Then no bypassing is needed.
However you may want to think about some means of balancing these cathodes in order to stop the two tubes falling out of DC balance as they age. Most PP transformers can tolerate some DC imbalance but beyond a certain point it will degrade the sound.
Simple DC balance adjustment can be achieved by using a 100R wire wound pot. Each side is attached to one of the cathodes and the wiper is attached to the CCS. This will introduce a small amount of degenerative feedback.
Gary Pimm uses a self balancing arrangement in his Tabor amp where he uses the OT as a current sensor and introduces a DC servo. A bit complex.
So you have three options;
-ignore DC imbalance and hope for the best
-introduce a balance pot
-introduce a DC servo
Shoog
First thing is to check if you have an issue. Place a 10R resistor on each cathode and see if the voltage drop matches on each side. Then, if there is a mismatch - scope up your output and look for signs of output waveform distortion in the bass region.
Once you know whats happening you can make a more informed decision.
I have moved a long way beyond this type of arrangement and operate self adjusting DC coupled two stage amps. I have always used toroidal transformers as outputs and these will not tolerate anything more than a few mA of imbalance. I want an amp that I don't have to adjust so I have used separate CCS for each leg and bypassed the cathodes to each other with large back to back caps. This ensures that there will never be an imbalance at the price of a slight degradation in the sound quality. I have subsequently moved the CCS to on top of the transformers (requires split primary windings) with a cap bypassing after the CCS to earth which eliminates all caps in the cathode. Not exactly useful to yourself though.
I would be inclined to include a balance pot for safeties and simplicities sake.
Shoog
Once you know whats happening you can make a more informed decision.
I have moved a long way beyond this type of arrangement and operate self adjusting DC coupled two stage amps. I have always used toroidal transformers as outputs and these will not tolerate anything more than a few mA of imbalance. I want an amp that I don't have to adjust so I have used separate CCS for each leg and bypassed the cathodes to each other with large back to back caps. This ensures that there will never be an imbalance at the price of a slight degradation in the sound quality. I have subsequently moved the CCS to on top of the transformers (requires split primary windings) with a cap bypassing after the CCS to earth which eliminates all caps in the cathode. Not exactly useful to yourself though.
I would be inclined to include a balance pot for safeties and simplicities sake.
Shoog
You don't need to hope for that, you can just use a proper output transformer that can easily withstand 5-20% of total DC current, depending on the devices and design. It will still have quite enough inductance to guarantee high load and very low distortion at 20Hz, surely far better than the equivalent single ended. Having a huge inductance with very little or no capability of dealing with DC imbalance is rather pointless. Moreover having a gap will linearize things and cause less variation of inductance as function of signal especially with certain core types, like amorphous, where such variation in presence of a gap becomes rather small.
This requires good accuracy in putting gaps in a reproducible way and knowledge of permeability as function of the actual gap. Lundahl, for example, can provide this quite easily. For example if one gets a LL1623 5.6K gapped for 30mA and uses it for PP, primary inductance would be equal or better than 92 H with such imbalance and a bit better with less imbalance. The equivalent PP version has got 150H but I don't think will tolerate more than 10-15 mA DC and as soon as DC is present inductance will drop anyway. In the latter case as you get close to the limit of DC imbalance you'll have higher distortion at the low end.
I don't know the KT120 but if you consider that a KT88 PP will have about 5.6K Zout in UL mode and about 2K in triode mode, even not considering that 92H is the primary inductance at 50Hz (which means higher at 20Hz because of core losses), the ratio XL/Req at 20Hz will be 4 and 8, respectively. Surely decent for the UL even without fb and pretty good for triode!
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As I said, test it before making a decision.
Most situation will be fine without special precautions.
Let me just say, that my transformers have good response down to 10hz, but that would disappear if there were more than 3-4mA of imbalance. Many disagree, but if you are prepared to go the extra mile the performance potential of Toroidal Output Transformers is significantly better than most of the competition.
Shoog
Most situation will be fine without special precautions.
Let me just say, that my transformers have good response down to 10hz, but that would disappear if there were more than 3-4mA of imbalance. Many disagree, but if you are prepared to go the extra mile the performance potential of Toroidal Output Transformers is significantly better than most of the competition.
Shoog
In the above example for the LL1623, with the gap for 30 mA imbalance, the response will be 1dB down at about 10 Hz for the UL without any feedback and 5 Hz for the triode. The undistorted power at 30 Hz will be reduced from 62 W of the PP with no imbalance to 52 W with 15 mA imbalance. However the PP version with 10-15 mA imbalance will only handle about 20-25W!
In my experience toroidals without a gap are only good for power supplies primarily because their huge inductance and headroom drop like rocks as soon as little imbalance occurs. Transformer distortion is not pleasant at all and is the main reason for poor bass performance in most valve amplifiers. Damping factor is really irrelevant in comparison. Servos and caps in the output stage just make things worse in comparison to a properly designed output stage. Really!! I know that the result is quite subjective. However it is subjective not because of the listener but mostly influenced by the rest of the chain and the room that are far more critical than the amplifier itself....
