triode/UL switch 2
I think I got the response for my question about disabling the UL/triode switch on the Music Angel KT88 on this diyaudio thread
"remove the wire(s) from pin 4 (1 wire per tube). Connect pin 3 and 4 with a 100 ohm resistor (or with the resistor on the way from relay to pin 4). Also remove the 2 power connections to the relays".
I think I got the response for my question about disabling the UL/triode switch on the Music Angel KT88 on this diyaudio thread
"remove the wire(s) from pin 4 (1 wire per tube). Connect pin 3 and 4 with a 100 ohm resistor (or with the resistor on the way from relay to pin 4). Also remove the 2 power connections to the relays".
I see the Music Angel KT88 is similar to XiangSheng Music Cruve D2020-KT88.
http://www.xshn.com/en/display.asp?id=5
http://www.xshn.com/en/display.asp?id=5
Hi All
I am now starting to "cure" onr of these amps... It is in stock form and I must draw a schematic before anything because I believe these amps circuits are very variable.
I am not an experieced tube modder so I will need all the help available.
Is it possible to remove the feedback without major surgery ?
Hope I am not beeing too boring... again....
Ricardo
I am now starting to "cure" onr of these amps... It is in stock form and I must draw a schematic before anything because I believe these amps circuits are very variable.
I am not an experieced tube modder so I will need all the help available.
Is it possible to remove the feedback without major surgery ?
Hope I am not beeing too boring... again....
Ricardo
this link might be helpful to you, not sure how good it is tho. http://lampizator.eu/AMPLIFIERS/CHINA/KT88/Music Angel KT88 reworking.html
Hi guys .... I'm new here and I too have an Music Angel KT88 which I'm modifying.
things I did;
- disconnected the switch at the back
- done the hardwired triode mod between pins 3 and 4 on all KT88's
- set the bias on all 4 to 35mA as a test but now I notice that the power-supply transformers are getting a bit too warm .... and I don't like it really.
before the bias was set to 25mA ... everything stayed a lot cooler.
But now ....
because I started to get .... only after a while, say about an hour or 2 when the tubes are heated up very well ..... a kind of high whistling noise in the right channel only when I'm playing the amp on really low volume.
Even when I mute the input ... I can still hear it ans it's a rather irritating high pitched tone. It differs a bit in volume too .... somethimes it even disappears for a second or 2. It looks like a kind of oscillation ...
I also had a lot of "microfonie" on the small tubes. When I gave a very little tick on the tubes .... with a pen or so .... it was very audible in the speakers.
So I thought .... old tubes .... let's change them.
I already changed valves 1 and 4 ... the phase turners ...the ones labelled on the amp and in the schematics as 12AU7.
I swapped them with a pair of balanced NOS Sylvania 5963 tubes and this worked fine .... at least the microfonie problem is gone.
I was supprised a lot when I noticed that the originals were 2 different tubes .... one was a 5963 ... the other one a 5965 ... can you imagine ..
With the swap of the other 2 valves ... 12AT7 ones .... something went wrong.
12AT7 printed on the amp and in the schematics .... seemed to be that the original ones were Shuguang 6N1 tubes ... printed that way on the tubes.
I swapped them with a balanced pair NOS Philips JAN 12AT7WC and things started to go wrong. Didn't look on the prrint on the tubes themself ... my mistake ...
From the first moment there was a difference in volume between left and right channel ... after about 2 hours orso the right channel stopped working.
I bought the tubes in holland .... I myself live in Belgium .... and the man from the shop mailed me and said that the 6N1 and the 12AT7 are not the same .... they are not 1 one 1 swappable !
So the philips tubes are send back and I'm now waiting for a pair of russian 6n1p-EV tubes. I hope everything will work fine then.
But ... in the mean time ... I plugged the old 6N1's back in place and still .... when the amp is heated up well .... I do still hear that irrating high noise in the right channel .... so if somebody can help me with this problem ....
you're wellcome
things I did;
- disconnected the switch at the back
- done the hardwired triode mod between pins 3 and 4 on all KT88's
- set the bias on all 4 to 35mA as a test but now I notice that the power-supply transformers are getting a bit too warm .... and I don't like it really.
before the bias was set to 25mA ... everything stayed a lot cooler.
But now ....
because I started to get .... only after a while, say about an hour or 2 when the tubes are heated up very well ..... a kind of high whistling noise in the right channel only when I'm playing the amp on really low volume.
