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

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Subject: [AMPS] Caveat on Dentron Filaments + MLA-2500B Notes
From: (Tom Rauch)
Date: Sat, 29 Jul 2000 21:51:15 -0400
> This model, and most of the Dentron power transformers
> were wound for 234 volt mains. (Alpha's were also, for that matter) I
> measured 6.75 volts on the tube pins of those expensive 8875's in the
> 2500B. I wound six turns of #22 teflon covered wire on a one inch form and
> placed it in series with the side of the filament winding that goes to
> ground. This reduced the voltage at the tube pins to a comfortable 6.18
> volts. This will also serve as a "bump" for any in-rush current that may
> or may not be there. (I don't have anything to measure inrush with)

Dentron transformers were never spec'ed in any detail. So the 
transformer manufacturer(s) just considered voltages "nominal".  

> I am very interested to chat with anyone that has played with MLA-2500B's.
> The padder door knob caps vary from model to model, and even within a
> model. 

Parts were often bought surplus. Tank coils were in two flavors, 
high power and low power. The same coil went in amplifiers with 
anode impedances from 1000 ohms to 4000 ohms.   

> "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.
> 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.

2.) Add a swamping resistor on the input

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.

4.) Add a glitch diode on the negative rail of the supply.

5.) Add a glitch resistor in the anode system.

6.) Many Dentrons used 1/4 watt 470k resistors across the diodes, 
remove them.

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.

Other than that, it's a pretty nice amp. It's really compact.

73, Tom W8JI

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