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Coytee

Jeffrey D. Medwin... OTL question

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Too much nitpicking on parts like transformers and caps on this thread. I have heard many amplifiers and probably right now have more than 20 and the differences in the better ones are subtle and subjective at the most. It is the design of the circuit that is more important and not whether there are coupling caps or transformer. A poorly designed OLT amplifier circuit will not sound good. When you start comparing the best of any design, again the differences are very subtle and subjective. If one kind was superior over the other we would only have one amplifier to buy and everyone would agree, right? Right now all l see on this thread is members disagreeing. So there you are.  

 

Time to go and listen to some music. 

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Dude.  You are in the tube section.  It's what they do.  Enjoy your system and stay out of this den of vipers.

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Thanks, Backfire.  Mine have been described as very esoteric (they probably all are!).  Jud called them  "class A SET" (they may not be SETs). They do sound like class A as well as like the OTLs I have heard.                                                                                                                      

 

Where are you located?  I'd be glad to have someone stop by for a look-see if desired.

 

Below is Jud Barber's writeup of the design of the circuit you wanted.  Do you know of another amp designed like this one?  There are those, like Harry Pearson, who have stated this is one of the best amps they have ever heard:

 

"The Joule Electra VZN-100 is a single ended triode power amplifier that does not need an output transformer.  The output stage consists of asymmetrical pairs of paralleled triodes, three tubes per half, which are driven out of phase and then paralleled resulting in cancellation of the DC component in the output of the amplifier.  This type of design requires very careful layout and attention to the multiple power supplies required for proper operation of this circuit.  The circuit is referred to as a bridge configuration, but the similarity stops there.  The overall circuit design takes advantage of numerous tweaks.

 

The circuit is biased full Class A at 100 watts into 8 ohms.  The design goal was to drive specified speakers with an average input impedance of only 3.2 ohms."  (The K-horn bass driver has 3.2 ohms impedance as I recall.)

 

Other circuit details are provided in narrative form.

 

I doubt that I can answer any questions, but I can refer you to a capable tech who worked with Jud for over a decade.

 

 -- Larry

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11 hours ago, oldtimer said:

Dude.  You are in the tube section.  It's what they do.  Enjoy your system and stay out of this den of vipers.

M'n better than yo'arn.

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14 hours ago, LarryC said:

                                                                                                                                                                                                                                                                                                                                                                                                                                                           

 

Thanks, Backfire.  Mine have been described as very esoteric (they probably all are!).  Jud called them  "class A SET" (they may not be SETs). They do sound like class A as well as like the OTLs I have heard.                                                                                                                      

 

Where are you located?  I'd be glad to have someone stop by for a look-see if desired.

 

Below is Jud Barber's writeup of the design of the circuit you wanted.  Do you know of another amp designed like this one?  There are those, like Harry Pearson, who have stated this is one of the best amps they have ever heard:

 

"The Joule Electra VZN-100 is a single ended triode power amplifier that does not need an output transformer.  The output stage consists of asymmetrical pairs of paralleled triodes, three tubes per half, which are driven out of phase and then paralleled resulting in cancellation of the DC component in the output of the amplifier.  This type of design requires very careful layout and attention to the multiple power supplies required for proper operation of this circuit.  The circuit is referred to as a bridge configuration, but the similarity stops there.  The overall circuit design takes advantage of numerous tweaks.

 

The circuit is biased full Class A at 100 watts into 8 ohms.  The design goal was to drive specified speakers with an average input impedance of only 3.2 ohms."  (The K-horn bass driver has 3.2 ohms impedance as I recall.)

 

Other circuit details are provided in narrative form.

 

I doubt that I can answer any questions, but I can refer you to a capable tech who worked with Jud for over a decade.

 

 -- Larry

 

I hunted around with Google, but I couldn't track down a schematic for the Joule-Electra.  However, as far as I can make out, it is a circlotron.  They can be very good indeed.  I have one that I built a few years ago, using EL509 tubes in the output stage.

