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[KT0DD]

Just read on W8JI's site that it's possible to substitute the 4 811a's in and AL811H amp with just 3 572B tubes, leaving the socket closest to the fan empty. Is this a good idea and what are the downsides to this. I know I won't get extra power but more durability. Thanks.

Todd - KT0DD

[VE7RF]

Just read on W8JI's site that it's possible to substitute the 4 811a's in and AL811H amp with just 3 572B tubes, leaving the socket closest to the fan empty. Is this a good idea and what are the downsides to this. I know I won't get extra power but more durability. Thanks.

Todd - KT0DD

sure, that will work good.  The 811 is a poor excuse for a tube to begin with.  65 watts anode diss...gimme a break.  Upgrade the fan with his replacement fan... if u have the under rated fan in there.

[KT0DD]

Just read on W8JI's site that it's possible to substitute the 4 811a's in and AL811H amp with just 3 572B tubes, leaving the socket closest to the fan empty. Is this a good idea and what are the downsides to this. I know I won't get extra power but more durability. Thanks.

Todd - KT0DD

sure, that will work good.  The 811 is a poor excuse for a tube to begin with.  65 watts anode diss...gimme a break.  Upgrade the fan with his replacement fan... if u have the under rated fan in there.

Thanks so much. I only expect 500 - 600 watts PEP out of the AL811H using 3 572b tubes. I think the 800 watts is a sales lie from Ameritron if you expect any life out of the tubes at all. I also run an AL80B as my main amp and cringe at their 1000 Watt rating. I run mine no more than 600-700 watts and the tube likes it fine. Of course, I am a renter, and only have 120V oulets. Landlord will not allow me to set up 240V for my shack and the dryer outlet is on the opposite end of the house. For what I run, 120V seems to work fine.
The new Chinese 811A's are lucky to have 50-60 watts plate dissipation from what I understand. Chinese JUNK! and that's not a boat I'm talking about...LOL.

[W7CXC]

Well here is my opinion and it is just a valid as RF's  The 811a is just fine for what it was designed for and when it was made by RCA here in the states. There were many home brew amps built with a pair of 811a's that made tens of thousands of contacts in the 50's and 60's. It's not a 4-1000a but it was not intended to be.

W8JI is almost always spot on with his information although others may disagree with that assessment.

Enjoy the hobby!

[KT0DD]

Thanks all, wish I could sit down with Tom-W8JI for 1/2 an hour and pick his brain...lol. I'll probably just get 4 572B's and replace all 4 811A's. I appreciate the help everybody.

Todd-KT0DD

[VE7RF]

Thanks all, wish I could sit down with Tom-W8JI for 1/2 an hour and pick his brain...lol. I'll probably just get 4 572B's and replace all 4 811A's. I appreciate the help everybody.

Todd-KT0DD

Tom has his own Ameritron  Facebook group.

[W6JAK]

Thanks all, wish I could sit down with Tom-W8JI for 1/2 an hour and pick his brain...lol. I'll probably just get 4 572B's and replace all 4 811A's. I appreciate the help everybody.

Todd-KT0DD

I  considered doing this as well but after thinking it over I decided to buy a set of four 811a tubes from Tom to keep as spares.  I just couldn't justify the additional cost of the 572b tubes.  I've been running my 811H with the original 811a tubes for 16 years and they're just as strong as ever.  Tom does rigorous testing and selecting of his tubes to weed out the bad ones coming from the Chinese suppliers.  Tom also matches the tubes at no extra cost, and his prices are very competitive.  With the spare set from Tom I will probably never need to buy additional tubes for the life of the amp as long as I keep running the amp within its limitations.

Just my two cents.

Good luck and 73

W6JAK

[W3SLK]

The 811's (and their kin), were originally designed to be the audio compliments for the 812's when RCA introduced them. There were a ton of them after WWII. Being that they were a medium mu tube and cheap, (at the time), companies started using them for class B amps because of the price and availability. Also because they were plentiful they also used 1624's (with some modifications) for amps back in the day. I'm surprised the audiophools never jumped on the 811's. 

[W9MT]

I believe that Tom (W8JI) is being misquoted on the matter of replacing the FOUR 811a's in an AL-811H amplifier with THREE 572B's.

