Vintage Tube Amp Restoration Walk Through

[jorjen]

 

Marty, I wasn't really accusing you of shilling, honest. So I apologize for that - I didn't expect the comment to be taken seriously.

I personally have no problem with a credible company coming out here and showcasing their work.

Most of the units in their store are very reasonably priced - at least, I think they are. They're beautiful, and if they back their work - this is good for anyone here looking for a turn-key tube solution.

Signed,

Cranky

 

 

and you know they are credible how? They appeared out of the clear blue sky less then a week ago.... there is ZERO customer comments anywhere to be found on the internet... The domain name was filed for about 8 months ago and the website went live just recently...They maybe the cats meow but honestly folks we have all been down this road before tread lightly and do not believe everything you read just because it has nice icing on it...the cake could be sour.

 

    Many of the comments in his reply to Marty are flat out ludicrous....any seasoned vintage technician will agree with me 100%...messing around inside a vintage piece of iron without just cause is asking for disaster... anyone that doesn't believe me just let me know I'll ship you a few to tear apart and you can see for yourself what they look and feel like on the inside after 50 years of hot/cold cycles and poor storage conditions! I'll even pay the shipping.... If left alone they are just fine.... if disturbed you can easily destroy them. Can you tear apart a transformer and replace the leads....absolutely....should you is an entirely different matter.

 

  I for one would like to see links to all this reference material available on the internet to backup his submersion approach to cleaning. He mentions the Ham Radio crowd...well my father and grandfather were both WW2 era ham radio restorers and they never did anything of the kind without 100% tear down to the raw chassis.

 

 Another absolute here is the Fisher he did in this post was already in darn nice condition and could have been restored to absolute beautiful condition without submersion.... I mean come on folks all it had was a little bit of grime on the chassis around the transformers.....

 

      The parts installed under the chassis on this rebuild cost about $30..... they are not the cheapest available but darn close....

 

And I know you remember how well my 296 cosmetic restoration turned out Craig. I am just some Joe Blow off the street and got better cosmetic results on a worse looking amp without the use of submersion or Ultrasonics.

[AEA Audio]

I thought it might be good to walk through the whole recording and playback process to get a better understanding of the differences between analog and digital recording sources, as well as understand if analog recordings are better for tube amps.

 

As we all know, acoustic music sources (those not electronically synthesized) start out as sound waves. These are then converted to electrical signals (usually with a mic) so they can be amplified and processed (mixed, equalized, digitized etc.). These original acoustic sound waves are then changed yet again to another medium so they can be recorded, copied and distributed in large numbers. 

 

The available recording mediums have expanded over the years, from vinyl to tape to digital. But the objective is always the same: to convert the original analog electrical signals into some form that can be stored, copied, and then converted back into something close to the original electrical signal, allowing us amplify them so they can be converted back into sound waves. The point is that it’s always analog voltage in— analog voltage out, no matter what the recording medium.

 

Of course there is a lot of difference between how these different storage methods work, but they are all used to recreate an analog signal that is (hopefully) very close to the original analog signal from the mic(s). The difference between recording methods, and their merits, is another subject, but you always end up with an analog signal. There is no inherent difference between vinyl and a FLAC file that would favor either tube or SS for playback. No matter if they were recorded on an analog board or a digital board, they all start with analog voltage from a mic and end up with an analog voltage going into your amp.

 

That brings us the question of what the difference between tube and SS amps is, and why they sound different. I don’t want to open that debate too wide, but I do want to say that the difference is irrespective of the recording format itself. The difference in sound between tube amps and SS amps is something I have spent a lot of time analyzing, and so have quite a few other people. The short answer is that it’s not frequency response, or how much distortion they produce,  or what source material works best. The difference is mostly in how they produce distortion, and how they interact with speakers. They both do these things in highly different ways. 

 

SS amplifiers did not replace tubes because they sound better. It was economics. SS gear is a lot cheaper to make, especially on a watt for watt basis. SS is also more user friendly (no tubes to replace, lighter, smaller and less heat).

