Hello fellow audiophiles...
As much as I am enjoying the Citation II and Fisher 400C combo, I feel that it is time for me to move up the chain now. It's been an awesome two years with them, and still, they sound better than anything else I've heard, save some DHT rigs I've been introduced to.
I recently gained access to some 6336A twin power triodes, some BEEFY power transformers for the 5a heaters, all the signal tubes I need, etc. Now its time to start the planning phase.
I will be the first to admit, I am addicted to the raw horsepower of the Citation II. It drives these Realistic Mach One's with ease and clarity. I can't imagine, at this point in my life, building a low power high sensitivity rig. I was stunted on the OTL idea (seeing about 25w max on a reasonable unit) until I came to realize that higher impedance results in higher power with OTL.
So here is the introduction to the plan:
The OTL itself I would like to build as inverse futterman, and all triode. I have found several plans floating around the web, they just need to be adapted a little to accept a couple 6336A's instead of the 6080/6082 or 6c33c.
The speakers is where everything goes out the window. To better the OTL's performance, I intend to modify a set of speakers to give me either a 32 or 64ohm load. The intended victim of this retrofit is a pair of Boston Acoustics VR975 towers. (see picture below for explanation)
Your thoughts, or should I get back to studying?
As much as I am enjoying the Citation II and Fisher 400C combo, I feel that it is time for me to move up the chain now. It's been an awesome two years with them, and still, they sound better than anything else I've heard, save some DHT rigs I've been introduced to.
I recently gained access to some 6336A twin power triodes, some BEEFY power transformers for the 5a heaters, all the signal tubes I need, etc. Now its time to start the planning phase.
I will be the first to admit, I am addicted to the raw horsepower of the Citation II. It drives these Realistic Mach One's with ease and clarity. I can't imagine, at this point in my life, building a low power high sensitivity rig. I was stunted on the OTL idea (seeing about 25w max on a reasonable unit) until I came to realize that higher impedance results in higher power with OTL.
So here is the introduction to the plan:
The OTL itself I would like to build as inverse futterman, and all triode. I have found several plans floating around the web, they just need to be adapted a little to accept a couple 6336A's instead of the 6080/6082 or 6c33c.
The speakers is where everything goes out the window. To better the OTL's performance, I intend to modify a set of speakers to give me either a 32 or 64ohm load. The intended victim of this retrofit is a pair of Boston Acoustics VR975 towers. (see picture below for explanation)
Your thoughts, or should I get back to studying?
Attachments
Hi,
You can't series the mids and bass without dire consequences.
FWIW you can't swap a 10"+12"PR to two driven units.
The result will sound dire, nevermind ruining a decent pair of speakers.
rgds, sreten.
I'm no fan of line arrays, but its an obvious way of getting high impedance.
http://www.humblehomemadehifi.com/download/Humble Homemade Hifi_Pipeline_copy.pdf
In theory a minimum of 112 ohm and 192 ohm >150Hz is possible.
You can't series the mids and bass without dire consequences.
FWIW you can't swap a 10"+12"PR to two driven units.
The result will sound dire, nevermind ruining a decent pair of speakers.
rgds, sreten.
I'm no fan of line arrays, but its an obvious way of getting high impedance.
http://www.humblehomemadehifi.com/download/Humble Homemade Hifi_Pipeline_copy.pdf
In theory a minimum of 112 ohm and 192 ohm >150Hz is possible.
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Well--25W Isnt the limit for an OTL!
The amount of power relates to the number of output valves you can (Tolerate the heat of) or have at your disposal.
100W is reasonably easy with a bunch of sweep-tubes for instance.....
I believe Alan Kimmel got around 200W with 8 PL/EL519/40KG6 tubes for instance in his Sweep-Tube Monoblock....
Hans (TubeTVR) I believe gets 100W from a quad of 6C33c-b with total reliability running into 8 ohm loads in his inverted Futterman......
You Dont need to use the 6C33C-B or expensive NOS series-pass Triodes for a good OTL. The original Futtermans used Pentode tubes....
If I was you--I would look at the range of high power sweep-tubes that are available reasonably priced (Read, Cheap) and consider those--Hint--Compactron....
IF you go your route of series-wiring speakers--Make sure they are all the same make/model/type--Or (As I found out) they'll sound horrible.
The amount of power relates to the number of output valves you can (Tolerate the heat of) or have at your disposal.
100W is reasonably easy with a bunch of sweep-tubes for instance.....
I believe Alan Kimmel got around 200W with 8 PL/EL519/40KG6 tubes for instance in his Sweep-Tube Monoblock....
