That was my feeling as well. I am fairly conservative when it comes to making changes. I will definitely purchase high quality replacement for the 0.22 UF coupling capacitors, but I will not change their values
on the topic of the coupling capacitors I have another question. I will replace the 4-0.22uf capacitors that are coupled to the Kt88s with the recommended Audyn True Copper Cap, but I see there are 2 more 0.22uf capacitors in the preamp section. Do these need to be replaced as well? If so do they need to be replaced with the same Audyn True Copper Cap or can I use a cheaper one like a Solen?
Hi,
You'd want to replace all caps for best results, the preceding caps in the chain invariably colour the end result.
That said I'm not familiar with the original caps or the Audyn True Copper ones. In general I found Audyn caps to be good value for money though.
Cheers, 😉
You'd want to replace all caps for best results, the preceding caps in the chain invariably colour the end result.
That said I'm not familiar with the original caps or the Audyn True Copper ones. In general I found Audyn caps to be good value for money though.
Cheers, 😉
Does anyone know of any documentation on how this tube amp works? I build a simple 2 tube SET amplifier and understand it fairly well. I also understand a little bit about push pull. In this amp, looking at the schematic, it looks like the 12AX7 performs basic pre amplification. Next are the 6SN7s and I don't understand what they are doing at all. The cathode of the last 6Sn7 is connected to the grid of they kt88 power amp. What is that all about? I thought that usually the plate of the 6SN7 would be connected to the grid of the kt88. I am really confused
There is nothing unusual in this amp. SRPP front stage, long tail phase splitter, cathode follower drivers, and finally ultra-linear push-pull output stage. All well known tube circuits. You need to read a few books on analogue electronics to understand how it all works.
Thanks avp, I researched each of the modules you described and now I have a better understanding of how this amp works.
Something is not working correctly with this amp. I am going to take it apart and see if there are any obvious problems (like burned out parts). One thing I was wondering, I understand that the 10ohm resistor is a 5W, not 3W as listed. Are the other resistors listed as 3W actually 3W. Also are the remaining resistors 1W or 2W? FInally what type of resistors do i need to get to replace the 3W, 2W or 1W?
Something is not working correctly with this amp. I am going to take it apart and see if there are any obvious problems (like burned out parts). One thing I was wondering, I understand that the 10ohm resistor is a 5W, not 3W as listed. Are the other resistors listed as 3W actually 3W. Also are the remaining resistors 1W or 2W? FInally what type of resistors do i need to get to replace the 3W, 2W or 1W?
Even if the 100b was biased 'hot' at 60mA per tube, the voltage across the 10 Ohm resistors would be a mere 0.6V and the dissipated wattage would be 36 milli-Watts.
Why such a huge wattage resistor is fitted in the first places begs the question Why?
Perhaps it is there to survive any nasty currents due to a tube defect?
I think the parameter that is more important here is the tolerance rating, a 1% resistor would give a more accurate set of results when comparing the currents of all 4 tubes.
So perhaps a 2W resistor could suffice here? That would survive almost half an amp through a KT88!
Why such a huge wattage resistor is fitted in the first places begs the question Why?
Perhaps it is there to survive any nasty currents due to a tube defect?
I think the parameter that is more important here is the tolerance rating, a 1% resistor would give a more accurate set of results when comparing the currents of all 4 tubes.
So perhaps a 2W resistor could suffice here? That would survive almost half an amp through a KT88!
Could you give more details as to what has gone wrong? We may be able to help you more with some specific details e.g. dead in one channel may be just a B+ fuse on the rear panel though of course many earlier 100b's did not have fuses fitted here.
If he was alble to set idle current, then B+ supply should be good. It is possible that one of low power tubes is dead.
Sorry if I missed something but why talk about replacing the bias monitor resistors if they are measuring correctly? My question was well intentioned as I am (or rather was) trying to help him having serviced a few of these amplifiers. But with alternative superior knowledge available I guess I am wasting my time here.
The reason that I was asking about the resistors was as follows. When I first got this amplifier it blew a .5 A fuse on the right channel and it smelled like something was burning. I discovered the problem was that my house voltage is 125V. I bought a voltage power regulator that cuts the voltage to 110V I have not had any more blown fuses since then.