 

Tubes don’t “color” the audio any more than SS does. Neither type of amplifier is perfect. A good design engineer can make a great sounding amp with either. However, having designed both, I will say it is generally easier to make a good sounding amp with tubes. On the other hand, it’s also easier to make a good sounding high power amp with transistors…  

 

These technical differences between SS and tube amps are at the heart of why vintage tube amps represent such outstanding performance for their relatively low cost. No manufacturer today can build a SS or tube amp that has the quality of components and sound that a properly restored high-end vintage tube amp has for anywhere near what they are going for. (Which is why vintage tube amps are steadily going up in price as more people realize they have a huge bang-for-the-buck advantage.)

[Tarheel]

Thanks for your post.  One question....when wholesale parts swaps are implemented during a repair or updating isn't much of the "vintage" sound lost.  Are you saying restoration means all the "old" parts are rebuilt to OE specs?

[tube fanatic]

Thanks for your post.  One question....when wholesale parts swaps are implemented during a repair or updating isn't much of the "vintage" sound lost.  Are you saying restoration means all the "old" parts are rebuilt to OE specs?

As someone who has spent decades repairing/restoring vintage gear (including when it wasn't vintage!), I'll give you my thoughts while you're waiting for Ken to get back on here.  The sound of the vintage equipment has more to do with the circuit parameters (tube operating conditions), and the quality of the output transformers in amps and receivers, than anything else.  Replacing the original capacitors, for example, with modern units is not going to prevent the equipment from delivering sound which is comparable to its factory state (keep in mind that others will not necessarily agree with my philosophy and that their premise may be just as valid).  Same for the original carbon resistors found in the vintage gear.  Replacing them, if found to be out of spec, with a modern metal film resistor is not going to change the sound.  Generally, the weak link in vintage equipment is capacitors with most needing replacement.  Following this is out of spec resistors which can cause the tubes to operate under different conditions from those originally intended.  That can certainly affect sound in a negative manner.  It's not possible to rebuild either component, although some restorers, in an effort to preserve the original look as closely as possible, will remove the "guts" of old capacitors and install modern replacements within the shell or case!  I encounter less of a call for that among users of audio equipment than old radios, and in either case if that's what they want, I tell them to look elsewhere!  That's a bit more than I want to handle in my "old" age.  I look forward to reading Ken's view on this as well.

And, Ken, I don't know if you saw my post in which I quoted Aaron Newman's comment on SS vs. tube amplifiers.  He really floored me with that as his LA-550 amp is considered one of the finest sounding tube amps ever made (at full output, its distortion was ridiculously low, thanks in part to its custom made output xfmrs).

Maynard

[AEA Audio]

Thanks for your post.  One question....when wholesale parts swaps are implemented during a repair or updating isn't much of the "vintage" sound lost.  Are you saying restoration means all the "old" parts are rebuilt to OE specs?

 

If it were possible to use the old original caps, and get the original sound quality (and reasonable reliability) from a vintage amp, then I would recommend leaving the caps (and other components) in place.  However, most (but not all) types of caps used back then have a service life (and/or shelf life) less than the 40 to 60+ years since they were manufactured. So, leaving them in place does not necessarily preserve the original sound at all. In actuality, the original sound quality has been degraded by component aging and by tarnish (I think of tarnish as ‘metal aging’). 

 

 

Certainly all electrolytics, and almost all film types, should be replaced due to their performance being quite different now than what it was when they were new. It’s not just that they become leaky, they also change a lot in the way they perform. That’s because their “characteristic operating parameters” also change over time. An example in the case of a capacitor is that even a new cap’s ESR and effective capacitance reactance changes with both frequency and with applied voltage. These operating parameters are taken into consideration in the original design, but after so many years these operating curves have changed quite a bit, causing the amp’s sonic qualities to change, even if the old cap checks good on a simple cap meter and shows no leakage. All out of tolerance components, including caps, resistors and tubes, etc. need to be changed for these same reasons: They can no longer function as they were originally designed to do.

 

That’s why I think a restoration needs to include replacing all components that have exceeded their operational lifespan (as well as gone out of compliance). Unless you want to restore an old amp using all original vintage components for “historical reference,” you end up listening to something that has less sound quality than it originally had.

