With this amplifier 8 ohms seems to work fine in most rooms if the efficiency is 90db 1/watt/1 meter or better.
Keep looking- but I understand if you don't like OTLs. This one has proven to be one of the most reliable ever made- if it were not, we would not have become the oldest OTL manufacturer in the world. You don't stay in business long if you have reliability problems.
Actually we used to run separate heaters for the driver. We found though over the years that we could get lower noise by combining the driver tubes and power tubes on the same filament circuit and there was no downside. The filament circuit does not have to be biased either. If we were running miniature tubes we would not be able to do this, but we are running 6SN7s and apparently there are no worries (we've been using 6SN7s since 1996).
The output tubes will last over 10,000 hours in the right installation. If used to drive 4 ohms they won't last so long as about 50% of the output power is dissipated in the tube itself (the amp will run cooler and draw less power when driving a higher impedance like 16 ohms). The longest lifespan we have seen without a single failure has been over 22,000 hours. The fuse for the B+ transformer for the outputs has proven to be effective speaker protection over the decades. We would never have been able to stay in business if the amp damaged speakers!
FWIW We keep a log of speakers reported damaged- in the last 32 years since the log was started, we have documented 2 cases where the amplifier legitimately damaged the speaker (the others being cases where the user installed alternate power tubes 😱). In those two cases the speaker only handled about 20 watts (and was being used with a 60-watt amp
). In such cases a series fuse is recommended.
Keep looking- but I understand if you don't like OTLs. This one has proven to be one of the most reliable ever made- if it were not, we would not have become the oldest OTL manufacturer in the world. You don't stay in business long if you have reliability problems.
Actually we used to run separate heaters for the driver. We found though over the years that we could get lower noise by combining the driver tubes and power tubes on the same filament circuit and there was no downside. The filament circuit does not have to be biased either. If we were running miniature tubes we would not be able to do this, but we are running 6SN7s and apparently there are no worries (we've been using 6SN7s since 1996).
The output tubes will last over 10,000 hours in the right installation. If used to drive 4 ohms they won't last so long as about 50% of the output power is dissipated in the tube itself (the amp will run cooler and draw less power when driving a higher impedance like 16 ohms). The longest lifespan we have seen without a single failure has been over 22,000 hours. The fuse for the B+ transformer for the outputs has proven to be effective speaker protection over the decades. We would never have been able to stay in business if the amp damaged speakers!
FWIW We keep a log of speakers reported damaged- in the last 32 years since the log was started, we have documented 2 cases where the amplifier legitimately damaged the speaker (the others being cases where the user installed alternate power tubes 😱). In those two cases the speaker only handled about 20 watts (and was being used with a 60-watt amp
). In such cases a series fuse is recommended.
Yes, that's higher impedance and efficiency than what I currently use.😀 My next speaker project will be different in that respect (if I can clear enough time to do it!).
The thing to remember with any tube amp is that tube power is expensive. So efficiency and the ease of drive pays off in spades. There really isn't any point in making any amplifier work hard, not if high quality sound is your goal anyway...
like current peeks of 500mA in tubes rated 125mA max. 
Perhaps your amp is more reliable than I think, after more than 40 years of repairing all kinds of electronic stuff, one gets the impression that not breaking down is the exeption 😱
Never the less this amp is not for me
Mona
The tube is rated for that but in service never sees anything like that!
I started out in the service industry as well in 1974. That is how I put myself through engineering school and also funded the creation of my company. One gets to know pretty quick when something is meant to be serviced and when it is not. Perhaps not so ironically, I find that the stuff that is meant to be serviced is usually a lot more reliable than that which is not.
The M-60 was (the first ones were built in 1990) and is built to hold up well and not require a lot of service (it is also designed to be easy to service should it need it). So the reliability issues really come down to the tubes and as long as the amp is not subjected to severe loads (or an air-tight cabinet...). They hold up so well that we have been able to put a one-year warranty on the power tubes- 4x longer than most manufacturers.
We had an additional problem many years ago: At that time the Futterman amplifier was really the only commercially available OTL in the US. It was not made by Futterman at the time, and by all accounts without the master's touch was quite failure prone. So when we appeared on the scene people were already convinced that 'if its an OTL it blows up'. We had to fight that legacy for 20 years or more. As a result we knew that the amps had to be more reliable than other tube amps and they are. They are quite stable too; at audio shows I like to demonstrate how you can pull power tubes out of the amp while it is playing, and the amp acts like nothing happened at all. I have swapped power tubes from one channel to the other while both channels were playing music. In addition I demonstrate that the amp just sits there if you short out the speaker connections, and that it acts like nothing had happened at all when you remove the short. A lot of amps will not take that kind of tampering and abuse.
IOW reliability is much better than you seem to know. It is why we are the oldest and also the largest OTL manufacturer worldwide.
I started out in the service industry as well in 1974. That is how I put myself through engineering school and also funded the creation of my company. One gets to know pretty quick when something is meant to be serviced and when it is not. Perhaps not so ironically, I find that the stuff that is meant to be serviced is usually a lot more reliable than that which is not.
The M-60 was (the first ones were built in 1990) and is built to hold up well and not require a lot of service (it is also designed to be easy to service should it need it). So the reliability issues really come down to the tubes and as long as the amp is not subjected to severe loads (or an air-tight cabinet...). They hold up so well that we have been able to put a one-year warranty on the power tubes- 4x longer than most manufacturers.