Hans (TubeTVR) I believe gets 100W from a quad of 6C33c-b with total reliability running into 8 ohm loads in his inverted Futterman......
You Dont need to use the 6C33C-B or expensive NOS series-pass Triodes for a good OTL. The original Futtermans used Pentode tubes....
If I was you--I would look at the range of high power sweep-tubes that are available reasonably priced (Read, Cheap) and consider those--Hint--Compactron....
IF you go your route of series-wiring speakers--Make sure they are all the same make/model/type--Or (As I found out) they'll sound horrible.
I agree. Roughly speaking, the power of an OTL is limited by the maximum current the output tubes can pass. This means that to a first approximation, the power output grows as the square of the number of tubes. So a pair of 6C33C give 25W max into 8 ohms, four 6C33C could give 100W, and so on. I bulit a "lite" version of the Alan Kimmel OTL, and although I lost my nerve cranking up the power beyond some point, I saw no sign of it giving out.
Of course, if you increase the impedance of the speaker load, you also might run up against a voltage limitation. With +/- 150V supply rails, which is typical in OTL amplifiers, the absolute maximum power you could achieve, even with an assumed 150V peak voltage to the speaker, is about 175W with a 64 ohm load. This, on the other hand, would easily be achieved, as Alastair alluded to, with a sweep tube OTL driving into 8 ohms, in the style of Alan Kimmel's design.
Chris
Of course, if you increase the impedance of the speaker load, you also might run up against a voltage limitation. With +/- 150V supply rails, which is typical in OTL amplifiers, the absolute maximum power you could achieve, even with an assumed 150V peak voltage to the speaker, is about 175W with a 64 ohm load. This, on the other hand, would easily be achieved, as Alastair alluded to, with a sweep tube OTL driving into 8 ohms, in the style of Alan Kimmel's design.
Chris
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Hi,
And for a fixed current power output increases proportional
to the impedance of the output load, with enough voltage.
rgds, sreten.
And for a fixed current power output increases proportional
to the impedance of the output load, with enough voltage.
rgds, sreten.
Current is not really a problem. A 6C33C can pass 2.5A at peak. Two in parallel will pass 5A. No need to make special high-impedance speaker configrations. How many watts do you want?
Chris
Chris
Well to clear some things up:
The VR975 BA's are free... They all suffer from foam rot. Not ruining a nice pair of speakers, rather repurposing a broken pair of speakers that are headed for a land fill.
I can fix the passive radiator bits by either closing off chambers to the 10" or 12" woofers (separating them), or open up the 10" woofer section and put a 12" in. It is a pin cushion woofer so modification is already needed. I'm a HORRIBLE carpenter, so I really just wanted a good box to start with. The air space around the 4" line drivers is already sealed off from the larger woofers so their Q is volume limited to higher frequencies anyway. If I do have to run a cross over, I can still achieve 32ohms rather than 64.
I want an all triode amp, and I already have a quad of 6336A's. All near matched, and same date codes.
I would love a big, grandiose OTL with about 20 tubes in it, but build time and my lack of knowledge in big/complex tubing would probably end up with a half finished amp that I won't touch again. Want to keep it simple, and break it up into a couple different projects. I can accomplish a couple parallel output tubes, but I believe I would get frustrated trying to put more than 3 output tubes per channel as the complexity really starts to break my spirits down. Also, I live in Florida. Tons of heat would drive my power bill through the roof.
Edit for CNPOPE> I would be happy with about 75w, but more is always better.
The VR975 BA's are free... They all suffer from foam rot. Not ruining a nice pair of speakers, rather repurposing a broken pair of speakers that are headed for a land fill.
I can fix the passive radiator bits by either closing off chambers to the 10" or 12" woofers (separating them), or open up the 10" woofer section and put a 12" in. It is a pin cushion woofer so modification is already needed. I'm a HORRIBLE carpenter, so I really just wanted a good box to start with. The air space around the 4" line drivers is already sealed off from the larger woofers so their Q is volume limited to higher frequencies anyway. If I do have to run a cross over, I can still achieve 32ohms rather than 64.
I want an all triode amp, and I already have a quad of 6336A's. All near matched, and same date codes.
I would love a big, grandiose OTL with about 20 tubes in it, but build time and my lack of knowledge in big/complex tubing would probably end up with a half finished amp that I won't touch again. Want to keep it simple, and break it up into a couple different projects. I can accomplish a couple parallel output tubes, but I believe I would get frustrated trying to put more than 3 output tubes per channel as the complexity really starts to break my spirits down. Also, I live in Florida. Tons of heat would drive my power bill through the roof.
