Radu, Ale, myself and several others have been building 01A preamps for at least 10 years, the reason being that used in the right context it's probably one of the 5 best sounding tubes we've ever used. Hence the motivation to explore if and how it could be used to drive an output tube.
One way to re-formulate the question would therefore be "what thoriated filament DHTs with a mu of at least 8 could be used at higher currents as a driver tube". One answer would be "use a 10Y", which I have already done, but there may be others. I think most of us could name 50 driver tubes that on paper would satisfy the usual requirements for a driver tube, but unless they had the potential to sound as good as an 01A they would be irrelevant to this thread.
I've used resistor loading with the 01A several years back, but a good plate choke sounded better - the issue isn't with the plate choke, which is amorphous and has good inductance. The issue isn't with the OPT either, since the Ri of the EL12n in triode is around 1K and 3K is fine. The issue lies with "how could you modify the circuit to enable the 01A to be used at more than low volumes". To which you might add "without losing the tonal qualities of the 01A or introducing unacceptable colourations".
Another possible way to re-formulate the question might be "what output tube or tubes with a mu of 18 or more could be driven more easily by an 01A, e.g by having lower a-g capacitance".
One way to re-formulate the question would therefore be "what thoriated filament DHTs with a mu of at least 8 could be used at higher currents as a driver tube". One answer would be "use a 10Y", which I have already done, but there may be others. I think most of us could name 50 driver tubes that on paper would satisfy the usual requirements for a driver tube, but unless they had the potential to sound as good as an 01A they would be irrelevant to this thread.
I've used resistor loading with the 01A several years back, but a good plate choke sounded better - the issue isn't with the plate choke, which is amorphous and has good inductance. The issue isn't with the OPT either, since the Ri of the EL12n in triode is around 1K and 3K is fine. The issue lies with "how could you modify the circuit to enable the 01A to be used at more than low volumes". To which you might add "without losing the tonal qualities of the 01A or introducing unacceptable colourations".
Another possible way to re-formulate the question might be "what output tube or tubes with a mu of 18 or more could be driven more easily by an 01A, e.g by having lower a-g capacitance".
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But with 2v of input signal maxed out from your DAC you are only 1/3 of the max signal you could be supplying the grid when biased at 7v. So you are hardly asking it to do anything as it is. At the plate voltage you are supplying you could be nearer to 9v bias and input signal for max output. No scope or sig. gen. for a FR sweep? If the tube is underperforming to your expectation under normal operating conditions then perhaps it is a tube from the old days that is just crying out to be in an amp with some feedback.
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Andy,
I use 841s in my Unity-Coupled amp. They can handle higher currents and voltages and have a mu of 30. If you run them at higher currents you will probably have to drive some grid current, though. I used op amps to do it, and come to think of it, I sent you a picture of that board in one of the emails I sent you.
Zout is kind of high for the 841 but I CCS loaded them and took the output from the source of the CCS mosfet to get a low Zout.
Problem is, 841s are hard to find sometimes. I don't think they made very many.
I use 841s in my Unity-Coupled amp. They can handle higher currents and voltages and have a mu of 30. If you run them at higher currents you will probably have to drive some grid current, though. I used op amps to do it, and come to think of it, I sent you a picture of that board in one of the emails I sent you.
Zout is kind of high for the 841 but I CCS loaded them and took the output from the source of the CCS mosfet to get a low Zout.
Problem is, 841s are hard to find sometimes. I don't think they made very many.
I think the solution is clear - buy Ales modules to enable current drive - seems it was meant to be that way.
Could have just sent his '10 year veteran' mate an email - "tube sucks swinging max volts into heavy grids, reckon I should try a MOSFET follower?"
Response would have been "Ah yeah, why wouldnt you?"
But after ten years, you'd know that, right? - apparently not.
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Read between the lines.. 'an hour later' someone replied with a commercially available solution, just happened to be his mate, and who is not listed (for some reason) as manufacturer (read: commercial interest).
Best to ignore (or report).
Could have just sent his '10 year veteran' mate an email - "tube sucks swinging max volts into heavy grids, reckon I should try a MOSFET follower?"
Response would have been "Ah yeah, why wouldnt you?"
But after ten years, you'd know that, right? - apparently not.
-
Read between the lines.. 'an hour later' someone replied with a commercially available solution, just happened to be his mate, and who is not listed (for some reason) as manufacturer (read: commercial interest).
