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[1HAMWANNAB]

My name is Eric, KD0SRD.

 

I am somewhat new to tube rf amplifiers.  I have built several GUITAR amplifiers

 

I recently picked up a nearly perfect Ameritron AL-811 (3 tube), with an isolation cable, original box, and an extra set of new tubes…  I got it for a song…

 

First thing I did was converted the AL-811 to run on 240 volts…  this bumped this idle voltage up to 1850 volts…  which is a little high for 811 tube but should be fine…  I started looking at the circuit and parts list and realized that this was the same power supply, tuning  system and filters as the AL-811h, meaning conversion to 4 tube would be possible.  (on another note I had decided that because the power supply was a NON regulated type the easiest way to lower the voltage was simply to add an additional tube, for a total of 4 tubes)

 

However I read that I need to do something called neutralizing the tubes, and that Ameritron had decided this was NOT needed for 3 tubes, but is needed for 4 tubes

 

It seems after I read more people say that grounded grid 811 tubes simply MUST be neutralized

 

Here’s the problem…  I don’t fully understand how this is accomplished

 

Here is what I do understand… these tubes will oscillate starting as low as 14mhz!!!

 

I read thateutralizing the tubes will stop this… it will also clean up the spurious admission to an extent

 

I (vaguely) understand that I need to add capacitance to the grid…  and something about sending signal from the plate, back to the grid?  (Similar to the negative feedback system I use in guitar amplifiers???  I think so anyway)

In case anyone was wondering both the al-811 & the al-811h both use the same transformer and power supply section

Eric - KD0SRD - W0BZT VICE President

[W8JX]

I would not waste my time doing it for two reasons. First it will at best gain 1 db of signal which will make no difference down range. Second a far better investment would be to simply install 3ea 572 and they have over twice the continuous safe plat dissipation as 4ea 811's and the HV power supply in a AL811 amp does not have enough guts to really hurt 3 572's  as they can safely handle 125 more ma or current than 4ea 811's. That being said while 4ea 811's look more impressive than 3, 3 ea 572's are far more bullet proof and can safely make more power than 4ea 811's and not even come close to max ratings while all AL811 amps exceed tube rating a good deal for their rated outputs. BTW, 4ea 572's will not make any more power than 3 in that amp because power supply cannot begin to exploit four of them.

[KOP]

Times two :-) , with one additional comment . tuning may vary significantly with the switch from 811 to 572B . Also find what W8JI Tom Rauch has written about the 811 tube and amplifiers .

~kop

[VK3BL]

2 things.

1) 1850 V is too high for the filter caps.  You have 4 in a string, each rated at 450v for a total of 1800V.

2) The AL-811H has a different (deeper) chassis than the AL-811, there is no room for a 4th tube, especially if you want to neutralise them properly.

Chuck in a pair of trio of 572Bs like W8JX suggested.

[G3RZP]

I wouldn't worry about the idle voltage being too high for the 811As but I would worry about an 1800 volt capacitor string with 1850 on it. OK, it's only 12.5 volts extra per capacitor - 2.8% high - but it is never recommended to push electrolytics to full voltage.

Neutralisation was recommended back in the 1960s by RCA for a four times 811A amplifier: it was implemented in the UK built KW1000 which was a two tube 572B amplifier. It is done by adding a tertiary winding to cathode choke and coupling via a small HV capacitor to the plate - the KW1000 just used an aluminium plate insulated from ground relatively close to the tubes. It is said that the 30L1, which doesn't have neutralisation, is far more stable on bands above 20m with it: some loads can make it distinctly touchy apparently.

Is it worth going to four tubes? From the viewpoint of more power, as the others say, no. From the consideration of making life easier on the tubes, yes, but if there isn't room or if the cooling is decreased, no. I would suggest you measure the heater voltage of the 811As to start with. There will be some drop in the filament choke, so measure the voltage at the tube socket and at the transformer end of the filament choke. Then check the transformer end of the choke with the amplifier producing full output, and remember that a DVM may then give wild answers because of RF getting in, so use an old fashioned moving coil meter! RCA generally reckoned on +/-10% on filament voltage, but Eimac seemed to go for +/-5%, and that's what I would go for, too. If the filament voltage is too high, then I would suggest a 'buck' transformer, which would also reduce the HV.

[N3QE]

The 4-hole AL-811H has and needs a neutralization circuit that the 3-hole AL-811 does not have or need.

There is literally no room in the AL-811 chassis to add a fourth tube and the neutralization network.

[W1QJ]

A wise old ham once told me that if you can't make the power you want to run with two tubes, then you picked the wrong tube for your amplifier.  IMHO two tubes should be the maximum amount of tubes that any amp should run.  Once you can't get as much power as you would like from 2 3-500 tubes, then your next bet is not another 3-500 but a single 8877.  Want more power than one 8877?  Then two. After that go to a 3cx3000.  That's just good advice.

