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here is a link with GOOD(!!!) info on the 811 & 811h. http://www.eham.net/articles/18364 pay particular attention to what Tom (W8JI) says. he knows these amps quite well (for a reason).

if you are going to put 572b's in an 811H, only put 3 in.
use 3 572's replace the four 811's, leave the socket nearest the fan (right rear socket) empty and remove that plate cap.

OK, but if I want to go 572b`s in my 3 tube 811 amp then do I use the same princpal and put only 2 tubes in it?

If they are direct replacements why would you put all but one in the amp?

the 811 and the 811h share the same power supply. the 811h is neutralized, the 811 is not. 4 572's in an 811h will kill the power supply if they are loaded to the max. 3 572's in an 811 will be fine

What does neutralizing have to do with killing the power supply?

If you replaced the 811A's with 572B's you would still have to run the plate current at the same as with the 811A's and there would be no problem. The power output will remain about the same but the tubes will last forever.572B's need a LOT more voltage to produce their full output,something the AL-811(H) power supply just will not provide.

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Front/back ratio comes from the antenna. Forward gain comes from the wall socket.

i never said that neutralization would kill the ps,

i said "4 572's in an 811h will kill the power supply if they are loaded to the max." and you agreed with that by saying : "572B's need a LOT more voltage to produce their full output,something the AL-811(H) power supply just will not provide".

as far as the neutralization issue, : "If you remove the four 811's and replace then with three 572B's, there will be virtually no change in SWR or neutralization" and " The 811H is better with three tubes because the stray capacitances are lower and the input match is slightly better. It's also a closer match to the neutralizing circuit."

i'll just take W8JI's words on it, who knows these amps better than Tom?

Ok, I been doing searches on the net of 572b`s and hearing some pretty bad things about blowing up amps, getting bad tubes that come apart and how taylor tubes are just cheap china made and no different than the cheap tubes for sale.

I am using the 811 tubes that I over heated and just running less watts in after tuning, I tune for full out put of the amp which is about 550w on ritty with about 35w in and then I swich to SSB but tunrn the input wats down till I see peaks of about 350-400w out of the amp. Seems to be running cooler.

I would love to switch to the 572b`s but I think a new set of 811`s and just being more careful tuning running it with less drive after tuning will work. I just dont want to spend a lot on 572b`s and have more problems.

But you know, I guess it can happen with the 811`s too, getting bad ones.

Any how, Thanks for all the info.

I was just going by what you said here. It reads as if you are saying the the 811H is neutralized and therefore having 4 572's in an 811H will kill the power supply if they are loaded to the max whereas 3 572's in the unneutralized 811 would be fine.

I now understand what you were actually getting at.

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Front/back ratio comes from the antenna. Forward gain comes from the wall socket.

Taylor tubes are in fact just regular tubes with the name silkscreened on them,nothing special. All the 572B's are made pretty much in the same place to the same standards. Yes there was a problem a few years ago with the plate caps coming unglued from the top of the tubes and hanging by the plate connection lead but that has been corrected. It happened to the 811A's and 807's as well.The glue used would dry out and become brittle in short order.There is nothing wrong with 811A's as long as you understand their operating limits,in fact they are a very good tube when run within their limits. The suggestion to upgrade to 572B's is just an option that a lot of guys take.

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Front/back ratio comes from the antenna. Forward gain comes from the wall socket.

OK, I know this thread is old but one more question. What is is neutralized mean? in one of the posts it is said the 811H amp is is neutralized and the 811 amp is not.

please enlighten me.

Thanks,
AP