After that incident in addition to lowering my voltage I also swapped all the tubes to Gold Lion Kt88, EH 6SN7, and Gold Lion 12AX7. The problem is that every since I made the changes to the tubes and power, I am getting a random popping sound. It occurs in both channels, but not at the same time. Each pop is about 15 minutes apart on average.
Now back to my original point. I bought new KT88 coupling caps and I am going to take the amp apart. While having it apart, I thought that if any of the resistors burned out (the burning smell) I could also replace them while the amp is taken apart. Thus that is why I am asking about the wattage and types of the resistors. I want to buy a spare set (since resistors are inexpensive) so that I can fix any of them that may have burned out.
Sorry for the long winded response, but I couldn't think of a shorter way to explain my question
After that incident in addition to lowering my voltage I also swapped all the tubes to Gold Lion Kt88, EH 6SN7, and Gold Lion 12AX7. The problem is that every since I made the changes to the tubes and power, I am getting a random popping sound. It occurs in both channels, but not at the same time. Each pop is about 15 minutes apart on average.
Now back to my original point. I bought new KT88 coupling caps and I am going to take the amp apart. While having it apart, I thought that if any of the resistors burned out (the burning smell) I could also replace them while the amp is taken apart. Thus that is why I am asking about the wattage and types of the resistors. I want to buy a spare set (since resistors are inexpensive) so that I can fix any of them that may have burned out.
Sorry for the long winded response, but I couldn't think of a shorter way to explain my question
Are you sure the 125 V is the problem? an increase in voltage should not touch the bias. I would not trust products that can't deal with 10% increase and it should not affect the bias since as the operating point increase due to B+ increase the bias voltage increase in the grid (see the -70 volt).
Btw changing resistors on a pcb can be very difficult. I did that so many times 🙁.
A benign pop sound means a tube is failing and should be replaced in both channels !
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I am fairly certain that the 125V power was a problem because:
1) at one point it threw the circuit breaker in my house for that circuit and I did not have anywhere near 15 amps running on that circuit.
2) while my voltage was running at 125V, I am fairly certain that there were peaks of at least 130V or more. The circuit breaker blew after running the amp for 20-30 minutes. The fuse blew on the next session after about 1 hour.
3) the filaments of all of the tubes were exceptionally bright. After installing the 110V regulator, the filaments seem to have a more normal brightness
4) since installing the voltage regulator (about 6 weeks ago) I have never had another circuit breaker or fuse issue
I was wondering about the tubes, and based on your suggestion I will try putting the old tubes back in and see if that stops the popping sound. I was thinking first swap the 12ax7 and try that for an hour to see if there are any pops, if there are I would put the new ones back in and swap out the 6SN7 and if that doesn't work, finally swap out the KT88.
Next I will open the bottom of the amp and swap out the KT88 coupling capacitors. There are 2 other 0.22 UF capacitors in the preamp. Is there value at swapping them out as well. Looking at the circuit diagram, they don't seem to be coupling caps. I bought the recommended "AUDYN TRUE COPPER CAP 0.22UF 630V COPPER FOIL CAPACITOR" for the Kt88s, but they were $25 each so I only bought 4 of them. If I do need to replace the other 2 can I replace them with cheaper upgrades such as the Jantzen, which are only about 10-15 dollars each.
1) at one point it threw the circuit breaker in my house for that circuit and I did not have anywhere near 15 amps running on that circuit.
2) while my voltage was running at 125V, I am fairly certain that there were peaks of at least 130V or more. The circuit breaker blew after running the amp for 20-30 minutes. The fuse blew on the next session after about 1 hour.
3) the filaments of all of the tubes were exceptionally bright. After installing the 110V regulator, the filaments seem to have a more normal brightness
4) since installing the voltage regulator (about 6 weeks ago) I have never had another circuit breaker or fuse issue
I was wondering about the tubes, and based on your suggestion I will try putting the old tubes back in and see if that stops the popping sound. I was thinking first swap the 12ax7 and try that for an hour to see if there are any pops, if there are I would put the new ones back in and swap out the 6SN7 and if that doesn't work, finally swap out the KT88.
