Members The Boogie Man Posted March 20, 2006 Members Share Posted March 20, 2006 What temperature should 6L6's run at? I know how to bias amps and have done it several times before but wanted to measure the actual temperature of the tubes, so I tested my Twin Reverb and the tubes were running at about 185 F with NOS Westinghouse tubes and the bias at -48 then I tested my Reverend Hellhound, it has no bias control, with new 6L6GCMSTR Ruby Tubes and they were running at 225+ F just off the thermometer's scale, this seems a little high to me, so I put two of the Twin's tubes in and it ran at about 180 F. I haven't checked the bias on the Reverend yet due to time but plan on it next weekend when I get home, I think the bias should be about -45 to 49, my guess that the bias for these tubes is to high and it's cooking the plates, or do these tubes just run hotter? Thanks Link to comment Share on other sites More sharing options...
Members potaetoes Posted March 20, 2006 Members Share Posted March 20, 2006 how are you measuring the bias to get those numbers? Link to comment Share on other sites More sharing options...
Members cap'n'crunch Posted March 20, 2006 Members Share Posted March 20, 2006 I'm going to put a temperature gauge on my amp. Link to comment Share on other sites More sharing options...
Members tommythelurker Posted March 20, 2006 Members Share Posted March 20, 2006 Originally posted by The Boogie Man ... I know how to bias amps and have done it several times before If you're biasing based on the grid voltage, you don't know how to bias amps and you're doing it wrong. If it's biased properly, the temp probably depends more on the heaters and ventilation than anything. I think you're barking up the wrong tree looking at the temp. Link to comment Share on other sites More sharing options...
Members The Boogie Man Posted March 20, 2006 Author Members Share Posted March 20, 2006 Originally posted by tommythelurker If you're biasing based on the grid voltage, you don't know how to bias amps and you're doing it wrong. If it's biased properly, the temp probably depends more on the heaters and ventilation than anything. I think you're barking up the wrong tree looking at the temp. Okay if you don't use voltage to measure bias what do you use? And as far as the temperature I measured it with a probe thermometer touching the side of the tube, not the best tester but better than by hand . Link to comment Share on other sites More sharing options...
Members potaetoes Posted March 20, 2006 Members Share Posted March 20, 2006 Originally posted by The Boogie Man Okay if you don't use voltage to measure bias what do you use? And as far as the temperature I measured it with a probe thermometer touching the side of the tube, not the best tester but better than by hand . idle current Link to comment Share on other sites More sharing options...
Members tommythelurker Posted March 20, 2006 Members Share Posted March 20, 2006 Originally posted by The Boogie Man Okay if you don't use voltage to measure bias what do you use? There are several methods. You can measure it with a volt meter, but you typcially measure the voltage across a 1 ohm resistor connected between the cathode and ground, NOT THE GRID VOLTAGE. (measuring across the OT primary can work too, but you have to know it's DC resistance). The point is to measure the current flowing through the plate (either directly or indirectly). The grid voltage controls the plate current, but the relationship between the two isn't consistent between different tubes, that's the WHOLE reason for adjusting the bias when replacing tubes in the first place. I dunno, maybe you are reading the voltage across a cathode resistor which is fine. But that voltage usually isn't referred to as negative and usually isn't in the range you're talking about for 6L6's (unless the resistor is shared with by 2 tubes?). It sounds like you're talking about grid voltage which isn't what you should be measuring. Read the links at the end of my sig if you want to know more. Link to comment Share on other sites More sharing options...
Members the russ Posted March 20, 2006 Members Share Posted March 20, 2006 those EL34 tubes are running way hotter in temperature than a 6L6 because they draw way more heater current than a 6L6. it's OK (possibly. bias the thing correctly and then report back). Link to comment Share on other sites More sharing options...