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I would always set good bass response as one of the primary objective of any of my amps, as I often like techno at loud volumes, and jazz with detailed double bass passages. Texture is what you loose if your output transformer poops out on you and that is totally unacceptable for me.
I used back to back caps and CCS in the cathodes to ensure both perfect balance and perfect differential operation. It works well and sound nice and tight all the way down. As I said I have recently moved to using CCS in the plate to ensure perfect DC balance, but also to get rid of the caps in the cathode. So I now have effective fixed bias with perfect and adjustment free DC balance.
Was it worth it ? Probably as the bass is a bit tighter and cleaner - but the difference is fairly marginal and its only the fact that this amp only uses local feedback and two stages which allows me to hear the difference. Slap a bit of gNFB around it and I can guarantee they would sound identical.
I think the discipline of working with DC intolerant toroidals offers nothing but advantages overall.
Shoog
I used back to back caps and CCS in the cathodes to ensure both perfect balance and perfect differential operation. It works well and sound nice and tight all the way down. As I said I have recently moved to using CCS in the plate to ensure perfect DC balance, but also to get rid of the caps in the cathode. So I now have effective fixed bias with perfect and adjustment free DC balance.
Was it worth it ? Probably as the bass is a bit tighter and cleaner - but the difference is fairly marginal and its only the fact that this amp only uses local feedback and two stages which allows me to hear the difference. Slap a bit of gNFB around it and I can guarantee they would sound identical.
I think the discipline of working with DC intolerant toroidals offers nothing but advantages overall.
Shoog
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You should also say how critical is making your amp work, how easily you can smoke it, you have 102 dB speakers and the average power you use is rather small. How good your system really is I don't know but I wouldn't be so categorical. Still have to listen to 102 dB speakers that sound good enough.....
It all takes work, but they have impressed many ears in their time.
I had two musicians through the other day and I was listening to some techno-jazz with plenty of bass. They both commented on how clean, detailed and powerful the system sounded. This is not an unusual situation.
I don't think I am kidding myself at this stage. I use a PP preamp (5687) with input phase spitting chokes and toroidal balanced outputs. The main power amp has input transformers doing phase splitting into a differential pentode long tailed pair and DC coupled to PP outputs using Schade type feedback through the LTP plate load resistor and the OT are toroidals. Minimum feedback - lots of iron balanced most of the way, three gain stages in total. I can hear any changes to any component.
Once I got it setup (which indeed was somewhat hard work initially) it will run for about two years before the output valves fall out of balance so badly that it starts to oscillate, at this point just throw in another set of outputs and your good to go for another year. It runs for about 12hrs a day, day in day out and never lets me down in operation (unless I mess with its guts).
The Speakers are Open Baffle with minimal crossovers and field coil main drivers.
Its an approach which certainly works if you pay attention to the details.
............
However in Tim's particular case, you are probably entirely right in saying that he will not have to apply any particular precaution for his particular Output Transformers.
Shoog
I had two musicians through the other day and I was listening to some techno-jazz with plenty of bass. They both commented on how clean, detailed and powerful the system sounded. This is not an unusual situation.
I don't think I am kidding myself at this stage. I use a PP preamp (5687) with input phase spitting chokes and toroidal balanced outputs. The main power amp has input transformers doing phase splitting into a differential pentode long tailed pair and DC coupled to PP outputs using Schade type feedback through the LTP plate load resistor and the OT are toroidals. Minimum feedback - lots of iron balanced most of the way, three gain stages in total. I can hear any changes to any component.
Once I got it setup (which indeed was somewhat hard work initially) it will run for about two years before the output valves fall out of balance so badly that it starts to oscillate, at this point just throw in another set of outputs and your good to go for another year. It runs for about 12hrs a day, day in day out and never lets me down in operation (unless I mess with its guts).
The Speakers are Open Baffle with minimal crossovers and field coil main drivers.
Its an approach which certainly works if you pay attention to the details.
............
However in Tim's particular case, you are probably entirely right in saying that he will not have to apply any particular precaution for his particular Output Transformers.
Shoog
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Open baffle in a typical living room, even worse in a small room, will only create more issues. The bigger the speaker the worse. One will have to work a lot harder to decouple the room contribution from the original musical signal and a good result is not even guaranteed. The room treatment one needs in this case is simply massive!! If one doesn't do this masking will occur because the typical listening room in 99.9999% of cases will have different characteristics respect to the "original" room and the net result will be less dynamics despite of the higher SPL one can achieve with such speakers when the average level will be enough to substantially excite room modes. Such average level is not that high. That's why I am not a fan of big high efficiency speakers in normal domestic conditions. In a really large room (and I mean it in the real sense) things can be quite different but room control/treatment still plays a crucial role.
I have quoted the word original because it can be both a natural recording room and artificial but that is and one cannot do anything about it except extracting the maximum amount of information that is related to maximum SPL only to a certain extent.
Anyway we are really OT now. So I will stop here.
I have quoted the word original because it can be both a natural recording room and artificial but that is and one cannot do anything about it except extracting the maximum amount of information that is related to maximum SPL only to a certain extent.
