----- Original Message -----
From: "Joe Subich, W4TV" <w4tv@subich.com>
To: <n9aa@arrl.net>; <Amps@contesting.com>
Sent: Wednesday, April 09, 2008 10:58 PM
Subject: Re: [Amps] Replacing 811A with 572B
>
>> The AL-811H runs about 1600-1800 volts HV, not 1400, which is
>> why it's not a bad idea to run the 572Bs in that amp.
>
> If the AL-811A plate voltage runs that high it is exceeding the
> "Maximum Plate Voltage" rating for the 811A tube and is a bad
> design. The RCA data sheet calls for 1500 V maximum in ICAS
> service and I would not want to push an 811A harder than that.
It really depends upon what the plate voltage is at the rated plate current.
If that 1800V is at rest then it is not excessive.
Even in operation a few hundred volts is meaningless as long as the
dissipation is not exceeded. Since the 811A is rated for plate modulated AM
at 1250V there is way more than enough insulation reserve at 1800V on SSB
voice peaks.
What kills 811's is exceeding the dissipation during tuning or misstuning.
The thin tantalum plates simply collapse.
>
> Unless the newer tubes are designed for higher plate voltages
> the 572B would be a safer choice. However, I don't see any
> really reliable 572B. I would certainly would not use a generic
> $39 Chinese 572B.
Since they are the same tube as you buy at RF Parts with their name, or
Taylor, on it they are are a good choice. There are/were only two Chinese
factories making the 572B and one is almost fully involved in audiophool
tubes now. Its no different than when they were built in the USA; one or two
plants provided as many brand names as were ordered.
My success with generic or Penta brand 572B's is 100% over the past several
years and this is at 6M. I buy 20-30 a year.
Carl
KM1H
>
> 73,
>
> ... Joe, W4TV
>
>
>
>> -----Original Message-----
>> From: Scott Manthe [mailto:scott.manthe@gmail.com] On Behalf
>> Of Scott Manthe
>> Sent: Wednesday, April 09, 2008 10:08 PM
>> To: Joe Subich, W4TV; Amps@contesting.com
>> Subject: Re: [Amps] Replacing 811A with 572B
>>
>>
>> The AL-811H runs about 1600-1800 volts HV, not 1400, which is
>> why it's
>> not a bad idea to run the 572Bs in that amp.
>>
>> While the cost difference seems substantial, you'll probably
>> never have
>> to replace a 572B, imported or otherwise, when run in an AL-811H. RF
>> Parts lists the Marshall 811A at $29.45 each and Chinese 572Bs at $39
>> each, so the price difference at this point is substantial,
>> but not so
>> much so, if you consider the increased life expectancy.
>>
>> The size difference between the two tubes does not present a
>> problem in
>> the AL-811H amp.
>>
>> 73,
>> Scott, N9AA
>>
>>
>> Joe Subich, W4TV wrote:
>> > Why waste the money ... four 572Bs are half again to twice the
>> > 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
>> >
>> >
>> >
>> >>
>
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