I have constructed a driver stage and I would like to share the results. It is a variation on the mu-stage. Inspiration for this came from Alan Kimmel and from several threads on diyAudio recently. http://www7.taosnet.com/f10/mustage.html
Though not an SRPP, the circuit resembles one. It has a pentode as the upper device. Rather than mirroring the lower device in the way an SRPP does, it behaves more like a constant current source due to the increased gain of the pentode.
Some may not like the idea of using a pentode. I was hesitant too.
The voltage gain device I am using is the 46 recommended to me by i_should_coco. I have grown quite fond of this device and it doesn't let me down here.
The pentode is the 6CM5/EL36 which I chose for it's transconductance of around 14,000uS, and because of its wide Vhk capability.
Here is the circuit (grid stoppers have been omitted for clarity).
Though not an SRPP, the circuit resembles one. It has a pentode as the upper device. Rather than mirroring the lower device in the way an SRPP does, it behaves more like a constant current source due to the increased gain of the pentode.
Some may not like the idea of using a pentode. I was hesitant too.
The voltage gain device I am using is the 46 recommended to me by i_should_coco. I have grown quite fond of this device and it doesn't let me down here.
The pentode is the 6CM5/EL36 which I chose for it's transconductance of around 14,000uS, and because of its wide Vhk capability.
Here is the circuit (grid stoppers have been omitted for clarity).
Attachments
Constant current
One thing that makes this circuit stand apart from the SRPP is its relatively constant current. Here is an oscillogram showing the voltage across the triode versus the voltage across the pentode's cathode resistor. It is to scale and the levels have been adjusted to make them proportional.
For those who are interested, I figured that at 8.37mA of quiescent current, the triode with a plate voltage of 177V has an equivalent DCR of 21,150ohms. Since this is 7.8 times higher than the pentode's cathode resistor, I figured the Rk would be that much less sensitive to displaying the current via its voltage. I turned it up on the scope by that much.
Unless I'm mistaken, this amounts to 30dB's worth of 'constant currentness'
One thing that makes this circuit stand apart from the SRPP is its relatively constant current. Here is an oscillogram showing the voltage across the triode versus the voltage across the pentode's cathode resistor. It is to scale and the levels have been adjusted to make them proportional.
For those who are interested, I figured that at 8.37mA of quiescent current, the triode with a plate voltage of 177V has an equivalent DCR of 21,150ohms. Since this is 7.8 times higher than the pentode's cathode resistor, I figured the Rk would be that much less sensitive to displaying the current via its voltage. I turned it up on the scope by that much.
Unless I'm mistaken, this amounts to 30dB's worth of 'constant currentness'
Attachments
Swing/Distortion.
According to Alan, the top device is also acting as a cathode follower. He says that this is where a pentode shines. I think this shows in the amount of swing I am able to get despite needing to drive output stage grids.
In this oscillogram, the driver's output is shown at 250Vp-p from a 360V rail, and the KT66's are biased at just 36V!
The distortion shown near the top of the waveforms is due to driving the output stage grids. The clipping near the bottom is the 46's themselves being driven into grid current as is confirmed by looking at the 6SN7's before them (not shown).
I've seen much worse looking driver distortions and at much lower voltages too.
According to Alan, the top device is also acting as a cathode follower. He says that this is where a pentode shines. I think this shows in the amount of swing I am able to get despite needing to drive output stage grids.
In this oscillogram, the driver's output is shown at 250Vp-p from a 360V rail, and the KT66's are biased at just 36V!
The distortion shown near the top of the waveforms is due to driving the output stage grids. The clipping near the bottom is the 46's themselves being driven into grid current as is confirmed by looking at the 6SN7's before them (not shown).
I've seen much worse looking driver distortions and at much lower voltages too.
Attachments
How does it sound?
The 46 for me was already a very articulate driver. Doing this seems to have brought it to a new level. The bass seems a smidge lighter, as if it has had some of the even order harmonics reduced. Bass articulation is better. I speculate that the improved PSRR may have a little to do with this. It really just sounds like there is more control.
But this does not sound like forced control. This circuit seems to have not added anything, just taken away. The mids and treble are also lighter and have improved articulation. There is less congestion. The spectrum remains balanced sounding.
Most importantly, the 46's have not been robbed of their charm. If anything, they sound as if they have just been allowed to perform under better circumstances.
If you've come this far, thanks for reading, I am interested in comments, criticism's and improvements.
The 46 for me was already a very articulate driver. Doing this seems to have brought it to a new level. The bass seems a smidge lighter, as if it has had some of the even order harmonics reduced. Bass articulation is better. I speculate that the improved PSRR may have a little to do with this. It really just sounds like there is more control.
But this does not sound like forced control. This circuit seems to have not added anything, just taken away. The mids and treble are also lighter and have improved articulation. There is less congestion. The spectrum remains balanced sounding.
Most importantly, the 46's have not been robbed of their charm. If anything, they sound as if they have just been allowed to perform under better circumstances.
