al-811h and 572b

[N9GGG]

Hi is it really worth it to change the 811 tubes in a al-811h to 4 572'b am I gaining anything.............thanks

[N4JTE]

I have made the revision and and very happy about it. Went with the grounded grid circuit as per W8JI who designed most of the ameritron amplifiers.
Bob

[KA2UUP]

Yes, better reliability because the 572B will be loafing and also smoother operation of the amp.  Don't expect more power because that is a function of the power supply.

My AL-811H works very well.  I did the change 10 years ago.

[W9KDX]

Wondering if the changes add to the reliability of just the tubes or if the change to 572s will extend the life of the 811H as well?

[N4CR]

Wondering if the changes add to the reliability of just the tubes or if the change to 572s will extend the life of the 811H as well?

The 572 tubes won't change the load on the amplifier components at all. Same gain, same voltages, same current.

The tubes themselves are more robust against abuse. And have slightly higher inter-electrode capacitance which may make them not neutralize properly since they might be out of range of the existing neutralization circuit. This is something you can probably fix with a minor circuit mod but you'd have to know what you were doing.

[W8JX]

The 572 tubes won't change the load on the amplifier components at all. Same gain, same voltages, same current.

Not exactly. A 572 can handle about 1000 more plate volts than a 811 and about 125 ma more plate current and nearly triple the dissapation. While you are limited by the power supply in a AL 811 you could in effect safely run a 3 tube one with 572's at same total plate current as a 4 tube 811 one. (they both use same power supply) Power supply and components are limiting factors here not tubes when 572's are used. Also at 811 voltage levels a 811 tube may have a bit more gain than a 572. With a proper power supply, two 572's can make as much power as four 811's. Its a shame that no one built a "modern" SB-200 amp. There is a possibility that 572's could actually have a shorter life than 811's in that tubes may not heat up enough in that amp to properly keep tube gettered.