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EL84 Bias Questions

6/21/2000 5:50 PM
Dean Hazelwanter	EL84 Bias Questions I converted a single-ended console stereo with 2 EL84s to a mono guitar amp power amp, and built a single 12AX7 preamp. It sounds extremely good. After I had it working, I started thinking about the bias and static power dissipation of the remaining power tube - since both power tubes shared the same cathode resistor and bypass cap. All I did to change the power amp section from stereo to mono, was to remove the 2nd channel's power tube. I later also removed that channel's OPT. I recalled reading that 12W is about right for an EL84. I calculated that at the moment, that mine was running at about (6.11/64.3 * (307-6.11)) = 28.6W!!! I changed the 68R (measured at 64.3R) cathode resistor to what I had on hand that was about twice the original - 150R. This brought the static dissipation to ~19.5W. Questions: - What is the optimum static power dissipation for an EL84, to get the best tone with reasonable life expectancy? - What is the maximum that one should be run at, before it: - Drastically reduces life expectancy - Starts on fire 8^) I also realized that another factor as to why it's running hotter is due to the fact that the load on the power supply was cut in half, which raised B+.

6/21/2000 5:50 PM

Dean Hazelwanter

EL84 Bias Questions

I converted a single-ended console stereo with 2 EL84s to a mono guitar amp power amp, and built a single 12AX7 preamp. It sounds *extremely* good. After I had it working, I started thinking about the bias and static power dissipation of the remaining power tube - since both power tubes shared the same cathode resistor and bypass cap. All I did to change the power amp section from stereo to mono, was to remove the 2nd channel's power tube. I later also removed that channel's OPT.

I recalled reading that 12W is about right for an EL84. I calculated that at the moment, that mine was running at about (6.11/64.3 * (307-6.11)) = 28.6W!!! I changed the 68R (measured at 64.3R) cathode resistor to what I had on hand that was about twice the original - 150R. This brought the static dissipation to ~19.5W.

Questions:
- What is the optimum static power dissipation for an EL84, to get the best tone with reasonable life expectancy?
- What is the maximum that one should be run at, before it:
- *Drastically* reduces life expectancy
- Starts on fire 8^)

I also realized that another factor as to why it's running hotter is due to the fact that the load on the power supply was cut in half, which raised B+.

6/21/2000 7:22 PM
MBSetzer	Hi Dean: I can't actually answer your questions, but I can give some data. In the GE book the example conditions for cathode bias 6BQ5 in single-end Class A service (the only operating point listed) have 250V on the screen & plate, cathode resistor of 135ohms, 48ma plate dissipation + 5.5ma screen diss. My 1967 Skylark has 250V B+ but a 150ohm Rk and does excellent, I tried 130ohms and found little difference in sound so I switched back. The original tube in this amp is still fine so these are probably good conditions for NOS tubes. If you continued with the setup you had, maybe you would find out most accurately whether there was reduced tube lifetime or a fire extinguisher was necessary. Since you have already removed the other OPT, I guess you didn't consider bridging the output of your preamp to both channels' inputs and using separate speakers to double the power and maybe the tone? This seems like a good idea with identical channels and signals to each one but different speakers for adding lushness. Mike

6/21/2000 7:22 PM

MBSetzer

Hi Dean:

I can't actually answer your questions, but I can give some data.

In the GE book the example conditions for cathode bias 6BQ5 in single-end Class A service (the only operating point listed) have 250V on the screen & plate, cathode resistor of 135ohms, 48ma plate dissipation + 5.5ma screen diss.

My 1967 Skylark has 250V B+ but a 150ohm Rk and does excellent, I tried 130ohms and found little difference in sound so I switched back. The original tube in this amp is still fine so these are probably good conditions for NOS tubes.

If you continued with the setup you had, maybe you would find out most accurately whether there was reduced tube lifetime or a fire extinguisher was necessary.

Since you have already removed the other OPT, I guess you didn't consider bridging the output of your preamp to both channels' inputs and using separate speakers to double the power and maybe the tone? This seems like a good idea with identical channels and signals to each one but different speakers for adding lushness.

Mike

6/21/2000 10:43 PM
Dean Hazelwanter	Thanks for the reply Mike. I think I'll try bringing it down a little more. I think I documented a few of the original voltages before I started screwing around with it. If I didn't, I should have. And even if I leave it as-is, I just bought 2 fire extinguishers about a month ago. 8^)

6/21/2000 11:05 PM
Peter S	Dean, I'm with Mike on this one. Maybe you can put the OT back in and run two output tubes and speakers. It would sound HUGE compared to the way it sounds now. Also normally, if you had two tubes running off the same cathode resistor and then removed one of them you have to double the value of the cathode resistor to keep the bias in the same range, however, most single ended Vox amps use a 150ohm cathode resistor and the push pull models use a 130ohm. So you're probably ok. I would put the amp in the dark with it idling and look for signs of stress.....ie. Glowing plates, screens etc. PS

6/21/2000 11:05 PM

Peter S

Dean,
I'm with Mike on this one. Maybe you can put the OT back in and run two output tubes and speakers. It would sound HUGE compared to the way it sounds now. Also normally, if you had two tubes running off the same cathode resistor and then removed one of them you have to double the value of the cathode resistor to keep the bias in the same range, however, most single ended Vox amps use a 150ohm cathode resistor and the push pull models use a 130ohm. So you're probably ok. I would put the amp in the dark with it idling and look for signs of stress.....ie. Glowing plates, screens etc.