 

As for which is better, tube to transistor, my experience is this: neither. They are different, tube distortion is “better tolerated” by the listener, but SS has a lot of other technical advantages that can be used to overcome that. However, the sad fact is that most SS gear (not all) that is aimed at the "mass market" is designed solely based on competitive profitability (price vs specs). And that’s why most (not all) of the mass market SS gear does not sound all that good (as we all know). I bring this up because the ubiquitous presence of SS gear tends to color our opinion of it.

 

 

In my experience looking into this “difference,” I have concluded that tubes don’t color sound any more than SS does. They both do it to some extent, but they do it with different "pallets." And to continue on that metaphor: A really good “artist” can design an amp that is quite transparent…  with either pallet.

[Ski Bum]

The difference is mostly in how they produce distortion, and how they interact with speakers.

 

QFT.  Key aspect: tube amps driving transducers.  The folks using tube buffers or line stages expressly in pursuit of "tube sound" should take note.  

[Tarheel]

Thanks AEA.....that is helpful. 

[AEA Audio]

 

The difference is mostly in how they produce distortion, and how they interact with speakers.

 

QFT.  Key aspect: tube amps driving transducers.  The folks using tube buffers or line stages expressly in pursuit of "tube sound" should take note.  

 

 

I would tend to agree. Just having a tube in the buffer or line stage would, in most cases, not make a great deal of a difference. It’s the driver and output stage where most of the difference between tube and SS is heard, and also some of that difference in sound is from the fundamentally different way in which they interact with the transducer (speaker).

[mikebse2a3]

Ken since there has been some discussion about Tube and SS amplifiers sound characteristics I would be very interested to read your thoughts on the "Reverb Effect" that Bob Carver and some others are claiming for the sound character produced by some types of amplifiers with high output impedance. Have you ever measured this effect as described by Bob Carver?

 

 

 

I have to state that I have very serious doubts about the claims of the "reverb effect" being audible and to date I haven't seen any actual Test Data that would make me believe it reaches audible levels..!!! 

 

I would also point out based on my understanding of the proposed "reverb effect" that Bob Carver claims to occur with high output impedance amplifiers the amplifier would also have to be designed with feedback regardless of output impedance before the amplifier could produce this effect and since Single Ended Tube amplifiers often are designed with no global feedback I don't see how the claimed "Reverb Effect" could be used to explain their sound qualities as some have claimed.

 

If anyone has actual test data showing the reverb effect I would very much appreciate them showing or linking to it please.

 

 

Thanks,

miketn

[tube fanatic]

Ken since there has been some discussion about Tube and SS amplifiers sound characteristics I would be very interested to read your thoughts on the "Reverb Effect" that Bob Carver and some others are claiming for the sound character produced by some types of amplifiers with high output impedance. Have you ever measured this effect as described by Bob Carver?

 

 

 

I have to state that I have very serious doubts about the claims of the "reverb effect" being audible and to date I haven't seen any actual Test Data that would make me believe it reaches audible levels..!!! 

 

I would also point out based on my understanding of the proposed "reverb effect" that Bob Carver claims to occur with high output impedance amplifiers the amplifier would also have to be designed with feedback regardless of output impedance before the amplifier could produce this effect and since Single Ended Tube amplifiers often are designed with no global feedback I don't see how the claimed "Reverb Effect" could be used to explain their sound qualities as some have claimed.

 

If anyone has actual test data showing the reverb effect I would very much appreciate them showing or linking to it please.

 

 

Thanks,

miketn

This is an interesting concept.  On occasion I've experienced a reverb, or echo-like, effect in some amplifiers which wrap the feedback loop from the opt secondary to the voltage amp cathode.  When removing the global loop and replacing it with local feedback, the effect disappears making me believe that it's caused by phase distortion.  Paradoxically though, in my amps which use frequency selective local feedback (from a typical series RC filter), there is no audible effect from the unquestionable phase distortion which is present.  I look forward to Ken's thoughts on this.

