OTL Reference Amp
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I don't really understand the 2u2 feedback capacitor from V1+ to 12K/4K7 at the driver-stage?
> I don't really understand the 2u2 feedback capacitor from V1+ to 12K/4K7 at the driver-stage?
Bootstrapping the plates of U1 U2, for better swing and gain.
With hi-Mu tubes you need very different drive to top and bottom of the output stage. Bootstrapping tends to fix that. With lo-Mu 6080, the drives are not so different, but they are large; bootstrapping is one way to improve drive voltage.
Bootstrapping the plates of U1 U2, for better swing and gain.
With hi-Mu tubes you need very different drive to top and bottom of the output stage. Bootstrapping tends to fix that. With lo-Mu 6080, the drives are not so different, but they are large; bootstrapping is one way to improve drive voltage.
Thanks PRR and Analag..!
I set the tubes in the OPT to conduct 45ma each. Place an ammeter in series with the 1 ohm resistor and the cathode of each tube to verify. Try to keep an average current draw of 45ma. You could get ingenious and mix an match to try and get an even current draw per pushpull side.
Analag
Analag
chrissugar
Well-known member
Analag,
Did you try any OTL with the 6C33 tube. It is very popular in this application. Any experience with it?
chrissugar
Did you try any OTL with the 6C33 tube. It is very popular in this application. Any experience with it?
chrissugar
chrissugar
Well-known member
[quote author="analag"]No, the 6AS7 is more readily available and cheaper I think.
[/quote]
Not for me.
I live close to Russia so 6C33 is the obvious choice for me. :grin:
chrissugar
[/quote]
Not for me.
I live close to Russia so 6C33 is the obvious choice for me. :grin:
chrissugar
Learner
Well-known member
[quote author="chrissugar"][quote author="analag"]No, the 6AS7 is more readily available and cheaper I think.
[/quote]
Not for me.
I live close to Russia so 6C33 is the obvious choice for me. :grin:
chrissugar[/quote]
Hey Chris,
Know any cheap source for 6C33 tubes? :green:
[/quote]
Not for me.
I live close to Russia so 6C33 is the obvious choice for me. :grin:
chrissugar[/quote]
Hey Chris,
Know any cheap source for 6C33 tubes? :green:
chrissugar
Well-known member
Hey Learner,
I have a source for the 6C33 russian tubes at the price of aprox 15-18 euros/piece depending on the quantity I buy.
chrissugar
I have a source for the 6C33 russian tubes at the price of aprox 15-18 euros/piece depending on the quantity I buy.
chrissugar
> these tubes are huge and suck up to 10A heater current when warming up!!!
6c33 works about like four 6080. About 4X heater power and about 4X the current and heat handling ability. (I don't have 6c33 data handy so it may not be quite "4", but I know it was nearly in proportion.)
So if you need two 6080 to do your job, try to find cheap 6336 (which is a double-size 6080). If you need four, you may want to consider the Russian Monster. Actually, 6c33 is selling cheaper than 6336. They all obey the same laws of physics, just different sizes.
6c33 works about like four 6080. About 4X heater power and about 4X the current and heat handling ability. (I don't have 6c33 data handy so it may not be quite "4", but I know it was nearly in proportion.)
So if you need two 6080 to do your job, try to find cheap 6336 (which is a double-size 6080). If you need four, you may want to consider the Russian Monster. Actually, 6c33 is selling cheaper than 6336. They all obey the same laws of physics, just different sizes.
> what loads can you amp drive?
It shows 5 sections of 6080 in series with 180V. Each section saturates like a 140Ω resistor; 5 sections like a 28Ω resistor. Maximum power will be into about 56Ω and possibly 120V peak (usually limited by drive voltage). Except that is WAY outside a 6080's rated peak current or plate dissipation. It may do this very happily with speech and music. Sine testing could fry the bottles.
Conservative design suggests a lower supply voltage on the output stage. But conservative design leads to small results.
Anyway: who cares how many watts? It's the sound that counts. Naked transformerless bottles have a different sound than either transformer-tube or sand-state amps.
> 8 and 16 ohms is as low as I go.
If you go less than 50 or 30 ohms, you might as well go as low as you want. The only real difference is lower power. Distortion and tube-abuse are nearly unchanged into a short circuit.
It shows 5 sections of 6080 in series with 180V. Each section saturates like a 140Ω resistor; 5 sections like a 28Ω resistor. Maximum power will be into about 56Ω and possibly 120V peak (usually limited by drive voltage). Except that is WAY outside a 6080's rated peak current or plate dissipation. It may do this very happily with speech and music. Sine testing could fry the bottles.
Conservative design suggests a lower supply voltage on the output stage. But conservative design leads to small results.
Anyway: who cares how many watts? It's the sound that counts. Naked transformerless bottles have a different sound than either transformer-tube or sand-state amps.
> 8 and 16 ohms is as low as I go.
If you go less than 50 or 30 ohms, you might as well go as low as you want. The only real difference is lower power. Distortion and tube-abuse are nearly unchanged into a short circuit.
chrissugar
Well-known member
PRR,
how many sections of 6C33 would produce optimal results (output impedance, damping, etc.) ?
chrissugar
how many sections of 6C33 would produce optimal results (output impedance, damping, etc.) ?
chrissugar
OK, turned out I had a similar design laying around in TubeCAD's Push-Pull Calculator.
