of them is shorted out, when on SSB mode. BOTH zeners are used when on CW mode.   On CW, the idle should be 75ma..and 150 ma on ssb.
On ssb, with one zener shorted out, the remaining zener provides the correct bias for ssb [150ma].  Since the fellow is seeing 250ma of idle, when on the ssb mode, his  ssb zener has shorted out internally [ IT'S DEFECTIVE].

## His problem is one or both zeners are bad..and have shorted out.  He currently has his amp in zero bias mode. 250ma @ 3 kv = 750w on idle = blast furnace.

Jim  VE7RF