From what I gather the EL34 will have a mighty hard job matching up to the sound of the 6080. I personally wouldn't bother.
Your efforts would be better spent on a 6080 PP with interstage phase splitting. This can be done cheaply with main toroidal outputs and phase splitters. I guarantee you won't be sorry you did. The bass will be a huge step up on the SE 6080. Alternatively lash it up with the self splitting to start with and see how it sounds - if its not as good as hoped add in the interstage phase splitting later.
Shoog
Hi Guys
I've only just caught this thread
Here's a schematic of the current version of my 6AS7 push pull amp.
It uses a transformer phase splitter on the input, a differential voltage amp and driver stage that runs at a very high voltage but low current.
The output stage is also a differential amp sitting on an EL34 constant current sink and feeds a custom wound Sowter 2k5 A-A output transformer.
The EL34 sink was changed this morning for an ITT 12E14 series pass regulator tetrode and it sounds very good indeed: dynamic, clean and detailed with a large soundstage and a tight, extended bass.
The whole scheme took a lot of thought and experiment over about a six month period before finally settling on the high voltage/ low current drive scheme. It is the total opposite to most drive setups, but that is because the 6AS7/6080 has quite different drive requirements than other more "sensible" choices of valve.
I get away with the low current because the very low mu and gain of the 6AS7 does not multiply up it's Miller capacitance to anything like the degree that most power triodes do: and of course chiefly because there are no step-up interstage transformers in the amp. The high voltage drivers negate the need for them.
The whole thing was inspired by Gary Dahl and Lynn Olsen's Amity PP project.
Each section has its own power supply; the power valves and tube constant current sink supply being solid state rectified, cap input PSUs and the voltage amp driver stages tube rectified with a CLC input using a pair of 8mA 150H Hammond chokes in parallel.
Steve
I've only just caught this thread
Here's a schematic of the current version of my 6AS7 push pull amp.
It uses a transformer phase splitter on the input, a differential voltage amp and driver stage that runs at a very high voltage but low current.
The output stage is also a differential amp sitting on an EL34 constant current sink and feeds a custom wound Sowter 2k5 A-A output transformer.
The EL34 sink was changed this morning for an ITT 12E14 series pass regulator tetrode and it sounds very good indeed: dynamic, clean and detailed with a large soundstage and a tight, extended bass.
The whole scheme took a lot of thought and experiment over about a six month period before finally settling on the high voltage/ low current drive scheme. It is the total opposite to most drive setups, but that is because the 6AS7/6080 has quite different drive requirements than other more "sensible" choices of valve.
I get away with the low current because the very low mu and gain of the 6AS7 does not multiply up it's Miller capacitance to anything like the degree that most power triodes do: and of course chiefly because there are no step-up interstage transformers in the amp. The high voltage drivers negate the need for them.
The whole thing was inspired by Gary Dahl and Lynn Olsen's Amity PP project.
Each section has its own power supply; the power valves and tube constant current sink supply being solid state rectified, cap input PSUs and the voltage amp driver stages tube rectified with a CLC input using a pair of 8mA 150H Hammond chokes in parallel.
An externally hosted image should be here but it was not working when we last tested it.
Steve
Very nice looking amp.
I have a few questions;
What is the voltage at the anode of the EL34 ? How do you find current balance over time. Are you using a EI output transformer ?
Have you tried any other type of CCS in the tail of the first two stages? I have used the LM317 but only fully bypassed. I was lead to believe that the its impedance was to low at high frequencies.
What voltage swing are you getting out of the second stage? Are the grids of the 6080's loading this stage down at all.
What voltage is across the 6080's, you seem to be running them a lot more conseratively than I am. What power output are you getting?
I was thinking of moving my input phase splitting to the front end with an input transformer as you have, and then turning my paralled 5687's into a differential pair. However the overload behaviour would likely suffer if I removed the interstage transformer driving the grids of the 6080's
Shoog
I have a few questions;
What is the voltage at the anode of the EL34 ? How do you find current balance over time. Are you using a EI output transformer ?
Have you tried any other type of CCS in the tail of the first two stages? I have used the LM317 but only fully bypassed. I was lead to believe that the its impedance was to low at high frequencies.
What voltage swing are you getting out of the second stage? Are the grids of the 6080's loading this stage down at all.