We had an additional problem many years ago: At that time the Futterman amplifier was really the only commercially available OTL in the US. It was not made by Futterman at the time, and by all accounts without the master's touch was quite failure prone. So when we appeared on the scene people were already convinced that 'if its an OTL it blows up'. We had to fight that legacy for 20 years or more. As a result we knew that the amps had to be more reliable than other tube amps and they are. They are quite stable too; at audio shows I like to demonstrate how you can pull power tubes out of the amp while it is playing, and the amp acts like nothing happened at all. I have swapped power tubes from one channel to the other while both channels were playing music. In addition I demonstrate that the amp just sits there if you short out the speaker connections, and that it acts like nothing had happened at all when you remove the short. A lot of amps will not take that kind of tampering and abuse.
IOW reliability is much better than you seem to know. It is why we are the oldest and also the largest OTL manufacturer worldwide.
This site seems to be populated with people who want *tons* of power, 300 to 500 watts of crappy raw power for PA use in their livingroom.
I have 75 watts per channel and it barely goes to 10w peak when it's very loud.
Quality over Quantity
I have 75 watts per channel and it barely goes to 10w peak when it's very loud.
Quality over Quantity
I'm not sure what I'm doing wrong then... I can easily drive the Citation into clipping when I am jamming out. My portable 6v6 SE can't barely even energize the Realistic Mach One's in a way thats enjoyable. If 10 watts is all you need then why am I clipping with 75? What am I doing wrong here?
Your speakers with 15" bass driver and a horn HF/mid driver, is what he means by PA equipment, in a living room I guess.
I have read somewhere that the Mach One has a sensitivity of just 90dB/W/M, perhaps that's why you can clip the Citation on high playback levels. How large is your listening space and how far are you from the speakers?
LOL
If hearing AND feeling is what he is implying, then +1 for it. I like my sound big - else I'd have book shelf speakers. I can't help that I love how they sound
I really like that M-60 design, although it does use quite a few tubes. I think I would want to retrofit mine to use the 2C52 and possibly do away with one of the gain stages, and maybe a 12SL7 for phase inversion. (not to mention I like the sound of them)
Looks like I've got some engineering to do
Could somebody please explain the highlighted part of the schematic?
If hearing AND feeling is what he is implying, then +1 for it. I like my sound big - else I'd have book shelf speakers. I can't help that I love how they sound
I really like that M-60 design, although it does use quite a few tubes. I think I would want to retrofit mine to use the 2C52 and possibly do away with one of the gain stages, and maybe a 12SL7 for phase inversion. (not to mention I like the sound of them)
Looks like I've got some engineering to do
Could somebody please explain the highlighted part of the schematic?
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V3 is CCS ( constant current source ) for upper cacsoded differential voltage gain stage V1+V2 .
10-4 banat thank you... with this being out of the audio signal path, would it be a huge loss to use solid state for this duty?
I'm about 11ft from each speaker when at the focal point, room is 14x19, speakers being on the corners of the long wall.
I'm about 11ft from each speaker when at the focal point, room is 14x19, speakers being on the corners of the long wall.
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Theoretically some SS CCS can be used instead of tube CCS without of any drawback ,
but personally I think that this excellent design deserve to be executed exactly as is in the original Atmasphere schematic, only with tube CCS .
7-3
but personally I think that this excellent design deserve to be executed exactly as is in the original Atmasphere schematic, only with tube CCS .
7-3
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I do love this design but I want to build it with these big fat triodes! I also like to adapt things so I can use what I have on hand. I don't have 4 SL7 tubes on hand but I have boxes of 2C52's and 4 of these 6336A's
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2c52 characteristic are much closer to 12sl7 , and is Not suitable for this M60 design ,
on the other hand Russian(6H8C) or Chinese 6sn7 are pretty cheap and available tubes ,
those 6336A power tubes can be used in M60 OTL, two tube for each amp instead of eight 6as7g , but with nominal 32 ohm LS-Z for best result I think .
More precise answers about 6336A application characteristic for OTL Circlotron you can get from Ralph ( Atmasphere ) I`m sure .
on the other hand Russian(6H8C) or Chinese 6sn7 are pretty cheap and available tubes ,
those 6336A power tubes can be used in M60 OTL, two tube for each amp instead of eight 6as7g , but with nominal 32 ohm LS-Z for best result I think .
More precise answers about 6336A application characteristic for OTL Circlotron you can get from Ralph ( Atmasphere ) I`m sure .
Generally speaking you can use those 12au7 for voltage gain stage (VAS ) which consist from V1+V2+V3 ,since they have pretty close characteristics as 6sn7 but Not exactly the same .
but for DC coupled driver stage (V4) you need there on the first place 6sn7 ,than 5692 ,or eventually 6cg7 or 12bh7 tube .
but for DC coupled driver stage (V4) you need there on the first place 6sn7 ,than 5692 ,or eventually 6cg7 or 12bh7 tube .
Klausmobile Mammoth I - An Atma-Sphere (r) Inspired OTL Amplifier
"replaced stock followers with Allen Wright constant current, constant voltage stage as Paul Speltz recommended"
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