Edit for CNPOPE> I would be happy with about 75w, but more is always better.
Hi,
Refoaming all the drivers is the way to go.
Repurposing is far more complicated than you think.
rgds, sreten.
I'm not familiar with the 6336A, and how far the plate current can be pushed beyond the nominal maximum given in the data sheet. In the case of the 6633C, it can pass up to about 2.5A with a cathode-plate voltage of about 130V, which is what is needed for 25W rms into 8 ohms with +/- 150V supply rails (130 = 150 minus the 20 V dropped across the 8 ohm load at the peak of the waveform). And in my experience, 25W is indeed achievable into 8 ohms. Of course the 2.5A is considerably beyond what is nominally allowable, but it is only reached at the peak of the waveform, and also one does not in practice listen to a steady 25W sinewave for long periods!
If you want 75W into 64 ohms, you'd need about 98V peak, which would be tricky with the usual kind of +/- 150V supplies, I would think. I suppose you could step up the supply voltages.
But, if you weren't constrained by your selection of tubes for the output stage, 75W would be really easy with a fairly modest number of sweep tubes, using a design like Alan Kimmel's. For high-power tube amplifiers, I think OTL is the easy way to do it.
Chris
The Kimmel OTL... it has 20 tubes in it!
I can do inverse futterman with 10! That's half of the tubes! Still, Kimmel won't accomplish an all triode amp.
The 6336A is limited to 400ma max per plate per data sheet. They supposedly replace 2-3 6080 (or equiv) tubes. I would love to stay within the data sheet limitations for prolonged tube life. I also prefer to stay with US made tubes vs Russia, as I have always found the sound to be more preferable with US NOS tubes.
I can do inverse futterman with 10! That's half of the tubes! Still, Kimmel won't accomplish an all triode amp.
The 6336A is limited to 400ma max per plate per data sheet. They supposedly replace 2-3 6080 (or equiv) tubes. I would love to stay within the data sheet limitations for prolonged tube life. I also prefer to stay with US made tubes vs Russia, as I have always found the sound to be more preferable with US NOS tubes.
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I made a "lite" version of the Kimmel OTL, with just 4 tubes (instead of 8 in the full version) in the output stage. I think it is claimed to give about 100W output. Yes, there are quite a few tubes in the preceding stages, there are ways to cut down that number, if one wants. The key point, I think, is that 4 output tubes is all it takes to drive 100W into 8 ohms. It is not difficult to push a lot of power into 8 ohms using tubes, contrary to what is sometimes said.
Chris
Chris
cbox83
This schematic can be easy modified for yours quad of 6336A ,
It is one variation of Sinclair & Peterson Amp. , it is not true OTL , it use autoformer to acomodate on standard 4-16 ohm LS , but this amp can be with very broad BW , 5Hz-100Khz depending from quality of autoformer ,
output power can be in region up to 60 W or even more , depending from values of bipolar supply for OPS , PSU is relative very simple to .
This schematic can be easy modified for yours quad of 6336A ,
It is one variation of Sinclair & Peterson Amp. , it is not true OTL , it use autoformer to acomodate on standard 4-16 ohm LS , but this amp can be with very broad BW , 5Hz-100Khz depending from quality of autoformer ,
output power can be in region up to 60 W or even more , depending from values of bipolar supply for OPS , PSU is relative very simple to .
Attachments
You might want to have a look at Luxman MQ-36 which used 6336A. It is just conventional Futterman with bootstrap, but there are the power supply which you can refer to and power output to drive 50ohms load. In order to output 50W 50 ohms, it needs 100V pk, so 50W =~ (70*70)/(2*50)
Here are some translations for that page, hope it helps.
"So it is a stereo unit, developing 2 x 25W / 16 Ohms, 0.8 Vrms input. The maximum power obtained, given the configuration of the output stage tube 6336A is 2 X40W / 50 ohms. This power was more than sufficient to properly driver speakers high efficiency of the time, and the type electrostatic panels Stax ESS6A, Quad ESL and other KLH 9, also very popular.
Here is the schematic, only one channel is represented:
Consider the diagram: We find entry into the 6267 pentode, better known as the EF86. Nothing to say when it is mounted to any conventional amount: A 140K resistor plate, and a resistive network stabilizing the voltage display contains a series 500K and 2M returning to ground, decoupled by a 0,1μF. In order to obtain the gain of the input stage, in this case of the order of 120 (42 dB) of the cathode 1K resistor is decoupled by a polarized capacitor of 30μF. This can be replaced by an electrolytic capacitor 47μF quality, or by a solid tantalum.