Anyway we are really OT now. So I will stop here.
We build and tune speakers to the room we use them in and tuning a system to a particular application is part of the deal with HIFI.
I have two OB speakers at this stage, one in the main living room and one in the bedroom. Both are high sensitivity. They certainly sound different depending on where you are in the room - but every speaker system has a sweet spot in my experience. I used the same speakers in a big mass loaded voight pipe at one stage. It always had a sibilant peak what ever I did - the same speakers in an OB are much better behaved and generally sound much cleaner.
I really think that many problems in HIFI are mostly theoretical in nature and aren't a big deal in practice. We follow a path which gets us to where we want to be, for me that high efficiency vintage speakers and low power class A PP amplifiers with plenty of Iron. You may have an entirely different mileage.
PS - I refoamed a pair of Tannoy HPD golds for a friend - when I listened I didn't like them at all - not capable of anything like the resolution of my main system. Horses for courses.
Shoog
I have two OB speakers at this stage, one in the main living room and one in the bedroom. Both are high sensitivity. They certainly sound different depending on where you are in the room - but every speaker system has a sweet spot in my experience. I used the same speakers in a big mass loaded voight pipe at one stage. It always had a sibilant peak what ever I did - the same speakers in an OB are much better behaved and generally sound much cleaner.
I really think that many problems in HIFI are mostly theoretical in nature and aren't a big deal in practice. We follow a path which gets us to where we want to be, for me that high efficiency vintage speakers and low power class A PP amplifiers with plenty of Iron. You may have an entirely different mileage.
PS - I refoamed a pair of Tannoy HPD golds for a friend - when I listened I didn't like them at all - not capable of anything like the resolution of my main system. Horses for courses.
Shoog
I think different. I thought like that many years ago.....
Real HiFi is fidelity to the original music signal. No other definition is correct.
Using the room acoustics to "equalize" speakers is one of the typical misconceptions (mistakes) of the typical high fidelity world.
Music happens in the space-time domain. It's a story in that four-dimensional space. Frequency is just the conjugate of time which comes handy for the description of the physical sound only. Between the physical sound and the musical sound there is the listener and unfortunately with a listener in the chain one cannot switch between frequency and time domains anymore in a predictable way. Frequency and time are no more equivalent when the listener is considered that's why no one can predict the sound of stereo system, or even a piece of it, even if it measures flawlessly. He can only judge when will listen to it.
The big benefit one gets is that even 50p records that will be nearly unlistenable in typical HiFi systems, even worse with the so-called Hi-End systems, can be really enjoyable building our own music system from the above statement. The number of "bad" records reduces drastically, except for certain musical tendencies where really there is no art (IMHO)......
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I would have to disagree, room acoustics is the reality of all music whether reproduced or live. It cannot be avoided and it has to be lived with. This is as applicable to a concert hall, recording studio and living room. This is what the sound engineers role is and he is the essential member of any band.
Can the amplifier - speaker move between locations and still sound excellent. Mine can so I believe I have done enough.
Shoog
Can the amplifier - speaker move between locations and still sound excellent. Mine can so I believe I have done enough.
Shoog
Happy for you but that doesn't mean anything to me.
The room is already present in the recording, artificial or natural doesn't matter at this stage. When you add yours you will loose information. Until you don't try you will never realize what this really means.
In a live event there is nothing to re-produce so you don't have such double event but still room acoustics is crucial!
I don't care about the sound of my system in another room if that is not my room. I just care about the best result in my specific situation. Otherwise I would just buy a ready made stereo system....
It is the same difference you have in a race in a proper racing circuit between a true racing car and common car, let's say, good for everything but best for nothing. There is no race! Who cares about the performance of the racing car on the road if winning the race is the only target?
I would add my very positive experience with using mains toroidals to what Shoog has already said. I have now built 1 mic amp, 1 line amp and 2 power amps using the differential topology. I have also used them in a parafeed headphone amp(with a ccs replacing the usual choke). It is not difficult to build matched current sinks for each cathode to negate the possibility of DC imbalance. In my case I use a single film cap to AC couple the cathodes.
I agree that sound quality is subjective and emotive. I am a classical pianist by training so I do at least have some experience on a daily basis of the sound of real music. Using these techniques I find that the sound quality leaves little if anything to be desired.
I do however use old Tannoy speakers requiring but little power. If a valve like 6S33S were used, there would be enough power to drive much less efficient speakers.
It is a cheap experiment worth trying if you don't mind using plenty of semiconductors in the support circuitry.
I agree that sound quality is subjective and emotive. I am a classical pianist by training so I do at least have some experience on a daily basis of the sound of real music. Using these techniques I find that the sound quality leaves little if anything to be desired.
I do however use old Tannoy speakers requiring but little power. If a valve like 6S33S were used, there would be enough power to drive much less efficient speakers.
It is a cheap experiment worth trying if you don't mind using plenty of semiconductors in the support circuitry.
Me too, although I don't play for living. I also play a bit the guitar.
However I think that even a dog would find a striking difference between the sound of instruments and instruments playing music. There is an abyss between the two and this is exactly where common HiFi fails!
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