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Author Topic: AL-811H Tubes  (Read 2758 times)
K4DZQ
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« on: April 21, 2013, 08:47:10 AM »

Is there any advantage in replacing the 811 tubes in my AL-811H amp with 572's?
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K0CWO
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« Reply #1 on: April 21, 2013, 09:15:55 AM »

Copy and paste "using 572 tubes in an 811" into Google and you will find a wealth of information.
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WB4CTX
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« Reply #2 on: April 21, 2013, 12:02:22 PM »

My advice for what it is worth is do not waste money putting 572b tubes in an 811 amp with the current Chinese tubes. You will be replacing them soon. They are not pumped down like the old USA made tubes and they will eventually arc over because they don't get hot enough for the getter to work well. I find 811s last much longer than 572b tubes in an 811 amp.
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W8JX
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« Reply #3 on: April 21, 2013, 12:08:51 PM »

Is there any advantage in replacing the 811 tubes in my AL-811H amp with 572's?

In a 811H, yes and no. With 4 of them your power supply is too weak to properly feed the tubes and they may not getter properly and fail early due to tube gassing. Dissipation wise you would never hurt them tuning. Two 572's can dissipate more energy than four 811's.  In a three tube amp it is more viable as you can run the harder. The 3 and 4 tube 811 amp use same power supply and max rated plate current is 175 per 811 tube or a total of 525 for 3 tube and 700 max for 4 tube. A 572 is rated at 275ma so you could run three of them safely  at 825ma or basically at same 700ma level as 4 811's without concern.  As far as Chinese tubes, their all made in china and some should stop knocking china tubes because if it was not for them then would be a lot of amps with no replacement tubes for them.
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N4CR
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« Reply #4 on: April 21, 2013, 12:15:45 PM »

I went through two sets of Chinese 572's in my Collins 30L-1 in less than a year. Sourced some real 811A's and they are working fine. The Chinese 572's went low on emissions after 90 days. They were not gassy.

Many believe they are not putting enough thorium in the filaments.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
WB4CTX
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« Reply #5 on: April 21, 2013, 12:30:05 PM »

I went through two sets of Chinese 572's in my Collins 30L-1 in less than a year. Sourced some real 811A's and they are working fine. The Chinese 572's went low on emissions after 90 days. They were not gassy.

Many believe they are not putting enough thorium in the filaments.

Same thing happened with my 811 amp. Decided to go back to 811s and got one set with two bad ones. Next set were all good and are still working fine. Stay away from any 572bs used in 811 amp.
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WB2EOD
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« Reply #6 on: April 21, 2013, 01:55:41 PM »

I own and 811H. 
A few things to consider
1.  Since the limiting factor is the power supply, the 572's won't provide any more output than the 811's.  With 811 tubes, the weak link is the tube itself.  Cheap and easy to replace, the 811 tube will fail before the power supply can destroy itself.  Slap in a set of 572's, and the weak link becomes the power supply the destruction of which is very expensive.
2.  Tune quickly and keep an eye on the grid current.  Overdrive is a frequent cause of 811 tube failures  I have a sheet of paper showing the load/plate settings for various frequency ranges.  I can tune up in a few seconds.
3.  As mentioned, the questionable quality of Chinese 572's raises the question do you really want to replace something that works with a questionable component.
4.  OTOH, if you are running power in high duty cycle modes such as RTTY/SSTV/PSK/DIGITAL, the 572's may be worth looking into

Hope this helps
73
WB2EOD
 

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W8JX
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« Reply #7 on: April 21, 2013, 02:38:03 PM »

if you are running power in high duty cycle modes such as RTTY/SSTV/PSK/DIGITAL, the 572's may be worth looking into

In a 811h, the tubes are not the only limitation for key down modes, the power supply a limiting factor too. They did not beef up power supply for fourth tube.
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N6AJR
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« Reply #8 on: April 21, 2013, 05:45:45 PM »

no, it only costs more money.
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VK2ANS
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« Reply #9 on: April 24, 2013, 03:33:15 PM »

Hi Guys, I have imported, upgraded and sold some hundreds of new Ameritron AL-811 series amplifiers into Australia over the past 15 years or so. The biggest problem with all the AL-811 series amplifiers (3x811, 4x811 and 4x572B) is their parasitic suppression circuit design. This is, in turn, the main cause of their inherent instability or tendency for self-oscillation. I have found, by far, the most effective way to greatly improve their stability is to firstly unwind each parasitic suppression coil by one turn (making each of them a 5-turn coil). Next, insert a 100 ohm 3W (5W is better) non-inductive carbon resistor inside each coil and solder each resistor to each end of each respective coil. This places each new resistor in parallel with each newly formed coil. The coils must not touch the body of each resistor (i.e. they must be air-wound). The original factory-fitted 100 ohm 3W resistors must remain in place. The factory sometimes forces the RF negative feedback U-shaped aluminium capacitance-forming plate hard up against (or even somewhat under) the bandswitch's shaft in order to improve stability by increasing the RF negative feedback level. This is absolutely the wrong way to reduce these amplifiers' inherent tendency to self-oscillate. In fact, once the parasitic suppression upgrade is completed, the RF negative feedback plate can be moved back as far as is mechanically possible, so as to maximise RF output up on the 12m and 10m bands. I agree with comments I just read here re it being not normally worthwhile to replace good 811 tubes with 572B tubes. I see consistently more peak output from 4 x 811 tubes than 4 x 572B tubes in these amplifiers.
Lee Andrews. VK2ANS.     
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VK2ANS
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« Reply #10 on: May 01, 2013, 04:44:53 PM »

Hi Guys, here are some more notes re AL-811 series amplifiers - based on what I've just read here;
1. The AL-811HD should have been released with 3 x 572B tubes (not 4 tubes). It would deliver slightly more output with only 3 x 572Bs - for several reasons. Total plate dissipation would still be relatively high at about 480W (i.e. 3 x 160W for each 572B) - or about DOUBLE the total plate dissipation of 4 x 811 tubes (i.e. 4 x 60W = 240W in total).
Ameritron already has the heavier duty AL-572 amplifier model - which has 4 x 572B tubes fitted, anyway.
2. I've often had to re-tap the 80m tapping one or two turns down on the low band PA coil (i.e. increasing inductance) to make the 80m tuning work properly (i.e. so it's not usually at zero for loading) - especially re the AL-811 3-tube amplifier.
3. I always remove the two 10/12m band plate coil wire taps and replace them with a 10mm wide silver-plated shim (thin) copper strap, going directly from the original 10m PA coil tapping point to the bandswitch. This greatly reduces stray capacitance and inductance. AL-811 series amplifiers generally deliver about 40~50W more output for the same 50W drive on 12m/10m, along with much smoother and more stable plate tuning. It's best to use only 50W drive for most tuning and tests, btw. 100W of drive should only be used for quick peak power output tests - especially re 811 tubes, of course.  
4. I often have to retap the 20m/30m PA coil tapping an additional half-turn in order to improve 30m output/performance, without reducing any 20m output/performance. The already-removed 10m wire tap from the bandswitch does the job nicely. Some retuning of the 30m/20m input coil helps in achieving a good input swr compromise on both 30m and 20m bands. By doing all this work the amplifier return rate quickly dropped from around 10~15% (depending upon the model) to virtually zero!
Regards, Lee.        
« Last Edit: May 01, 2013, 05:08:39 PM by VK2ANS » Logged
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