Maynard

[AEA Audio]

 

Ken since there has been some discussion about Tube and SS amplifiers sound characteristics I would be very interested to read your thoughts on the "Reverb Effect" that Bob Carver and some others are claiming for the sound character produced by some types of amplifiers with high output impedance. Have you ever measured this effect as described by Bob Carver?

 

 

 

I have to state that I have very serious doubts about the claims of the "reverb effect" being audible and to date I haven't seen any actual Test Data that would make me believe it reaches audible levels..!!! 

 

I would also point out based on my understanding of the proposed "reverb effect" that Bob Carver claims to occur with high output impedance amplifiers the amplifier would also have to be designed with feedback regardless of output impedance before the amplifier could produce this effect and since Single Ended Tube amplifiers often are designed with no global feedback I don't see how the claimed "Reverb Effect" could be used to explain their sound qualities as some have claimed.

 

If anyone has actual test data showing the reverb effect I would very much appreciate them showing or linking to it please.

 

 

Thanks,

miketn

This is an interesting concept.  On occasion I've experienced a reverb, or echo-like, effect in some amplifiers which wrap the feedback loop from the opt secondary to the voltage amp cathode.  When removing the global loop and replacing it with local feedback, the effect disappears making me believe that it's caused by phase distortion.  Paradoxically though, in my amps which use frequency selective local feedback (from a typical series RC filter), there is no audible effect from the unquestionable phase distortion which is present.  I look forward to Ken's thoughts on this.

Maynard

 

 

Sorry to have taken so long in responding.

 

I believe the ”reverb effect” Bob Carver is talking about is caused by what is commonly known as “damping factor.” I’ve known Bob since the Phase Linear days, and he has a way of using terms to describe technical things that can be a bit confusing…( I won’t say misleading ). Its part salesmanship and part trying to use “layman” terms to make things more understandable by the average customer.

 

The concept of using a speaker as a microphone to capture some “echo” while it is playing music cannot be done, for many reasons. The biggest reason is that the sensitivity of the speaker (ability to pick up sound) is almost zero when the cone is extended under force from the voice coil. Think of the effect of putting your hand firmly on the cone while trying to use the speaker as a mic. The pressure will keep the cone from vibrating, so it can’t pick up sound. The same thing happens when the cone is under force (being  pushed) by the voice coil, causing any ability to pick up ambient sound to be greatly dampened.

 

What Bob is actually talking about is the effects of damping factor, and more specifically, the ability of feedback to affect an amp’s output impedance, and therefor damping factor. One way to think of the 

effects of damping factor is its effect on controlling speaker cone “rebound.” Bob used the term “echo effect” to describe the effects of damping factor. The ability of his amp to respond to and control this “rebound” voltage with increased damping factor through the use of feedback is what he was touting. Bob is a very creative and innovative design engineer, and an equally creative salesman.

[Frzninvt]

Sounds similar to the designs of Louis W. Erath (LWE speaker fame) that utilized an amplifier's negative feedback circuit to enable better and more precise control of the woofer.  He could place a 15" woofer in a cabinet no larger than a Heresy and get it to reproduce solidly to 20Hz.  I am surprised this technology never really took off.  I had a pair of his LWE-1's but never had an amplifier with a clip kit or the custom amps he made.  There was a CM Labs 911 amplifier that came with the cinch-jones connectors so that you could plug them directly into the LWE line of speakers. 

 

In car audio an amplifier's damping factor is extremely important as it was an indicator of how quickly the amplifier could start and stop the woofer.  Important when listening to bass heavy tracks like those with dual kick drums so the woofers would not stumble over themselves and sound sloppy.

 

Shame he passed away a number of years ago.

