Hello fellow DIY'ers:
I have built a number of PP amps, EL34 and 300B primarily, along with a few similar design PP preamps. Few were copies of designs found here, few were my own design (but obviously nothing new).
Happy with what I have, but with the single-digit temperatures making me a hermit, the tube bug has hit me again. I have a shelf full of tubes, capacitors, and test equipment that has been calling my name.
Thought it would be time to try out a 300B SET, just for kicks and to be able to audition the differences between PP and SE here at home. I've heard some Cary amps that weren't too bad, but the construction quality was pathetic, and the noise floor was harsh (above 120Hz noise, real nastiness).
OPT not purchased yet, but will probably be Lundahl. Power transformer is One Electron (only shielded for me from now on). Tubes I would consider (not currently in use elsewhere) are 7119, 6922, 6SN7, 6W6. I have attached a schematic for consideration. My goal is to run 92dB speakers with no need for a rock concert.
A two stage amp is probably not where I would lean; I am a believer in having sufficient drive for the 300B, so I won't do a 6SL7 into a 300B. I have considered PowerDrive, but will leave that off for the second project if this first one works out. Those are the basic parameters.
Played around with trying to get at least one stage direct coupled, but found I just didn't have enough B+ available to get the operating points I wanted. Either that or I would have had to resort to a dual power supply configuration. Potentially possible with the One Electron transformer, but I don't throw huge capacitors right after my rectifiers. Tradeoffs, you know.
Thoughts ?
I have built a number of PP amps, EL34 and 300B primarily, along with a few similar design PP preamps. Few were copies of designs found here, few were my own design (but obviously nothing new).
Happy with what I have, but with the single-digit temperatures making me a hermit, the tube bug has hit me again. I have a shelf full of tubes, capacitors, and test equipment that has been calling my name.
Thought it would be time to try out a 300B SET, just for kicks and to be able to audition the differences between PP and SE here at home. I've heard some Cary amps that weren't too bad, but the construction quality was pathetic, and the noise floor was harsh (above 120Hz noise, real nastiness).
OPT not purchased yet, but will probably be Lundahl. Power transformer is One Electron (only shielded for me from now on). Tubes I would consider (not currently in use elsewhere) are 7119, 6922, 6SN7, 6W6. I have attached a schematic for consideration. My goal is to run 92dB speakers with no need for a rock concert.
A two stage amp is probably not where I would lean; I am a believer in having sufficient drive for the 300B, so I won't do a 6SL7 into a 300B. I have considered PowerDrive, but will leave that off for the second project if this first one works out. Those are the basic parameters.
Played around with trying to get at least one stage direct coupled, but found I just didn't have enough B+ available to get the operating points I wanted. Either that or I would have had to resort to a dual power supply configuration. Potentially possible with the One Electron transformer, but I don't throw huge capacitors right after my rectifiers. Tradeoffs, you know.
Thoughts ?
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Hi Zigzag,
I can't quite make out what the second tube is in your design, (6N6?) but I would use resistive loading on the 6SN7 (27K or so) with conventional cathode bias and a CCS (or choke) on the second stage with fixed bias (-22V?) where linearity with big signal swings is what is needed.
I can't quite make out what the second tube is in your design, (6N6?) but I would use resistive loading on the 6SN7 (27K or so) with conventional cathode bias and a CCS (or choke) on the second stage with fixed bias (-22V?) where linearity with big signal swings is what is needed.
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It says 6W6, but this is probably a mistake as that is a beam tube, not a triode.
6W6 is fairly commonly triode connected for driver use, and I think fairly good in that connection. The 6Y6 is very popular as a driver triode connected in Japan..
That of course is what I did initially, but I still could not read it. (I use FF in Linux) If I squint hard enough I can almost tell that is a "W."😀
If you want to go all-DHT you will get superb delicacy and transparency at the expense of a larger PSU to encompass the filament supplies - you'll need 6 in all. And easiest solution here is a seperate PSU box with all the transformers and chokes in it. So only DC in the signal chassis.
