I completely removed the CF wires, and the output impedance in Pentode went to 3.3R
It seems that distortion is also higher, for some unknown reason when I link between the CF wires it is still acting in the circuit for some unknown reason, maybe the fuse is not a perfect link, Not a clue.
What is sure is that the CF is properly wired, and that completely removing it degrades the performance, I will do basic THD tests to confirm that it is worse without CF completely removed from circuit.
It seems that distortion is also higher, for some unknown reason when I link between the CF wires it is still acting in the circuit for some unknown reason, maybe the fuse is not a perfect link, Not a clue.
What is sure is that the CF is properly wired, and that completely removing it degrades the performance, I will do basic THD tests to confirm that it is worse without CF completely removed from circuit.
Things achieved: works well without global feedback, sound is acceptable and THD is under 50db (0.4% without feedback) at 5 Watts (two signals) on all harmonics.
Things to work on: try kt120, change input capacitor to HIFI capacitor, augment global feedback which right now only reduces THD by 3 db. Adjust feedback capacitor if needed,
Change cathode resistors of 10R to 1R to reduce interaction with CF. (as Audio Research does)
In the future amp individually bias pentode screens with 15 mA regulators (4 per amp).
In the new amplifier isolate completely the gain and driver stage with dedicated PS, use all film caps, and possibly use regulators/choke for power tubes and large film decoupling caps.
Things to work on: try kt120, change input capacitor to HIFI capacitor, augment global feedback which right now only reduces THD by 3 db. Adjust feedback capacitor if needed,
Change cathode resistors of 10R to 1R to reduce interaction with CF. (as Audio Research does)
In the future amp individually bias pentode screens with 15 mA regulators (4 per amp).
In the new amplifier isolate completely the gain and driver stage with dedicated PS, use all film caps, and possibly use regulators/choke for power tubes and large film decoupling caps.
I will also measure any cross talk and voltage changes in the screen voltage , this voltage modulating the gain could explain some of the unwanted sound of pentode.
More measurements were taken... of X-talk.
A typical amplifier with semi mono supplies through separate chokes and resistors should have 85.5db at least of separation.
I measured around 36.5 db of separation, which means that at somewhere full volume there is a distortion background in the other loudspeaker and vice-versa creating a mess of harmonics which continue to grow.
I think I have to separate the screen voltage with separate capacitors, isolated through a 4 k resistor there should be little influence beside the main voltage of power supply...
On a final build there would be separate regulators for each phase and mono channels so that would not be an issue at all.
But I want to completely isolate why the amplifier has a lot of resolution but sounds with a flat background, sounds congested... I turned up a little bit the Global feedback. I changed the cathode resistors to 0.5Watts 1R, I know they are too small but I have nothing else at the moment.
A typical amplifier with semi mono supplies through separate chokes and resistors should have 85.5db at least of separation.
I measured around 36.5 db of separation, which means that at somewhere full volume there is a distortion background in the other loudspeaker and vice-versa creating a mess of harmonics which continue to grow.
I think I have to separate the screen voltage with separate capacitors, isolated through a 4 k resistor there should be little influence beside the main voltage of power supply...
On a final build there would be separate regulators for each phase and mono channels so that would not be an issue at all.
But I want to completely isolate why the amplifier has a lot of resolution but sounds with a flat background, sounds congested... I turned up a little bit the Global feedback. I changed the cathode resistors to 0.5Watts 1R, I know they are too small but I have nothing else at the moment.
Good results, my THD readings went very low after adding a separate 350uf capacitor for the 400V screen bias. The cross talk went from 36.5db to around 85db.
The lower cathode resistor removed interactions with the CF stage, which could be misleading because it rearrange the THD figures when the bias is unbalanced.
I changed bias to 57ma, (previous 52ma). and readjusted everything.
I removed the frost 12sl7 and installed a normal Haltron in each sections, I know that the frosted 12sl7 gives me a good 3db of harmonics reduction. I have no idea how the 6sl7 from tung sol would perform because of the heater, maybe 1 or 2 decibel worse.
