Thanks to all that replied, both on the list and privately.
As soon as I get my current (and hopefully last) PA77 project off the
bench, I'll open up a 76A for measurement and study.
My guess is that finding a place for a 5V 10.25A filament transformer
will be a challenge, but you never know. TCOM sells a
80VA toroidal kit with a 3.4" x 1.6" core that might work if something
more standard won't.
Sounds like an interesting winter project, assuming hobby time is
available this winter. :-)
73,
Jim N7CXI
On 8/2/2012 8:00 PM, Martin Sole wrote:
> I think the 8877 would be about the best of the possible choices, if it can
> be made to work. More effort required than a pair of 3CX800's but I think
> 8877 availability and relative costs will make it a better long term bet.
>
> I think it needs a new plenum box to start with, or at the very least a big
> cutout and a new top plate, but I hate butchery so I'd go with a new box and
> I think there will be a few others interested to make it worth having a
> bunch made up to get some economy. If the existing blower is good enough
> then best keep it for simplicity and lack of butchery.
>
> The power supply remains the stumbling block but certainly with some
> variants, 3 hole 78's it is quite possible to see them produce well over
> 1.5A key down! I would propose a doubler might be an effective solution,
> provided everything else can be kept in check. The stock power supply board
> sits over the meters and the electrolytics fill the space underneath. More
> modern caps would be typically half the height and allow enough space for a
> different filament transformer hopefully.
>
> Given that these amplifiers are a good desktop size and with pretty much all
> the work done, retrofitting a good single tube might be the best long term
> alternative.
>
>
> Martin, HS0ZED
>
>
>
>
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Dr. David Kirkby
> Sent: 03 August 2012 09:37
> To: 'AMPS'
> Subject: Re: [Amps] 8877 operation at 2KV anode voltage?
>
> On 03/08/2012 02:28, Jim Barber wrote:
>> Rick's question about refitting an Alpha 76/78 with a GS-35B got me
>> thinking again about about using an 8877 instead. I have several good
>> tubes that have been sitting around for years, and now that I'm trying
>> to wean myself away from PA77's they have no preordained future use.
>>
>> I haven't used the 8877 below 3500V myself, so I dug around and found
>> a Penta Labs data sheet that shows Class B operation with 2KV on the
>> anode, although the example given is for FM BC.
>> Anyway, pushing the tube up to maximum plate current (1A) results in
>> 60ma of grid current, which they don't seem concerned about. Output is
>> 1330 watts, at 66.5% efficiency when driven with 64 watts.
>>
>> So... Since the cooling requirements of the 8877 are relatively
>> modest, if one found a place for the heater transformer it looks like
>> it could be done. Any reason why not?
>>
>> Thanks,
>> Jim N7CXI
>
> I suspect IMD would be poorer at low anode voltages.
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