36 Watt EL34 Superlite TMB

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crgfrench
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Re: 36 Watt EL34 Superlite TMB

Post by crgfrench »

Josh how did you decide on the 1kuF value of the output cathode bypass cap?
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Re: 36 Watt EL34 Superlite TMB

Post by crgfrench »

Josh you might want to consider changing your plate load resistor to pin 1 of the PI to 82k.
I believe the one at pin 6 should stay at 100k but if you use 100k at pin 1 the PI will have unbalanced outputs.

I'm no expert on LTPs, I only build cathodyne, but I am pretty sure you want lower voltage at pin 6 than on pin 1. Because at pin 6 the signal has already been amplified by the first triode. Using 82k will raise the voltage at pin 1 (I believe it should be 5V to 15V higher) which will result in balanced signals from the PI.

I'm not sure why Trinity uses equal 100k load resistors there (Richie Hall's 18W designs also suffer from this error but his 36WTMB is correct. Ian's reference schematic has it wrong too. All of the Wattkins LTPs have this problem too), but Marshall, Soldano, Egnater, Aiken, Huss, Hoffman and Robinette all use 82k & 100k at the PI.

You could also use 68k at pin 1 and 82k at pin 6 to run it hotter but still balanced.

This might all stem from the original schematic derived from Colin Macfarlane's amp, which shows twin 100k resistors here. That could be a mistake.
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Re: 36 Watt EL34 Superlite TMB

Post by Daviedawg »

Twin 100ks are sometimes deliberately specified to get more harmonic response from the power stage by creating a slightly out of phase signal. There have been discussions in the past about what if any benefits accrue from this. I don't recall ever seeing a side by side audio comparison.

It is another of those discussions which are never resolved. Like the benefits of having a standby switch.

Dd
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Re: 36 Watt EL34 Superlite TMB

Post by JMPGuitars »

What Dd said. You can look at theory all day long based on other amps, but that doesn't tell you anything. Every 18 watter I've built has had 100K PI plates. 99% of the variants do too. If it wasn't a great tone, somebody would have changed that by now.

Here's my actual PI voltages:
1: 177
2: 25.5
3: 34.3
6: 175
7: 25.5
8: 34.3

You can unbalance it or balance it based on the tube itself. An unbalanced tube can give a significant voltage difference. If that's what you want.

If you think that idea is a good one, then build an amp with an LTP pair. Record a demo with both resistor values in the same amp, and see what people think.

Thanks,
Josh
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Re: 36 Watt EL34 Superlite TMB

Post by crgfrench »

Yep it seems like both ways are common -- I noticed the WEM dominator and the Matchless amps use unbalanced PIs as well. This would make for a cool A/B experiment...

Actually an extra resistor and a switch is all one would need to A/B it in the same exact amp.
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Re: 36 Watt EL34 Superlite TMB

Post by crgfrench »

Daviedawg wrote:
Tue 09/03/19 2:09 am
Twin 100ks are sometimes deliberately specified to get more harmonic response from the power stage by creating a slightly out of phase signal.
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?
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Re: 36 Watt EL34 Superlite TMB

Post by colossal »

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.

18W LTPI Balance.png

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

Post by crgfrench »

colossal wrote:
Tue 09/03/19 7:49 am
Here is a chart showing voltage gain for the 18W LTPI.
Thank you -- that was a phenomenal explanation. I learned something today thanks to you!
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Re: 36 Watt EL34 Superlite TMB

Post by Daviedawg »

crgfrench wrote:
Tue 09/03/19 6:59 am
Daviedawg wrote:
Tue 09/03/19 2:09 am
Twin 100ks are sometimes deliberately specified to get more harmonic response from the power stage by creating a slightly out of phase signal.
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?
Careless terminology. Sorry. Substitute "slightly out of balance".

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Re: 36 Watt EL34 Superlite TMB

Post by crgfrench »

...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

Post by geoff 1965 »

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

Post by crgfrench »

geoff 1965 wrote:
Tue 09/03/19 8:32 am
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.
It was a "colossal" explanation indeed!
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Re: 36 Watt EL34 Superlite TMB

Post by JMPGuitars »

crgfrench wrote:
Tue 09/03/19 8:25 am
...so, since Josh's tail resistor is 15k, this amp might benefit from the 82k load resistor. Perhaps?
"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

Post by crgfrench »

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".
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Re: 36 Watt EL34 Superlite TMB

Post by crgfrench »

Here I drew it out by hand
20190903_111039.jpg
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Re: 36 Watt EL34 Superlite TMB

Post by colossal »

crgfrench wrote:
Tue 09/03/19 8:25 am
...so, since Josh's tail resistor is 15k, this amp might benefit from the 82k load resistor. Perhaps?
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

Post by JMPGuitars »

crgfrench wrote:
Tue 09/03/19 9:16 am
Sonically, probably would sound about the same either way.

To make a long story short, "nevermind".
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

Post by crgfrench »

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

Post by colossal »

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

Post by JMPGuitars »

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. ;)
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