It's unclear to me how the signal would become out of phase. Wouldn't the imbalance result in asymmetric amplitudes of the top and bottom peaks? How would it shift the phase?
36 Watt EL34 Superlite TMB
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Re: 36 Watt EL34 Superlite TMB
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Re: 36 Watt EL34 Superlite TMB
Here is a chart showing voltage gain for the 18W LTPI. The inverting side resistor was varied from 80 - 100k while the non-inverting side was held constant at its usual 100k value. The bias resistor is 820Ω and the tail is 47k, per the standard 18W values.
The 82k/100k pair, as seen in a Plexi, is typical used where the tail resistor value is <15k. When the tail resistance is low, then the imbalance between the plate voltages is increased and so the inverting side plate resistor value is lowered to compensate. Bassmans had 6k8 to 10k tail resistors with 82k/100k plate loads. Plexis used 10k. The tail resistance in the 18W is high and so the best balance is achieved when the plate resistors are matched. The best match with the 47k tail is around 96.5k/100k but is of course rounded up to the nearest available value, 100k. Some Plexis running high plate voltages on the output tubes have problems with red-plating the inverting side.
The 82k/100k pair, as seen in a Plexi, is typical used where the tail resistor value is <15k. When the tail resistance is low, then the imbalance between the plate voltages is increased and so the inverting side plate resistor value is lowered to compensate. Bassmans had 6k8 to 10k tail resistors with 82k/100k plate loads. Plexis used 10k. The tail resistance in the 18W is high and so the best balance is achieved when the plate resistors are matched. The best match with the 47k tail is around 96.5k/100k but is of course rounded up to the nearest available value, 100k. Some Plexis running high plate voltages on the output tubes have problems with red-plating the inverting side.
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Re: 36 Watt EL34 Superlite TMB
Thank you -- that was a phenomenal explanation. I learned something today thanks to you!
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Re: 36 Watt EL34 Superlite TMB
Careless terminology. Sorry. Substitute "slightly out of balance".
Dd
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Re: 36 Watt EL34 Superlite TMB
...so, since Josh's tail resistor is 15k, this amp might benefit from the 82k load resistor. Perhaps?
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Re: 36 Watt EL34 Superlite TMB
you learn something every day on this forum! i love it! there's a lot of theory's on the 82/100K plates but colossal's is the best explanation i've seen and makes absolute sense.
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Re: 36 Watt EL34 Superlite TMB
It was a "colossal" explanation indeed!geoff 1965 wrote: ↑Tue 09/03/19 8:32 amyou learn something every day on this forum! i love it! there's a lot of theory's on the 82/100K plates but colossal's is the best explanation i've seen and makes absolute sense.
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Re: 36 Watt EL34 Superlite TMB
"Some Plexis running high plate voltages on the output tubes have problems with red-plating the inverting side." My schematic, as it is, produces healthy voltages.
Now, if you reeeeally want to change the plate resistance, it isn't an arbitrary number. You would actually need to calculate it. The PI is driving EL34s, not EL84s.
Then you would still need to do the experiment yourself, A/B demo the different values, and compare the voltages.
...and again, a small difference in resistor value could be overcome by a difference in the two halves of the PI.
Thanks,
Josh
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Re: 36 Watt EL34 Superlite TMB
Ahhhhhh...
I just used Kuehnel's LTP balance calculator, and with your present setup (100k&100k), your inverted side puts out 28.71dB while your noninverted side puts out 28.01dB for an imbalance of only 0.70dB.
If you swapped in an 82k as I suggested (82k&100k), your inverted side would be 27.80dB while your noninverted side would be 28.32dB for an imbalance of 0.52dB. That would be very slightly better.
Sonically, probably would sound about the same either way.
To make a long story short, "nevermind".
I just used Kuehnel's LTP balance calculator, and with your present setup (100k&100k), your inverted side puts out 28.71dB while your noninverted side puts out 28.01dB for an imbalance of only 0.70dB.
If you swapped in an 82k as I suggested (82k&100k), your inverted side would be 27.80dB while your noninverted side would be 28.32dB for an imbalance of 0.52dB. That would be very slightly better.
Sonically, probably would sound about the same either way.
To make a long story short, "nevermind".