PS

6/22/2000 8:12 PM
Dean Hazelwanter	Thanks for the input, Peter - much appreciated! I'd rather not change it back to stereo, or bridge the channels for the following reasons: 1) I had the chassis sitting around for over a year, until I found an old CariVoice(sp?) solid state PA combo with 10" alnico speaker at a surplus sale (including a Shure UniDyneIII mike) for $10. I gutted the 'three-legged-fuse stuff', mounted the power amp chassis from the console stereo in the bottom, and added the (Vox style) preamp circuitry. This circuit and this combo style cabinet are a match made in heaven, and it sounds so good as is. 2) As mentioned, one 10" in the cabinet limits to one channel, and with the size of the speaker, I don't think it could handle the increase in power by bridging. 3) This amp project distracted me from what I was working on before - modifying my Traynor YBA-1 head (pretty much done - just waiting for new tubes from LV), and mounting it in a combo cabinet with a Celestion 12". The YBA-1 find at a garage sale distracted me from finishing a 4 channel PA amp for my Son's band. The PA amp distracted me from finishing a rack mount stereo effects system with 3 distortions, 2 delays, 2 EQs, 2 different pitch shifters, 2 phase shifters, mixer etc. I've built all the boards up (including all the switching stuff) so it's at the point of starting the metal work for the enclosure. Oh yeah, there was also a pedal board with power supply and ABY box in there too - but they're done. So, as you can see, I really should get this finished so I can get back to the previous distractions 8^). It sounds great the way it is, and it accomplished the goal of getting be back to actually playing instead of building exclusively. If I do anything else to this little tone machine, it will only be the final tweaks of changing the cathode resistor or lower B+ to reduce static power disipation. BTW, I have successfully quite a few projects before (pedals, amps etc). I've learned my lesson that I shouldn't build something so massive as the rack mount FX system, because it takes so damned long it's all too easy to get distracted with other projects. My wife keeps saying that I'm building 'an electronic woman' in our basement - I keep saying I'm waiting for the nipple-chips to come in. ;^)

6/22/2000 8:12 PM

Dean Hazelwanter

Thanks for the input, Peter - much appreciated!

I'd rather not change it back to stereo, or bridge the channels for the following reasons:

1) I had the chassis sitting around for over a year, until I found an old CariVoice(sp?) solid state PA combo with 10" alnico speaker at a surplus sale (including a Shure UniDyneIII mike) for $10. I gutted the 'three-legged-fuse stuff', mounted the power amp chassis from the console stereo in the bottom, and added the (Vox style) preamp circuitry. This circuit and this combo style cabinet are a match made in heaven, and it sounds so good as is.

2) As mentioned, one 10" in the cabinet limits to one channel, and with the size of the speaker, I don't think it could handle the increase in power by bridging.

3) This amp project distracted me from what I was working on before - modifying my Traynor YBA-1 head (pretty much done - just waiting for new tubes from LV), and mounting it in a combo cabinet with a Celestion 12". The YBA-1 find at a garage sale distracted me from finishing a 4 channel PA amp for my Son's band. The PA amp distracted me from finishing a rack mount stereo effects system with 3 distortions, 2 delays, 2 EQs, 2 different pitch shifters, 2 phase shifters, mixer etc. I've built all the boards up (including all the switching stuff) so it's at the point of starting the metal work for the enclosure. Oh yeah, there was also a pedal board with power supply and ABY box in there too - but they're done.

So, as you can see, I really should get this finished so I can get back to the previous distractions 8^). It sounds great the way it is, and it accomplished the goal of getting be back to actually playing instead of building exclusively. If I do anything else to this little tone machine, it will only be the final tweaks of changing the cathode resistor or lower B+ to reduce static power disipation.

BTW, I *have* successfully quite a few projects before (pedals, amps etc). I've learned my lesson that I shouldn't build something so massive as the rack mount FX system, because it takes so damned long it's all too easy to get distracted with other projects.

My wife keeps saying that I'm building 'an electronic woman' in our basement - I keep saying I'm waiting for the nipple-chips to come in. ;^)

6/22/2000 11:00 PM
Peter S	Dean, I almost fell off my chair laughing at that last comment. Very funny! Well sounds like you have your hands full. Sounds kind of like me, only I have chassis materials,caps, trannies, resistors, etc. for three of my "Kimerik" amps ready to start the next batch of amps, but I keep getting interrupted by repairs,repairs,and more repairs. Guess I shouldnt complain too much since I couldnt afford to build the amps without the income from the repairs. The overhead on the amp building is too high to make any money on them, especially with 150+ man hours going into each unit and a lifetime warranty on them. Sure is fun building this stuff! Could you possibly e-mail me the schematic for your electronic woman? I may need one of those when my wife gets tired of my working 18 hours every day. Just kidding.....actually she really likes my amp line......and she knows if I'm building or repairing.....I'm not playing one of my 50 watters cranked on 12. Peter Schmitt Kimerik Amplifiers

6/22/2000 11:00 PM

Peter S

Dean,
I almost fell off my chair laughing at that last comment. Very funny! Well sounds like you have your hands full. Sounds kind of like me, only I have chassis materials,caps, trannies, resistors, etc. for three of my "Kimerik" amps ready to start the next batch of amps, but I keep getting interrupted by repairs,repairs,and more repairs. Guess I shouldnt complain too much since I couldnt afford to build the amps without the income from the repairs. The overhead on the amp building is too high to make any money on them, especially with 150+ man hours going into each unit and a lifetime warranty on them. Sure is fun building this stuff! Could you possibly e-mail me the schematic for your electronic woman? I may need one of those when my wife gets tired of my working 18 hours every day. Just kidding.....actually she really likes my amp line......and she knows if I'm building or repairing.....I'm not playing one of my 50 watters cranked on 12.

Peter Schmitt
Kimerik Amplifiers

6/22/2000 11:19 PM
Carl Z	150 hours/amp? Good Lord man, that's almost two weeks!!! Carl Z

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