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Vacuum Tube OTL power amp!!

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atmasphere

atmasphere

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Joined 2007
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M Gregg said:
I have no connection to this link..

I just find it interesting..shame there is no schematic...:D

http://www.audioclassica.de/files/Dokumente/Test-Image-Hifi.pdf

Regards
M. Gregg
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From the description it sounds like a Futterman spinoff. Interestingly, with 4 power tubes does 30 watts into 8 ohms. You can get 35 watts into 8 ohms with 5 6AS7Gs; obviously the pentode tube does not offer that much more power for the package. This is why I have leaned towards the 6AS7G as it is more linear, easier to obtain and less expensive.
 
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preludefan

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Joined 2011
there is talk of the 6as7 only being made by one or two manufacturers- is this true? and also, if you know, will it continue to be produced in future?




atmasphere said:
From the description it sounds like a Futterman spinoff. Interestingly, with 4 power tubes does 30 watts into 8 ohms. You can get 35 watts into 8 ohms with 5 6AS7Gs; obviously the pentode tube does not offer that much more power for the package. This is why I have leaned towards the 6AS7G as it is more linear, easier to obtain and less expensive.
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Dimitris AR

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Joined 2011
atmasphere said:
OTLs are not more sensitive in particular but Circlotrons can be, depending on how the power supply is built and the application. In an OTL application there is an incentive to keep the series resistances of the power supply rail to a minimum, to make maximum current available. People don't think about this very much, but OTLs have to directly deliver the current that is part of the power that moves the speaker, so they can have a lot in common with transistor amps in that regard.
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I agree with you , as I mention that in the folowing thread post # 118 at 17th March 2012
http://www.diyaudio.com/forums/tubes-valves/204960-new-tim-mellows-otl-project-12.html , and yes there is people who think about this seriously .
 
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Dimitris AR

Member
Joined 2011
atmasphere said:
From the description it sounds like a Futterman spinoff. Interestingly, with 4 power tubes does 30 watts into 8 ohms. You can get 35 watts into 8 ohms with 5 6AS7Gs; obviously the pentode tube does not offer that much more power for the package. This is why I have leaned towards the 6AS7G as it is more linear, easier to obtain and less expensive.
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Maybe it use the pentodes in triode connection , and this dosen't mean that the pentodes are inferior to the triodes that you mentioned before ( 6AS7G ) , IMO if this amp use the output tubes like pentodes then the power will be almost double , and you miss something that in a Futterman amp you can't DC coupling the output tubes with the drivers , like you can do in Circlotron amps .
 
atmasphere

atmasphere

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Dimitris AR said:
Maybe it use the pentodes in triode connection , and this dosen't mean that the pentodes are inferior to the triodes that you mentioned before ( 6AS7G ) , IMO if this amp use the output tubes like pentodes then the power will be almost double , and you miss something that in a Futterman amp you can't DC coupling the output tubes with the drivers , like you can do in Circlotron amps .
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If I'm not mistaken you get more power out of the pentodes wired in triode on account of the output impedance being lower with triodes. There is a real trade-off having to do with pentode power vs output impedance.
 
atmasphere

atmasphere

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If you run a lot of feedback, yes, but I think you will find something quite curious. If you add a lot of feedback, it will not increase the power available into lower impedances. This is why I believe that increasing negative feedback does not actually decrease the output impedance.

IOW the term 'output impedance' as it is used in amplifiers today does not refer to the actual output impedance of the amplifier- it refers to the servo gain combined with the actual output impedance. Its confusing! But this is why I say that triode operation is more advantageous, because if you run the amplifier open-loop (no feedback) then you get the truth of what the actual output impedance is.
 
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Dimitris AR

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Joined 2011
O course NFB will not increase the output power it will only decrease the output impedance having higher damping factor etc ... , I am not talking about this case , but I believe that if we have a little bit higher G2 voltage than the anode voltage then the Plate resistance of the pentode will be lower therfore the output impedance of the amp will be lower too without applying any NFB ( actual output impedance as you called it ) .
 
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banat

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Joined 2009
Hi
My experience with PL519 tube is that specified Philips data is valid but for only very short time of peak cathode current , of few milisecond duration , in real life tube PL519 will blown internal cathode to tube pin link easy and fast , that is actually the weak point of this tube , similar case is with tubes PL509 , EL519 and El509 , regardless to tube manufacture brand .
PL504 tube don`t suffer from this internal over current peak contact link failure , it is pretty rugged tube .
Best Regards for all !
 
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