[Amps] Replacing 811A with 572B

[jeremy-ca]

----- Original Message ----- 
From: "Joe Subich, W4TV" <w4tv at subich.com>
To: "'Roger (K8RI)'" <sub1 at rogerhalstead.com>; <Amps at contesting.com>
Sent: Thursday, April 10, 2008 12:22 AM
Subject: Re: [Amps] Replacing 811A with 572B


>
>> OTOH the 572s are much more rugged and tolerant.
>
> The only difference is plate dissipation.  Operated at 1250 volts
> on the plate and not overdriven, 4 x 811A have more than enough
> plate dissipation for service at 800W PEP output.
>
>> They should run until they die of old age.
>
> The most likely failure is of the cathode - not the plate.


In the real world its the plate that usually fails from incorrect tuning 
procedure. It may also be grid failure from the same set of circumstances.

The early Russian 811A's that MFJ used had soft glass and easily melted down 
and lost vacuum.

Since 100W will not overdrive 4 811's thats not an issue


Since
> the cathodes are the same, there will be no difference in life
> between the two tubes unless someone tries to put a brick on the
> key.
>
>> Of course you can't get that cherry glow as quickly out of
>> the 572s as with the 811s<:-))
>
> Sure you can ... if the 572B is operated at or above its maximum
> ratings like some 811As.

There is more than one rating parameter, dont be obsessive about voltage 
alone.

Carl
KM1H


>
> 73,
>
>   ... Joe, W4TV
>
>
>
>
>> -----Original Message-----
>> From: amps-bounces at contesting.com
>> [mailto:amps-bounces at contesting.com] On Behalf Of Roger (K8RI)
>> Sent: Wednesday, April 09, 2008 10:36 PM
>> To: Amps at contesting.com
>> Subject: Re: [Amps] Replacing 811A with 572B
>>
>>
>> Joe Subich, W4TV wrote:
>> > Why waste the money ... four 572Bs are half again to twice the
>> >
>>
>> Waste?  When compared to most tubes in the 1KW to legal limit
>> either the
>> 811 or 572s are down right inexpensive.  OTOH the 572s are much more
>> rugged and tolerant.   They should run until they die of old age. So,
>> when I look at  it that way the 572s are less expensive over
>> time than
>> the 811s.  Of course you can't get that cherry glow as quickly out of
>> the 572s as with the 811s<:-))
>>
>> To me it's a good choice (IF they'll fit)
>>
>> 73
>>
>> Roger (K8RI)
>> > price of four 811s.  The 572B will not provide any more gain or
>> > power output than the 811 unless you modify the power supply for
>> > about 40% more voltage (2 KV vs. 1400 V).
>> >
>> > The 572B is taller and wider which might represent a problem
>> > (moving the plate caps too close to the metal case or overly
>> > restricting air flow between the tubes).
>> >
>> > If you are really concerned about reliability pick one of the
>> > better 811 suppliers - perhaps Marshall (made in the USA) -
>> > but I would not waste the money and effort on imported 572Bs.
>> >
>> > 73,
>> >
>> >    ... Joe, W4TV
>> >
>> >
>> >
>> >
>> >> -----Original Message-----
>> >> From: amps-bounces at contesting.com
>> >> [mailto:amps-bounces at contesting.com] On Behalf Of Bert Almemo
>> >> Sent: Wednesday, April 09, 2008 7:37 PM
>> >> To: amps at contesting.com
>> >> Subject: [Amps] Replacing 811A with 572B
>> >>
>> >>
>> >> Hi guys,
>> >>
>> >> I have a friend who wants to replace 4x811A with 4x572B in an
>> >> Ameritron amp. I don't think it's a good idea but he insists
>> >> a lot of hams do it. What do you think?
>> >>
>> >> 73 Bert, VE3OBU
>> >>
>> >>
>> >
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>> >
>>
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