 

http://www.lweloudspeakers.com/index.html

 

http://www.audioreview.com/cat/speakers/floorstanding-speakers/lwe/1/prd_119983_1594crx.aspx

Edited May 20, 2015 by Frzninvt

[mikebse2a3]

Sorry to have taken so long in responding. I believe the ”reverb effect” Bob Carver is talking about is caused by what is commonly known as “damping factor.” I’ve known Bob since the Phase Linear days, and he has a way of using terms to describe technical things that can be a bit confusing…( I won’t say misleading ). Its part salesmanship and part trying to use “layman” terms to make things more understandable by the average customer. The concept of using a speaker as a microphone to capture some “echo” while it is playing music cannot be done, for many reasons. The biggest reason is that the sensitivity of the speaker (ability to pick up sound) is almost zero when the cone is extended under force from the voice coil. Think of the effect of putting your hand firmly on the cone while trying to use the speaker as a mic. The pressure will keep the cone from vibrating, so it can’t pick up sound. The same thing happens when the cone is under force (being pushed) by the voice coil, causing any ability to pick up ambient sound to be greatly dampened. What Bob is actually talking about is the effects of damping factor, and more specifically, the ability of feedback to affect an amp’s output impedance, and therefor damping factor. One way to think of the effects of damping factor is its effect on controlling speaker cone “rebound.” Bob used the term “echo effect” to describe the effects of damping factor. The ability of his amp to respond to and control this “rebound” voltage with increased damping factor through the use of feedback is what he was touting. Bob is a very creative and innovative design engineer, and an equally creative salesman.

 

 

 

Well I'm confused by Carver's comments for sure..!!!!

 

 

Thanks Ken for your comments and I agree with your statement "The concept of using a speaker as a microphone to capture some “echo” while it is playing music cannot be done, for many reasons. The biggest reason is that the sensitivity of the speaker (ability to pick up sound) is almost zero when the cone is extended under force from the voice coil."

 

 "he has a way of using terms to describe technical things that can be a bit confusing"  Boy you can say that again  .... I understand damping and also that feedback effects damping but it seems to me Carver is talking about something different with his Speaker-Mic Reverb comments and what he is trying to accomplish with it..

 

What I don't believe is all Tube Amps via "speaker-microphone" pickup and amplify the rooms reverb at a perceptible level let alone that we would perceive it as reverb and an enhancement of the acoustical space.

 

 

I've high lighted in Red and Blue his comments that make me believe he is talking about artificial reverberation enhancement of the recording via the "Speaker-Microphone" reverb effect.

 

 

 

 http://www.enjoythemusic.com/superioraudio/equipment/0512/bob_carver_cherry_180_tube_monoblock_amplifier.htm

 

 

 

Speaker-Microphone Effect

 

It is well known that a loudspeaker can work backwards, converting sound pressure into an electrical signal at its input terminals. Many years ago, it was Roger West who mentioned having used a Sound Lab ESL as a microphone, though I'm not sure with what degree of success. Carver believes that his amps can take advantage of this effect and says that:

 

"The extra cool thing about this amp is that it has the ability to listen to the room via the principal of reciprocity. The loudspeaker the amp is connected to behaves like a microphone and "listens to the room." That signal is then sent back to the input to be mixed with the forward signal to help generate a more delicious sense of acoustic space by including the room in the final sound in a way other amps can't."

 

 

 

I expressed my doubts to Carver about the audibility of this effect, noting that the speaker microphone effect couldn't amount to more than a few millivolts. Carver's response was as follows:

 

"You are right - about 3.1 mV at 90 dB SPL at our listening position, then multiplied by the gain of the amp, 32 X = 0.0992 volts rms. With a high quality speaker about 89dB or so sensitive at 14 feet away we have about 8.5 forward volts at the speaker, and 0.0992 "speaker microphone" volts. Finally, 0.00992/8.5 = -38.6 dB, just barely noticeable, but quite significant after we get used to listening to it."

 

"As low as it is. All tubes amps do this a little bit by their nature, transistor amps don't do it at all, and this amp does it a lot by intentional design. It's easy to hear the "oom signal if we hook up a small speaker on the end of about 150 feet of wire and go outside. Hook it across the main speaker in your living room with the amp turned on, but no input. Then have a friend come into your living room and clap his or her hands, stomp around, make noise and sing happy birthday. While you are 150 feet away outdoors with the door closed so you can't hear your friend from the room, hold the small test speaker in your hand and listen to it. You will hear the sound of the room. With a solid state amp, you will not hear a thing, just silence. All tube amps that have a non-zero output source impedance do it, at least a little bit as you surmised. Transistor amps don't do it at all because the almost zero source impedance shorts out any "speaker microphone" signal. Yes, my amps do it a lot by virtue of the current feedback loop. With this amp, lower idle current (as read on the meter), makes the amp do it more. Higher idle current makes it do it less."

