I am reading Jones' book. It talked about triode has more 2H and pentodes have broader odd harmonics. I can understand people use triode for SE as it has more 2H or higher even harmonics that sounds good for ears. But I read people here that suggested connecting EL34 as triode for PP output. I don't see why that is better. From the waveform, the asymmetrical nature of the output waveform of each tube is cancelled when you sum the two tubes together in the OT. So the waveform is compressed on both ends and become odd harmonics.
So what other reason for using triodes for push pull amp?
So what other reason for using triodes for push pull amp?
Yes, low source impedance and low distortion (if run class A or nearly). The SE might have dominant 2nd harmonic but look at the numbers. PP wins hands down even if you compare PP to a parallel SE. Feedback is not necessary anymore.
In my opinion the only other configuration that can compete with EL34 PP triode is the EL34 PP Ultralinear + cathode feedback that will have all the advantages of the triode with 2x power. As always there is price to pay and that's the transformer with dedicated windings which is not precisely common and cheap...
In my opinion the only other configuration that can compete with EL34 PP triode is the EL34 PP Ultralinear + cathode feedback that will have all the advantages of the triode with 2x power. As always there is price to pay and that's the transformer with dedicated windings which is not precisely common and cheap...
Alan, this is a picture of a kit amp you can buy from B. Aloia in Italy (and possibly from everywhere if one asks...). It's called VTA The Last. It's mono-block. 8 x EL34 per channel in triode connection. You can buy it as a kit for less than 3000$ (only the chassis is excluded). The guaranteed performance is: 60W in class A at 1% THD and FR (-3dB) from 3 Hz to 60 KHz. Not enough? If you connect a 4R speaker to the 8R secondary (or just have an nominal 8R speaker with 4R minimum impedance) it will deliver up to 90W at 3% THD. Not enough? If the load drops to 2R, as above, it is still capable of 70W at 3% THD.
Of course there is the smaller version with 4x EL34 per channel (all the same except power is 1/2). The basic kit is a bit different but a lot cheaper and has the usual 15W. When I say class A it is pure class A, even for the basic amp in fact each tube runs at about 400V/50 mA and the primary load is 10K plate-to-plate.
Of course there is the smaller version with 4x EL34 per channel (all the same except power is 1/2). The basic kit is a bit different but a lot cheaper and has the usual 15W. When I say class A it is pure class A, even for the basic amp in fact each tube runs at about 400V/50 mA and the primary load is 10K plate-to-plate.
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Do you mean triode has mainly 2H and not much of the higher harmonics. So if the 2H cancels out, you have the lowest distortion?
Thanks
Ok, but the price you pay is for a guaranteed result which is not obvious for an amplifier that uses power 8 tubes per channel and zero feedback.
yes
In theory...In practice a triode amp with no feedback is not the same as a pentode amp with feedback. I would say that pentode gives more power being more efficient but the overall gain you need is more if need to apply feedback. You can almost make a true integrated amp with just 1xECC83 and 2xEL34 in triode, you can't do that in pentode + fb unless you give up quite some power.
What is source impedance? Do you mean plate resistance here?
Source impedance is the internal resistance of the amplifier. Triode connected KT88’s 2A3’s, 300B’s and more have low plate resistance; I think triode connected KT88’s are the lowest. Pentodes need lots of feedback to reduce the amplifiers internal resistance. Low internal resistance swamps the variation in Impedance of the loud speaker. If you use a high source impedance amp make sure you use an overdamped speaker like the Altec 515
With modern speakers an amp with high internal impedance will sound BOOMMYY
Phil
Source impedance is the internal resistance of the amplifier. Triode connected KT88’s 2A3’s, 300B’s and more have low plate resistance; I think triode connected KT88’s are the lowest. Pentodes need lots of feedback to reduce the amplifiers internal resistance. Low internal resistance swamps the variation in Impedance of the loud speaker. If you use a high source impedance amp make sure you use an overdamped speaker like the Altec 515
With modern speakers an amp with high internal impedance will sound BOOMMYY
Phil
In my experience,
Triode in PP often sounds better than Triode SE..
Triode SE sounds different to both pentode or true triode.
I prefer local feedback to global feedback.
Shade FB or cathode FB or no FB is topology dependant.
Each to their own I guess.
Its how to get the "Sound" you prefer without it sounding sterile or clinical.
Most of the equipment I have built in the past has been switchable for FB and triode pentode U/L
And it normally spends most of its time in Triode.
Of course you have to watch the bias settings..
I have run PPP ie 4 EL34 per channel in Triode.
I prefer U/L using the electron stream diode idea..YMMV
If you switch back to Triode there is a difference but its very close.
Regards
M. Gregg
Triode in PP often sounds better than Triode SE..
Triode SE sounds different to both pentode or true triode.
I prefer local feedback to global feedback.
Shade FB or cathode FB or no FB is topology dependant.
Each to their own I guess.
Its how to get the "Sound" you prefer without it sounding sterile or clinical.
Most of the equipment I have built in the past has been switchable for FB and triode pentode U/L
And it normally spends most of its time in Triode.
Of course you have to watch the bias settings..
I have run PPP ie 4 EL34 per channel in Triode.
I prefer U/L using the electron stream diode idea..YMMV
If you switch back to Triode there is a difference but its very close.
Regards
M. Gregg
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Triodes have low distortion when used with a high impedance load. An output triode does not have a high impedance load, so it does not have low distortion. It does have low output impedance (due to its built-in local feedback), so further feedback can be avoided by those who wish to avoid further feedback and are willing to cope with a somewhat poor damping factor.