Next I will open the bottom of the amp and swap out the KT88 coupling capacitors. There are 2 other 0.22 UF capacitors in the preamp. Is there value at swapping them out as well. Looking at the circuit diagram, they don't seem to be coupling caps. I bought the recommended "AUDYN TRUE COPPER CAP 0.22UF 630V COPPER FOIL CAPACITOR" for the Kt88s, but they were $25 each so I only bought 4 of them. If I do need to replace the other 2 can I replace them with cheaper upgrades such as the Jantzen, which are only about 10-15 dollars each.
FUSE: Ok, I tried to make sense of the fuse blowing up, by the right channel you mean the right transformer, I presume each transfo input has a fuse. You say it is a 0.5 amps fuse, the fact it didn't blew up and tripped the main breaker is impossible. Also a 300 watt power with two power transfo must make quite a transient current and would sure blow a 0.5 A fuse. On the schematic it is a 3amps250V fuse which make sense if you have 2 = 6 amps.
In any case a fuse of 3 amp should have prevented the main breaker tripping in all cases. Your idea to use a transformer step down is clever and costly as you could have reach the same result with a line in-series load of a few ohms. The only way I can explain the failure and main breaker tripping is a bad house breaker, overloading with many appliances the main breaker, or serious shorts in the power transformer.
The filaments:
check with a voltmeter the voltage, an easy way is to remove a pre tube and stick the voltmeter in and power up.
CAPS: (source : http://www.angelfire.com/electronic/funwithtubes/Testing_caps.html
Changing coupling caps because they could be failure inside them is just crazy on a new amp. You can check the internal resistance with a voltmeter if it helps. the two .22uf caps if you mean C5 , they are critical and receive the full music signal 🙂 I would say more critical than the coupling caps. Since they don't receive high voltage the should not cause any pop noises and I would not touch them.
General comments on the design:
the feedback seems very low, cant help but comment for a KT88 amp.
If you used a lm334z for the longtail you could get rid completely of the cathode follower driver stage as it would stabilize the long tail pair for driver operation. Right now with the resistor it is unstable and unable to drive much so they use a cathode follower to buffer it. If you look at tube4hifi amps they use the lm334z and it sounds very good. That is easy to do and should improve the amp way more than capacitors (1/100 difference). I would also increase the feedback as it must sound muddy even in ultralinear. The feedback increase should stabilize the amp more than the small feedback and improve the bass response from impedance matching and transformer deficiencies.
In any case a fuse of 3 amp should have prevented the main breaker tripping in all cases. Your idea to use a transformer step down is clever and costly as you could have reach the same result with a line in-series load of a few ohms. The only way I can explain the failure and main breaker tripping is a bad house breaker, overloading with many appliances the main breaker, or serious shorts in the power transformer.
The filaments:
check with a voltmeter the voltage, an easy way is to remove a pre tube and stick the voltmeter in and power up.
CAPS: (source : http://www.angelfire.com/electronic/funwithtubes/Testing_caps.html
Changing coupling caps because they could be failure inside them is just crazy on a new amp. You can check the internal resistance with a voltmeter if it helps. the two .22uf caps if you mean C5 , they are critical and receive the full music signal 🙂 I would say more critical than the coupling caps. Since they don't receive high voltage the should not cause any pop noises and I would not touch them.
General comments on the design:
the feedback seems very low, cant help but comment for a KT88 amp.
If you used a lm334z for the longtail you could get rid completely of the cathode follower driver stage as it would stabilize the long tail pair for driver operation. Right now with the resistor it is unstable and unable to drive much so they use a cathode follower to buffer it. If you look at tube4hifi amps they use the lm334z and it sounds very good. That is easy to do and should improve the amp way more than capacitors (1/100 difference). I would also increase the feedback as it must sound muddy even in ultralinear. The feedback increase should stabilize the amp more than the small feedback and improve the bass response from impedance matching and transformer deficiencies.
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I can't explain the circuit breaker blowing either. All I can say is that it did happen and I checked and the only thing running on the circuit is this amp, my CD player, Tuner, Laptop and a few CFL light bulbs. That is no where near 15A. I can tell you that the second time this happened, when it blew the fuse, I also smelled a burning smell. By they way, this version of the amp has two different fuses. The first is a 3A and it is the one in the circuit diagram. It did not blow. The second is a 0.5A fuse, not on the circuit diagram and that is the one that blew. There are two of each fuse. The 0.5A fuse is under the speaker terminals.
