[Amps] AL-811 & 572's

[W7MJM]

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" <w7mjm at arrl.net>
To: "mitch cox" <ww4cox at embarqmail.com>
Cc: <w8ji at contesting.com>; <w8ji at w8ji.com>
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" <ww4cox at embarqmail.com>
> To: "W7MJM" <w7mjm at arrl.net>
> 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 but....you 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" <w7mjm at arrl.net>
>> To: amps at contesting.com
>> 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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>