 

I am a bit sceptical about the Joule-Electra claim that is is biased for full Class A at 100 watts into 8 ohms, though.  That would mean 5 amps maximum current.  With three tubes per side in the circlotron, that means a maximum of about 1.7 amps per tube.  That is certainly not a problem for the 6C33C, as a maximum current.  However, if it were truly running in class A up to 100 watts, that would mean the quiescent current in each tube would need to be about 0.83 amps.  With a plate supply voltage of probably about 150 volts, that would be about 104 watts tube dissipation, which is well in excess of the rated 60 watts maximum for the 6C33C.  It's not a problem to have the plate dissipation exceed 60W instantaneously, but it is a problem to have 104 watts steady dissipation.  Usually in an OTL, 6C33C tubes are biased for about 200 mA quiescent current.

 

All of which is a long-winded way of saying that I think it is almost certainly running in class AB, and not class A.  At low powers, up to a couple of watts, it will effectively be running in class A, but transitioning to AB at higher powers.  Nothing at all wrong with that, though!  It's just a little bit naughty, I think, when manufacturers make overstated claims about class A operation.  (Joule-Electra would not be the only one!)

 

I really like the circlotron design; a truly symmetrical type of output stage.  I'm sure the Joule-Electras must sound superb! 

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Very interesting, Backfire!  What db output would be generated by a couple of watts into Class A?

 

I don't believe any schematics are available for a Joule OTL, which makes them very difficult to repair.  I use Rich Brckich of Signature Sound in Liverpool, NY, the one who worked with Jud for many years.

 

Yes, they do sound superb.

 

 -- Larry

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47 minutes ago, Backfire said:

 

 

All of which is a long-winded way of saying that I think it is almost certainly running in class AB, and not class A.  At low powers, up to a couple of watts, it will effectively be running in class A, but transitioning to AB at higher powers.  Nothing at all wrong with that, though!  It's just a little bit naughty, I think, when manufacturers make overstated claims about class A operation.  (Joule-Electra would not be the only one!)

 

 


Class AB refers to a specific set of operating conditions which are usually somewhere between class A and class B.  So unless the amp has provision to change that, it is running in AB at idle through its maximum power output. It combines the advantages of both classes while minimizing their respective disadvantages.
 

 

Maynard

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Very interesting and good to hear.  No, no provision to change the class.  What wattage output at 8 ohms do you estimate for mine?  What do you think might be the sound difference between A's and mine?

 

- Larry

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22 minutes ago, tube fanatic said:


Class AB refers to a specific set of operating conditions which are usually somewhere between class A and class B.  So unless the amp has provision to change that, it is running in AB at idle through its maximum power output. It combines the advantages of both classes while minimizing their respective disadvantages.
 

 

Maynard

 

Yes, I was speaking a bit loosely.  People sometimes speak of a class AB amplifier as "operating in class A up to X watts," where X is the maximum power it can put out before the tubes essentially stop conducting at the bottom half of the audio cycle.  It can give a useful characterisation of how much power it takes before the tubes go into cutoff. 

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On 9/11/2020 at 12:50 PM, Coytee said:

Having heard what I heard....  I'd have to conclude that it's more out of group ignorance.  Most might not have heard them themselves.

 

I'd bet you lunch (at a slightly higher grade than McDonalds.....  hmmm.....did I just call myself cheap??!!)

 

Anyway, I'd bet you lunch that if you built one and someone nabbed it.....  it would get good reviews and a second, third might follow.

 

What surprised me was not my wife, not my sister in law, but BOTH of them just stopped cold turkey while eating dinner (which was NOT turkey) and said paraphrased "OMG, what are you listening to in the living room......it sounds SO REAL like the singer is in there with us"

 

The ONLY time I ever (ever ever ever) heard any comments like that is when the OTL's were playing.

 

 

What caused you to sell your four SE-OTLS ??

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On 9/14/2020 at 1:37 PM, Jeffrey D. Medwin said:

 

 

What caused you to sell your four SE-OTLS ??

 

Power or more accurately, lack of.

