Ebay AL-811H, What Have I done?

[543_Dallas]

IMO the 572b tubes may not be worth the money in this amp. Running the 811s at a more realistic output would be better.

The 572b more plate dissipation but gain will be lower since they will be running at 1000 volts lower than they would be in an Al-572. Lower gain means you'll drive it harder to get the same and have higher grid current.

I haven't looked at the schematics but another question is what conduction angle will the 572b be running at in this amp with no other changes? Will they be biased enough to be linear and have good IMD?

Im curious what the idle current will be with the 572b tubes vs the 811s. If they use dynamic bias on that unit it would have to be bypassed to test that. The plates will run cooler but if the conduction angle ends up more towards class C why not just change the bias and take some load off of the 811s since no power is being gained anyway.

Just thinking out loud.

 

[Shockwave]

The truth is there is not a quality 811A or 572B tube in production today at any price. No one even tried to make a good 811 after RCA since every one after them thought is was OK to leave the 6 radiator fins off the plates so any overload could fold them into the grid now.

The 572B does not suffer from that problem but it has another dealing with the ridiculously thin layer of thorium applied to the filament. This is what determines the tubes life and there is no comparison whatsoever to an original Cetron 572B. Every Chinese tube is way behind the Cetron 572B or RCA 811A in terms of life expectancy.

Unless it's an Eimac, I stopped buying news tubes long ago. Better luck has been found buying good old tubes off eBay. Ones made in the 60's and 70's are of better quality than those made from the 1980's and after. The good news is at least the Chinese tubes are usable now. When they first came out, many would arc over on the first application of B+.

 

[Captain Kilowatt]

IMO the 572b tubes may not be worth the money in this amp. Running the 811s at a more realistic output would be better.

The 572b more plate dissipation but gain will be lower since they will be running at 1000 volts lower than they would be in an Al-572. Lower gain means you'll drive it harder to get the same and have higher grid current.

I haven't looked at the schematics but another question is what conduction angle will the 572b be running at in this amp with no other changes? Will they be biased enough to be linear and have good IMD?

Im curious what the idle current will be with the 572b tubes vs the 811s. If they use dynamic bias on that unit it would have to be bypassed to test that. The plates will run cooler but if the conduction angle ends up more towards class C why not just change the bias and take some load off of the 811s since no power is being gained anyway.

Just thinking out loud.

The output will not be lower. The gain is the same. The 572Bs are capable of operating with an additional 1000 volts and will provide more p9wer at that voltage simply because VxI=P. The fact they are being run at a lower than max. Voltage has nothing to do with the gain they will provide when measured in dB. Power will be the same whether 811As are used or 572Bs but the 572B is a MUCH better tube that will take higher duty cycle modes MUCH better.

 

[543_Dallas]

The output will not be lower. The gain is the same. The 572Bs are capable of operating with an additional 1000 volts and will provide more p9wer at that voltage simply because VxI=P. The fact they are being run at a lower than max. Voltage has nothing to do with the gain they will provide when measured in dB. Power will be the same whether 811As are used or 572Bs but the 572B is a MUCH better tube that will take higher duty cycle modes MUCH better.

Ok. I had read a post by someone on another forum saying he swapped the 811s for 572b's and had to drive the amp harder to get the same output as with the 811s. Maybe he had some soft 811s....who knows

https://www.eham.net/ehamforum/smf/index.php?topic=110822.15#msg946076

I have to disagree that running a tube at lower anode voltage does not reduce gain of the tube. Maybe I misunderstood you.

All that aside, what about the bias? Sure the 572b's will fit the socket and make watts but should we at least look at idle current to see what happens there. I've never heard of anyone doing anything more than plugging in the tubes and talking on them.

 

[Shockwave]

Think of the old SB-220 and its CW / SSB switch that increases plate voltage and output power on the higher voltage SSB setting. These changes in output happen with the same drive level so we have to assume the voltage change is affecting the gain of that stage.

While I can't remember where I heard it from, I do recall trusting the source. The claim was that 572B tubes have slightly worse IMD performance when using them in an 811A amplifier. I still have trouble figuring out why because these tubes are identical in every aspect other than the thicker graphite plate on the 572B.

It makes me question if they were comparing how a 572B operates at 1000 volts below it's rated operating voltage as opposed to being run at full voltage. In this case I could easily see how the IMD for that tube at a given power level would be worse at lower plate voltage. However, I would expect it to be the same for the 811A at that same voltage.