I could not get a reasonable plate dissipation with the +/-180V supplies until I turned the grid bias WAY down. Assuming the tube will live 20% over RCA's plate dissipation ratings, I got -76V grid bias. P-P Calc gives these "answers":
The 100 Watts in 16 ohms is great. The 33 watts of heat at full power in a 13W-14W plate is not great, but may be allowable in speech/music use. The 4% THD is fine, can be much lower under feedback.
What really bothers me is the 744mA peak current in a tube rated for 125mA DC. I could accept 2X or 250mA peak in audio use, but 6X really seems unlikely to be good for the tube.
Linearity shows the reason for even 4% THD: the tubes are under-biased. Gain (red) is low at idle and increases on peaks. This is confirmed by the waveforms which shows huge peak/idle ratio. The idle current can't be increased because we are allready at/past the plate rating.
A supply lower than +/-180V would reduce the too-high peak current and also the obscene drive voltage (135V peak!), and allow higher idle current for better peak/idle current ratio. This would of course reduce power, except that at 180V and 744mA peak, the tube life may be too to enjoy the power.
Changing to +/-100V supplies gives only 20W output, but a less unhealthy 362mA peak per tube and an easier 54V peak grid drive. THD is 1.4% 3rd, 0.06% 5th. Dissipation at full power sine-test is only 14.6 watts, utterly safe for speech/music work. +/-120V gives 450mA peak, 35 watts at 16 ohms at 2.3%THD, 70V drive, and only 18W dissipation in full-power testing.
I could not get a reasonable plate dissipation with the +/-180V supplies until I turned the grid bias WAY down. Assuming the tube will live 20% over RCA's plate dissipation ratings, I got -76V grid bias. P-P Calc gives these "answers":
The 100 Watts in 16 ohms is great. The 33 watts of heat at full power in a 13W-14W plate is not great, but may be allowable in speech/music use. The 4% THD is fine, can be much lower under feedback.
What really bothers me is the 744mA peak current in a tube rated for 125mA DC. I could accept 2X or 250mA peak in audio use, but 6X really seems unlikely to be good for the tube.
Linearity shows the reason for even 4% THD: the tubes are under-biased. Gain (red) is low at idle and increases on peaks. This is confirmed by the waveforms which shows huge peak/idle ratio. The idle current can't be increased because we are allready at/past the plate rating.
A supply lower than +/-180V would reduce the too-high peak current and also the obscene drive voltage (135V peak!), and allow higher idle current for better peak/idle current ratio. This would of course reduce power, except that at 180V and 744mA peak, the tube life may be too to enjoy the power.
Changing to +/-100V supplies gives only 20W output, but a less unhealthy 362mA peak per tube and an easier 54V peak grid drive. THD is 1.4% 3rd, 0.06% 5th. Dissipation at full power sine-test is only 14.6 watts, utterly safe for speech/music work. +/-120V gives 450mA peak, 35 watts at 16 ohms at 2.3%THD, 70V drive, and only 18W dissipation in full-power testing.
bcarso
Well-known member
Hehe. Though you didn't ask me how many tubes would be optimal Chris, I'd be inclined to joke that if you have to ask, you can't afford it.
> how many sections of 6C33 would produce optimal results (output impedance, damping, etc.)?
As Many As You Can Afford. Clearly output impedance and damping increase with number of parallel sections.
Who needs damping? Get DF anywhere near 1, and then EQ-out the slight error. Incredible obsessive damping factors are for sand-state guys with WAY too much gain on their hands.
Using old/reliable triode loading rules: tube output impedance should be 1/2 to 1/5th of load impedance. 1/2 maximizes power per tube. 1/5th is a little less power per tube but significantly less distortion.
Rp for 6c33 under full heat is about 40 ohms. To meet the "1/2" rule for an 8 ohm load, we need 10 tubes per side, 20 tubes, 800 Watts just for heaters, 1,600 watts of heater for stereo, surely over 2,500 Watts (or nearly 4 HorsePower) of total dissipation and electric-bill. (Not counting any air-conditioning you may need to cancel what amounts to two good electric fires.) Damping Factor is "only" 2 to 4; if you want to explore the "1/5Rp" rule for better damping, multiply your electric bill by 2 or 3.
As Many As You Can Afford. Clearly output impedance and damping increase with number of parallel sections.
Who needs damping? Get DF anywhere near 1, and then EQ-out the slight error. Incredible obsessive damping factors are for sand-state guys with WAY too much gain on their hands.
Using old/reliable triode loading rules: tube output impedance should be 1/2 to 1/5th of load impedance. 1/2 maximizes power per tube. 1/5th is a little less power per tube but significantly less distortion.
Rp for 6c33 under full heat is about 40 ohms. To meet the "1/2" rule for an 8 ohm load, we need 10 tubes per side, 20 tubes, 800 Watts just for heaters, 1,600 watts of heater for stereo, surely over 2,500 Watts (or nearly 4 HorsePower) of total dissipation and electric-bill. (Not counting any air-conditioning you may need to cancel what amounts to two good electric fires.) Damping Factor is "only" 2 to 4; if you want to explore the "1/5Rp" rule for better damping, multiply your electric bill by 2 or 3.