What voltage is across the 6080's, you seem to be running them a lot more conseratively than I am. What power output are you getting?
I was thinking of moving my input phase splitting to the front end with an input transformer as you have, and then turning my paralled 5687's into a differential pair. However the overload behaviour would likely suffer if I removed the interstage transformer driving the grids of the 6080's
Shoog
Hi
I'm getting around 55V at the anode of the EL34
The current balance seems to be spot on.
I'm currently looking into adjustable grids so that the voltages can be balanced between the sections. However there is little DC offset at the optx primary and the amp sounds so good that I can't seem to be bothered to mess about too much.
The output trafos are EI cored from Sowter, capable of handling 250mA
The LM317 CCSs I'm using are unbypassed to keep the Ra as high as possible on both stages of the driver circuitry.
Because I'm only operating the plates of the second stage at 2.75mA each the flat load produced by the CCS is capable of allowing the plates to swing from approx 20V to 500V with no problem. Halving this for the two sides and then adding back together at the output of the stage means I can get around 480V peak to peak.
The driver stage is cruising at 300V p to p when it is under full drive
I certainly can't detect any loading down of the driver by the grids of the 6AS7s. As I mentioned the whole idea behind the way the amp works is about hi-voltage drive, which is only possible in this instance due to the lack of interstage transformers, which of course need current if they are going to overcome the Miller capacitance caused by the transformer magnification.
The whole philosophy of the amp is to exploit the apparent disadvantage of the low gain of the valve by arranging the drivers to swing lots of volts without the need for accompanying high current. This keeps the noise right down to almost silent operation at idle.
There is about 120 volts across the valves and yes they are running very conservatively. Though many people would argue otherwise, I am convinced I can swing lots more volts through the outputs, without entering class AB because of the flattening load effect of the pentode CCS.
In fact with the amp being differential end to end It was necessary to be extremely careful at each stage of the design that there was no possibility of any of the stages entering grid current or straying out of class A operation even for an instant. Under these conditions the clipping behaviour would be extremely nasty.
So to summarize the main design considerations
a) I needed massive voltage swing to get the 6AS7s to move without an interstage transformer between.
b) there had to be strictly enforced class A operation throughout the amp if it was going to be differential from input to output otherwise overload operation would be nasty. This by necessity would limit output power but it was a compromise worth making in this particular instance.
Running the driver stage at high voltage took care of the first consideration
The low current operation went a long way to addressing the second, made possible, ironically, due to the low gain of the 6AS7 itself. As for power output, all I know is that it will go very loud indeed with no signs of distress.
The way this thing has been put together has caused a lot of raised eyebrows on another forum but the bottom line is it works beautifully. It seems to possess much of the treble air, space and midrange presence of a single ended amp combined with the bass slam of a push pull amp.
Here's a pic of the amp in situ
Steve. 🙂
I'm getting around 55V at the anode of the EL34
The current balance seems to be spot on.
I'm currently looking into adjustable grids so that the voltages can be balanced between the sections. However there is little DC offset at the optx primary and the amp sounds so good that I can't seem to be bothered to mess about too much.
The output trafos are EI cored from Sowter, capable of handling 250mA
The LM317 CCSs I'm using are unbypassed to keep the Ra as high as possible on both stages of the driver circuitry.
Because I'm only operating the plates of the second stage at 2.75mA each the flat load produced by the CCS is capable of allowing the plates to swing from approx 20V to 500V with no problem. Halving this for the two sides and then adding back together at the output of the stage means I can get around 480V peak to peak.
The driver stage is cruising at 300V p to p when it is under full drive
I certainly can't detect any loading down of the driver by the grids of the 6AS7s. As I mentioned the whole idea behind the way the amp works is about hi-voltage drive, which is only possible in this instance due to the lack of interstage transformers, which of course need current if they are going to overcome the Miller capacitance caused by the transformer magnification.
The whole philosophy of the amp is to exploit the apparent disadvantage of the low gain of the valve by arranging the drivers to swing lots of volts without the need for accompanying high current. This keeps the noise right down to almost silent operation at idle.
There is about 120 volts across the valves and yes they are running very conservatively. Though many people would argue otherwise, I am convinced I can swing lots more volts through the outputs, without entering class AB because of the flattening load effect of the pentode CCS.