Now the feedback is a little higher, I didn't use a scope to check for resonances, it seems it doesn't have much, I ran only once into instability during the THD tests up to 25 Watts.
At 14 Watts I obtained 0.076 and 0.083% THD with KT88 at lower voltage, the output impedance is under 1ohm.
The gain is 0.7db matched at worse.
It is one of the best amplifier and the best that I ever built.
The thing with tubes is that they are noisy and produce a lot of distortion in the output stage, the supplies are not regulated and it goes through an output transformer.
It does provides accuracy, tone, details, flow, harmonics and insight into the performance. But, it doesn't have the ambiance and analytic accuracy of the top solid state amplifier. Especially in terms of clarity, neutrality, attack, and composure.
The solid state amplifier is way more defined around each sound. Without going into ultralinear flaws of shouting or exacerbating a particular sound, the tube amp still lack the ability to resolve the full attack of each sound, and unravel the logic plan of a piece. The solid state lays bare every sound used in the song to the limit of confusion... I don't know how to describe more.
Both amplifiers have bass, extension, musicality and words are clear in choir music.
The lower cathode resistor removed interactions with the CF stage, which could be misleading because it rearrange the THD figures when the bias is unbalanced.
I changed bias to 57ma, (previous 52ma). and readjusted everything.
I removed the frost 12sl7 and installed a normal Haltron in each sections, I know that the frosted 12sl7 gives me a good 3db of harmonics reduction. I have no idea how the 6sl7 from tung sol would perform because of the heater, maybe 1 or 2 decibel worse.
Now the feedback is a little higher, I didn't use a scope to check for resonances, it seems it doesn't have much, I ran only once into instability during the THD tests up to 25 Watts.
At 14 Watts I obtained 0.076 and 0.083% THD with KT88 at lower voltage, the output impedance is under 1ohm.
The gain is 0.7db matched at worse.
It is one of the best amplifier and the best that I ever built.
The thing with tubes is that they are noisy and produce a lot of distortion in the output stage, the supplies are not regulated and it goes through an output transformer.
It does provides accuracy, tone, details, flow, harmonics and insight into the performance. But, it doesn't have the ambiance and analytic accuracy of the top solid state amplifier. Especially in terms of clarity, neutrality, attack, and composure.
The solid state amplifier is way more defined around each sound. Without going into ultralinear flaws of shouting or exacerbating a particular sound, the tube amp still lack the ability to resolve the full attack of each sound, and unravel the logic plan of a piece. The solid state lays bare every sound used in the song to the limit of confusion... I don't know how to describe more.
Both amplifiers have bass, extension, musicality and words are clear in choir music.
The major things to consider right now are to remove the transformer case to listen to any improvement in the sound.
It is a R-core transformer, which performs way ahead of anything I heard in other transformers, the sound is a good league clearer, without phase issues and leakage.
The excitation current is very little, the loss very low, no magnetic loss due to symmetrical perfectly aligned windings.
Magnetic flux is high, (inductance), it is a big improvement in vocal intelligibility it rivals my best solid state amplifier.
The other thing to consider is to plug in the kt120 (heater is at the limit 7A) to see if it yields any sonic miracle.
And either leave it as it is as secondary system (to be used with high sensitivity speakers with low current needs) or build the big mono block with doubling the power supply current and regulating the B+ as well as the screens.
After hearing this I will never return to other types of transformer and power tubes other than kt88 and kt120.
It is a R-core transformer, which performs way ahead of anything I heard in other transformers, the sound is a good league clearer, without phase issues and leakage.
The excitation current is very little, the loss very low, no magnetic loss due to symmetrical perfectly aligned windings.
Magnetic flux is high, (inductance), it is a big improvement in vocal intelligibility it rivals my best solid state amplifier.
The other thing to consider is to plug in the kt120 (heater is at the limit 7A) to see if it yields any sonic miracle.
And either leave it as it is as secondary system (to be used with high sensitivity speakers with low current needs) or build the big mono block with doubling the power supply current and regulating the B+ as well as the screens.
After hearing this I will never return to other types of transformer and power tubes other than kt88 and kt120.