To keep it cheap and simple use DHTs with low filament current, but nevertheless enough to keep them out of the range of microphony. As I write I'm listening to a 300b SET with 30sp (VT-67) into 31 driver. Cheap and just a lovely sound in all ways. To take it up a notch go for 01a into 71a - but that's more expensive. In any case, in my opinion having tried all this out, all these all-DHT options will put you way ahead of conventional tubes - 6SN7 or whatever. There's a "26 pre amp" thread recently which will give you just about all the information you need for the first two stages. In fact a 26 is great as an input tube, and even marginally usable as a driver.
If you listen to classical music a lot or acoustic jazz and vocals, you'll notice the difference in the timbre of acoustic instruments, which to my ears is significantly more lifelike with DHTs. With rock and electronic music I can't really comment since that's not my thing.
andy
To keep it cheap and simple use DHTs with low filament current, but nevertheless enough to keep them out of the range of microphony. As I write I'm listening to a 300b SET with 30sp (VT-67) into 31 driver. Cheap and just a lovely sound in all ways. To take it up a notch go for 01a into 71a - but that's more expensive. In any case, in my opinion having tried all this out, all these all-DHT options will put you way ahead of conventional tubes - 6SN7 or whatever. There's a "26 pre amp" thread recently which will give you just about all the information you need for the first two stages. In fact a 26 is great as an input tube, and even marginally usable as a driver.
If you listen to classical music a lot or acoustic jazz and vocals, you'll notice the difference in the timbre of acoustic instruments, which to my ears is significantly more lifelike with DHTs. With rock and electronic music I can't really comment since that's not my thing.
andy
Thanks guys for the quick responses.
First, Andy: I am with you, DHT territory might be the way to go. However, I already have a PP amp where 46's drive 300B's, so in some sense, been there done that. Lots of interstage transformers (2 per monoblock plus Tribute input splitter). If I like SET (jury is still out), I may make a second amp with DHT's in the input sections, go for the gusto.
The idea here is to use the tubes I have, which is very little selection, I admit.
Kevin, better schematic attached.
Wow, what an obvious option ! Might even give me the ability to direct couple. I'll play around with this; anyone used the mu output of the DN2540 cascode with good results? I see Gary Pimm uses it often.
First, Andy: I am with you, DHT territory might be the way to go. However, I already have a PP amp where 46's drive 300B's, so in some sense, been there done that. Lots of interstage transformers (2 per monoblock plus Tribute input splitter). If I like SET (jury is still out), I may make a second amp with DHT's in the input sections, go for the gusto.
The idea here is to use the tubes I have, which is very little selection, I admit.
Kevin, better schematic attached.
Wow, what an obvious option ! Might even give me the ability to direct couple. I'll play around with this; anyone used the mu output of the DN2540 cascode with good results? I see Gary Pimm uses it often.
ZZF,
Your will not get the full μ of 20 from the 1st gain block. The 221 KOhm 6W6 grid leak resistor is very much part of the 'SN7 triode's NET AC load. You will come close, but less than 20X is the result.
Your will not get the full μ of 20 from the 1st gain block. The 221 KOhm 6W6 grid leak resistor is very much part of the 'SN7 triode's NET AC load. You will come close, but less than 20X is the result.
Agreed, I'm just sketching out a gameplan at this point. I'll test each individual stage for performance and distortion at the o/p. In the past, I've just built my amps after straining over the design. This time, I'll build on a large piece of plywood; if I don't care for what I hear, I may just disassemble.
Next version: I think this might have some potential. $10 in LED's per channel. I read from Gary Pimm's site the mu output has about a 500 ohm output impedance. Looked like DC coupling stages still wasn't very obtainable with one power supply; nbd.
Use of a 5.1V zener diode looks like a good choice, too; impedance around 7 ohms is comparable to the LED string. Anyone compared the two? 5.1 to 6.2V zeners are somewhat quiet with good TC.