I wasn't replacing the coupling caps because I thought they were burned out. I was replacing them because several people posted input to this and other forums saying that the sound quality could be improved by replacing the "cheap" stock 0.22UF caps with audiophile caps.
If the popping is caused by bad tubes, would it more likely be the 12ax7, 6SN7 or KT88?
As for your design changes that you mentioned, I am afraid that I am not enough of an expert to understand what you are saying. Do you have specific instructions for what you are talking about?
Thanks for all your help. I am new to tube amps. I love the sound and hope to learn more by reading books and having discussions over forums like this one.
I wasn't replacing the coupling caps because I thought they were burned out. I was replacing them because several people posted input to this and other forums saying that the sound quality could be improved by replacing the "cheap" stock 0.22UF caps with audiophile caps.
If the popping is caused by bad tubes, would it more likely be the 12ax7, 6SN7 or KT88?
As for your design changes that you mentioned, I am afraid that I am not enough of an expert to understand what you are saying. Do you have specific instructions for what you are talking about?
Thanks for all your help. I am new to tube amps. I love the sound and hope to learn more by reading books and having discussions over forums like this one.
The tube with wear should be the 12ax76sn7 first suspects, than kt88 which has strong construction to withstand surges.
But i am thinking the fuse of 0.5 might be to protect the kt88 over current. But... 500mA of current is crazy and only happens when oscillation and instability which again reinforce the theory that the 125V is not the real cause of failures. If kt88s are damaged I am sorry for you. If you trust the fuse you can wait for the tube to fail and blow something it could cost less than buying 2 new tubes for nothing.
''Thanks for all your help. I am new to tube amps. I love the sound and hope to learn more by reading books and having discussions over forums like this one.'' tube amps are fun and costly!
But i am thinking the fuse of 0.5 might be to protect the kt88 over current. But... 500mA of current is crazy and only happens when oscillation and instability which again reinforce the theory that the 125V is not the real cause of failures. If kt88s are damaged I am sorry for you. If you trust the fuse you can wait for the tube to fail and blow something it could cost less than buying 2 new tubes for nothing.
''Thanks for all your help. I am new to tube amps. I love the sound and hope to learn more by reading books and having discussions over forums like this one.'' tube amps are fun and costly!
If you want to change the sound more I suggest to change C2 and C3 with film caps of good quality and appropriate voltage +20%, this I discovered opens up the top end. Consider this a 5 times greater improvement than changing signal capacitors. Beyond that there is nothing much to do if you don't want to change the circuit. (ah yes, changing the 6sn7 to a vintage matched section tube 🙂.
Sdiesel77 - in the picture I see where you replaced the 4 coupling caps. It looks like you took out the .22uf preamp caps as well, but I don't see where you replaced them in the picture. Did you replace 4 or 6 caps?
This weekend I experimented with my tubes to try to fix the popping sound that I was having. I had been using Gold Lion 12ax7 and Kt88 and EH 6sn7 instead of the stock tubes. First, i put the stock 12ax7 and 6sn7 tubes back in (leaving the gold lion Kt88). I played a classical CD. The popping sound was gone, but the sound of the amp was not good - way too much on the high end. Then I put back the gold loin 12ax7. The popping was still gone and the sound was significantly better (not nearly as much high end). Then I decided to put back the EH 6sn7. Now I forgot to label how I had them placed in terms of V1,2,3,4. And so when I reinserted them I am fairly certain that I did not put them back in, in the same order as I originally had them. After putting them in and listening to another cd, for some unexplainable reason, the popping was still gone! Could the order of the tubes have caused this? I did also notice some improvement in sound from the EH 6sn7, but most of the improvement came from the 12ax7.
Next week I plan on replacing the .22uf capacitors with high end capacitors and I will see if that provides any further improvement. At this point the amp sounds good, but not great. My main complaint is that the dynamic range as music goes from soft to loud, is too large. Is that a characteristic of tube amps in general or is this specific to my tube amp?
Next week I plan on replacing the .22uf capacitors with high end capacitors and I will see if that provides any further improvement. At this point the amp sounds good, but not great. My main complaint is that the dynamic range as music goes from soft to loud, is too large. Is that a characteristic of tube amps in general or is this specific to my tube amp?
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