Even when I mute the input ... I can still hear it ans it's a rather irritating high pitched tone. It differs a bit in volume too .... somethimes it even disappears for a second or 2. It looks like a kind of oscillation ...
I also had a lot of "microfonie" on the small tubes. When I gave a very little tick on the tubes .... with a pen or so .... it was very audible in the speakers.
So I thought .... old tubes .... let's change them.
I already changed valves 1 and 4 ... the phase turners ...the ones labelled on the amp and in the schematics as 12AU7.
I swapped them with a pair of balanced NOS Sylvania 5963 tubes and this worked fine .... at least the microfonie problem is gone.
I was supprised a lot when I noticed that the originals were 2 different tubes .... one was a 5963 ... the other one a 5965 ... can you imagine ..
With the swap of the other 2 valves ... 12AT7 ones .... something went wrong.
12AT7 printed on the amp and in the schematics .... seemed to be that the original ones were Shuguang 6N1 tubes ... printed that way on the tubes.
I swapped them with a balanced pair NOS Philips JAN 12AT7WC and things started to go wrong. Didn't look on the prrint on the tubes themself ... my mistake ...
From the first moment there was a difference in volume between left and right channel ... after about 2 hours orso the right channel stopped working.
I bought the tubes in holland .... I myself live in Belgium .... and the man from the shop mailed me and said that the 6N1 and the 12AT7 are not the same .... they are not 1 one 1 swappable !
So the philips tubes are send back and I'm now waiting for a pair of russian 6n1p-EV tubes. I hope everything will work fine then.
But ... in the mean time ... I plugged the old 6N1's back in place and still .... when the amp is heated up well .... I do still hear that irrating high noise in the right channel .... so if somebody can help me with this problem ....
you're wellcome
Why not throwing out the PCBs and see it as a punched and finished chassis, fully fitted with mains and output transformers and valve sockets, knobs, binding posts, etc. so to wire a compatible proven circuit of choice for KT88 p2p with good components? It can be less, clean, & predictable work.
X-ploder, You have one of the later models of the MA KT88 amp with 6N1P input tubes. This is roughly equivalent to the 12AT7, but with different heater wiring. The amp can easily be modified to 12AT7 if some good old tubes of this type should come your way.
ECC85 will work nicely instead of the Chinese, or Russian, 6N1P. An odd substitute would be PCC88 which is also said to work nice in this amp.
I have heard about others also having some noise problems with this amp. If tube substitution does not help, make an extra check for good connection in tube socket and other bad connections or poor soldering.
You have permanently rewired to triode mode, this make the amp less likely to oscillation, but if you have access to a scope, look for any odd waveforms on the output. If the noise is more of a buzzing sound it could be caused by noise on the heater supply. Adjust the heater balance pots for minimum noise with your ear to the speaker.
A bit odd that the PT gets to hot with 35mA per tube, are you sure you have measured and calculated correctly? It expect it should be able to supply 50mA/tube without problem, but lower current usually sound just as good, and increases tube life.
--
Svein B.
ECC85 will work nicely instead of the Chinese, or Russian, 6N1P. An odd substitute would be PCC88 which is also said to work nice in this amp.
I have heard about others also having some noise problems with this amp. If tube substitution does not help, make an extra check for good connection in tube socket and other bad connections or poor soldering.
You have permanently rewired to triode mode, this make the amp less likely to oscillation, but if you have access to a scope, look for any odd waveforms on the output. If the noise is more of a buzzing sound it could be caused by noise on the heater supply. Adjust the heater balance pots for minimum noise with your ear to the speaker.
A bit odd that the PT gets to hot with 35mA per tube, are you sure you have measured and calculated correctly? It expect it should be able to supply 50mA/tube without problem, but lower current usually sound just as good, and increases tube life.
--
Svein B.
Hey Sven B,
thx. for al the helpfull tips and info
I do think to that I have a recent MA with KT88 tubes.
Lot's of things are not present or changed on my MA.
One of them is that I don't have al the resistors on the input of the amp.
The cathode capacitor on the input stages is also a lot bigger then what you see on most schematics. Mine's are 1000µF . Should be gooso I leave them in place.
The heater balanced is been done ... allthough I noticed ... when I did the adjustment before because of a 50Hz noise that I heard in the left channel ... that the variable resistors aren't in a very good shape. Lot's of cracks were hearable when I adjusted them but anyway ... those two are easy to replace.
Anyway .... that kind of noise, a very loud hum ... is gone and isn't the kind of noise which I was talking about earlier.
