SV: [AMPS] MLA-2500 MOD FOR GG 4CX-800'S CONFIKGURATION

[sm5ki]

I want again raise the question I raised many months ago and did not get any
reaction at all. Maybe I should have included the word nichrome,hi!: 

Why not use triode/or supercathode connected 8122s or 8072s instead of the
8875s? Same socket. Cheap price and you can still get abt 50 of the 8072s in
Scandinavia. The G1  can take 100 mA and the G2 using the same cup
construction should be able to handle even more current as it has a thermal
connection with the outside world and larger diameter. 

You have eventually to make a new bifilar cathode coil with a rf tap to
divide grid currents equal. The 8122 needs a squirrel cage blower. For the
8072 you can make with a lace a cooler similar to the 8875 for transveral
cooling. Could be used in the SB230 as an replacement for the 8873 as well
with a similar cooling system. 

By the way: can the cooler from the 8875 be moved over to the 8072 if the
8875 anode eventually has the same diameter.

Can anybody familiar with those tube curves look into the 8122/8072 curves
if they are linear enough run in my suggested application?

PROSIT DE HANS
----------
>Från: measures <2@vc.net>
>Till: "Phil Clements" <philk5pc@tyler.net>, "MEL" <k3ml@bellsouth.net>,
"AMPS" <amps@contesting.com>
>Ämne: Re: [AMPS] MLA-2500 MOD FOR GG 4CX-800'S CONFIKGURATION
>Datum: lör 5 aug 2000 05.46
>

>
>>
>>> >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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