Since the element spacing in both tubes are the same, bias requirements for both tubes should be the same if they are run at equal plate voltage.

 

[338_MtRushmore]

Think of the old SB-220 and its CW / SSB switch that increases plate voltage and output power on the higher voltage SSB setting. These changes in output happen with the same drive level so we have to assume the voltage change is affecting the gain of that stage.

While I can't remember where I heard it from, I do recall trusting the source. The claim was that 572B tubes have slightly worse IMD performance when using them in an 811A amplifier. I still have trouble figuring out why because these tubes are identical in every aspect other than the thicker graphite plate on the 572B.

It makes me question if they were comparing how a 572B operates at 1000 volts below it's rated operating voltage as opposed to being run at full voltage. In this case I could easily see how the IMD for that tube at a given power level would be worse at lower plate voltage. However, I would expect it to be the same for the 811A at that same voltage.

Since the element spacing in both tubes are the same, bias requirements for both tubes should be the same if they are run at equal plate voltage.

Does the voltage affect gain? It seems to me that saying increased output due to higher voltage increases the gain of the tube would be like saying my antenna has more gain because I am putting more power through it. Wouldn't gain be a change of power output with all else equal? I read somewhere that said the 572b had lower gain with lower voltage. It mentioned lower Mu I think, but I have no idea what Mu is.

It is interesting all of the differing opinions on this subject. There are so many variables and the facts from years past may, or may not, apply today. It still strikes me as odd that there seems to be zero people complaining about failures of today's 811a.

 

[loosecannon]

i've worked on a few of these amps, and they each had the same problems.

I can't say its a weakness of the design, it's really about the hands that have been in them and how hard they were ridden.

I have had to replace the high power choke (the big coil that connects to the caps of the tubes) in two of them.

when people get in there and start swapping tubes, that little magnet wire that is soldered to the small PC board gets wiggled one too many times and breaks.

one i got lucky and could re-solder it. the other needed replacement.

the other big replacement item is the filter caps.
many of those amps are old now and the caps and diodes need replacing.
be prepared to spend 100 bucks doing this job.

LC

 

[Shockwave]

These questions are interesting since amplifier gain has to do with both the circuit and device being used. If we measure gain by how many db increase there is from input to output, that number does go up as more voltage is applied to the device. Is the mu of a tube not measured at some specific plate voltage? I've seen different manufacturers rate the mu on a 572B between 170 and 190 while the RCA 811A only states a mu of 160.

Since the grid structure and placement are identical in both tubes and this controls the mu, other than a higher plate voltage what other reason could the 572B have a higher mu rating? If the gain is not related to plate voltage and the 572B is listed as having the higher mu, it should make more power in an 811A amplifier if the plate voltage has had no impact on gain.

For more information on why today's 811A tubes are junk, check out this link posted by Tom: https://www.w8ji.com/811a_tube.htm

 

[wavrider]

Gonset had a four tube amplifier using 811A tubes. GSB-201
That was back in the day of real power supplies, Heavy beast.

Had a local op who used to abuse it on a daily basis, talking holes melted in the plates on the 811 tubes.

I finally talked him into putting some 572B tubes in it.
Performance was the same, and no more cherry red burnt holes in the plates as the 572 b is a much more rugged tube. (as all ready stated).

 

[338_MtRushmore]

It arrived today, and I think it could have been worse. I found broken solder joints where 20, 15 an 10m tie into the high power choke. The band switch has a little discoloring, but all of the contacts still have spring to them. It also has 2 blown grid resistors. I assume I should get rid if the grid resistors as W8JI recommends, but I will have to do some more research on that.

 

[338_MtRushmore]

I can't figure out why the fan is outside the case. Maybe a lousy attempt to increase air flow?

 

[Road Squawker]

I can't figure out why the fan is outside the case. Maybe a lousy attempt to increase air flow?
View attachment 26927 View attachment 26928 View attachment 26929

Thats the factory installed fan location

 

[338_MtRushmore]

Thats the factory installed fan location

All of the other pics I find look like this, but maybe the older units were mounted on the outside?

 

[Road Squawker]

thats an 811 H.
perhaps I misread the thread, I thot you got an 811 three holer.

DUH!!!! I just went back and L@@KED at the thread title,.... you can smak me now

 

[338_MtRushmore]

Oh, I didn't realize the 811 had the fan mounted outside. That just makes the situation even stranger.