I connected the amp to loudspeakers 3 ways and scope to see if I could find anything.
At 3Vrms at 20 hertz the woofer is bottoming out, zero distortion, it does attempt at 10khz to make a square wave.
There is no ringing, nada, I thought lowering the feedback resistor from 2.2k to 2k would cause ringing , none,
It poorly does square waves past 10khz but this is not an issue, I tried to clip the amp at 6 Vrms at 10khz , no clipping.
At 3Vrms at 20 hertz the woofer is bottoming out, zero distortion, it does attempt at 10khz to make a square wave.
There is no ringing, nada, I thought lowering the feedback resistor from 2.2k to 2k would cause ringing , none,
It poorly does square waves past 10khz but this is not an issue, I tried to clip the amp at 6 Vrms at 10khz , no clipping.
Placing or removing the cover had no effect on the waves or resonance, or anything visible through the scope.
I debunked a myth: that placing kt120 in kt88 amps can be done.
I debunked a second myth: kt120 at low voltages and around 55mA bias sound very good.
This is all false, kt88 are superior, I would say, don't use Kt120 unless you bias them over 75mA and over 500V,
At 450V and 60mA kt88 are superior.
The only thing that KT120 did better is a lower noise floor in bass signals, cleaner, but all THD figures went up.
In my designed mono Kt120 amplifier the KT120 are way better because they can be biased properly and have 550V, 85mA and the power supply can pump the heater at required max value and the current without flinching.
The power supply of a well designed kt88 amp is just not sufficient to operate kt120s and it is better to NOT upgrade to a bigger tube.
The only exception would be from EL34 to Kt88 because the step is not as huge, it should in most case be an upgrade.
I debunked a second myth: kt120 at low voltages and around 55mA bias sound very good.
This is all false, kt88 are superior, I would say, don't use Kt120 unless you bias them over 75mA and over 500V,
At 450V and 60mA kt88 are superior.
The only thing that KT120 did better is a lower noise floor in bass signals, cleaner, but all THD figures went up.
In my designed mono Kt120 amplifier the KT120 are way better because they can be biased properly and have 550V, 85mA and the power supply can pump the heater at required max value and the current without flinching.
The power supply of a well designed kt88 amp is just not sufficient to operate kt120s and it is better to NOT upgrade to a bigger tube.
The only exception would be from EL34 to Kt88 because the step is not as huge, it should in most case be an upgrade.
Even if the kt120 has more THD (at least 5 db more ) placed in the amp, it does have some more resolution of complex wave forms.
For pleasure and curiosity, I took two pictures of 670hz wave patters which repeats and are asymmetrical.
It is connected to 3 ways loudspeakers to take measurements at normal listening volume, guess which one is the solid state and which one is the tube, which one should sound best 🙂, note the pattern has a lot of irregularities and no oscillations, the amplifiers have a hard time reproducing the voltages in the loudspeakers, a lot of problem.
For pleasure and curiosity, I took two pictures of 670hz wave patters which repeats and are asymmetrical.
It is connected to 3 ways loudspeakers to take measurements at normal listening volume, guess which one is the solid state and which one is the tube, which one should sound best 🙂, note the pattern has a lot of irregularities and no oscillations, the amplifiers have a hard time reproducing the voltages in the loudspeakers, a lot of problem.
This is a preliminary test that I did to compare pentode + cathode feedback vs ultralinear, since then the pentode circuit performs a lot better. It is obvious that the sound of the P+CF is way superior, as easy confirmed by ear, I had the amp play some music and it was obvious that the UL was very 'good' for some standards but that the pentode had that clarity.
This test is done with two signals, main 1khz is at 1.5 watts, the 2khz, is at 1watt, approximately
I don't know why the values of THD are so high in that picture, but it gives a comparison idea.