This stage is coupled directly to a phase shifter Schmitt, which is a differential phase shifter. But contrary to what is usually done, it's not a double triode is used, but two pentodes, namely two 6CL6. The 6CL6 is a miniature pentode high slope and high dissipation power (anode dissipation 8,2W and 1,9W screen) that allows it to be loaded with a low resistance, typically 7.5 K, 6K here. These conditions are necessary to attack the 6336A, which are tubes that may be a slight grid current. A set of 4 resistors and said capacitor 50μF adjuster to the upper portion of the phase shifter required for the driver 6336A.
Note in passing that each 6336A 1/2 since we are in a double push-pull, has its own link capacitor, Mylar originally composed of 47 nF in parallel with a 0,22μF followed by 1M for to prevent oscillations. The series resistance of 100 ohms and 4 ohms 3W are intended to avoid any collision, any instability at high frequencies. Also note the original bias circuit with a coil voltage by branch of push pull, the potentiometer 5K adjusting the polar of each branch of the push, which should be in the range of -60V, voltage taken between gate and cathode .
The loop against classical reaction is the rate of around 25 db."
......
"
Critics of OTL amplifiers, arguing that they are killers speaker, and would have done better to learn a little electronics, will have obviously not noticed that in this type of scheme, the pass DC in series in the power tubes, without going through the speaker coil, paired this with or without tubes. As you have noticed, of course, (I expected no less from you
) That there is no middle point on the secondary winding of the power transformer 250V. This is what is called a put at the fictitious mass, to avoid any destruction of the speaker in case of imbalance of power tubes."
Here are some translations for that page, hope it helps.
"So it is a stereo unit, developing 2 x 25W / 16 Ohms, 0.8 Vrms input. The maximum power obtained, given the configuration of the output stage tube 6336A is 2 X40W / 50 ohms. This power was more than sufficient to properly driver speakers high efficiency of the time, and the type electrostatic panels Stax ESS6A, Quad ESL and other KLH 9, also very popular.
Here is the schematic, only one channel is represented:
Consider the diagram: We find entry into the 6267 pentode, better known as the EF86. Nothing to say when it is mounted to any conventional amount: A 140K resistor plate, and a resistive network stabilizing the voltage display contains a series 500K and 2M returning to ground, decoupled by a 0,1μF. In order to obtain the gain of the input stage, in this case of the order of 120 (42 dB) of the cathode 1K resistor is decoupled by a polarized capacitor of 30μF. This can be replaced by an electrolytic capacitor 47μF quality, or by a solid tantalum.
This stage is coupled directly to a phase shifter Schmitt, which is a differential phase shifter. But contrary to what is usually done, it's not a double triode is used, but two pentodes, namely two 6CL6. The 6CL6 is a miniature pentode high slope and high dissipation power (anode dissipation 8,2W and 1,9W screen) that allows it to be loaded with a low resistance, typically 7.5 K, 6K here. These conditions are necessary to attack the 6336A, which are tubes that may be a slight grid current. A set of 4 resistors and said capacitor 50μF adjuster to the upper portion of the phase shifter required for the driver 6336A.
Note in passing that each 6336A 1/2 since we are in a double push-pull, has its own link capacitor, Mylar originally composed of 47 nF in parallel with a 0,22μF followed by 1M for to prevent oscillations. The series resistance of 100 ohms and 4 ohms 3W are intended to avoid any collision, any instability at high frequencies. Also note the original bias circuit with a coil voltage by branch of push pull, the potentiometer 5K adjusting the polar of each branch of the push, which should be in the range of -60V, voltage taken between gate and cathode .
The loop against classical reaction is the rate of around 25 db."
......
"
Critics of OTL amplifiers, arguing that they are killers speaker, and would have done better to learn a little electronics, will have obviously not noticed that in this type of scheme, the pass DC in series in the power tubes, without going through the speaker coil, paired this with or without tubes. As you have noticed, of course, (I expected no less from you
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Koonw> Thank you very much for that share! It was VERY informative! I could easily match a lot of this to triode pre stages. Im never surprised how inventive people can get with circuitry
and yours as well Banet> That schematic is amazingly simple, and by my understanding of OTL so far, that auto former looks easily removable.
and yours as well Banet> That schematic is amazingly simple, and by my understanding of OTL so far, that auto former looks easily removable.
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cbox83
Yes it is pretty simple design , but I will not remove output autoformer from many reason , and yes , input tube can be 1/2 of 6SL7 and phase inverter tube can be 1/2 of 6SN7 .
