Crossover distortion with my 6L6 from Eurotubes

[tpawlowskis]

Hey guys. I have been playing my b-52 at100 head for a while now with some new 6l6's and all the 12ax's in the pre. also put a new recto tube in. There's like this harsh grainy sound that makes leads not enjoyable on the amp and rythem sections sound like theyre played on a squire strat when i'm really playing them on a les paul.

When I biased the amp I put my voltimeter on the chassis ground, and on pin 5 of the P-tubes. All read a perfect 34-increased to 39. Eurotubes specifies to touch pin 3, but I did not get a reading on pin 3 and a youtube instructional on this amp specified pin #5. Eurotubes specifies not to exceed 44 on your bias reading.

I think this distortion is crossover distortion. Does anyone know what typically causes this? Am I just an idiot and doing something wrong?

If it helps at all - There is some grounding issue on one of my pots on my les paul nothing serious, just an issue where if you don't touch the strings while you play the amp will hum a bit. I want to say last time i tried a different axe on it it still sounded harsh, so i don't think the ground prob. on the les paul has anything to do with this issue. Thanks and hope all is well people of the forum.

[olddawg]

How are you setting the amp up? Is this a new problem since a retube? If you have the master v turned way down and the pres turned way up it will sound very thin with a distortion most people find unpleasant.

[tpawlowskis]

How are you setting the amp up? Is this a new problem since a retube? If you have the master v turned way down and the pres turned way up it will sound very thin with a distortion most people find unpleasant.

Hey and thank you for the quick reply. I play with it at a volume that's satisfiable to my neighbors but still loud enough to shake the floor and piss my mom off. I'd say the master at half and the channel vol at 3-4. I did get the chance to turn it up all the way and the gain's just really lacking. It's got no oomph or clarity to the palm mutes. No pres knob on this amp, it's got treb, bass, mid, contour, gain, and low res for this amp.

Edit: I also can tell you that the retube has made the tone better, but i can tell that this amp is still not playing how it is supposed to play since i used to have one a while back. I'm just thinking that this has to do with the increase in specs a 6L6 p-tube has, and my amp might not be running them high enough? I don't know about tubes, i just know how to set the bias on this beast.

[stokes]

"All read a perfect 34-increased to 39".I assume you mean you have -39vdc on pin 5.That really tells nothing about where the tubes are idling.If the problem is truly crossover distortion it would indicate that the tubes are biased too cold,but without knowing the current draw,I couldnt advise any change.You need to determine the current draw and then multiply that by your plate voltage to determine the idle watts.6L6's have to be very cold to cause crossover distortion,I have to doubt that is the problem,but without knowing what the idle dissipation is you really cant tell.

[acorkos]

you need one of these to go w/your meter to determine current draw...

Eurotubes - Bias Probes for vacuum tube valve amps!

[Jazz P Bass]

"All read a perfect 34-increased to 39".I assume you mean you have -39vdc on pin 5.

And to add to that thought, going from -34Vdc to -39Vdc set the bias 'colder'.
So if there was a xover notch at -34, at -39 it will be worse.

[tpawlowskis]

alright alright everyone thank you very much, so we're really getting something here. I'm still quite noobish at this biasing procedure. could you guys tell me what i need to actually be pulling the reading off of in order to get the bias? Are you saying I need to be moving in the OPPOSITE direction? like -30?ish for example? and are you also saying that reading the voltage from pin 5 is incorrect on a 6l6? because when I read pin 3 as instructed by eurotubes, as i said it was reading "1" I am pulling the amp out of the chassis right now as we speak. sorry for the late response, I'm working overnight, so i had to sleep.

[stokes]

Reading the dc voltage on pin 3 will give you the plate voltage.Reading pin 5 gives you the bias supply voltage,apparently negative 39 or -39 in this case.If you actually see no voltage at pin 3 you have a serious problem somewhere.You should be seeing over 300 vdc at the plate,or pin 3.Not sure what the exact plate voltage on that amp is,but being a modern 6L6 amp,it should be more than likely over 400 v's.Check that pin 3 again before you go any further.

[tpawlowskis]

Hey man, I checked pin #3, it's reading 2 volts. it read "1 volt" before I turned the bias to -34. the PV on this amp on average according to eurotubes is 460. I think there's some sort of mis reading between my voltimeter and I, because the amp is plenty loud and the tubes glow etc. I just cant seem to read what you guys want me to read from pin #3. If i turn the bias pot slightly in the "+" direction which is counter clockwise it would seem, the reading on pin #5 decreases to the lower 30's, pin #3 raised 1 volt "2 volts" and my reading from both power tube fuse leads reads 454V

so to answer your question, YES, pin 3 is increasing proportionally it would seem, but I am not getting a reading of 300+v as you say. I think everything is the way it should be, the voltimeter may just not be the right kind? it's one of those meters that doesnt have the big dials in the middle, just AC, DC, OHM. and a 4 character meter on the screen. it's called a FLUKE T5 1000.

[JoeM]

Are you sure you're reading the correct pin numbers? If you are reading 454V at the fuse, pin 3 should be the same.

[tpawlowskis]

it says "pin 3" on the circuit board. along with the others labeled as well.

[tpawlowskis]

i mean, when i set the bias from -39 to -34 thats increasing i would assume as ive now learned, and I saw the voltage on the fuse increase from about 445 to 454 and the tone went from kweef to beef. but for some reason pin three is just not reading this lol. unless the translation on the circuit board is wrong. cause i measured the fuse voltage at 454, 25/454 = 0.055 x.7 = .38, so that would mean my bias is set at 38 i assume lol?

[JoeM]

Sure it's not an "8"? If you check all the 6L6 socket pins, do you find a pin that's close to the 454V you measure at the fuse. You should have HV at pin 3 and pin 4.

Voltage will go up and down with the bias, since your changing the current through the output stage. Why not just set it at where it sounds best? Just don't redplate your tubes!

[tpawlowskis]

well, i can check. will it fry my p-tube if i try picking up the ground, and touching the other pins with the +lead?

[JoeM]

You have the meter set for DC volts, and not DC amps, right? Then ground your negative lead, and check the socket pins with the positive lead. Does the negative lead have a clip? I dont like using two hands when measuring.