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Re: 36 Watt EL34 Superlite TMB
Here I drew it out by hand
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Re: 36 Watt EL34 Superlite TMB
Possibly. Josh's posted voltages indicate a good DC balance, but the AC balance is another story. EL34s require a little more of a push than EL84s too.
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Re: 36 Watt EL34 Superlite TMB
Exactly! The intrigue of any theoretical discussion has to outweigh my laziness. Since any potential reward is likely unnoticeable, I'll let somebody else experiment if they want to.
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Re: 36 Watt EL34 Superlite TMB
I realize this may seem OCD but you could optimize it with a Dale 68.1k and a Dale 22.1k in series, for a nearly perfect balance, just sayin'...
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Re: 36 Watt EL34 Superlite TMB
Dumble used a trimmer pot in the middle of the pair of PI plate loads and attached B+ to the wiper. The Inverting side might get 91k and balance trimmer at 10k. The AC balance is set by dialing the trimmer and using a scope with a target of about 6V difference being desirable. Some imbalance is desirable for tonal consideration. The 18W PI is more center biased than a Plexi PI too and that will change the sound as well.
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Re: 36 Watt EL34 Superlite TMB
I have a difference of 2 VDC at the plates, all else matching. I could swap the tube and probably get 10 or 15V difference at the plates, or an exact VDC match.
Theory is great, but tube variance is reality. If you want to tweak that to make your OCD happy, you'll want to do it like colossal said re: dumble, and rebias every time you change the tube. I could see some people being entertained by that...but I'd rather play if I ever get any free time.
Theory is great, but tube variance is reality. If you want to tweak that to make your OCD happy, you'll want to do it like colossal said re: dumble, and rebias every time you change the tube. I could see some people being entertained by that...but I'd rather play if I ever get any free time.
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Re: 36 Watt EL34 Superlite TMB
I would take a very similar approach:
The AC balance is set by dialing the trimmer and using a stratocaster with a target of the juiciest tone and sweetest sustain being desirable...
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Re: 36 Watt EL34 Superlite TMB
I am bumping this as I'm interested as well. Josh, did you try A/Bing the stock cap and a 1kuF or just go with the big one to get more of the fixed bias stiffness right from the start?
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Re: 36 Watt EL34 Superlite TMB
I've built many superlites. I've built them with 100, 250, 1000, and 2200uF caps. They all sound good, but I think I remember liking the 1000 more... Who knows, it's been a long time since I thought much about that cap. I actually drew that circuit a couple years ago, and I'm not getting any younger.
But also keep in mind that my build has a 5AR4 tube rectifier, and I have a dropping resistor on the B+, so it's not going to sound too stiff anyway. If anything I miiiiight have been thinking about how it affects the bottom end.
Thanks,
Josh
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Re: 36 Watt EL34 Superlite TMB
I know I tried a 1000uF cap once, but it was so long ago I can't quite remember its effects. I want to say I remember it affecting the feel of the amp and I preferred the stock 47uF cap better. Oh, and I meant stock 18W, not a Superlite...JMPGuitars wrote: ↑Tue 09/03/19 1:51 pmI've built many superlites. I've built them with 100, 250, 1000, and 2200uF caps. They all sound good, but I think I remember liking the 1000 more... Who knows, it's been a long time since I thought much about that cap.
Man, none of us are {=[JMPGuitars wrote: ↑Tue 09/03/19 1:51 pmI actually drew that circuit a couple years ago, and I'm not getting any younger.
For sure. The screen supply dropping resistor will dump some voltage on big transients, giving some great sag and compression. That's why we love these amps! Very easy under the fingers and they sustain forever.JMPGuitars wrote: ↑Tue 09/03/19 1:51 pmBut also keep in mind that my build has a 5AR4 tube rectifier, and I have a dropping resistor on the B+, so it's not going to sound too stiff anyway. If anything I miiiiight have been thinking about how it affects the bottom end.
Thanks,
Josh
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Re: 36 Watt EL34 Superlite TMB
I was playing earlier on a Lite 2b(ish) variant today, with a 250uF cap on the cathode. The amp sounded a little farty playing in the lower register, and especially with my looper pedal playing backing tracks. I rebiased the amp, swapped the 250uF for a 1000uF cathode cap, and it sings now.
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