Members The Boogie Man Posted March 20, 2006 Author Members Share Posted March 20, 2006 Originally posted by tommythelurker There are several methods. You can measure it with a volt meter, but you typcially measure the voltage across a 1 ohm resistor connected between the cathode and ground, NOT THE GRID VOLTAGE. (measuring across the OT primary can work too, but you have to know it's DC resistance).The point is to measure the current flowing through the plate (either directly or indirectly). The grid voltage controls the plate current, but the relationship between the two isn't consistent between different tubes, that's the WHOLE reason for adjusting the bias when replacing tubes in the first place.I dunno, maybe you are reading the voltage across a cathode resistor which is fine. But that voltage usually isn't referred to as negative and usually isn't in the range you're talking about for 6L6's (unless the resistor is shared with by 2 tubes?). It sounds like you're talking about grid voltage which isn't what you should be measuring.Read the links at the end of my sig if you want to know more. Thanks. According to my limited info the bias voltage it should be about 47 on pin 5. I've never seen tubes run so hot in any amp without frying them in a short time. This amp has a fixed bias, I quess I should measure the bias and plate voltage as well when I open it up next weekend. I have never used Ruby tubes before, I've been told it's a very good tube, they claimed it's a matched set but I don't know the rating #, the tubes test good on my Tester no problems. When I swap out the Rubys for the NOS Westinghouse from my Twin the amp runs way cooler that's why I measured the temperature just as a reference. I may just stick with the Westinghouse, they run cooler and seem to last forever. Next weekend I'll try some used black face RCA's I've been saving from my Twin and check the bias and plate voltage on this little Hellhound with several different tube sets, I may have to put in a bias control if I want to run the Rubys. Link to comment Share on other sites More sharing options...
Members potaetoes Posted March 20, 2006 Members Share Posted March 20, 2006 Originally posted by The Boogie Man Thanks. According to my limited info the bias voltage it should be about 47 on pin 5. I've never seen tubes run so hot in any amp without frying them in a short time. This amp has a fixed bias, I quess I should measure the bias and plate voltage as well when I open it up next weekend. I have never used Ruby tubes before, I've been told it's a very good tube, they claimed it's a matched set but I don't know the rating #, the tubes test good on my Tester no problems. When I swap out the Rubys for the NOS Westinghouse from my Twin the amp runs way cooler that's why I measured the temperature just as a reference. I may just stick with the Westinghouse, they run cooler and seem to last forever. Next weekend I'll try some used black face RCA's I've been saving from my Twin and check the bias and plate voltage on this little Hellhound with several different tube sets, I may have to put in a bias control if I want to run the Rubys. you're demonstrating to yourself exactly why the bias voltage is meaningless, as far as measuring what a tube is doing. different tubes operate differently with the same voltages. Link to comment Share on other sites More sharing options...
Members The Boogie Man Posted March 21, 2006 Author Members Share Posted March 21, 2006 Originally posted by potaetoes you're demonstrating to yourself exactly why the bias voltage is meaningless, as far as measuring what a tube is doing. different tubes operate differently with the same voltages. Sure, I knew that but a tubes life depends a bit on it's tempature, plate voltage, bias and quality, the Ruby is running way hotter so I would guess it's life would be shorter. All else being equal, it's sounds just as good but not better than the other tubes I've used. Link to comment Share on other sites More sharing options...
Members potaetoes Posted March 21, 2006 Members Share Posted March 21, 2006 Originally posted by The Boogie Man Sure, I knew that but a tubes life depends a bit on it's tempature, plate voltage, bias and quality, the Ruby is running way hotter so I would guess it's life would be shorter. All else being equal, it's sounds just as good but not better than the other tubes I've used. the ruby is probably running way hotter because it's not biased properly. thermal cycles hurt tubes much more than constant heat does. heat alone isn't really a problem if the tube is operating within its normal ranges. they have heaters in them for a reason. if you're at all concerned about tube life, you *need* to measure the idle dissipation in relation to the plate voltage, and adjust the bias accordingly. measuring the bias voltage at pin 5 tells you absolutely nothing about what the tube is actually doing. Link to comment Share on other sites More sharing options...
Members The Boogie Man Posted March 21, 2006 Author Members Share Posted March 21, 2006 Originally posted by potaetoes the ruby is probably running way hotter because it's not biased properly. thermal cycles hurt tubes much more than constant heat does. heat alone isn't really a problem if the tube is operating within its normal ranges. they have heaters in them for a reason. if you're at all concerned about tube life, you *need* to measure the idle dissipation in relation to the plate voltage, and adjust the bias accordingly. measuring the bias voltage at pin 5 tells you absolutely nothing about what the tube is actually doing. The heater should be running at about 6.3 volts same as any 6L6, according to Reverend the plate voltage should be 450 and bias is fixed no adjustment and about 47 from what they claim. I haven't had the time to measure plate or bias voltage on this amp yet since my test equipment is in one state and my amp and I in another, it worked good last weekend with the new Rubys but I noticed how hot it was just touching the chassis with my hand after the first 1/2 hour. I checked and found some info on Rubys saying they can handle 490-500 volts, makes me think they are drawing more curent than the NOS and are over biasing their tube at the factory something like that? Maybe just this pair but it makes me wonder if I should use them in this amp since I can't adjust them down a little, but they were cheap and the only tubes I could get in a pinch on the road. Link to comment Share on other sites More sharing options...