The noise is realy high in frequency ... big guess ... let's say above 10Khz.
The bias is measured and adjusted correctly.
It's easy .... the resistors where you measure the bias on are 100ohm resistors.
So 350mV is 35Ma ... easy.
But maybe you are right ... I think I am going to turn the bias back to 25mA ... it sounded good that way before so ....why not.
One thing ... the hardwire didn't change anything on the high noise that I hear. It was there before the triode mod.
And about the heater wiring of the 12AT7 tubes .... between pins 4 and 5 of the AT7 sockets ... on my MA ... the pcb-connection has been cut ... the copper has been scratched away.
Give me some time ... I'm gonna put a lot of inside pictures online of my amp.
thx. for al the helpfull tips and info
I do think to that I have a recent MA with KT88 tubes.
Lot's of things are not present or changed on my MA.
One of them is that I don't have al the resistors on the input of the amp.
The cathode capacitor on the input stages is also a lot bigger then what you see on most schematics. Mine's are 1000µF . Should be gooso I leave them in place.
The heater balanced is been done ... allthough I noticed ... when I did the adjustment before because of a 50Hz noise that I heard in the left channel ... that the variable resistors aren't in a very good shape. Lot's of cracks were hearable when I adjusted them but anyway ... those two are easy to replace.
Anyway .... that kind of noise, a very loud hum ... is gone and isn't the kind of noise which I was talking about earlier.
The noise is realy high in frequency ... big guess ... let's say above 10Khz.
The bias is measured and adjusted correctly.
It's easy .... the resistors where you measure the bias on are 100ohm resistors.
So 350mV is 35Ma ... easy.
But maybe you are right ... I think I am going to turn the bias back to 25mA ... it sounded good that way before so ....why not.
One thing ... the hardwire didn't change anything on the high noise that I hear. It was there before the triode mod.
And about the heater wiring of the 12AT7 tubes .... between pins 4 and 5 of the AT7 sockets ... on my MA ... the pcb-connection has been cut ... the copper has been scratched away.
Give me some time ... I'm gonna put a lot of inside pictures online of my amp.
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I believe you mean 10 ohm. There may however be two in parallell, the otherr on the other side of the PCB, making the cathode resistors 5 ohm.
In that case 350 mV would be 70 mA, which would run a bit hot.
--
Svein_B.
You start to confuse me Svein_B ...
I'm gonna checkt it again for sure ... with my Fluke ... anyway .... today I am planned to do a re-adjustment of the bias anyway.
About the pics ... thx. Salas for checking the problem with the pics
You're right Svein_B
I measure 5 ohm on each of the 4 cathode resistors ... oeps ... so indeed I'm running them now at 70mA .....
Gonna turn the bias voltage on the resistors .... which are indeed 10 Ohm ... back to 250mV right away.
That should correspond with a bias of 50mA.
I have a bias calculator here;
it shows me that with an anode voltage of 444Volt, and a WA max of 42Watt each KT88 .... I should be running now at a bias of 55% of what the tubes can handle.
This should be a nice Class A/B setting.
Thx. Sven_B ... I wasn't aware running my bias so high
I measure 5 ohm on each of the 4 cathode resistors ... oeps ... so indeed I'm running them now at 70mA .....
Gonna turn the bias voltage on the resistors .... which are indeed 10 Ohm ... back to 250mV right away.
That should correspond with a bias of 50mA.
I have a bias calculator here;
it shows me that with an anode voltage of 444Volt, and a WA max of 42Watt each KT88 .... I should be running now at a bias of 55% of what the tubes can handle.
This should be a nice Class A/B setting.
Thx. Sven_B ... I wasn't aware running my bias so high
Another thing I discovered today ....
The grounding of the chassis which is missing on the MA.
I connected it ... grounded the chassis with the ground pin of the AC input ...
well ... I immediately noticed weired kind of noises in my speakers ... I even heard mediacenter PC activity in the speakers.
When I turned my mediacenter PC off .... gone was the noise.
The noise went also away when I switched input on the MA.
Defintely ground loops ..... so if you wonder why they didn't connect the chassis to the ground ....
The grounding of the chassis which is missing on the MA.
I connected it ... grounded the chassis with the ground pin of the AC input ...
well ... I immediately noticed weired kind of noises in my speakers ... I even heard mediacenter PC activity in the speakers.
When I turned my mediacenter PC off .... gone was the noise.
The noise went also away when I switched input on the MA.
Defintely ground loops ..... so if you wonder why they didn't connect the chassis to the ground ....
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