To save you the trouble of working out the load lines, you can use this excellent Interactive Valve Data Sheet. But don't forget to subtract the plate and screen grid voltages from the cathode voltage when working out the plate dissipation and output power. Of course, I would go with fixed bias for push pull amps, since no power is wasted in the cathode resistor, but then again you may prefer to have a slightly more compreassive feel with the cathode bias.
Yes, that is closer than your previous post. It would do a huge amount of good to get familiar with those data sheets. A single-ended 6L6GC will only put out around 11 watts. With 350v plate, 250v screen and -18v bias. See this link. See the top row, Application Data A,S/E, class A single-ended: TDSL Tube data [6L6-GC] Note only 2.5 mA screen current. Generally, on a class A single ended amp, your input p-p is twice your bias voltage. If you bias at -18 a positive 18v swing will get you to 0v and a neg. 18v swing gets you to -38v, alternating between saturation and cutoff. So 36v p-p signal. Of course, you can drive it less if you want. On the data sheet, look at Peak AF grid #1 voltage. This is peak, not p-p, so double it for p-p.
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Jazbo, cool! Thanks. I thought I had seen all the online calculators, but that one is new to me.
boobtube, yes, I was only expecting about 11 or 12 watts out of this.
Thanks for the info. This clears up some of my understanding.
What does "Peak AF grid #1 voltage" mean? Specifically the "AF" part...
That chart (and you referenced the value) had 350V at the plate and 250V at the screen. I was planning on 320V at the plate and 315V at the screen. Is there any reason to choose a screen voltage that much lower than the plate? I have to admit some confusion in following valve wizard's explanation of choosing screen voltage in SE amps and specifically his discussion of the "sliding screen". I mean, I kind of get what he's talking about, but I don't really get his explanation of picking the screen voltage, particularly the paragraph starting with, "Firstly we must know what voltage we need the screen to sag down..."
boobtube, yes, I was only expecting about 11 or 12 watts out of this.
Thanks for the info. This clears up some of my understanding.
What does "Peak AF grid #1 voltage" mean? Specifically the "AF" part...
That chart (and you referenced the value) had 350V at the plate and 250V at the screen. I was planning on 320V at the plate and 315V at the screen. Is there any reason to choose a screen voltage that much lower than the plate? I have to admit some confusion in following valve wizard's explanation of choosing screen voltage in SE amps and specifically his discussion of the "sliding screen". I mean, I kind of get what he's talking about, but I don't really get his explanation of picking the screen voltage, particularly the paragraph starting with, "Firstly we must know what voltage we need the screen to sag down..."
The AF to the best of my knowledge stands for Audio Frequency. This is the AC signal that is impressed on to the control grid(grid #1). Grid #2 is the screen grid.. This is why we need to know the voltage gain of our preamp to know what voltage is needed to drive the power tubes to full power.
I used the 350v and 250v because this was the example given in the data. That is why we either need to take the easy way out and use a bias calculator or know how to interpolate the plate curves to fit what we have for our amp.
I haven"t read that whole page on Merlin's site but I believe he is talking about the voltage the screen drops to when screen current flows through the screen grid resistor. As more current flows, more voltage is dropped and that lowers the actual voltage on the screen itself. We need to find out what that voltage will be so we can use the correct voltage when doing wattage computations.
The family of curves will compress downward with a drop in screen voltage since the screen has more of an influence on current flow than the same change in plate voltage.
See this and notice amperage and screen voltage. This is what he is talking about, see pp.43-44. I know this is for small signal pentodes, but the effect is the same:
https://books.google.com/books?id=9... screen grid voltage family of curves&f=false Do you have his book? I strongly recommend that you buy it. It is superb! I emphasized the 11 watt output because in post #39 you spoke of 27 watts.
I used the 350v and 250v because this was the example given in the data. That is why we either need to take the easy way out and use a bias calculator or know how to interpolate the plate curves to fit what we have for our amp.
I haven"t read that whole page on Merlin's site but I believe he is talking about the voltage the screen drops to when screen current flows through the screen grid resistor. As more current flows, more voltage is dropped and that lowers the actual voltage on the screen itself. We need to find out what that voltage will be so we can use the correct voltage when doing wattage computations.
The family of curves will compress downward with a drop in screen voltage since the screen has more of an influence on current flow than the same change in plate voltage.
See this and notice amperage and screen voltage. This is what he is talking about, see pp.43-44. I know this is for small signal pentodes, but the effect is the same:
https://books.google.com/books?id=9... screen grid voltage family of curves&f=false Do you have his book? I strongly recommend that you buy it. It is superb! I emphasized the 11 watt output because in post #39 you spoke of 27 watts.
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Awesome. Thanks. That makes much more sense... Won't be able to work on things too much tonight and probably not tomorrow night. Hopefully I can make some progress on Wednesday. I'd like to get started on the component layout pretty soon as well.
My bad, all this time, I thought you were working on a PP amp. For SE, it makes more sense to just use cathode bias... If you play with the Interactive Valve Data Sheet a bit, I think you will soon learn what Merlin meant by setting the screen voltage, basically you want the load line to go through the knee of the Eg1=0V curve, depending on the OPT that you are using, the slope of the load line will intersect the plate curve at different points - thus the interactive part comes in handy.
Also an important point to remember is your amp is single ended, class A, so the max. power is at zero signal(quiescent). The power drawn from the power supply is virtually constant, dropping slightly when signal is applied. This is in contrast to class AB1 where more power is drawn when signal is applied.
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