I got the following verbiage from the following URL: https://www.w8ji.com/al811h_schematic.htm

Quote:

"It is perfectly acceptable to replace the 811 tubes with three or four 572B tubes in the AL811 or AL811H respectively to increase duty cycle and tube reliability."

Note the words "in the AL811 or AL811H respectively". In plain, Americanized English this means "3 811a's replaced with 3 572b's" in the AL811 amp, and "4 811a's replaced with 4 572b's" in the AL811H amp.

If you take an amplifier whose output pi-network is matched to transform the parallel impedance of four tubes and match that to a 50 Ohm output load while having only 3 tubes populated, you're going to have problems, as your design impedances won't match the design's criteria. Ever try to drive a 4 cylinder car with only three spark plugs firing? It doesn't work so well.

It's just another design criterion that 572b's are plug-in compatible with 811a's, albeit with greater maximum plate Voltage and RF power output, if the DC power supply of the amp is capable of supporting this. The power supplies of the AL811 and AL811H are NOT capable of providing this higher power level.

The only thing you get by substituting 572b's for 811a's is more ruggedness for longer tube life...nothing else. But the replacement has to be "3 tubes for 3 tubes" or "4 tubes for 4 tubes" depending on your 3 or 4 socketed AL811 "series" amp.

Sorry.....but no free lunch.

[KM4AH]

I'm surprised the audiophools never jumped on the 811's. 

Love it when people who wouldn't now good sound if it bit them in the ass talk about audiophools.

[AC2RY]

I'm surprised the audiophools never jumped on the 811's. 

Love it when people who wouldn't now good sound if it bit them in the ass talk about audiophools.

In audio use tube that require grid current is limited. Too much trouble when there are other options available. If grid current is acceptable there are better available tubes still - like 805 or 211 which are equivalent to 572 in their ruggedness.

[VE7RF]

I believe that Tom (W8JI) is being misquoted on the matter of replacing the FOUR 811a's in an AL-811H amplifier with THREE 572B's.

I got the following verbiage from the following URL: https://www.w8ji.com/al811h_schematic.htm

Quote:

"It is perfectly acceptable to replace the 811 tubes with three or four 572B tubes in the AL811 or AL811H respectively to increase duty cycle and tube reliability."

Note the words "in the AL811 or AL811H respectively". In plain, Americanized English this means "3 811a's replaced with 3 572b's" in the AL811 amp, and "4 811a's replaced with 4 572b's" in the AL811H amp.

If you take an amplifier whose output pi-network is matched to transform the parallel impedance of four tubes and match that to a 50 Ohm output load while having only 3 tubes populated, you're going to have problems, as your design impedances won't match the design's criteria. Ever try to drive a 4 cylinder car with only three spark plugs firing? It doesn't work so well.

It's just another design criterion that 572b's are plug-in compatible with 811a's, albeit with greater maximum plate Voltage and RF power output, if the DC power supply of the amp is capable of supporting this. The power supplies of the AL811 and AL811H are NOT capable of providing this higher power level.

The only thing you get by substituting 572b's for 811a's is more ruggedness for longer tube life...nothing else. But the replacement has to be "3 tubes for 3 tubes" or "4 tubes for 4 tubes" depending on your 3 or 4 socketed AL811 "series" amp.

Sorry.....but no free lunch.

Good catch  on the 811  vs  811H.

However,  3 x 811A's could easily be replaced with just 2 x 572B's.
And 4 x 811A's  could easily be replaced  with just  3 x 572B's.

811A  rated for 175 ma
572B rated for 275 ma.

The number of tubes installed has nothing to do with the tank circuit.  It's loaded B+ and plate current, that's it.   
Run the same OEM  plate current with either 2 or 3 x 572B's  in the  AL811   OR run the same OEM plate current  with either 3 or 4  x 572B's in the AL-811H.... and plate load Z stays the same, and PI net stays the same.

However, after saying that, if only 2 x 572B's were stuffed into the 3 tube amplifier, the fil V would probably rise a bit, cuz of the lower fil current.  The tuned input would be outa whack, with one less tube installed, since the input Z would increase a bunch.   And ditto with stuffing 3 x 572B's  into the 4 tube amplifier.

Tom cautions that some of the 811 amps have the wrong fan installed, that only blows 30% as much air as what was originally in there.  Dunno if all new production amps have the wrong fan or not.  With the wrong fan in there, you will end up having a bad day, regardless of tube choice.