Sorry to bring this one back, but i have a question and didn't want to start yet another thread. Someone stated the books say 6L6 require a impedance between 3.8k and 8.5k. So my OT is past the lowest recommendation at 3.6k. Would setting the OT to 4 ohm which would be 7.2k truly replicate the same tone as a OT with a 7.2k primary? I ask because if i set it to 4 the tone doesn't change much, but i would think such a OT would sound very different. Or does it somehow just change the safety factor but not the tone. Somehow i just cannot wrap my head around this stuff so excuse any downright stupidity.
Also, I have a 4k heyboer and might consider trying that even if the tone doesn't change if running such a low impedance tranny could cause reliability issues, which is another thing i'm wondering about.
sub 6L6 for EL34?
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iknowjohnny
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Re: sub 6L6 for EL34?
The design of OPT's and their characteristics is something that might be learned from many books and much research. Changing the output load will stuff with the designer's intent. From what I read on the net, for example, putting 4 ohm speaker on 8 ohm output to halve the primary impedance is generally safe, but going the other way, 8 ohm load on 4 ohm tap to double the primary impedance may or may not work as desired due to the inductance of the primary winding. Something happens to the primary inductance (halved?) which will affect frequency response I guess. This is simply what I have read from a respected and highly experienced guitar amp and hifi amp dude.
I accidentally tried 2 x EL34 PP on 10K (8 ohm speaker on 4 ohm tap) and it sounded very middy, dull, no balls, no top, no bottom. I was about to crank it up hard when I realized my mistake.
I ordered 2 OPT's recently, one 4K2 and one 3K6, both Magnetic Components, will try both with EL34 and 6L6.
A lower primary impedance is going to encourage the tubes to pull more current for the same B+ and screen grid voltage, i.e. work harder. To get an idea of what sort of working conditions you are going to force upon your tubes, you need to draw a load line, to draw a load line you need the values for B+, screen grid voltage and OPT primary impedance. The best place to learn this that I know of is the valvewizard website. When you do a load line, you can see how much it crosses the allowable dissipation limit of the tube, that is a measure of how close you will get to red-plating. In PP amps it is common to cross the dissipation curve, since the tube only crosses it on positive cycles and gets a rest on the negative cycle (i.e. 50% duty cycle)
You may or may not find that any one primary impedance is the magic bullet, because there are many other factors - different types/brands of tubes, different speaker loads, different screen resistors, different screen voltages, different OPT manufacturers.
Attached a loadline. It looks right on the edge for a screen voltage of 400V, but thats the curves for fixed 400V screen voltage, and maybe not what the voltage is *at the screens* after the 1K resistor. I have seen an 80V peak voltage drop across 1K screen grid resistors on an amp I built, the first guitar amp I ever built which was recently, so a newbie here.
I accidentally tried 2 x EL34 PP on 10K (8 ohm speaker on 4 ohm tap) and it sounded very middy, dull, no balls, no top, no bottom. I was about to crank it up hard when I realized my mistake.
I ordered 2 OPT's recently, one 4K2 and one 3K6, both Magnetic Components, will try both with EL34 and 6L6.
A lower primary impedance is going to encourage the tubes to pull more current for the same B+ and screen grid voltage, i.e. work harder. To get an idea of what sort of working conditions you are going to force upon your tubes, you need to draw a load line, to draw a load line you need the values for B+, screen grid voltage and OPT primary impedance. The best place to learn this that I know of is the valvewizard website. When you do a load line, you can see how much it crosses the allowable dissipation limit of the tube, that is a measure of how close you will get to red-plating. In PP amps it is common to cross the dissipation curve, since the tube only crosses it on positive cycles and gets a rest on the negative cycle (i.e. 50% duty cycle)
You may or may not find that any one primary impedance is the magic bullet, because there are many other factors - different types/brands of tubes, different speaker loads, different screen resistors, different screen voltages, different OPT manufacturers.
Attached a loadline. It looks right on the edge for a screen voltage of 400V, but thats the curves for fixed 400V screen voltage, and maybe not what the voltage is *at the screens* after the 1K resistor. I have seen an 80V peak voltage drop across 1K screen grid resistors on an amp I built, the first guitar amp I ever built which was recently, so a newbie here.
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iknowjohnny
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Re: sub 6L6 for EL34?
Thanks. Thats what i figured, so i'm going to swap in the 4k heyboer today and see how i like it. if for no other reason than to play it safe.
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iknowjohnny
- Posts: 1070
- Joined: Thu Apr 24, 2008 2:10 am
- Location: los angeles
Re: sub 6L6 for EL34?
Well, i did this and i heard the heyboer's tone that it's always had. I like the mag comp OT better. And i'm not sure it's a matter of 4k vs the mag comp's 3.6k primary. So the question is, should i buy a mag comp fender style OT or is there a way to make the 3.6k OT safer? I don't play out anymore with rare exception and even then only small bars where i never get over 4 on the master, usually less. Is there anything i can do or is a 3.6k OT fine with 6L6's?