In fact with the amp being differential end to end It was necessary to be extremely careful at each stage of the design that there was no possibility of any of the stages entering grid current or straying out of class A operation even for an instant. Under these conditions the clipping behaviour would be extremely nasty.
So to summarize the main design considerations
a) I needed massive voltage swing to get the 6AS7s to move without an interstage transformer between.
b) there had to be strictly enforced class A operation throughout the amp if it was going to be differential from input to output otherwise overload operation would be nasty. This by necessity would limit output power but it was a compromise worth making in this particular instance.
Running the driver stage at high voltage took care of the first consideration
The low current operation went a long way to addressing the second, made possible, ironically, due to the low gain of the 6AS7 itself. As for power output, all I know is that it will go very loud indeed with no signs of distress.
The way this thing has been put together has caused a lot of raised eyebrows on another forum but the bottom line is it works beautifully. It seems to possess much of the treble air, space and midrange presence of a single ended amp combined with the bass slam of a push pull amp.
Here's a pic of the amp in situ
An externally hosted image should be here but it was not working when we last tested it.
Steve. 🙂
It look lovely, much nicer than mine. I see you also have the Russian coke bottle valves - nice sounding but a bit softer than the RCA's which to my ear sound the best. I also have some Mullards which are nice, but a bit thinner on the bass.
The only thing I'm still a little puzzled over is the need for the high voltage driver stage. In mine the driver is running off a 170V B+ with a CCS on top of the 5687's. This gives me a swing of about 60V. This is more than adequate because I have the 6080's biased at 30V. Of course my 6080's are running at 100V and 100mA so I am effectively dragging twice the power output out for about the same voltage across the 6080. I can't imagine that your second stage is ever asked to deliver more than a fraction of its potential voltage swing - which I suppose keeps it nice and clean.
As you observe the use of an interstage transformer massively amplifies the Miller capacitance, and I push 40mA single ended into the phase splitter transformer - this gives 20mA into each transformer/grid.
I run my amp for about 18hrs per day. I found that my first set of 6080's lasted about 1yrs before they fell so badly out of balance that the amp started to oscillate. Because I am using toroidal output transformers I can not tolerate any DC imbalance and therefore I put LM317 CCS's in the tails of each triode section. I keep it differential by capacitively coupling the cathodes together.
I have zero noise issues just as yourself.
I know what you mean about people not coming on board with these unusual designs, there is a lot of prejudice against the 6080 and running these valves at low voltages. Still thats their problem not ours.
My next amp is going to be based on Gary Pimms Tabor amp, but there are some design issues which I have been scratching my head over for a few months and they just won't quite resolve themselves !
Good work.
Shoog
The only thing I'm still a little puzzled over is the need for the high voltage driver stage. In mine the driver is running off a 170V B+ with a CCS on top of the 5687's. This gives me a swing of about 60V. This is more than adequate because I have the 6080's biased at 30V. Of course my 6080's are running at 100V and 100mA so I am effectively dragging twice the power output out for about the same voltage across the 6080. I can't imagine that your second stage is ever asked to deliver more than a fraction of its potential voltage swing - which I suppose keeps it nice and clean.
As you observe the use of an interstage transformer massively amplifies the Miller capacitance, and I push 40mA single ended into the phase splitter transformer - this gives 20mA into each transformer/grid.
I run my amp for about 18hrs per day. I found that my first set of 6080's lasted about 1yrs before they fell so badly out of balance that the amp started to oscillate. Because I am using toroidal output transformers I can not tolerate any DC imbalance and therefore I put LM317 CCS's in the tails of each triode section. I keep it differential by capacitively coupling the cathodes together.
I have zero noise issues just as yourself.
I know what you mean about people not coming on board with these unusual designs, there is a lot of prejudice against the 6080 and running these valves at low voltages. Still thats their problem not ours.
My next amp is going to be based on Gary Pimms Tabor amp, but there are some design issues which I have been scratching my head over for a few months and they just won't quite resolve themselves !
Good work.
Shoog
Shoog said:
Thanks for the comments Shoog. 🙂
The driver stage as you have observed is merely cruising at normal listening levels. I deliberately over engineered it so that there was no chance of it straying out of class A or running into grid current. Also the design keeps the drive signal nice and clean.
