Regarding the Music Angel KT88 Amplifier, there is a version in production which has 4 bias adjust pots., (two for each channel) There is some interaction between them, but it does mean that each KT88s grid bias can be independantly adjusted. I have set them so that there is approx. 950mV on each Cathode, with the HT+(B+) line reading 415v, this beig 33watts, using the formula P divided by V x Rk, as explained by 'gingertube' in his post dated 11-02-05
Erm sorry if this departs from normal protocol but I'm too new to email you direct. I'm a newbie to the whole valve amp thing but keen to learn. I recently purchased a Music Angel KT88 which I know you're pretty knowledgable about. I've downloaded various schematics, taken pictures of the inside of mine and quite frankly don't know where to begin with tweaking/modding it. I live in Gillingham Kent and couriering it back forth to Oxford could be dear, would you be able to do schematic and mod reccomendations from photos? Happy to pay, may even be worth sending to an old pro and getting it done properly! Please contact me if you can help. 
Music Angel XD800 KT88 Amplifier
'Valvenewbie', as you know, I have already emailed you with regard to modifying your XD800. The following may be of interest to other forum members. One of the points you raised concerns the use of ceramic valve(tube)holders, which I personally think is a good idea, along with remounting the 4 1k 'gridstopper' resistors as close to the grid pins of the KT88s as practicable, connecting them by the shortest possible leads to the
PCB. Ideally, perhaps, the whole output stage should be 'hardwired', rather than using a PCB, or is that going a bit too far. It would certainly be very time consuming!! As to the 'dreaded' Triode/Pentode switch I guess it would be possible to do away with(or at least disconnect) the relays J2 & J3, and repacing the exsisting switch with a 4pole 2way type capable of handling the KT88 anode currents directly, thus getting rid of the source of 'pop' caused by the relays 'dropping out', although some signal degradation might still be present, and it doesn't alter the fact that the output transformers used aren't, IMHO, designed for true 'Ultra Linear' Pentode operation
'Valvenewbie', as you know, I have already emailed you with regard to modifying your XD800. The following may be of interest to other forum members. One of the points you raised concerns the use of ceramic valve(tube)holders, which I personally think is a good idea, along with remounting the 4 1k 'gridstopper' resistors as close to the grid pins of the KT88s as practicable, connecting them by the shortest possible leads to the
PCB. Ideally, perhaps, the whole output stage should be 'hardwired', rather than using a PCB, or is that going a bit too far. It would certainly be very time consuming!! As to the 'dreaded' Triode/Pentode switch I guess it would be possible to do away with(or at least disconnect) the relays J2 & J3, and repacing the exsisting switch with a 4pole 2way type capable of handling the KT88 anode currents directly, thus getting rid of the source of 'pop' caused by the relays 'dropping out', although some signal degradation might still be present, and it doesn't alter the fact that the output transformers used aren't, IMHO, designed for true 'Ultra Linear' Pentode operation
Just an update regarding these amps....I've posted about my XD-SE KT88 on other threads, and in summary I still think its a fantastic bargain for the price......however, regarding the triode/UL switch I would say that its an absolutely necessary mod it you run the amp in Triode mode....
I've recently had to replace the transformers in my electrostatics as one of them burned out. The manufacturer suggested the xfrmr had suffered a severe spike, which I was at first sceptical about, until I remembered the constant splat that came from the KT88 when it was turned off....that could be the cause of my transformer failure....
The fact that the triode switch defaults to UL means that as soon as the power is turned off the relay drops out and the amp switches to UL mode, which presumably dumps all residual power into a more efficient configuration, and hence the dying click.
I wish I had done the switch mod when I was putting the earth in now....ho hum, we live and learn
Ed
ps is this of any use valvenewbie??
http://www.lampizator.eu/AMPLIFIERS/CHINA/KT88/Music Angel KT88 reworking.html
I've recently had to replace the transformers in my electrostatics as one of them burned out. The manufacturer suggested the xfrmr had suffered a severe spike, which I was at first sceptical about, until I remembered the constant splat that came from the KT88 when it was turned off....that could be the cause of my transformer failure....