The real numbers I will redo that picture with only the Pentode in Cathode feedback. I will post it tomorrow the final pictures of THD
The real numbers I will redo that picture with only the Pentode in Cathode feedback. I will post it tomorrow the final pictures of THD
I did another arbitrary test with the kt88 amp and the solid state amplifier, this time at 4Vrms and a 100, 120 hertz type of signal, lower, obviously the control signal cannot be generated or intercepted properly by the ADC\DAC so I will show the control picture, the measured picture, it could simply a matter of capacitance. It looks like the solid state amplifier has a bit more digging and power for the previous tests connected to loudspeakers, now on the 8 ohm dummy load, both tube and solid state seems the same amplifier, for your viewing pleasure and utter confusion : (solid state left, tube, right)
I did an IMD 1kz,2khz test at same voltage, the Solid state amplifier has 5db more THD in second harmonics, then 2db more in other harmonics (3,4,5etc
You can see a picture of 46 hz + 1khz, I drew on the picture to simplify the comparisons... at around 8 watts.
You can see a picture of 46 hz + 1khz, I drew on the picture to simplify the comparisons... at around 8 watts.
at almost 2 watts, I did a single 1khz signal test, the tube kt88 gets 0.023% thd , the solid state amplifier 0.066, this represents 10db more second harmonics.
the solid state has -3db of 3rd harmonics.
But then for 4, 5, 6, 7 it has way more, the tube amp has -92db of 5, no 4, 6, 7 etc, all below 100db+
The solid state has -73.4 db relative of 4th harmonics then a cascade of , 5, 6, 7, 8 etc going -80db te o -90
The only other anomaly is that the SS has a regulated stable PS with independence of mains fluctuations, it has a lot less noise and no hum.
The tube amp has a fluctuating power supply with the main voltage which has quite an effect during measurements, it also has a lot of hum in the 120hz region.
Perhaps this is causing the sound to not sound open.
For sure the kt120 is a serious contest for the solid state amplifier, but it will need a proper regulated PS , and mono block with more than double the current capacity of the actual transformer and a double sized choke, double sized capacitor supply
the solid state has -3db of 3rd harmonics.
But then for 4, 5, 6, 7 it has way more, the tube amp has -92db of 5, no 4, 6, 7 etc, all below 100db+
The solid state has -73.4 db relative of 4th harmonics then a cascade of , 5, 6, 7, 8 etc going -80db te o -90
The only other anomaly is that the SS has a regulated stable PS with independence of mains fluctuations, it has a lot less noise and no hum.
The tube amp has a fluctuating power supply with the main voltage which has quite an effect during measurements, it also has a lot of hum in the 120hz region.
Perhaps this is causing the sound to not sound open.
For sure the kt120 is a serious contest for the solid state amplifier, but it will need a proper regulated PS , and mono block with more than double the current capacity of the actual transformer and a double sized choke, double sized capacitor supply
My tube amp has developed a high pitch cavitation noise on the Transformer, this is spread to the power tubes.
I think the heater winding is damaged from the kt120 heavy current, or a main input capacitor is damaged, no clue but looks like it is not going into the speakers.
I think the heater winding is damaged from the kt120 heavy current, or a main input capacitor is damaged, no clue but looks like it is not going into the speakers.
Actually it is a very clear signal, like in the sample, and I can hear it through all my tube gears,
My main power amp has the resonance, and the tubes of my pre amp and tubes of my dac.
My pre-amp has a complex input filter for RFI and others So it should not be affected.
I know that today is very sunny, and many neighbor have solar panels on their rooftop which could send some EMI pollution and RFI, so it is weird that the main tubes catch it so strongly, in the basement it is less. I observed this before but today it is very very strong...
My main power amp has the resonance, and the tubes of my pre amp and tubes of my dac.
My pre-amp has a complex input filter for RFI and others So it should not be affected.
I know that today is very sunny, and many neighbor have solar panels on their rooftop which could send some EMI pollution and RFI, so it is weird that the main tubes catch it so strongly, in the basement it is less. I observed this before but today it is very very strong...
I can strongly hear it in the basement at lightbulbs base connections nearest the electrical panel.
It is very strange.
It is very strange.
Yes! It must be solar panels disturbing the ground and inserting ghz mhz crap in out mains... such a shame.
It subsidized, now I can only hear the buzz of all the CFLs and Diodes, which is lower, and it is more intense at night.