If you've come this far, thanks for reading, I am interested in comments, criticism's and improvements.
I did something quite similar using 801A+choke+807. 807 was wired as a pent initially to mimick a CCS load for the lower tube (the 801A). I took the output from the upper cathode follower and from the lower plate: as a matter of facts the lower anode sounded better but square waves were sharper when the signal was taken from the upper cathode.
Now the 807 is triode strapped (no more a quasi-CCS then...) and the output is from the upper cathode. I'd say it sounds better to me now. I can switch back and forth (cathode/anode) on the fly but I didnt notice such a difference as in the previous set-up. Again square waves are noticeable better from the upper cathode.
Next I will get rid of the 807 retaining just the choke or using a IXYS Chip a la tubelab. Those staked valves seem too complex for me... I like simplicity.
OH! For your info ... the SRPP is driving a hi-L interstage tranny.
Ciao
Gianluca
Now the 807 is triode strapped (no more a quasi-CCS then...) and the output is from the upper cathode. I'd say it sounds better to me now. I can switch back and forth (cathode/anode) on the fly but I didnt notice such a difference as in the previous set-up. Again square waves are noticeable better from the upper cathode.
Next I will get rid of the 807 retaining just the choke or using a IXYS Chip a la tubelab. Those staked valves seem too complex for me... I like simplicity.
OH! For your info ... the SRPP is driving a hi-L interstage tranny.
Ciao
Gianluca
Very cool! I've been tempted by the mu-stage for a while, but never got around to building one - I ended up transformer coupling. I suspect I'm going to have to try this out!
I agree with the use of a pentode - they make better CCSs and cathode followers that triodes. I always thought that the mu-stage had relatively limited swing, but I gues this isn't the case ;-)
BTW, have you read the article on the "Choke Assisted Mu-Stage" in Vacuum Tube Valley a while ago? Very interesting.
Cheers,
Pete
I agree with the use of a pentode - they make better CCSs and cathode followers that triodes. I always thought that the mu-stage had relatively limited swing, but I gues this isn't the case ;-)
BTW, have you read the article on the "Choke Assisted Mu-Stage" in Vacuum Tube Valley a while ago? Very interesting.
Cheers,
Pete
Not directly that article ... I had the chance to read those ideas reported here and there.
My first amp was transformer coupled (LL1660PP a la Olson) and after a while I tried with a mustage driver 6H30 or 6N6P plus EF184 ... much better IMHO. Measurements were OK in both the amps but I liked the mu-stage more. I am not going back to IT. And as I said, it sounded better (but measured poorer) when the signal was taken from the lower anode.
OH! The swing of the mu-stage is limited as a matter of facts.
The interstage I am using now in this choked-SRPP is actually an autoformer to step-up the signal... stepping up the signal is not my favorite choice but I need that +6db extra gain.
Ciao
Gianluca
My first amp was transformer coupled (LL1660PP a la Olson) and after a while I tried with a mustage driver 6H30 or 6N6P plus EF184 ... much better IMHO. Measurements were OK in both the amps but I liked the mu-stage more. I am not going back to IT. And as I said, it sounded better (but measured poorer) when the signal was taken from the lower anode.
OH! The swing of the mu-stage is limited as a matter of facts.
The interstage I am using now in this choked-SRPP is actually an autoformer to step-up the signal... stepping up the signal is not my favorite choice but I need that +6db extra gain.
Ciao
Gianluca
One little question:
you say you're running the 46 at 8mA, this current obviously has to pass into the EL36. It can't be very happy for such low current, so you might try (see if the sound or measurements get better) to increase the current throught the EL36 without changing 46's current. How? Look at figure 4 of the link you've posted: http://www7.taosnet.com/f10/mustage.html
you say you're running the 46 at 8mA, this current obviously has to pass into the EL36. It can't be very happy for such low current, so you might try (see if the sound or measurements get better) to increase the current throught the EL36 without changing 46's current. How? Look at figure 4 of the link you've posted: http://www7.taosnet.com/f10/mustage.html
Perhaps the current should be increased. Running the 46 up seems the first logical step. Adding the extra CF cathode resistor to ground seems to be more than just an extra current path, it appears to somehow make the current more constant. One circuit I have seen has a ccs here instead of the resistors.
The higher current is going to mean a smaller upper Rk. I am concerned this will decrease the signal to the pentode. I have a 90H choke that I made from two small power xfmr's that has a good FR. I fear it will saturate. Is there a way to tell while trying it?
Geek,
DC screen driving sounds like a great idea. I'm set up for triode now and it may need higher rails so I feel I'm more than one step away from it. Maybe if I manage to learn to use the mu stage well, I'll head that way
I have a copy of Morgan Jones' book on order right now so I guess anything's possible.
The higher current is going to mean a smaller upper Rk. I am concerned this will decrease the signal to the pentode. I have a 90H choke that I made from two small power xfmr's that has a good FR. I fear it will saturate. Is there a way to tell while trying it?