[1HAMWANNAB]

Whats the realistic output on 3x 572b with that power supply section?

[N9MXY]

Whats the realistic output on 3x 572b with that power supply section?

I have never done it but I read somewhere that the output was about identical, I think the advantage is that the 572 is a more robust tube.

[KM1H]

I wouldn't worry about the idle voltage being too high for the 811As but I would worry about an 1800 volt capacitor string with 1850 on it. OK, it's only 12.5 volts extra per capacitor - 2.8% high - but it is never recommended to push electrolytics to full voltage

.

Agree and Ive had those exact same part # caps fail at ~430-450V in the past in the Senior series amps which also push the caps to the limit. Adding a 5th cap and resistor is not hard or change the original 4 to 500V Snap In type.

Neutralisation was recommended back in the 1960s by RCA for a four times 811A amplifier: it was implemented in the UK built KW1000 which was a two tube 572B amplifier. It is done by adding a tertiary winding to cathode choke and coupling via a small HV capacitor to the plate - the KW1000 just used an aluminium plate insulated from ground relatively close to the tubes. It is said that the 30L1, which doesn't have neutralisation, is far more stable on bands above 20m with it: some loads can make it distinctly touchy apparently.

Heath and Gonset soon had a 4X 811A amp on the market following that advice plus it showed up as a project in one of the ARRL's SSB Manuals.
The 30L1 is seriously unneutralized on 10/12/15M and barely stable on 17; and while 20 isnt sufficiently unbalanced to cause a problem it sure isnt perfect.

Even the SB-200 and 10M added SB-201 can take off violently at times on 10 and to a lesser degree on 15.

Is it worth going to four tubes? From the viewpoint of more power, as the others say, no. From the consideration of making life easier on the tubes, yes, but if there isn't room or if the cooling is decreased, no. I would suggest you measure the heater voltage of the 811As to start with. There will be some drop in the filament choke, so measure the voltage at the tube socket and at the transformer end of the filament choke. Then check the transformer end of the choke with the amplifier producing full output, and remember that a DVM may then give wild answers because of RF getting in, so use an old fashioned moving coil meter! RCA generally reckoned on +/-10% on filament voltage, but Eimac seemed to go for +/-5%, and that's what I would go for, too. If the filament voltage is too high, then I would suggest a 'buck' transformer, which would also reduce the HV.

I would say to go with just a pair of 572B's since they will be far from overloaded, will still put out ~600W, and will save $90-100 or so; while having a much better chance of full stability on all bands. Tweak the input networks if required. Eimac didnt build a 811 or 572 series tube and Eimac violated the +5% rule at times when it suited them.

Using just a basic neutralization I can get over 30dB of I/O isolation on 6M with any SB-200 conversion and brand of 572B tubes Ive tried.

[W8JX]

Whats the realistic output on 3x 572b with that power supply section?

.
Well 3 ea 811's are rated at 175ma per tube and times 3 that's 525 ma max safe plate current if you follow tube specs and ignore Ameritron ratings. (who recommend much higher limits) At 600 watts out with 811's you are almost 300% over tubes continuous safe ratings. 3 each 572's can handle 275 ma per tube times 3 is 825ma. That being said you can load/drive the trio of 572 a lot harder safely. That being said in theory if you safely load 700 ma and figure on about 1050 watts input and around 600 or so out. That  is about 450 watts dissipation and 572's are rated at 160 watts per tube so that's 480 so 450 is in the zone. By comparison 4 each 811 max out at 700 ma and only have 180 watts of continuous safe dissipation so when loaded to same 700 ma you are at well over 200% of rated safe dissipation for the tubes and way outside the safe zone. You are also well beyond beyond ICAS rating too which are short term and not continuous ratings. (and some still wonder why AL811's eats tubes)


Also on adding 500 volt caps, they are a lot harder to find and cost a lot more. I would replace all 4 450 volt caps with bigger ones and add a 5th in series to play it safe. (you need to increase capacitor size because as you add more in series it reduces the capacitance of the string and increasing the value of them offsets this)

[KA9UCN]

   What are the intended uses for the amp? If it just to give a bit more boost to a SSB signal then plate dissipation is not all that critical. If you plan on a high duty cycle, plate dissipation becomes a major concern.

  A few facts about the 811 and 572 tubes.
1 The 572 is a direct drop in replacement for the 811
2 The 752 has the same gain and frequency characteristics as an 811.
3 The only advantages to the 572 in an 811 Amp is plate dissipation. It will not last any longer than an 811.
4 The 572 cost considerably more!