BTW ,you can check DIY Atmasphere M60 thread, which is pure OTL : http://www.diyaudio.com/forums/tubes-valves/161112-what-tubes-tube-amp.html
Yes it is pretty simple design , but I will not remove output autoformer from many reason , and yes , input tube can be 1/2 of 6SL7 and phase inverter tube can be 1/2 of 6SN7 .
BTW ,you can check DIY Atmasphere M60 thread, which is pure OTL : http://www.diyaudio.com/forums/tubes-valves/161112-what-tubes-tube-amp.html
Hi,
If it were me designing an OTL amp nowadays I wouldn't use 6336As unless I had a few hundreds of them and I was sure they weren't pulls.
Mind you, it's a very good valve but it's never been cheap either.
6080 are cheap however and there are still tons of them floating around. That's what I use in my own OTL amp.
Also, you need to keep in mind these regulator valves are all dual triodes so if you put them in // you'd better cater for a bias pot for every single tube.
Either that or match set out of hundred of them and they'll tend to run away once in use.
From that perspective 6C33s are easier as you'll need les of them. But they sure ain't pretty.
Many years ago I once built a set of Bruce Rozenblitt's original OTL (the one that appeared in Glass Audio in 92 (?) using twice the amount of power tubes and the 6SN7 splitter/driver in parallel.
(I'd use a 6CG7 instead)
It sounded quite good and it's still used by that customer to this very day.
Anyhow, just some thoughts.
Cheers, 😉
If it were me designing an OTL amp nowadays I wouldn't use 6336As unless I had a few hundreds of them and I was sure they weren't pulls.
Mind you, it's a very good valve but it's never been cheap either.
6080 are cheap however and there are still tons of them floating around. That's what I use in my own OTL amp.
Also, you need to keep in mind these regulator valves are all dual triodes so if you put them in // you'd better cater for a bias pot for every single tube.
Either that or match set out of hundred of them and they'll tend to run away once in use.
From that perspective 6C33s are easier as you'll need les of them. But they sure ain't pretty.
Many years ago I once built a set of Bruce Rozenblitt's original OTL (the one that appeared in Glass Audio in 92 (?) using twice the amount of power tubes and the 6SN7 splitter/driver in parallel.
(I'd use a 6CG7 instead)
It sounded quite good and it's still used by that customer to this very day.
Anyhow, just some thoughts.
Cheers, 😉
Hi,
6528 if I could lay my hands on them, sure. It at least has the advantage to have a mu. 😉
The 37 is an aviation version I guess. Like the 6082.
Didn't check really.
Cheers, 😉
6528 if I could lay my hands on them, sure. It at least has the advantage to have a mu. 😉
The 37 is an aviation version I guess. Like the 6082.
Didn't check really.
Cheers, 😉
6337 is what 6336 wants to be when it grows up. 100W anode, much larger envelope and base. I only have 4, alas.
I have a decent stock of 6528, awaiting the day when I make the right speaker for an OTL.
I have a decent stock of 6528, awaiting the day when I make the right speaker for an OTL.
Hi,
No idea this 6227 even existed. Never came across it in any of my valve books.
Pretty impressive stuff tough.
Personally I use 92dB, first order three way with a ribbon tweeter. All 8 Ohm.
No problems to mention unless you'd want to run Jimi Hendrix full blast. That tends to pop a fuse occasionally
Then again, I'm well aware of where corners have been cut. PS isn't all that great, too many corners cut.
And yes, a set of 16 Ohm speakers would help in the bass department but then again I think an OTL should be capable of driving a bog standard 8 Ohm nominal speaker just the same.
I know how to, just too bloody lazy to actually make one. 😀
That said, the sound of these things is truly an ear opener. No OPT comes close IME.
One big veil lifted if you'd ask me.
Cheers, 😉
No idea this 6227 even existed. Never came across it in any of my valve books.
Pretty impressive stuff tough.
Personally I use 92dB, first order three way with a ribbon tweeter. All 8 Ohm.
No problems to mention unless you'd want to run Jimi Hendrix full blast. That tends to pop a fuse occasionally
Then again, I'm well aware of where corners have been cut. PS isn't all that great, too many corners cut.
And yes, a set of 16 Ohm speakers would help in the bass department but then again I think an OTL should be capable of driving a bog standard 8 Ohm nominal speaker just the same.
I know how to, just too bloody lazy to actually make one. 😀
That said, the sound of these things is truly an ear opener. No OPT comes close IME.
One big veil lifted if you'd ask me.
Cheers, 😉
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