 

If they would have had say, 30 or more watts, I'd kept them forever.  As it is/was, they just couldn't keep up with how I turned things up (especially while working in other parts of house when system was pounding.....  this was on Khorns)

 

Edited to add:  I only had two amps, not four.

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Coytee, 

 

I know you're not technical, and I don't think you realize this, but likely every single tube amp you have ever owned, and heard at your home, has had negative feedback ( NFB ) in it .  

 

I believe, ( am not 100% sure ) that all the OTLs you have asked me about,  need to use NFB.   The Rosenblit SE-OTL , for sure, uses NFB.

 

The George Wright 2A3 amp didn't need it, but it has NFB, as part of the topology. 

 

A Loftin - White type amp, typically, has zero NFB.   

 

                 *****    With zero NFB, we hear the music, in the proper timing of the music, not in the amp's timing.     ***** 

 

A zero NFB tube amp can be like a breath of fresh, natural, air !  

 

The first dictate, from my musician-audio mentor in 1982, was " Be sure to design the amp so it does not need to use negative feedback ".   The second dictate was, " Be sure the chokes, in a 2A3 amp, are 20 Ohms or less, IF you can find them. " 

 

Since 1983 Coytee, all my tube amps have had zero NFB, and the chokes were all 24 Ohms or less. 

 

The G.W. 3.5 2A3 amp, in addition to having NFB added to it's circuit,  employs  a 195 Ohm Hammond ( 158M ) choke.  That is approximately ten times too high in DCR - from ideal !!!  

 

I have brought up this discussion of NFB and DCR, ( in relation to tube amp design )  to point out to you, and all others, that there exists a big gap, between commercial tube amps typically available and used in homes, and ..........what can be achieved. 

 

A Hammond 159ZA choke, an available part for about two decades, is 6 Ohms in DCR.  Employed in a double pi series B+ filter ( L1/C1/L2/C2 ),  there exists a total of only 12 Ohms of inductor resistances -  to power the output stage.  After about 75 hours, ( to break them in ), it does a wonderful job, particularly in delineating the bottom-end of a well recorded grand piano.

 

Jeff

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1 hour ago, CECAA850 said:

Haven't read yet, I will. @mikebse2a3 is very knowledgeable on this subject and messed around with it a lot, he has a clear preference (cannot recall what), but to the point that if someone can tell him what their preference is he can advise/warn a particular amp may not be to their likeing because of that one parameter alone.

 

I assume that article is going to tell me it's a preference thing, not a signal compromise. I'm going to also guess the fantasy part come is because of how things are recorded, and so you are fooling yourself if you think there is some advantage to having, or not having it, for sound reproduction at home.

 

 

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1 minute ago, dwilawyer said:

I assume that article is going to tell me it's a preference thing

Then we'll wait for you to read it.  It's informative and not that long.  They didn't use too many big words that I didn't understand.

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I'm an electronics tech with 40+ years manufacturing/ test engineering experience.(oh and I enjoy Klipsch horn loaded speakers). The way I understand it is All AMPLIFIERS have some degenerative element(s) otherwise they would have too much gain. I know with simple tube amplifiers it is possible to control the gain with cathode or grid degeneration and no output to input global feedback. But with high gain operational amplifiers this is not possible. The only available method to control the amplifier is NFB. Without it the only circuits you could make would be comparators or oscillators.

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For your information, I hope some enjoy this person, a perspective, and that some get something out of it .   

This was taken from Sound Practices Magazine, Joe Roberts editor, in 1992,  from Joe's very first issue:                    

 

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                       cent genius,gave us the vibrations on foil.      ty.  The minute we did that we were re-

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Note please Robert Fulton was a musician.  He played piano, coronet and trumpet, and designed trumpet mouthpieces, by ear.  Doc Severinsen used Robert's mouthpieces on his instruments.           https://en.wikipedia.org/wiki/Doc_Severinsen

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4 hours ago, CECAA850 said:

Then we'll wait for you to read it.  It's informative and not that long.  They didn't use too many big words that I didn't understand.

Great article, I was totally wrong on what I thought it would say.

 

Explains NFB very well, and also problems without if not compensated for correctly. 

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