I'm captivated by the Octode design and van Dorn's tube choices for the standard version are some of my favorites as well. I happen to have E-P 10W OPTs that are 8200R:8R so they are perfect for an Octode.
I wish to keep the 6922 as the input tube since I have some nice 6N23Pi and I love the sound of the 6V6 family of tubes so I'm considering the 6V6 in triode mode for that portion of the amp, but I'm not sure what to use for the pentode tube to keep the power down around 7W-9W for the pentode since the 6V6 will deliver about 2W in triode. Would the 6W6 be a good choice or would the EL84/6BQ5 be a better choice?
Thanks!
I wish to keep the 6922 as the input tube since I have some nice 6N23Pi and I love the sound of the 6V6 family of tubes so I'm considering the 6V6 in triode mode for that portion of the amp, but I'm not sure what to use for the pentode tube to keep the power down around 7W-9W for the pentode since the 6V6 will deliver about 2W in triode. Would the 6W6 be a good choice or would the EL84/6BQ5 be a better choice?
Thanks!
How about a schematic and/or a link to more information on the Octode. This is the first time I have even heard of it, and have little time at the moment for searching.. 😀
Octode Topology
Seems to be a standard buffer circuit, but using the LTP triode's plate for the feedback instead of it's grid, so the reverse Mu acts as the feedback attenuator. Net effect, a buffer with gain of Mu.
Seems to be a standard buffer circuit, but using the LTP triode's plate for the feedback instead of it's grid, so the reverse Mu acts as the feedback attenuator. Net effect, a buffer with gain of Mu.
The article is in the Nov. 2009 audioXpress. It ties a triode to a pentode for the output tubes. So the power of the two is added together. I believe the triode/pentode portion was the new portion of the circuitry. John's article in Tubecad isn't quite the same as the amp, as he was modifying the circuit a bit and wouldn't fully disclose van Dorn's design.
The article isn't on the aX site, so if you really want to read it Kevin to help me out, I can scan it in and email it to you.
I tried to find contact info for A.J. van Doorn, but only came up with a lady that appears to be in the medical field. So either JB is assuming A.J. is a man, or there are two A.J van Doorns.
The article isn't on the aX site, so if you really want to read it Kevin to help me out, I can scan it in and email it to you.
I tried to find contact info for A.J. van Doorn, but only came up with a lady that appears to be in the medical field. So either JB is assuming A.J. is a man, or there are two A.J van Doorns.
Since I'm a noobie, especially WRT understanding design variations, what would JB's question below do to the sound and/or performance of the Octode?
"I should stop right now, as I want to encourage you to read Mr. van Doorn’s article first. I will, however, leave with one small thought experiment: What if we attached the second triode’s grid in the Octode amplifier not to ground but to the pentode’s unbypassed cathode resistor?" - JRB
"I should stop right now, as I want to encourage you to read Mr. van Doorn’s article first. I will, however, leave with one small thought experiment: What if we attached the second triode’s grid in the Octode amplifier not to ground but to the pentode’s unbypassed cathode resistor?" - JRB
Unfortunately AX goofed and the major schematic is found as errata in the Jan 2010 edition.
Adriaan is the first name, which I found out when I contacted Pieter at Tribute to see how much transformers would cost.
The pentode action seems quite similar in concept to Nelson's Aleph current source since it's trying to present a negative impedance to the power triode, making it's life easier.
I'm definitely going to build one at some point. Need to finish other projects 1st.
I was surprised no one mentioned this circuit before, I find it quite clever.
Yes, I have the schematic from the Jan '10 issue too.
I do plan to eventually build the amp as designed, but for now, I have the OPTs and 2/3s of the tubes and I could even use 6V6s for the pentodes if I knew what values to tweak to bring the NFB up to reduce their output to 7W-8W. In that case, I'd only need PS iron.
Quite a clever circuit, but I doubt if it will sound like a triode because the cathode-coupled amp is not a triode. The triode curvature of the first triode is largely cancelled by the second triode, which will increase the proportion of odd-order products.
Putting the grid of the second triode to an unbypassed pentode cathode resistor introduces an extra feedback path which will raise the output impedance - the opposite of what the first triode connection will do, and generally the opposite of what people want.
Putting the grid of the second triode to an unbypassed pentode cathode resistor introduces an extra feedback path which will raise the output impedance - the opposite of what the first triode connection will do, and generally the opposite of what people want.
Now, let's expect the guy who enlighten us about how to create crossover distortions by the presence of feedback, and how presence of feedback means "without any feedback".
Show must go on! 😀
Show must go on! 😀
I'm not sure what he means by "without any negative feedback" - the connection to the first triode anode is negative feedback, although not of the usual linear kind. This will reduce output impedance, as he says. It is the pentode cathode resistor connection which will raise impedance, but this might be someone else's addition.
It won't sound like a triode, but it should sound like a pair of triodes in push-pull. As I recall the pentodes are used essentially as current boosters where the triodes handle a small fraction of the load and the pentodes handle the rest. The triodes can be operated in class A which has no crossover distortion, and the pentodes can be operated in class AB. It's an interesting approach.
PS My experience with current boosters and current mirrors etc, is that they have some qualities of feedback e.g. sharp cutoff but being unity gain do not suffer from slew rate limitations etc. as do loop NFB designs.
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Exactly! If you move the triode plate connection from the pentode plate to B+, you've essentially made a grounded grid driver feeding a pentode. I have trouble believing a little feedback magically transforms the pentode into a triode!
I don't see how it could universally. Do you mean the curves shift or the noise distribution shifts?
Awesome! Thanks for the link to the Fest, Erik.
But, back to my main question, as I'm too interested in the design not to build it, but I want to build it at 10W/ch first.
Which tube is a great choice for the pentode portion? I'm a noobie, so I can't figure out how to mod the existing schematic to throttle back a 6V6GT to 7W-8W, or should I not even worry about exceeding the 10W rating of the OPTs since they will only peak above 10W on transients?
But, back to my main question, as I'm too interested in the design not to build it, but I want to build it at 10W/ch first.
Which tube is a great choice for the pentode portion? I'm a noobie, so I can't figure out how to mod the existing schematic to throttle back a 6V6GT to 7W-8W, or should I not even worry about exceeding the 10W rating of the OPTs since they will only peak above 10W on transients?
What is noise distribution shift?
Of course, I mean dependance of anode current on anode voltage! And you may turn triode into pentode using negative feedback by current, but it will be a low gain pentode.
I meant a change in the noise spectrum. What I was trying to get get across is my thinking that even if you did get triode curves out of a pentode by use of feedback, you would still have partition noise as an artifact, no?