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  •  » Wow, I picked up a Magnavox era V4B and the original tubes all tested

#1 2011-06-13 12:22:09

zone47
New member
Registered: 2011-01-31
Posts: 8

Wow, I picked up a Magnavox era V4B and the original tubes all tested

really strong!   They are all Magnavox branded 7027s.  It sounds great and I should just leave well enough alone, but I am curious if you swap a 6L6 in there just to see if the tone changes for better or worse, or does it need a rebias to do that?   I have a bias right meter so would just have to know what amperage to set them at.   Any help is appreciated. smile

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#2 2011-08-24 16:04:55

Liquids
Member
From: CT
Registered: 2010-08-01
Posts: 491

Re: Wow, I picked up a Magnavox era V4B and the original tubes all tested

No one replied to this?  Bummer.

You sound handy - is the original bias resistor in there (75k) and/or measure out accurately, or was it replaced with a resistor and/or variable resistor combo of some kind?

Either way If your bias resistance is around 75k, plug in some suitable (not just any) 6L6GCs and you'll be set.  I messed a lot with biasing, and in the end, I biased by ear with a trimmer and measured the total resistance....and it was uncanny how close to 75k it was!  Tubes differ, sure, but that resistor provides a stable and sweet sounding bias for dare I say the whole range of 6L6GC/7027 tubes.  Doesn't hurt to measure current across the plate resistors if you can, however.   

In that case, I'd be sure NOT to exceed 30mA idle, and it's hard to imagine the 75k resistor would put you there or higher. Please forget what you've heard about 70% idle bias (38mA here!), at least with this amp! I think that my method ended up setting mine near 25mA.  When I was dialing it in by ear, I think I even dug the tone down to ~20mA which is about 35%, and just slightly past cut-off! I was concerned about crossover distortion from a theoretical standpoint so I split the difference more or less and it sounds the same-ish between there and under 50% or so. 

The Amp runs pretty cool the way I have it, too = reliability.  SED 6L6GCs aint cheap but they sound good and should last forever at that bias.

Last edited by Liquids (2011-08-24 16:07:42)


Matthew

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