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Converting from 6volt to 31volt tubes.

jtrouter

Active Member
Jul 7, 2015
77
34
28
62
Hello,
I want to rebuild/convert a D&A MDX200/Mavrick. I plan on removing the 4 final 6lq6's tubes and replace them with
31lq6 tubes and wire the heaters in series. I Know it can and has been done in the past using 4x31 volt tubes. But i am not
smart enough to know what else i need to do in support of this? I did post this elsewhere on the internet and it almost started
a war between a bunch of guys so i just said thank you and ended it. The tubes i am using are indirect heaters that did come up
and that makes a good start or so i was told. From what i understand i just have to pull from the 120 volt going into the Amp and
just run the 4 heaters in series? I do plan on running the power thru a power switch so the power does not run 24x7 but what else do
i need to do to 1. make it safe. 2 to do it right! I have read about it being done many times infact i even read about it in a bearcat? amp
on this forum. Please help me do it right and not burn down my house. Thank you all.
 

nomadradio

Analog Retentive
Apr 3, 2005
5,207
7,392
573
Louisville, KY
www.nomadradio.com
The transformers in that amplifier serve to isolate the circuits in the amplifier from the 120-Volt power line. May not sound like a big deal, but it is. Any direct circuit path to the hot side of your wall socket is a deadly hazard.

There's a name for amplifiers that connect the 120-Volt line directly to circuit elements inside.

Widow maker.

The way this is done is to unhook pins 4 and 5 from the 12.6 (yes, 12.6)-Volt secondary of the transformer. D&A powers pairs of 6.3-Volt heaters in series to the 12.6-Volt transformer secondaries. Wiring each set of four tube's heaters in series would add up to 120 Volts for each set of four tubes. Sounds neat and easy.

And dangerous.

What I would recommend is to find room for a 6.3-Volt transformer with at least a 5 Amp rating. Might end up bolting it to the back panel and feeding wires through grommets. The Maverick's original two 12.6-Volt windings now get wired in series-aiding, and the new transformer in series with those. This will add up to the 31 Volts AC needed for this trick with a minimum of rewiring, and with transformers isolating the wall socket from the innards of the Maverick. This way you can minimize the tube-socket rewiring on the heater pins 4 and 5.

Mavericks are not all wired the same. I have seen them with one heater pin grounded on all eight sockets, and I have seen them wired in series pairs, only every-other socket's heater with a ground connection.

You should probably upgrade the line cord to a 3-prong line plug if this has not already been done.

And if you're hell-bent on powering four 31-Volt heaters in series directly from the AC line cord, that third prong could save your life.

73
 

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