In your second stage you could even eliminate the leds and use the negative bias supply to provide -22V to the grid of the 6W6. This IMO is even better than an LED - no dynamic impedance to contend with at all - and saves a heck of a lot of LEDs in the bargain. IMO at 14 LEDs you are beyond the point of diminishing returns and conventional fixed bias with a grounded cathode is better.
Excellent suggestion. Simpler is good.
Question about the gainstage, Kevin. You suggested using a plate resistor in place of the CCS. Morgan Jones recommends at least 8mA, 150V for optimal linearity of the 6SN7, though I admit I see few designs use the tube this hot. For my o/p, I end up with a 20K resistor, which seems to load the tube heavily (<3x Rp). I see a tradeoff between taking the first stage to extremes with the LED/CCS, versus a lower o/p with plate resistor. Is your preference based on simplicity/cost or performance?
Question about the gainstage, Kevin. You suggested using a plate resistor in place of the CCS. Morgan Jones recommends at least 8mA, 150V for optimal linearity of the 6SN7, though I admit I see few designs use the tube this hot. For my o/p, I end up with a 20K resistor, which seems to load the tube heavily (<3x Rp). I see a tradeoff between taking the first stage to extremes with the LED/CCS, versus a lower o/p with plate resistor. Is your preference based on simplicity/cost or performance?
Another thought -- the Western Electric connection on the parafeed transformer keeps the biasing mechanism out of the current loop path. Cost is that you need a transformer that can handle the big signal swing, and maybe a high B+ ... to eliminate the high B+, you could use a plate choke on the second stage instead.
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Hi Zigzag,
I run the 6SN7 at 8 - 10mA and completely agree with MJ on that issue. I just like the way they sound resistively loaded, however I have not tried the CCS approach so that colors my suggestions. Irrationally perhaps I have some aversion to solid state in the signal path - SY and I have talked about this, and my conclusion is that you should perhaps try both if you are motivated to do so. The distortion spectra will be slightly different with the resistive load resulting in significantly more distortion - however overall the distortion should still be relatively low. (My driver stage only requires 15Vpp in for full output so that helps with preceding stage's linearity.) Note for best distortion performance you do need to bypass the cathode of the 6SN7, and you need to find some very good capacitors to do so - in this case I would probably try to find some surplus low voltage film caps to save money. (I just don't like electrolytics in the signal path, and contrary to popular opinion I find most modern electrolytics sonically inferior to their ancestors - that is why I no longer use them if possible. Sorry for the rant.. 😀 )
Note that with a 400V rail and 8mA a 27K - 30K plate resistor would be in my recommended range.. (Cathode resistor from 750 - 1K)
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Yet another advantage of CCS loading is the fact that cathode bias resistors don't need bypassing. 😀 Who cares about degeneration, when tens of Megohms of AC impedance are loading the anode?
Given the fact that the 6SN7 triode will pass pretty stringent distortion tests, without any help at all, the CCS loaded version will be scary good.
Given the fact that the 6SN7 triode will pass pretty stringent distortion tests, without any help at all, the CCS loaded version will be scary good.
Don't forget the effect of the grid bias resistor on the AC load line which is on just the other side of the coupling capacitor - and that should be on the order of a couple of hundred K at most.. The cathode resistor should be bypassed in this application even with a CCS. Alternately you can use led bias (4 red leds should be just about right) and skirt this issue entirely.
I'm still in favor of trying several approaches particularly since this amplifier is still in the breadboard phase of development, Zigzag will know what he likes best upon hearing it, and IMHO it is a matter of personal preference numbers aside...
Excellent. I will try both and report back if I can hear any differences. My suspicion is that the two capacitors will color the sound more than the input stage, and possibly more than the driver stage. This will be my first no-feedback design with coupling capacitors. I have another no-feedback amp, but with all interstage iron.
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