Geek,
DC screen driving sounds like a great idea. I'm set up for triode now and it may need higher rails so I feel I'm more than one step away from it. Maybe if I manage to learn to use the mu stage well, I'll head that way
I have a copy of Morgan Jones' book on order right now so I guess anything's possible.
Pete, here is a link to an italian forum where my project is being documented. Still on the breadboard and still to be tweaked. Unfortunately you'll see only an old version of the schematic
looking at the schematics, note that
-the output is taken from the 801A's anode
-there is no coupling cap between 801A and 807
-the 807 is biased by the choke DCR + that 820 resistor
The 807 is now triode strapped (get rid of that 47kR and connect the screen to the anode via the 100R and forget the 100uF screen bypass cap). The 807/801 are now wired as SRPP but the choke (and the 820R) are biassing and loading the tubes. Thats what I called choke assisted SRPP
http://www.audiofaidate.it/forum/topic.asp?TOPIC_ID=224&whichpage=1
Do I like that mu-stage? Yes I liked it. I am considering to use a different driver (eimac 100TH) and IXYS chips to make the whole thing simpler.
I hope it can help.
Ciao
Gianluca
looking at the schematics, note that
-the output is taken from the 801A's anode
-there is no coupling cap between 801A and 807
-the 807 is biased by the choke DCR + that 820 resistor
The 807 is now triode strapped (get rid of that 47kR and connect the screen to the anode via the 100R and forget the 100uF screen bypass cap). The 807/801 are now wired as SRPP but the choke (and the 820R) are biassing and loading the tubes. Thats what I called choke assisted SRPP
http://www.audiofaidate.it/forum/topic.asp?TOPIC_ID=224&whichpage=1
Do I like that mu-stage? Yes I liked it. I am considering to use a different driver (eimac 100TH) and IXYS chips to make the whole thing simpler.
I hope it can help.
Ciao
Gianluca
Hi Gianluca,
Nice! That looks *very* similar to the "choke assisted mu-stage" published in VTV. I feel it's a very elegant circuit, which removes one of my bugbears with the conventional mu-stage - the coupling cap. (I hate capacitors ;-) ), and provides a very high impedance load ot the lower tube.
So you're saying you're running basically the same circuit, but the 807 is triode strapped? What made you change from pentode operation?
Cheers,
Pete
Nice! That looks *very* similar to the "choke assisted mu-stage" published in VTV. I feel it's a very elegant circuit, which removes one of my bugbears with the conventional mu-stage - the coupling cap. (I hate capacitors ;-) ), and provides a very high impedance load ot the lower tube.
So you're saying you're running basically the same circuit, but the 807 is triode strapped? What made you change from pentode operation?
Cheers,
Pete
I just wanted to try it (LOL). The basic idea was to load the 801A with the highest load ... a ccs possibly. But now I really want to get rid of the iron+heater for the sake of simplicity.
Speaking of "sound quality" ... well if you go for a pentode load take the signal out of the lower plate (I liked it more) ... triode mode ... not such a big difference (cathode/plate).
OH! As Indm was suggesting, an higher mu pentode would work even better than the 807 (mu=8). 807 was just cheap, sturdy and reliable.
OH! If you elect to use the choke ... you need to pay attention to its DCR before buying it as it will bias your tubes.
Ciao
Speaking of "sound quality" ... well if you go for a pentode load take the signal out of the lower plate (I liked it more) ... triode mode ... not such a big difference (cathode/plate).
OH! As Indm was suggesting, an higher mu pentode would work even better than the 807 (mu=8). 807 was just cheap, sturdy and reliable.
OH! If you elect to use the choke ... you need to pay attention to its DCR before buying it as it will bias your tubes.
Ciao
Hi Gluca,
That's a neat schema. For for a mu follower variant or srpp, Paul Joppa also likes it better when the signal is taken from the lower plate. He also cited that a choke at the upper cathode has its advantages too (he listened to this type of set up).
Maybe the new srpp version of the Seth PP (2A3 and 300B) amp could be modified in this way too for the 6H30pi or other good driver tubes with the same heater voltages.
regards,
fred
OH! I made it with a 6H30 + EF184 ... lovely! And I used also the EXO173 to drive the 2A3s. I didnt use a choke ... but it sounded really great ... I could not find any choke with a suitable H/RDC combination. Probably using other penthodes with higher bias (the EF184 was at 2V or so).
Now that amp is dismantled and parts recycled ... but I do miss it.
Now that amp is dismantled and parts recycled ... but I do miss it.
Hi guys,
Did one of you ever looked on www.triodedick.com?
There are a lot of mu-stage designs availlable, some also with an english project desciption.
The designs use a variation of tubes for V1 in pre-amp and power amp setup.
regrads.
Did one of you ever looked on www.triodedick.com?
There are a lot of mu-stage designs availlable, some also with an english project desciption.
The designs use a variation of tubes for V1 in pre-amp and power amp setup.
regrads.