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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