[AMPS] Caveat on Dentron Filaments + MLA-2500B Notes

[Phil Clements]

> > "window" for matching various loads on 160 meters, as most of the
> > capacitance is fixed in the padders. Also, various values of padders on
> > the tune cap are used from model to model depending on what value it
> > is...140 pf on the older models and 170 pf on the "B" model.
>
> That's because the early models were "designed" and tested with
> the covers off. Later, when customers couldn't get resonance on
> 160, someone looked at one with a lid on. The lid reduced the
> inductance in a coil that was already too small for reasonable
> operating Q.

The coil in the "B" model measures 19 uh...very adequate for a Q of
10-12 on 160m. (1100 ohms plate load)
>
> > All in all, the MLA-2500B is a sweet, compact, legal-limit
> > amp with no bad habits and very smooth tune-up. Too bad
> > that the 8875's are becoming extinct!
>
> 1.) Make sure it has a grid-trip circuit.

Always a good idea with high-dollar tubes!
>
> 2.) Add a swamping resistor on the input

You don't have to....a 100 ohm swamping resistor is standard,
unless some "HFer" has cut it loose!
>
> 3.) Check the bleeders. Dentron used carbon (an absolute no-no)
> resistors and sized them, in some models, so the 2 watt resistor
> dissipated 2.2 watts. Bleeders always should be metal, not carbon.

This must be a late "B" model I have...it has 120k ohm MOF's in it.

>
> 4.) Add a glitch diode on the negative rail of the supply.
>
> 5.) Add a glitch resistor in the anode system.

Denny used two 1 ohm 1 watt resistors in parallel at the
base of the plate choke.
>
> 6.) Many Dentrons used 1/4 watt 470k resistors across the diodes,
> remove them.

No resistors on the "B" model...just .01 @ 1kv caps.
>
> 7.) The bandswitch wafer has the ten meter tap next to a grounded
> screw. That's the highest voltage part of the switch. An Ameritron
> wafer will fit, and move the highest voltage lug away from the
> grounded screw if you ever have a bandswitch failure.
>
> 8.) The Dentron suffers from filament inrush problems, because the
> filament winding is on a big transformer and the filament winding is
> wound with needlessly heavy wire. Do yourself a favor and step-
> start the transformer...or add a second small filament transformer
> that is rated to barely supply the tube's operating current.

I agree 100%...along with grid-trip, this can only preserve the
high-dollar tube life. Running at over 6.3 volts is not a good idea
either. My dropping resistor approach is probably not enough ohms
in my case to reduce inrush. A better approach would be to step-start
the entire transformer. There is a big "thunk" when you turn it on.
>
> Other than that, it's a pretty nice amp. It's really compact.

I am totally impressed with it, especially the 6 band capability.
Nobody from that era did any tube protection.Tubes were much
cheaper then.

(((73)))
Phil, K5PC




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