Members potaetoes Posted March 21, 2006 Members Share Posted March 21, 2006 Originally posted by The Boogie Man The heater should be running at about 6.3 volts same as any 6L6, according to Reverend the plate voltage should be 450 and bias is fixed no adjustment and about 47 from what they claim. I haven't had the time to measure plate or bias voltage on this amp yet since my test equipment is in one state and my amp and I in another, it worked good last weekend with the new Rubys but I noticed how hot it was just touching the chassis with my hand after the first 1/2 hour. I checked and found some info on Rubys saying they can handle 490-500 volts, makes me think they are drawing more curent than the NOS and are over biasing their tube at the factory something like that? Maybe just this pair but it makes me wonder if I should use them in this amp since I can't adjust them down a little, but they were cheap and the only tubes I could get in a pinch on the road. again, you will not know what the tubes are actually doing unless you actually measure the idle current and plate voltage. there's no way around it. Link to comment Share on other sites More sharing options...
Members The Boogie Man Posted March 21, 2006 Author Members Share Posted March 21, 2006 Thanks! I can only assume your right, do you know what the correct idle current for 2 6L6GC in class AB at 450 plate volts would be and where would you measure it? If I remember right, I've always measured bias at pin 5 on most amps and I think pin 3 for cathode/plate but it's been a few years and I could be wrong. I think these Rubys are running way too hot, maybe the tubes are rated for 400 plate volts not 500 as was claimed, that would explain the high temp. When I get home I'll research it more. If they are okay working at this tempature I wouldn't recommend these tubes for any amp with the sockets mounted to the PC board, you would melt something. Link to comment Share on other sites More sharing options...
Members potaetoes Posted March 21, 2006 Members Share Posted March 21, 2006 measuring current involves interrupting the circuit, either with a bias probe socket, or by soldering a 1-ohm 10-watt resistor in series with the plate (pin 3). measuring the voltage drop across that resistor when there's no signal to the amp, will tell you how much current is passing through the plate at idle. bias probes do the same thing (place a resistor in series) without soldering, and cost about $20 or so. they really make life easy. any 6L6GC *should* be able to handle at least 500V on the plate. a normal bias range for a typical 6L6 at 450v is 30-38ma per tube, or a little more or less per taste. the voltage at pin 5 is pretty much irrelevant - adjust it to whatever value gets the plate current into the right range. if there's no adjustment trimpot, you can either add one (common, makes things easier in the future), or adjust the value of the bias resistor (less practical). Link to comment Share on other sites More sharing options...
Members the russ Posted March 21, 2006 Members Share Posted March 21, 2006 oops, sorry, didn't see you were talking about the Ruby 6L6 not the EL34 Link to comment Share on other sites More sharing options...
Members potaetoes Posted March 21, 2006 Members Share Posted March 21, 2006 one more thing... if the tube is getting too hot, it doesn't mean voltage is too high. voltage doesn't create heat. current creates heat. if the tube is biased improperly and passing too much current, the plate will get extremely hot and eventually (possibly) begin to melt. this can happen even with very conservative voltages, like 325v on the plates. Link to comment Share on other sites More sharing options...
Members The Boogie Man Posted March 21, 2006 Author Members Share Posted March 21, 2006 Originally posted by potaetoes one more thing... if the tube is getting too hot, it doesn't mean voltage is too high. voltage doesn't create heat. current creates heat. if the tube is biased improperly and passing too much current, the plate will get extremely hot and eventually (possibly) begin to melt. this can happen even with very conservative voltages, like 325v on the plates. yea I thought of that too. These tubes look okay when working, a little more orangeish glow than some but no burn spots on the tube filliments after 6 hours of steady use, I've seen worse. Maybe there just made to run hotter. Link to comment Share on other sites More sharing options...
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