Edited May 21, 2015 by mikebse2a3

[AEA Audio]

 

Sorry to have taken so long in responding. I believe the ”reverb effect” Bob Carver is talking about is caused by what is commonly known as “damping factor.” I’ve known Bob since the Phase Linear days, and he has a way of using terms to describe technical things that can be a bit confusing…( I won’t say misleading ). Its part salesmanship and part trying to use “layman” terms to make things more understandable by the average customer. The concept of using a speaker as a microphone to capture some “echo” while it is playing music cannot be done, for many reasons. The biggest reason is that the sensitivity of the speaker (ability to pick up sound) is almost zero when the cone is extended under force from the voice coil. Think of the effect of putting your hand firmly on the cone while trying to use the speaker as a mic. The pressure will keep the cone from vibrating, so it can’t pick up sound. The same thing happens when the cone is under force (being pushed) by the voice coil, causing any ability to pick up ambient sound to be greatly dampened. What Bob is actually talking about is the effects of damping factor, and more specifically, the ability of feedback to affect an amp’s output impedance, and therefor damping factor. One way to think of the effects of damping factor is its effect on controlling speaker cone “rebound.” Bob used the term “echo effect” to describe the effects of damping factor. The ability of his amp to respond to and control this “rebound” voltage with increased damping factor through the use of feedback is what he was touting. Bob is a very creative and innovative design engineer, and an equally creative salesman.

 

 

 

Well I'm confused by Carver's comments for sure..!!!!

 

 

Thanks Ken for your comments and I agree with your statement "The concept of using a speaker as a microphone to capture some “echo” while it is playing music cannot be done, for many reasons. The biggest reason is that the sensitivity of the speaker (ability to pick up sound) is almost zero when the cone is extended under force from the voice coil."

 

 "he has a way of using terms to describe technical things that can be a bit confusing"  Boy you can say that again  .... I understand damping and also that feedback effects damping but it seems to me Carver is talking about something different with his Speaker-Mic Reverb comments and what he is trying to accomplish with it..

 

What I don't believe is all Tube Amps via "speaker-microphone" pickup and amplify the rooms reverb at a perceptible level let alone that we would perceive it as reverb and an enhancement of the acoustical space.

 

 

I've high lighted in Red and Blue his comments that make me believe he is talking about artificial reverberation enhancement of the recording via the "Speaker-Microphone" reverb effect.

 

 

 

 http://www.enjoythemusic.com/superioraudio/equipment/0512/bob_carver_cherry_180_tube_monoblock_amplifier.htm

 

 

 

Speaker-Microphone Effect

 

It is well known that a loudspeaker can work backwards, converting sound pressure into an electrical signal at its input terminals. Many years ago, it was Roger West who mentioned having used a Sound Lab ESL as a microphone, though I'm not sure with what degree of success. Carver believes that his amps can take advantage of this effect and says that:

 

"The extra cool thing about this amp is that it has the ability to listen to the room via the principal of reciprocity. The loudspeaker the amp is connected to behaves like a microphone and "listens to the room." That signal is then sent back to the input to be mixed with the forward signal to help generate a more delicious sense of acoustic space by including the room in the final sound in a way other amps can't."

 

 

 

I expressed my doubts to Carver about the audibility of this effect, noting that the speaker microphone effect couldn't amount to more than a few millivolts. Carver's response was as follows:

 

"You are right - about 3.1 mV at 90 dB SPL at our listening position, then multiplied by the gain of the amp, 32 X = 0.0992 volts rms. With a high quality speaker about 89dB or so sensitive at 14 feet away we have about 8.5 forward volts at the speaker, and 0.0992 "speaker microphone" volts. Finally, 0.00992/8.5 = -38.6 dB, just barely noticeable, but quite significant after we get used to listening to it."