  Placing a 4th tube in a 3 tube amp will not drop the plate idle voltage. Will you get more gain from 4 tubes than 3 with the same drive? Very little, to get full output from  4 tubes over 3 will require 1/3 more drive. This is not usually a problem as 3 tubes drive at approx. 45 watt and 4 requires 60 watt for full output.
The 4th tube will put more strain on a power supply. Is this a problem? It can de but does not have to be. Again depending on output and duty cycle. A single 811 is good for around 150 watt output.

   It all comes down to intended use of the amp. If it is SSB. I would not change a thing and just run the 3 tubes. If it is for higher duty cycles. I would use 3 572s, 3  tubes will put a lot of strain on the 811 amp power supply.

  The 811 amp seams to hold its value and for that reason I would use it as it is or sell it and try to find something like a sb220. It all deoends on use. For just short of 500 watt on ssb. It is a good amp that will help you during rough times. If it is used for something like RTTY or AM. It is a terrible amp that will be a repair bench money sucker.

   If you just want to tinker and improve the amp. You might consider a Russian tube conversion. This is something that is getting off topic for this thread, but something that might be fun at a fraction of the cost of a 4 tube 572 conversion.

Joe KA9UCN

[KM1H]

Whats the realistic output on 3x 572b with that power supply section?

Also on adding 500 volt caps, they are a lot harder to find and cost a lot more. I would replace all 4 450 volt caps with bigger ones and add a 5th in series to play it safe. (you need to increase capacitor size because as you add more in series it reduces the capacitance of the string and increasing the value of them offsets this)

1. 500V Snap In caps are readily available and far cheaper than the ancient and obsolete type Ameritron has used since the 80's.

2. Ameritron uses the same identical cap in strings of 4 to 8 without needing a higher value.

5. Read a power supply book to understand what is really needed. Many uninformed hams think more C is always better and since cost has dropped dramatically over the years the manufacturers oblige rather than fight a losing battle with mostly non technical hams these days.

[W8JX]

2 The 752 has the same gain at the gain and frequency characteristics as an 811.

Yes but misleading because they can be safely driven a lot harder

3 The only advantages to the 572 in an 811 Amp is plate dissipation. It will not last any longer than a 811.

Simply not true. There are some 572's out there 40 years old still working. A tuning error that will melt 811's thin metal plates quickly will not phase the 572's heavy graphite anode. Far more rugged tube. Suggesting that a 811 tube will last as long at same power level as a 572 is like saying you only need a 1/2 ton p/u to pull a big 5th wheel travel trailer and it will last as long at a 1 ton model doing it. Such is the big difference between a 572 and a 811.

4 The 572 cost considerably more!

A lot cheaper when you factor in one or two replacement sets of 811's.

   It all comes down to intended use of the amp. If it is SSB I would not change a thing and just run the 3 tubes. If it is for higher duty cycles. I would use 3 572s, 3  tubes will put a lot of strain on the 811 amp power supply.

No more than 4ea 811's

  The 811 amp seams to hold its value and for that reason I would use it as it is or sell it and try to find something like a sb220. It all intends on use. For just short of 500 watt on ssb. It is a good amp that will help you during rough times. If it is used for something like RTTY or AM. It is a terrible amp that will be a repair bench money sucker.

I would not be so generous. A SB200 hold value better even at its advancing age and is far better built too.

   If you just want to tinker and improve the amp. You might consider a Russian tube conversion.
This is something that is getting off topic on this thread, but something that might be fun at a fraction of the cost of a 4tube 572 conversion.

If you want to do some serious tinkering without to much effort, remove stock transformer and squeeze in one for a AL572 which will provide a much higher plate voltage and you will easily be able to drive three 572's to 800 watts out with higher plate voltage.

[W8JX]

1. 500V Snap In caps are readily available and far cheaper than the ancient and obsolete type Ameritron has used since the 80's.

Not that I have seen. When I have shopped for replacement 500v caps for my Dentron Clipperton QRO (it came with them) They cost a lot more than 450 volt versions

2. Ameritron uses the same identical cap in strings of 4 to 8 without needing a higher value.

And what is the surprise? Its cheaper for Ameritron to use same part and less capacitance when more are in series.

5. Read a power supply book to understand what is really needed. Many uninformed hams think more C is always better and since cost has dropped dramatically over the years the manufacturers oblige rather than fight a losing battle with mostly non technical hams these days.

It is a very arguable subject with no simple answer. (just like the size/value of bleeder resistors too) Less capacitance is not wise and higher capacitance tends to provide a higher average voltage when modulating. BTW when most amps were built many years ago it was costly and took a lot of space to provide a lot of capacitance for corners were cut. Different game today.