The fact that the triode switch defaults to UL means that as soon as the power is turned off the relay drops out and the amp switches to UL mode, which presumably dumps all residual power into a more efficient configuration, and hence the dying click.
I wish I had done the switch mod when I was putting the earth in now....ho hum, we live and learn
Ed
ps is this of any use valvenewbie??
http://www.lampizator.eu/AMPLIFIERS/CHINA/KT88/Music Angel KT88 reworking.html
Music Angel XD-SE KT88
19 June 08 0858GMT
We do indeed, live and learn. I hadn't until now heard of output transformers being burnt out on these amplifiers, but I can see that a severe spike, possibly caused by the switching to UL at switch off could do this. If any Music Angel Amp comes my way for repair/mod. in future I shall certainly recommend that the Triode/UL Switch be bypassed. Output Transformers are too expensive to risk damaging!! Incidentally, and I must stress I haven't tried this, I wonder if output transformers made for the original Williamson Amplifier design,some versions of which have true pentode(UL) connections via taps in the primary windings, could be fitted to these amplifiers. Sowter in the UK can supply them, but they cost abt £181 + TAX(17.5%VAT in GB) EACH!(About $400 at current exchange rates) Bear that in mind, folks
Mike
vitalstates said:
19 June 08 0858GMT
We do indeed, live and learn. I hadn't until now heard of output transformers being burnt out on these amplifiers, but I can see that a severe spike, possibly caused by the switching to UL at switch off could do this. If any Music Angel Amp comes my way for repair/mod. in future I shall certainly recommend that the Triode/UL Switch be bypassed. Output Transformers are too expensive to risk damaging!! Incidentally, and I must stress I haven't tried this, I wonder if output transformers made for the original Williamson Amplifier design,some versions of which have true pentode(UL) connections via taps in the primary windings, could be fitted to these amplifiers. Sowter in the UK can supply them, but they cost abt £181 + TAX(17.5%VAT in GB) EACH!(About $400 at current exchange rates) Bear that in mind, folks
Mike
Cheers V, that's very useful to know and is one of the modifications I want Oxaudio to do once he has the time. I did look at the lampizator website when I was contemplating doing it myself but I'm such a beginner and there were too many anomolies on his website let alone the differences in my circuit which I couldn't get my head round. Still very interesting reading though.Interestingly lampizator has a different approach to some such as Oxaudio and Gingertubes, Lampy suggests increasing the size of the capacitors in various places but this is cautioned against in DIY forums because of the Williamson circuit. Double dutch to me but too much of a quagmire for me to muddle through quite yet!
Cheers again
Valvenewbie said:LOL, well, glad we got to the bottom of that one. Now you've got me nervous I think I'll disconnect my speakers from the back before I switch off my amp to be safe until "senior moment" Oxaudio upgrades my amp..........
Just switch to UL mode before powering down - my switchable amplifiers had almost inaudible pops when changing mode, is this not the case here?
I find it pops whatever mode it's in, in fact I read the opposite in a thread; that it shouldn't pop if your in Triode mode. Given that these amps tend to be quite unique in the way they're put together, from 1 to another, quantity, quality and even placing of components it may not be that easy to ascertain patterns in behaviour.Having said that I still think it's a cracking amp, very nice looking and the sounds great but rather than wait for some of the cheaper components to fail, or my speakers to pop, or get myself electrocuted with the groundless body I want to get it mod'ed a bit. All in all it will have probably cost me about £400 not including tube rolling which is pretty good all in methinks for what you get.