 

"As low as it is. All tubes amps do this a little bit by their nature, transistor amps don't do it at all, and this amp does it a lot by intentional design. It's easy to hear the "oom signal if we hook up a small speaker on the end of about 150 feet of wire and go outside. Hook it across the main speaker in your living room with the amp turned on, but no input. Then have a friend come into your living room and clap his or her hands, stomp around, make noise and sing happy birthday. While you are 150 feet away outdoors with the door closed so you can't hear your friend from the room, hold the small test speaker in your hand and listen to it. You will hear the sound of the room. With a solid state amp, you will not hear a thing, just silence. All tube amps that have a non-zero output source impedance do it, at least a little bit as you surmised. Transistor amps don't do it at all because the almost zero source impedance shorts out any "speaker microphone" signal. Yes, my amps do it a lot by virtue of the current feedback loop. With this amp, lower idle current (as read on the meter), makes the amp do it more. Higher idle current makes it do it less."

 

 

Yep, I also found it confusing when it was first mentioned in this thread. But after thinking it through, I’m still fairly certain Bob is really just talking about damping factor. The key references in the conversation are when he says, “All tubes amps do this a little bit by their nature…” and “Transistor amps don't do it at all because the almost zero source impedance shorts out any speaker microphone." Bob also says, “Listen… with the amp turned on, but no input…With a solid state amp, you will not hear a thing”  These comments all point to damping factor.

 

The microphone affect he mentions is indeed associated with the effects of damping factor because they are both caused by the same phenomena: That speakers produce their own “voltage/current” as the coil moves through the magnet’s field. This “counter current” is “shorted out” by a SS amp’s low output impedance, and acts like a “break” slowing down, or “damping” this effect, which slightly changes the way the speaker cone moves. You are also correct that feedback can change the amp’s "effective" output impedance, and Bob was famous for using inventive feedback methods to shape the way his amps sound.

[mikebse2a3]

Thanks Ken,

Ken have you heard Bob Carver's explanation of this "Tube Magic" sound effect in this video interview link below? If so please feel free to ignore this post but it is comments like these that are why I believe he is talking about more than just the low damping effect and the resulting amplitude variation tracking the loudspeaker impedance variations that results. He is also claiming a "delayed room reverb effect" via the speaker/microphone that gets amplified by the amplifier via the feedback loop wrapping around the amplifier to it's input (ie: Global Feedback).

Begin listening at 50 minutes into the video for him to describe how the loudspeaker-microphone/amp(tube type) is listening to the room and I believe you will see he is talking about more than just the damping factor effect that one would normally expect.

In Mr. Carver's own words he says: "So what happens in a vacuum tube amplifier is the amplifier makes another sound that's related to the sound that it heard, in other words the amplifier is able to listen to the room because it is hearing reverberation, echoes, time delays, all the of the components associated with a venue. So what happens is a loudspeaker speaks and the room speaks back to the loudspeaker, the amplifier hears it by the signal going around the feedback loop and out it comes again..............The real delay is the acoustic delay and that delay makes it sound spacious and big to our ear/brain system.


Video: Home Theater Geeks 29: Audio Legend Bob Carver Aug 6 2010

I have recently performed objective test that agree if the speaker is un-driven and connected to an amplifier with a high internal impedance (ie: low damping factor) this will allow a speaker-mic induced voltage to develop at the output of the amplifier but also due to my test I disagree that "all tube amplifiers" amplify this reverb effect to a perceivable level especially in a typical home listening environment with typical musical recordings.


miketn

 

edit: link to my test:  https://community.klipsch.com/index.php?/topic/157466-speaker-microphone-and-room-reverb-effects/#entry1884284

Edited May 28, 2015 by mikebse2a3

[Jim]

Another builder/restorer chased off?

[Thaddeus Smith]

By fellow builder/restores no less.

[JDJohnson]

I like what I see from them. Good prices too. I may send my 299C to them someday.

 

http://aeaaudio.com/restoration/

[jimjimbo]

Anyone have any current experience with these folks? AEA Audio?

[Wolfbane]

Anyone have any current experience with these folks? AEA Audio?

 

I know that they've sold some vintage tube amps on Audiokarma after posting equipment on the Vendor Forum last year you want to ask the same question there.