>
>> >I have an MLA-2500 with soft tubes. Seeking information on modification
>> >using a
>> >pair of 4cx800 tubes in grounded grid.thus eliminating a seperate bias
>> >supply and screen supply.
>> >
>> You will need a high pressure centrifugal blower. The tubes need to be
>> relocated near the Tune-C. A small 6v transformer is needed to increase
>> the heater potential.
>>
>> good luck, Mel
>
>You really only need one tube, as it will put out as much as the pair of
>8875's.
A pair of 8875s is rated at 700mA keydown. A 3cx800A7 is rated at 600mA
keydown.
>To take advantage of the potential of a pair will be nullified by a power
>supply melt-down.
Highly unlikly for SSB. The avg. current for a pair of 3cx800A7s in SSB
is c. 500mA.
>It is easy to get screen and bias voltage with a few simple mods
>on the existing transformer. Running tetrodes @ zero bias and screen voltage
>went out with the glass tube era.
>
An 8281 tetrode can deliver 23db gain.
>Instead of relocating the tube near the tune C, why not add a small
>profile 500 pf
>vacuum variable where the second tube was? This will kill two birds with one
>stone....better lay-out, and removal of all those door knob padders that
>are marginal @ 1500 watts.
>
Load padders are also used.
>Two Rotron fans in tandem might just do the cooling trick. Some heat
>sensitive paint on the tube will tell the tale.
(get a high pressure blower)
>GL!
>Phil, K5PC
>
>
>
>
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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