Most desirable 6L6GC tubes?

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Most desirable 6L6GC tubes?

Postby C_corie on Sun Sep 28, 2014 7:12 pm

I know a place that has a bunch, probably not "matched." At all. 2 questions what are THE types to look out for- specifics, and also how much does matching matter in my 64 6G6B Bassman? I've heard back in the day (since these were fixed bias people just stuck two of the same brand in) is this true? I have an amp tech...but in theory doesn't fixed bias mean it will just "work"? Please educate me.
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Re: Most desirable 6L6GC tubes?

Postby Baron Von Machinenmann on Sun Sep 28, 2014 7:19 pm

Stop sign logo RCA blackplates, but I wouldn't pass up on any old school production GE/RCA/Sylvania etc....
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Re: Most desirable 6L6GC tubes?

Postby OldSchoolDave on Sun Sep 28, 2014 10:22 pm

C_corie wrote:...but in theory doesn't fixed bias mean it will just "work"?


As I understand it, "fixed bias" means the bias is fixed - i.e., non-adjustable. It may or may not be proper for today's wall voltages and tubes.

C_corie wrote:...people just stuck two of the same brand in...


If that. I've seen plenty of vintage amps with power tubes of mixed brands. Not recommending that, just didn't seem to be as big a deal back then.

As the good Baron said, any of the good old "made in the U.S.A." 6L6GC's are worth having.

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Re: Most desirable 6L6GC tubes?

Postby Unit_1 on Sun Sep 28, 2014 10:53 pm

do a search for "cathode bias vs fixed bias"

fixed bias is adjustable, and MUST be adjusted (called biasing) when new power tubes are added

cathode bias is self adjusting, but produces less power per tube
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Re: Most desirable 6L6GC tubes?

Postby pdf64 on Sun Sep 28, 2014 11:00 pm

As I understand it, "fixed bias" means the bias is fixed - i.e., non-adjustable


Fixed bias (a separate constant negative voltage applied to the control grid) as opposed to self regulating cathode bias.
The fixed bias can generally be adjusted, whether by twiddling a trimmer or fitting different value resistors to the bias supply.

in theory doesn't fixed bias mean it will just "work"?


No, rather it's cathode bias that will tend to self regulate; even with that arrangement though, it's often the case that two or more power tubes may share a cathode bias resistor, which will tend to reduce the self regulation effect.

Fixed bias means that there's no self regulation, and so if the amp's operating conditions are too far removed from those suitable for the individual tube, tone or reliability will suffer.

I've heard back in the day (since these were fixed bias people just stuck two of the same brand in) is this true?


Then and now, that certainly goes on, and generally nothing terrible will happen.
It's not a good idea though, if you value tone and the amp's reliability and longevity.
Any tube (yes even the most holy grail golden age NOS) will sound bad if over biased, due to excessive crossover distortion.
Whereas if under biased, a tube may run too hot and short, ruining it and likely damaging the amp, possibly to a significant degree.
In the light of the above, I hope that it is clear that it can be very beneficial to check that the amp's operating conditions are suitable for its power tubes.
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Re: Most desirable 6L6GC tubes?

Postby C_corie on Mon Sep 29, 2014 4:02 pm

I get it it, I guess but... Did people in the 60s/70s change the bias resistors each time they changed tubes? I just feel like this is a "new idea" obviously it's correct and has always been needed but I mean when did it become "common knowledge" among just regular guitar players? Like Mesas are fixed bias and work because they know what tubes will run in them safely...is there an easy/inexpensive way to test tubes to see if they fall into the range my Bassman likes? I'm still a little hazy on this.
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Re: Most desirable 6L6GC tubes?

Postby Baron Von Machinenmann on Mon Sep 29, 2014 4:08 pm

The safest way would be to buy a biasing tool, not cheap but if you don't want to take it to your tech everytime you switch output tubes is a good investment. Something like this https://tubedepot.com/products/bias-king-pro
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Re: Most desirable 6L6GC tubes?

Postby Unit_1 on Mon Sep 29, 2014 8:25 pm

C_corie wrote:I get it it, I guess but... Did people in the 60s/70s change the bias resistors each time they changed tubes? I just feel like this is a "new idea" obviously it's correct and has always been needed but I mean when did it become "common knowledge" among just regular guitar players? Like Mesas are fixed bias and work because they know what tubes will run in them safely...is there an easy/inexpensive way to test tubes to see if they fall into the range my Bassman likes? I'm still a little hazy on this.


there used to be a LOT of tubes sold back then since just about everything was tube driven. the makers, when doing quality assurance testing, would simply through away the ones that were not in spec. if enough were out of range then they'd do an post mortem investigation on the tubes to find the problem and correct it.

so a simple non-adjustable resistor would be used to set the bias at a "fixed" standard position and everything was fine.

current production is limited and throwing away tubes that are out of spec is too expensive, so many people have added trim pots (a variable resistor) to the circuit to allow the tech adjust the bias to the lowest position, hook up a bias tester, and raise the bias until the tube is running in it's "best" position.

btw, Mesa is the only reseller (not a tube producer) that i know of that tests tubes and only sells the ones that will work without having to adjust the fixed bias resistor.