Best to ignore (or report).
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If you're looking for a conspiracy theory I can assure you that nothing could be further from the truth. I already said previously that I'm familiar with building with tubes and have no knowledge base in solid state, which takes me outside my comfort zone. I can build modules but I can't design them. Hence I use plate chokes rather than active loads, and the same applies to any other solid state solutions, whether Ale's designs or any other designs. I'm willing to entertain solid state solutions as long as I could build them successfully, and that applies to any and all solid state solutions. So yes, suggestions for solid state solutions are helpful under the above conditions.
I hope this answers your question, because there exists no other answer to it.
Hello Tony. Yes, absolutely I would consider a cathode follower. The condition would be that it doesn't add colourations that would invalidate the choice of the 01A in the first place. The same applies to a solid state follower. If either of these were sonically transparent or near-transparent I would be well pleased. So by all means suggest. As I said before, I've never used cathode followers but I can build one.
I don't remember the final schematics since I was just testing them on the bench. I ended up using SIT as a gain stage without follower because I find its impedance and gain is good enough by itself for my purpose. I think you can drive 300B either with EL803S CDDA or with high voltage Sony SIT but may not be enough gain/swing with Sony by itself. I'm not knowledgeable about tubes, and EL803S was actually chosen based on Thöress 300B amp. I think it is one of very linear tube.
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I've heard a SIT amp with Sony devices and it was marvellous - the nearest to a good tube amp I've ever heard. But I thought the Sony devices were long out of production. I happen to have a couple if I can find them. I think by large heatsink you're talking "very large heatsink", but that's not impossible.
If SIT devices are unavailable, can you suggest a current production alternative? Also interested in a circuit.
On another note, I changed the grid resistor on the EL12n output stage from 100k to 560k and I think I'm hearing a slight improvement in the sound - not massive but I think it's detectable as a slightly smoother sound to the upper strings - i.e. violins. They're still not right, though - the sound is thinner than it should be.
If SIT devices are unavailable, can you suggest a current production alternative? Also interested in a circuit.
On another note, I changed the grid resistor on the EL12n output stage from 100k to 560k and I think I'm hearing a slight improvement in the sound - not massive but I think it's detectable as a slightly smoother sound to the upper strings - i.e. violins. They're still not right, though - the sound is thinner than it should be.
I'm not using 4P1L any more - there's something slightly sharp about the sound and the mid frequencies are rather disembodied. It has many virtues - very easy to use, clean sound - but once you hear the drawbacks you can't get them out of your head. There's no point in using a 4P1L to drive a 300b in a 2 stage amp - not enough gain. I tried it driving an EL12n but the sound was not acceptable, while the 10Y worked fine. As stated the 26 and 01A are "unfinished projects".... but I prefer both just as tubes.
You can still get Tokin SIT, but it has high input capacitance, so it has to be driven by very low impedance driver. According to recent Nelson Pass interview, it seems like someone may start manufacturing new SIT.
A good example and explanation about SIT @high voltage is Michael Rothacher's LuminAria.
https://audioxpress.com/assets/upload/files/AXApri2014_LuminAria SIT Preamp.pdf
A good example and explanation about SIT @high voltage is Michael Rothacher's LuminAria.
https://audioxpress.com/assets/upload/files/AXApri2014_LuminAria SIT Preamp.pdf
Andy, why don't you accept Ale's offer? It won't take much effort soldering the components to the PCB and hooking it up.
I'm on a similar situation as I am building a system around the 01A and the 300B output tube. I have a high quality 2V8 pp output DAC (passive output stage). The system might get more complicated later on when/if I move on to a multi amp system.
i think design starts with the 300b g1 negative bias, say -55v, so then your driver should be able to swing at least 55 volts peak....
you then ask yourself, is my driver able to swing this?
if yes, then the next question to ask is, is your driver able to drive the 300b miller + stray caps to 20khz?...30pf stray is a common design assumption...
so if the answer to this is no....then you look at ways to assist the voltage amp stage....
anyway this is my thought process gleaned from reading the postings of tube gurus here...
you then ask yourself, is my driver able to swing this?
if yes, then the next question to ask is, is your driver able to drive the 300b miller + stray caps to 20khz?...30pf stray is a common design assumption...
so if the answer to this is no....then you look at ways to assist the voltage amp stage....
anyway this is my thought process gleaned from reading the postings of tube gurus here...