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Re: [Amps] AL-811 & 572's

To: "Joe Subich, W4TV" <>
Subject: Re: [Amps] AL-811 & 572's
From: "W7MJM" <>
Reply-to: W7MJM <>
Date: Wed, 9 Feb 2011 21:20:23 -0800
List-post: <">>

Just looked at the parts list for the AL-811 and AL-811H. The power supply 
transformer, T1, is listed for both versions of the amp as part #406-1532. 
So it's the same transformer. Which brings me back to my original question 
about running three 572B's in an AL-811 vs. an AL-811H. If the power supply 
is the same, and if the AL-811H can drive three 572B tubes to 800 watts 
output (fourth hole empty, as recommended by W8JI), then why can't the 
AL-811 do the same? Or perhaps it can; not that I need it to.

Am I missing something here? Perhaps W8JI can stop by and explain it.


----- Original Message ----- 
From: "Joe Subich, W4TV" <>
To: "W7MJM" <>
Cc: <>
Sent: Wednesday, February 09, 2011 7:47 PM
Subject: Re: [Amps] AL-811 & 572's

> I suggest you download the AL-811 and AL-811H from the Ameritron web
> site then compare schematics/parts lists.  That is what any other
> person would need to do to answer your question.
> If the schematics and parts are identical, you have your answer.
> If they are different, you have a different answer.  The power
> supply specifications are different between the two amplifiers -
> 700 mA for the AL-811H vs. 550 mA for the AL-811.  That's a hint
> right there.
> In addition, since the 572B is an 811A with a heavier anode and the
> 811 is only clean to about 200W PEP per tube, I would not expect
> three 572Bs to be clean at 800 W with only 1500V on the plates.
> Now, if one chose to raise the AL-811 plate voltage to 2250V (and
> the rest of the components could handle the increased voltage), I
> suspect one could easily see 1000W PEP from a 3 x 572B AL-811.
> The AL-811 is restricted as much by the limited plate current
> capability of the 811A tube at 1500V as it is by the plate
> dissipation (cooling) of the tubes.  Substituting a beefier anode
> does nothing for the plate current capability.
> 73,
>    ... Joe, W4TV
> On 2/9/2011 3:27 PM, W7MJM wrote:
>> Is there anyone on the list who can answer my question as to why three
>> 572B's in the three-hole AL-811 should only put out 600 watts whereas 
>> W8JI
>> has suggested (if memory serves), that when retubing the four-hole 
>> AL-811H
>> with 572B's, only three are needed and the fourth hole can be left empty?
>> If an AL-811H retubed in this manner puts out 800 watts PEP, why not the
>> AL-811? Is the power supply different?
>> Waiting to be enlightened, but without any arcing. :-)
>> 73,
>> Martin
>> W7MJM
>> ----- Original Message -----
>> From: "W7MJM"<>
>> To: "mitch cox"<>
>> Cc:<>;<>
>> Sent: Wednesday, February 09, 2011 10:36 AM
>> Subject: Re: [Amps] AL-811&  572's
>>> Very interesting, Mitch. I think I'll just stick with the 600 watts 
>>> rated
>>> output (my old Healthkit HM-2140 PEP meter shows 500 and my new MFJ-868
>>> shows 800); obviously I need to calibrate the meters, but the drive is
>>> about 70 watts so I'm probably putting out about 600 to 650 watts.
>>> With the three 572Bs in place, I don't have to worry as much about 
>>> tune-up
>>> mistakes cooking the anodes, and the tubes seem to take up to about 100
>>> watts of drive while staying under 165 mA on the grid current meter and
>>> there are no reports of bad audio or splatter.
>>> The reason I wondered whether the trio of 572B's in the AL-811 would put
>>> out 800 watts is because Tom Rauch, W8JI, who designed the amp, 
>>> suggests,
>>> when retubing an AL-811H with 572B's, that it's only necessary to 
>>> install
>>> three 572B's and leave the fourth hole empty. So I figure, if three 
>>> tubes
>>> in the AL-811H produce 800 watts of output, why not the same result in 
>>> the
>>> AL-811? Is the power supply different?
>>> I'll cc Tom and see if he has any thoughts about this.
>>> 73,
>>> Martin
>>> W7MJM
>>> ----- Original Message -----
>>> From: "mitch cox"<>
>>> To: "W7MJM"<>
>>> Sent: Wednesday, February 09, 2011 5:18 AM
>>> Subject: Re: [Amps] AL-811&  572's
>>>> If you wire the buck boost on the transformer to the highest voltage
>>>> setting, 2200 VDC, 800 watts is a breeze are also raising 
>>>> the
>>>> filament voltage to a dangerous level. I tested one of these 
>>>> transformers
>>>> in a home brew 2 months ago along with 3 Svetlana 572B tubes. At the
>>>> highest voltage setting I got 900 watts out with 75 watts of drive. I
>>>> also wondered about how stout this little transformer was so with good
>>>> cooling, unlike the factory amp, and a separate filament transformer, I
>>>> locked this home brew down at 900 watts CW for 1 hour, 20 minutes. No
>>>> problems were encountered with either the tubes or the tranformer. The
>>>> transformer was warm after the test but not hot and the amp will still
>>>> produce 900 watts so nothing was harmed. At 85 watts of drive this amp
>>>> will do a full KW with the same tubes as used in the test.
>>>> ----- Original Message -----
>>>> From: "W7MJM"<>
>>>> To:
>>>> Sent: Tuesday, February 8, 2011 11:57:18 PM
>>>> Subject: Re: [Amps] AL-811&  572's
>>>> "Thanks for the input guys. I'm aware of the limitations of the power
>>>> supply
>>>> on the AL-811. I was looking for performance information about the 572B
>>>> tube
>>>> itself."
>>>> Isn't the power supply in the 3-tube AL-811 identical to the power 
>>>> supply
>>>> in
>>>> the 4-tube AL-811H? If so, shouldn't you be able to run the 3-tube 
>>>> AL-811
>>>> at
>>>> 800 watts PEP output when it's been retubed with 572B's?
>>>> Anyone care to comment on this?
>>>> 73,
>>>> Martin
>>>> _______________________________________________
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