I have read a lot of comments about the 6080 saying "why bother with it?'
Well it is cheap and easily obtainable for a start. If one is prepared to put in the design work to support its requirements then it can be made to sound superb much better than its on-paper performance would suggest.
Good luck with the new amp.
Steve
Shoog said:
Thanks for the idea, Shoog.
I have done a self-splitting 832A PP amp and tried interstage splitting with the same amp, I found no sonic difference between the two, so I removed the interstage and use self splitting (less parts).
You suggested mains toroidal for PP OPT, what size should I get? I can also have a custom PP OPT wound.
How about this: B+ for the 6080 of around 250VDC, 75ma across the 6080, 80V bias, self-splitting, driver is 12at7 at 200VDC B+, plate choke loaded. Toroidal mains for OPT or a custom wound PP OPT.
Thanks.
Interesting comment. I have a quad of EL86's for just such a project I will have to build it.
I would guess that the 832A probably has higher gain than the 6080, which would make it a better candidate for self splitting.
I get about 5.500hrs out of my 6080's at 10Watts. 12.5Watts is a little harder run than I would choose.
If you can get custom wound toroidals then go for it.
I still think the 12at7 is a little weak for the job.
Shoog
Shoog said:
I can't get custom wound toroidals. I can get an EI core wound for a PP OPT.
In lieu of the 12at7, what would be a nice driver? Or should I go for a two stage driver?
Thanks.
Hi,
I have a suggestion. How about a cascode differential driver? I would thinking of using 6922, 6N1P or 5687 with 300 to 350V B+.
http://www.glass-ware.com/tubecad/DF3.gif
I have a suggestion. How about a cascode differential driver? I would thinking of using 6922, 6N1P or 5687 with 300 to 350V B+.
http://www.glass-ware.com/tubecad/DF3.gif
alexg said:
I used the differential cascode for my last project. In that case, I used 6BQ7As with a CCS as an active tail load. It works just great: enough gain to support gNFB without having to add a second gain stage. Low distortion as well.
However, you have to be very careful with the layout since this is a high gain design, and is more susceptable to going unstable. That means make certain you keep all leads as short as possible, just as if you were doing an RF amp. Also, that glass ware schemo is missing a bypass cacacitor across R2. Since the upper triode is operating as a grounded grid, you need to make the grid-to-ground impedance as low as possible, otherwise, it'll oscillate.
Hi,
Here's another ultra-linear cascode driver.
http://www.tubecad.com/january2000/page5.html
Would it be good enough to drive the parafeed SE 6080 output stage? For the driver tubes, I have 6DJ8, 6822, 6FQ7 and 6N6P.
Here's another ultra-linear cascode driver.
http://www.tubecad.com/january2000/page5.html
Would it be good enough to drive the parafeed SE 6080 output stage? For the driver tubes, I have 6DJ8, 6822, 6FQ7 and 6N6P.
I would say that either of the two schemes would be more than suitable for a high voltage low current design using the 6080.
However if you decided to follow the approach I took, as in a low voltage high current, then a different type of driver is needed. Somewthing with a medium Mu and higher current capability. The ECC88 family would be a good start (though I prefer to run these at below 8mA for long lifes sake) or the 5687 makes an exccellent candidate. Another good option for a two tube stereo parafeed design might be the 6H30 which is both meaty and good.
Shoog
However if you decided to follow the approach I took, as in a low voltage high current, then a different type of driver is needed. Somewthing with a medium Mu and higher current capability. The ECC88 family would be a good start (though I prefer to run these at below 8mA for long lifes sake) or the 5687 makes an exccellent candidate. Another good option for a two tube stereo parafeed design might be the 6H30 which is both meaty and good.
Shoog
Shoog said:
I did try your approach, 160V B+, 100V on the plate of the 6080, biased at 31V, 90ma through the tube. Am still using my 12at7 driver (with plate loaded with a choke).
Much better sound.
I don't have a 5687 nor a 6H30, but I will try to get them and use them.
Shoog, thanks for the low voltage, high current tip on the 6080. Makes a whole lot of difference.
Thanks.
Shoog said:
Thanks Shoog.
There is just one thing though, the power is a bit lower now. I will try to increase current across the tube.
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