'Valvenewbie'
ASsyou will now know it wasn't the Music Angel's Output Transformer which burnt out, but a Transformer in an Electrostatic Loudspeaker which failed. I will get in touch with you about modifying your Amp. as soon as I have reduced my workload to a more manageable level. In the meantime, the suggestion that, if you normally use the Music Angel in Triode Mode, switch it to Pentode(UL) Mode BEFORE Turning Off the Power Switch is a good idea, since the crackle is due to the relay switching back to the default - i.e. pentode - mode as power is removed.
(Hope all that makes sense)
As to the mains earth- it is a simple job to attach a short lead to the ground pin of the IEC power socket, with the other end connected to chassis via one of the adjacent screws. Having said that I must stress that you should only do this yourself if you are ABSOLUTELY CERTAIN that you know what to do
ASsyou will now know it wasn't the Music Angel's Output Transformer which burnt out, but a Transformer in an Electrostatic Loudspeaker which failed. I will get in touch with you about modifying your Amp. as soon as I have reduced my workload to a more manageable level. In the meantime, the suggestion that, if you normally use the Music Angel in Triode Mode, switch it to Pentode(UL) Mode BEFORE Turning Off the Power Switch is a good idea, since the crackle is due to the relay switching back to the default - i.e. pentode - mode as power is removed.
(Hope all that makes sense)
As to the mains earth- it is a simple job to attach a short lead to the ground pin of the IEC power socket, with the other end connected to chassis via one of the adjacent screws. Having said that I must stress that you should only do this yourself if you are ABSOLUTELY CERTAIN that you know what to do
I also have a music angel kt88 and in the manual it says not to switch modes while the amp in on! Mine developed a hum after about 20 hrs and I got a schmetic from the guy in canada who repairs them. He drew his own schmetic because the company would not send him one.He said they are very secretive and worried someone will copy thier design. Even with his schmetic I found the the filter caps were different than his values.
The filter capacitors in an amp. I repaired/modded a year or so ago were differnt values from those shown in the schematic posted on this forum. I fitted higher value capacitors, with balancing resistors accross them, amongst other mods.
As to the design, from what I can see the PSU may be original, although it's circuitry is of a fairly standard type, but the amplifier proper is, as is well known, a Williamson circuit not too different from that designed about 60 years ago, except that PCBs are used, so the PCB layout would be the only thing unique to the MA
in any case!!
As to the design, from what I can see the PSU may be original, although it's circuitry is of a fairly standard type, but the amplifier proper is, as is well known, a Williamson circuit not too different from that designed about 60 years ago, except that PCBs are used, so the PCB layout would be the only thing unique to the MA
in any case!!
Yes it's ironic that they copied another design and they are worried about someone copying thier amp. I guess they don't want you to know the values of resistors or caps they are using.I got the impression from the repair guy that there are many versions of this amp with different values for the parts.This makes it hard for anyone else to copy.
keithgreenhalgh said:
I find that changing modes with the amp on, as long as the volume is low doesn't produce a pop in the speakers. It definitely makes much less noise than the snap crackle of turning off the amp in triode mode. I thought it popped in UL as well but I'll try it out again Oxaudio.
For anyone looking for the schematic there's a thread which has an attachment of the schematic:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=91594&highlight=
Or should I say "a" schematic, don't think it accurately reflects the amp I have....
help!!
Hello all, I have a problem with my Music Angel KT88 XDSE resistors have burnt one quoting (PCBs KT88) I replace the 3 20ohms resistance 2.7 k x2 x2, there was a capacitor 470mf HS / 450V.
I reconnect and resistance to smoke again???? HS Tube?
Thank you for your help
I'm French and my English is not as good as the Roquefort! lol
Hello all, I have a problem with my Music Angel KT88 XDSE resistors have burnt one quoting (PCBs KT88) I replace the 3 20ohms resistance 2.7 k x2 x2, there was a capacitor 470mf HS / 450V.
I reconnect and resistance to smoke again???? HS Tube?
Thank you for your help
I'm French and my English is not as good as the Roquefort! lol
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