Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on August 28, 2016, 05:16:18 am
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Hi guys, I have a large number of 6DQ6A and 6DQ6B valves in various brands.
Because this are oddball valves you can't buy them as "matched pairs"
So I'm down to matching them via the 1ohm cathode resistor and then its a battle to get them reasonably close.
I have mainly RCA and GEC and a few local brands.
Is it acceptable to mix brands to get a closer match or even 'A' and 'B'. Thanks
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It's OK with me.
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To me also
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For those tubes that didn't match well you can double the bias supply circuit for fixed bias or adopt a balance circuit for cathode bias
(http://i.imgur.com/Fqztnd9.jpg)
Ciao
Franco
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I think as long as your don't exceed the tube specs, your ear is better than the #'s in this case
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It's sort of a judgement call as to how much of a mismatch you are or would be willing to accept.
I once bought a garage load of electronic junk with about 35 6L6GA Sylvania NOS WW2 era and as I didn't especially want to use them in amps since their max voltage rating is lower than your typical Fender, I decided to sell them on ebay, hopefully as matched pairs. Thus I needed to build a tube matcher, which I did. It simply applied a reasonable B+ and negative bias, same for both tubes, and showed the respective cathode currents on two big meters that I could photograph and put up in an ebay ad.
Well, the differences between two same brand same date code, lived in the same crumbling cardboard boxes next to each other for 60 years really surprised me. There were plenty of 50% differences. 30 ma vs 45 ma. 22 ma vs 36 ma. Now, nobody can say whether tube mfg tolerances were tighter then because the machinery was in better shape or was better for a 1964 6L6GC.
Anyway, I set about testing and matching them and arbitrarily decided that "matched" meant "within 2%". Of course, nobody can say whether and how any given pair would age, either.
Keep in mind that zillions of similar examples occurred with people just jamming two new 6L6 into Fender amps for 50 years. I don't think any bones were broken. If they are within 5-10 mils I wouldn't be at all concerned. Up at 15-20 mils difference, I would avoid using that pair.
(http://i172.photobucket.com/albums/w32/ttm4/ebay_tubes_4007.jpg) (http://s172.photobucket.com/user/ttm4/media/ebay_tubes_4007.jpg.html)
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... adopt a balance circuit for cathode bias ...
Hmmm... I feel like I've seen that schematic somewhere before. :grin:
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(http://www.diyitalia.eu/images/smilies/Trilly-18-39.gif) (http://www.diyitalia.eu/images/smilies/Trilly-18-39.gif) (http://www.diyitalia.eu/images/smilies/Trilly-18-39.gif)
I think you're right :icon_biggrin:
Franco