Most output valves are pentodes or tetrodes, so those who want a 'triode' output have the option of using them in triode connection. Output triodes tend to be rarer and more expensive, although some prefer them.
Most output valves are pentodes or tetrodes, so those who want a 'triode' output have the option of using them in triode connection. Output triodes tend to be rarer and more expensive, although some prefer them.
This is a broad generalization, and doesn't always apply. (Attached: 6CB6 loadline) Pents can make more of the nastier higher order harmonics, but it's not inevitable. It depends on the internal construction and loadlines. Some types like the 6L6-oids tend to make a lot of that h5 and higher. You can see this with the Twin-T test: after nulling a sinusoidal fundamental, the residual looks like a pretty good sawtooth at three times the frequency.
The 6BQ6GA/GTB, a TV HD final, has a residual that's almost pure sine wave at 3X the fundamental. Even though the magnitude is higher, the 6BQ6 sounds significantly better open loop.
The "other reasons" are that there aren't very many power triodes made for audio. The power pentode/beam former revolution happened pretty early, and obsoleted the audio power triodes like the 45, 50, 2A3, 300B. Most power triodes are RF types with high u-factors, high rp and require Class AB2. Other triodes were developed for series pass service or TV vertical deflection amps, but these are in the same power class as the 2A3 & Co. You want something falling between the extremes of the 2A3 and 845, you'll have to go with a pseudotriode connection with a pentode.
There is also the questions of linearity and rp. Triodes are more linear, and that helps, not just with harmonic distortion but intermod distortion as well. Reducing both distortion mechanisms could mean the need for less gNFB or none at all.
The lower rp of triodes helps with speaker damping in the same way that vertical deflection types like the 6CK4 with its low u-factor and rp damps the vertical deflection coils. That's the other thing gNFB buys you: a decreased Zo when using pents with their inherently higher rp's.
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Another way of looking at the beam pentode/tetrode is as a triode with separate output and feedback terminals. That's much more flexible if you want to do interesting things, like take the feedback from somewhere else. UL mode is one example. Driver stages can be configured with screen feedbacks as another example. One can even make an entire P-P amplifier mimic a SE amp by returning the feedback to the screen grid of the input tube.
The early output triodes are scarce, but the big TV sweep tubes can be configured for a Mu of 3 to 4 like the early triode outputs. Why pay $100s for a 300B when $12 can get you the same.
Then there is screen grid drive mode, which is very linear.
The early output triodes are scarce, but the big TV sweep tubes can be configured for a Mu of 3 to 4 like the early triode outputs. Why pay $100s for a 300B when $12 can get you the same.
Then there is screen grid drive mode, which is very linear.
There is another way to get around the boomy sound with highish impedance power amps. Amps are made to give flat response into a flat resistive load. Speakers are made to give a desired (usually flat) response when driven by a voltage (low impedance signal). But if you add a parallel RLC network, to an overall speaker's terminals, designed to make it present a flat resistive overall impedance to the amp, you get the same benefit as if the amp had a low output impedance -- the amp plays a flat voltage response and boominess and midrange/tweeter weirdnesses go away.
It's not as hard to do as it might seem, and certainly easier to do than making amp mods to lower output impedance. You just need a response curve of the speaker's impedance (using something like WT3 is easiest), and load the data into a speaker circuit simulator program (such my Xsim app). Then add components simulated across the given impedance to pull the higher impedance parts of the curve down to match the lowest parts... basically make lossy series-LC resonators to pull on the speaker's parallel resonators. Even just playing around without much circuit intuition, it's not very hard to see how it works and to get there. When you have a network that does what you want, get the parts and strap them at the amp's outputs or at the speakers' inputs. Then it doesn't matter what the amplifier's output impedance is, it just plays into its designed resistive load and is happy.
I did that with my horn speakers when used with a NFB SET amp that had things sounding very boomy and kind of strange through the midrange. A handful of components on each speaker system brought the impedance that originally varied between 4.5 ohms and as high as 30 ohms down to a 4 ohm curve with only about an ohm of variation (you can only bring impedances in places down, not up, though). Sound tightened right up with bass as solid as it was with a solid state amp, but with the SET HD2 detail enhancement.
If anyone out there wants to try this, get or borrow a woofer tester (WT2, WT3, DATS, or other) and measure your speaker's impedance into a "ZMA" file and email it to me. I'll be happy to toss together a network design for you to build up (not much work, it would just be a few Rs, Ls and Cs most likely). I'm getting convinced that it makes more sense to get speakers to have a flat impedance than to demand amplifiers be impedance immune. Why let speaker designers be sloppy and toss the grief onto amplifier designers? Though both being well behaved might be a good way to go, too!
I hope some of this made sense. I'm a bit sleep deprived tonight.
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There is some confusion here. Most speakers are designed to be fed from a very low impedance, well below an ohm. Your parallel RLC network would need to present this sort of impedance to properly compensate for a high amp output impedance. This would flatten the speaker frequency response, as you say, but also severely load the amplifier. Power goes down, distortion goes up, and LF extension takes a battering too. Not the way to correct a problem.bwaslo said:
It might work as a partial correction, provided you can put up with the downside. Much better to fix the amp, or buy more suitable speakers.
There are several commecial speakers that will work just fine with valve amps. The low frequency behaviour can be adjusted rather "easily" with ported speakers.
Low frequency tuning needs to be done anyway when placing speakers in a room regardless of the amplifier.....the (static) damping factor is really an overrated parameter.
Low frequency tuning needs to be done anyway when placing speakers in a room regardless of the amplifier.....the (static) damping factor is really an overrated parameter.
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