"best" is a subjective term. some people like the tubes set hot, and they don't last as long but that's the sound they like. other people prefer lower bias settings which means they don't have to buy tubes as often. it's the tone that counts so when setting bias you might want to try different settings (as long as you don't go too hot and blow up your new tubes) till you find the setting that sounds good to you.

and write that number down somewhere because after about 30 to 40 hours of playing you need to recheck the bias because tubes can sometimes change during the break-in and you might need to tweak the setting to get it back to the setting you liked best.
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Re: Most desirable 6L6GC tubes?

Postby C_corie on Tue Sep 30, 2014 5:12 pm

So would it matter if I bought the single tube one- or the two tube set? Could I just put in NOS and if they are SG the right ma? Level just call it good. Like do matched tubes even matter if I check the bias, like matched brand even?
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Re: Most desirable 6L6GC tubes?

Postby Unit_1 on Tue Sep 30, 2014 5:31 pm

C_corie wrote:So would it matter if I bought the single tube one- or the two tube set? Could I just put in NOS and if they are SG the right ma? Level just call it good. Like do matched tubes even matter if I check the bias, like matched brand even?


some amps have a bias adjustment trim pot for each tube, some have only one trip pot for *both* tubes

if your amp has individual adjustments then matched tubes are not so important. if you have only one trim pot, then it's important to buy a matched set of tubes!

i've seen brand new tubes be WAY WAY out of spec, and there's just no way a single trim pot could work on tubes so far apart from each other. in fact i bought one set of tubes that were so far out of spec that I had to change the resistor before the trim pot could get close enough to set the bias correctly.

remember having the bias too low can also cause component failure. very important to not be too high, and not be too low.

you cannot guess at bias, you must have a tool so you can adjust it and see when it's correct. to do otherwise could risk frying your amp. :Zap:

*not worth it* to save the cost of a bias tool!!!!!!!
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Re: Most desirable 6L6GC tubes?

Postby C_corie on Tue Sep 30, 2014 5:51 pm

I guess I'm saying if my amp is fixed bias, couldn't I just buy the one socket version and as long as the tubes were within a range of what.....4-6ma just call it close enough? I feel like I'm not understanding this in that there seems to be a safe "window" and if both of those tubes read in that window in the amp, can't you just call it close enough?
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Re: Most desirable 6L6GC tubes?

Postby Unit_1 on Tue Sep 30, 2014 6:46 pm

ah, ok. i misunderstood the question

you can just buy the one socket version, but testing both sides is important even if the tubes are matched

what i do is adjust the bias to the lowest setting, then test both sides to see which is the highest, then i adjust the bias with the tester on the highest side and then retest the lowest side to see if it's still lower

:cheers:

there is also a way of testing with just a multimeter, which is how i do it, but the potential for death from electrocution exists using this method if done carelessly :Zap:

buying a tester is MUCH safer and eliminates that potential
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Re: Most desirable 6L6GC tubes?

Postby C_corie on Tue Sep 30, 2014 8:09 pm

I saying....whatever the "safe" range of a 6L6GC in a Blonde Basssman is....if I get a set of two "random" maybe matching brand NOS tubes, and test them and if they both are within the window or whatever is safe.....lets say....38-46 or whatever it might be, wouldn't it be fine to just use them and not change the bias resistor? OR is it much common that one will be 29 and the other will be 49 so you have to mess with the bias. I just don't get why Fender designed these amps with a fixed part that has to be removed/soldered/changed each type you put tubes in. I know a bias pot can be installed but I'd rather keep it original.
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Re: Most desirable 6L6GC tubes?

Postby pdf64 on Tue Sep 30, 2014 8:33 pm

Did people in the 60s/70s change the bias resistors each time they changed tubes?


I think that tubes were regarded as user replaceable consumable, and that would often work out ok.
Potential issues with that are that some pre-amp tubes (which may even have checked out fine on a drug store tester) would be too noisey, microphonic etc in (certain) tube slots in a guitar amp.
As mentioned, some power tubes may not have been suitable for a particular amps' operating conditions, even if they checked out as good on a tester.
My guess is that such problems would have been blamed an a 'bad tube' and further tubes bought until something suitable was found.
If this happened too often then the owner might take the amp to a tech.

I just feel like this is a "new idea" obviously it's correct and has always been needed but I mean when did it become "common knowledge" among just regular guitar players?


I think that Groove Tubes have been marketed directly to musicians for around 30 years, with one of their USPs being that for their power tubes, once bias was set for a particular rating of a tube type, then it shouldn't need adjusted when the tubes were replaced with others of the same rating
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Re: Most desirable 6L6GC tubes?

Postby C_corie on Wed Oct 01, 2014 12:35 pm

So what is a safe level of mismatched current draw between two tubes in a 6G6B Bassman? Like where is the cut off between ok and ohhhhh no good- I'm planning on buying that bias socket thing. But don't know much about acceptable limits.
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