I did audition again with the modifications to the tube amp with the lower resistors, higher feedback and covers off (lol) and the dedicated single capacitors for the screens...
Well, the noise in the background lowered a lot and some atmosphere returned, the sound is more organic, more natural, the transistor has just a bit of lead in the bass definition but the tube amp is quite there, it is not as analytic but if you just listen carefully it has all the detail there that the solid state amp has.
I am sure that the kt120 tube properly placed in mono final design would be the best amplifier.
I am glad to have the solid state mono which showed me many faults in my concept and I could see the importance of a separate power supply, regulation of screen voltage, limitation and quality of cathode 1ohm resistor.
Also the right channel is impacted by the proximity of the power line inside the amp, wires go 2 mm and 5 mm , there was no other place and the cathode follower driver has its inputs right next to the power switches, like 1 cm. so it raise the noise and also reduce the gain of that channel, thd is also a bit more.
I found that capacitors didn't make any difference when the whole amp is working well. I started from UL kt120, went to no feedback, triodes, then new transformer, UL, Cathode feedback, then changed the circuit, built another el34, 6bl7, el84, and kt77 amps, then a good kt88 amp, and used finally the most prohibited and booed pentode mode! and voila!
In terms % of importance: 1. (40%) the new transformer gave me the clarity and phase which I always wanted.
2. (35%)The Pentode mode brought that clarity to whatever was achievable
3. (15%)The Cathode feedback windings improved the output resistance and corrected many odd harmonics,
4. (10%)the new circuit of paraphrase, cathode followers (old circuit is 6sn7 -, ecc99 LTP on top of pentode CCS) gave the amp more stability.
It subsidized, now I can only hear the buzz of all the CFLs and Diodes, which is lower, and it is more intense at night.
I did audition again with the modifications to the tube amp with the lower resistors, higher feedback and covers off (lol) and the dedicated single capacitors for the screens...
Well, the noise in the background lowered a lot and some atmosphere returned, the sound is more organic, more natural, the transistor has just a bit of lead in the bass definition but the tube amp is quite there, it is not as analytic but if you just listen carefully it has all the detail there that the solid state amp has.
I am sure that the kt120 tube properly placed in mono final design would be the best amplifier.
I am glad to have the solid state mono which showed me many faults in my concept and I could see the importance of a separate power supply, regulation of screen voltage, limitation and quality of cathode 1ohm resistor.
Also the right channel is impacted by the proximity of the power line inside the amp, wires go 2 mm and 5 mm , there was no other place and the cathode follower driver has its inputs right next to the power switches, like 1 cm. so it raise the noise and also reduce the gain of that channel, thd is also a bit more.
I found that capacitors didn't make any difference when the whole amp is working well. I started from UL kt120, went to no feedback, triodes, then new transformer, UL, Cathode feedback, then changed the circuit, built another el34, 6bl7, el84, and kt77 amps, then a good kt88 amp, and used finally the most prohibited and booed pentode mode! and voila!
In terms % of importance: 1. (40%) the new transformer gave me the clarity and phase which I always wanted.
2. (35%)The Pentode mode brought that clarity to whatever was achievable
3. (15%)The Cathode feedback windings improved the output resistance and corrected many odd harmonics,
4. (10%)the new circuit of paraphrase, cathode followers (old circuit is 6sn7 -, ecc99 LTP on top of pentode CCS) gave the amp more stability.
https://www.highdeftapetransfers.ca/products/handels-water-musick-royal-firework-s-stokowski
This recording is incredible by the size of the bass and clarity of harpsicord, the massive bassoon, caisse Claire, and all the flutes, piccolo, can hear the massive sound, it is so beautiful, Mozart symphony sound dwarf compared to this. I will rehear also gabrielli church Vienna chatedral recording wich has a splendid echo.
This recording is incredible by the size of the bass and clarity of harpsicord, the massive bassoon, caisse Claire, and all the flutes, piccolo, can hear the massive sound, it is so beautiful, Mozart symphony sound dwarf compared to this. I will rehear also gabrielli church Vienna chatedral recording wich has a splendid echo.