I have a 2 x Sovtek 6L6WXT+ (cannot find the datasheet ) ODS build and was wondering where to bias the power tubes. I've been reading that ODS's are quite happy with pretty cold bias settings.
Not familiar with that 6L6, but if they are similar to the 30w 6L6GC, here is a bias chart with different plate voltages.
Looks like between 30 and 42ma is the zone with the 42ma being around 70%. http://members.shaw.ca/house-of-jim/Htm ... ables.html
It really depends on the tube specs. Have never looked into 6L6s much, but the few sheets I've seen have a -wide- variation of plate dissipation; 19-30 watts!.
If your original table is based on 19 max watts, then 24.7 ma bias corresonds to about 63% with 485 volts.
Edited- That sheet also includes 5881s. Even at that, that bias is pretty low?? Use the second table,
gearhead wrote:It really depends on the tube specs. Have never looked into 6L6s much, but the few sheets I've seen have a -wide- variation of plate dissipation; 19-30 watts!.
If your original table is based on 19 max watts, then 24.7 ma bias corresonds to about 63% with 485 volts.
Edited- That sheet also includes 5881s. Even at that, that bias is pretty low?? Use the second table,
I just got answer where I bought the tubes and they are same as 6L6GC (30w).
Those are definitely 30W tubes (I had a case of them at one point) so you can go as high as 44ma at 485V. BUT I always thought they sounded better closer to 50% of max dissipation. Try 'em around 35ma too and see what you prefer.
Firestorm wrote:Those are definitely 30W tubes (I had a case of them at one point) so you can go as high as 44ma at 485V. BUT I always thought they sounded better closer to 50% of max dissipation. Try 'em around 35ma too and see what you prefer.
All right, I'll try comparing 35 to 44..
I red somewhere that these tubes has 20% more output than the averidge 6L6's.
I think that's New Sensor's claim that the 6L6WXT+ has 20% more output than the 5881WXT. My favorite new production 6L6 is the SED (Winged C) version from St. Petersburg, but I had problems with spotty QC a few years ago. I think the 6L6WXT+ is a close second (and it sure looks exactly like the 6L6EH, but cheaper). They're great in Fenders particularly. In JTM45s and Bluesbreakers, I prefer the SED, biased hot.
Out of a case (100) of the 6L6WXT+s, I had about 10 go bad (that's actually a pretty good rate) and about 60 were close enough to be considered "matched." The other ones were all over the place, but that's pretty consistent with anyone's tubes if they aren't burned in before they're shipped. For anybody thinking getting just a few of these, definitely have them matched to weed out the duds.
Firestorm wrote:I think that's New Sensor's claim that the 6L6WXT+ has 20% more output than the 5881WXT. My favorite new production 6L6 is the SED (Winged C) version from St. Petersburg, but I had problems with spotty QC a few years ago. I think the 6L6WXT+ is a close second (and it sure looks exactly like the 6L6EH, but cheaper). They're great in Fenders particularly. In JTM45s and Bluesbreakers, I prefer the SED, biased hot.
Out of a case (100) of the 6L6WXT+s, I had about 10 go bad (that's actually a pretty good rate) and about 60 were close enough to be considered "matched." The other ones were all over the place, but that's pretty consistent with anyone's tubes if they aren't burned in before they're shipped. For anybody thinking getting just a few of these, definitely have them matched to weed out the duds.
Id put a 1ohm 1% resistor in the chathode return
to get a ma. measurement and do the math
when you bias colder you end up with a shallower
cunduction angle towards B side of class AB
when bias hotter towards the max dissapation of the tube
you load the power supply dropping the plate volts
giveing you steeper conduction angle towards the A side of class AB
or farther away from B
the sweet spot is the compromise between that gives the max watts
and low harmonic distortion... or desired harmonic content
Andy Le Blanc wrote:Id put a 1ohm 1% resistor in the chathode return
to get a ma. measurement and do the math
when you bias colder you end up with a shallower
cunduction angle towards B side of class AB
when bias hotter towards the max dissapation of the tube
you load the power supply dropping the plate volts
giveing you steeper conduction angle towards the A side of class AB
or farther away from B
the sweet spot is the compromise between that gives the max watts
and low harmonic distortion... or desired harmonic content
know your load....
I have the 1ohms in there. I tried with 35mA and 44mA and did not notice any difference on the sound ..so I settled at 37mA.
if your measuring a single tube at a plate volts of 485
thats a watts dissipation of 17w and 21w
thats cold.... the 70% rule is around 24w for a tube that can be pushed
to 30w in rough terms
thats still really cold for some applications
it would be around .050 A or 50ma for the 70%
if your plate volts stays at 485v
A x V = W
you really should do the math every time... every amp is a different dynamic creature ....
charts might not represent the real voltage conditions your working with...
and every time you touch the bias the rest of the voltage conditions will change
Andy Le Blanc wrote:if your measuring a single tube at a plate volts of 485
thats a watts dissipation of 17w and 21w
thats cold.... the 70% rule is around 24w for a tube that can be pushed
to 30w in rough terms
thats still really cold for some applications
it would be around .050 A or 50ma for the 70%
if your plate volts stays at 485v
A x V = W
you really should do the math every time... every amp is a different dynamic creature ....
charts might not represent the real voltage conditions your working with...
and every time you touch the bias the rest of the voltage conditions will change
Yeah, as I mentioned on my first post I got 46.6mA doing the math. I'll try setting the iddle current nearer to 50mA and then measure the plates again + listen carefully
I get 30W x 0.7 = 21W (70% of max. plate disspation) With a 1 ohm cathode resistor, maybe 5% of your current will be screen current, so at 485 plate volts, 45.5 ma measured across your 1-ohm gets you to 70%). Mike mathews would love to have you bias hotter (you could join the tube of the week club) but I doubt that the sonic benefits outweigh the stress on the tubes.
As Andy points out, biasing hotter shifts you toward Class A, which will minimize crossover distortion, but will also significantly reduce output power -- first because Class A is way less efficient than Class B, and second (as Andy also points out) because loading the power supply like that will pull your plate volts down. If you treat 50% and 70% as the allowable range, you can set your bias for the sound you're looking for: fat and full, bias hot, "crispy," bias colder (a little crossover distortion gives you a touch of odd-order harmonics).
The other thing to consider is that a lot of amp manufacturers (like Fender in the '60s) biased their amps cold for tube life and adjusted the frequency response of the amps to compensate. Try biasing a blackface Super Reverb at 70% and it will not sound "right" (whatever that means).
) ODS build and was wondering where to bias the power tubes. I've been reading that ODS's are quite happy with pretty cold bias settings.
..so I settled at 37mA.