Phil Clements philk5pc@tyler.net
Fri, 4 Aug 2000 19:48:13 -0500

> >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.
To take advantage of the potential of a pair will be nullified by a power supply
melt-down. 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.

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.

Two Rotron fans in tandem might just do the cooling trick. Some heat sensitive
paint on the tube will tell the tale.

Phil, K5PC

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