# 6l6 = 31.493 Disapation Watts

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as you all well know i have been playing with this 6L6 monster lol! i took the voltage reading on pin 3 of the 6l6 (409v) i then measured the voltage across the cathode resistor(30.8v) which has no bypass cap (would adding say a 25uf cap here improve the bottom end?) anyways the cathode resistor=195ohms.

now that the stage is set. i then began my calculations. i took the 30.8v cathode voltage and divided it by the 195ohms cathode resistance and came up with .154amps or 154ma. i then multiplyed this by the 409v plate voltage and came up with 62.986w disapation. now this is a combined load of both output tubes as they are both useing the same cathode resistor. so i then divided the wattage by 2 and came up with 31.493w per tube. does this sound like the bias is a little hot for 6l6 and would acount for quicker break-up and lower headroom? or even still have i completely fouled the whole bias calc process up?

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as you all well know i have been playing with this 6L6 monster lol! i took the voltage reading on pin 3 of the 6l6 (409v) i then measured the voltage across the cathode resistor(30.8v) which has no bypass cap (would adding say a 25uf cap here improve the bottom end?) anyways the cathode resistor=195ohms.

now that the stage is set. i then began my calculations. i took the 30.8v cathode voltage and divided it by the 195ohms cathode resistance and came up with .154amps or 154ma. i then multiplyed this by the 409v plate voltage and came up with 62.986w disapation. now this is a combined load of both output tubes as they are both useing the same cathode resistor. so i then divided the wattage by 2 and came up with 31.493w per tube. does this sound like the bias is a little hot for 6l6 and would acount for quicker break-up and lower headroom? or even still have i completely fouled the whole bias calc process up?

I'm not exactly sure of your measurements, but wouldn't the voltage drop across the tube and not just the plate voltage be the value to multiply the cathode current by to determine dissapation?

Meaning if there were no cathode resistor then the voltage drop across the tube would equal the plate voltage. But with a cathode resistor, wouldn't the voltage drop across the tube be the plate voltage minus the cathode voltage?

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31.5mA sounds right in the ballpark to me. 6L6s usually bias somewhere between 30-35mA each, IIRC. Max. wattage for a 6L6 is 23 watts, which you'll want to stay below for longer tube life.

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well yeah that does make sence! but however that is the process i used and those are the figures i came up with so i dont really know iam new to this tube stuff but trying to get a grip on it. i bought gerald webers "tube guitar amplifier essentials" and it has been a really big help i would like another book that has more step by step proceedures do you know of any?

after looking again it would seem iam i little cool on my bias at 29.12mA this is useing the voltage drop across the tube.

thanx for the clarification.

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Generally, the higher it's biased the more distortion you'll get - at the cost of a shorter tube life. I like a matched set of tubes at about 33mA each in my '74 Twin Reverb.

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well this beast resembles a fender, but it truely is a design of its own. the tone stack is really wierd! there is a scheamatic in the gallery(harmony model h-535) i have gone through it. it is built well. point to point hand wired mallory caps RCA 12AX7 and two mesa/china 6L6GC's some of the resistors have drifted and need replaceing. while i am rambling. cuold i increase the plate voltages(present voltage 409V) on the plate and respectivly the grid to get more power out of the 6L6's. i have a marshall 8100 valstate that is 100w SS hybred and the harmony is just a hair under the volume output of it. so i would like to get the harmony up to the same volume level. i would like to run them in stereo. kinda get that early marshall tone out of the harmony and the later marshall tone out of the 8100. wierd but just me lol!

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I wouldn't try to increase the plate voltage anymore than it is. 409V is probably pushing the tubes to their limits as it is...

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i have read of tube testing done on the 6L6's with a bias at 35mA and a plate voltage being around 470VDC. this test included the EL37 and came out in favor of the valve art 6L6GC! but i have also read of some amps useing the 7581A or a few other tubes for sheer power rateings. hence the old marshall "50 watt" head walking all over the fender tweed as far as output db is concerned. i dont know but i think some of the 6L6 line can be pushed up to 80watts rms total amplifier output. i think the did it by pushing the plate and grid voltage as high as safely possable.

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That's interesting. But, when was that test conducted? I personally wouldn't try it with new manufacture tubes. 5881s can handle a lot of abuse, though...

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it was in webers book guitar tube amplifier essentials it was on nos and new tubes i dont know how well the new tubes would hold up in real life. things are much different in some gurus shop than out in some dirty powered bar lmao! but i guess instead of wasteing my time and money on this harmony i should just go and buy a jcm800 and be done with it but being a poor electrician somewhat limits my gear budget.

thanx for the info though you guys are always a great help.

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OK, that makes sense. With NOS tubes, I could see that happening. You could try some 5881s, though - they can withstand some pretty high plate voltages.

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well i have 5881 mesa tubes whoever it is that mesa buys thier tubes from. but i dont want to do it if its going to fry them. i have one old sylvainia 6L6GC that could prob take such a voltage but i think im going to use it along with some rca 12ax7 tubes that i have to build a single ended amp. (first ever build) so i think i might purchase some nos sylvianias they were the original tubes in this monster and it had a little more volume level than the marshall with them before i busted on out of its base duh! stupid move but hey live and learn!

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Those Mesa tubes are probably made in the Sovtek factory. They're kind of expensive, but KT88s can also handle some pretty high plate voltages, IIRC. In any case, best of luck to ya!

Edited by Paul Marossy
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as always you guys have managed to keep me from mutilating my gear lol

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Glad to be of service. Guitar gear is a terrible thing to waste!

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