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Old 28th February 2007, 09:51 PM   #1
OXAUDIO is offline OXAUDIO  United Kingdom
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Default Music Angel Amplifiers

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
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Old 27th May 2008, 10:37 PM   #2
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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.
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Old 30th May 2008, 10:00 AM   #3
OXAUDIO is offline OXAUDIO  United Kingdom
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Default 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
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Old 19th June 2008, 08:27 AM   #4
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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/...reworking.html
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Old 19th June 2008, 09:15 AM   #5
OXAUDIO is offline OXAUDIO  United Kingdom
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Default Music Angel XD-SE KT88

Quote:
Originally posted by vitalstates
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/...reworking.html
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
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Old 19th June 2008, 09:20 AM   #6
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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
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Old 19th June 2008, 04:22 PM   #7
kevinkr is offline kevinkr  United States
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Default Re: Music Angel XD-SE KT88

Quote:
Originally posted by OXAUDIO


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

Transformers referred to in a previous post are in an electro-static speaker system, (Quad ESL or similar) not the OPTs in the amplifier.

Hard wiring in triode mode would seem sensible if the additional power of UL mode is not required.
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Old 19th June 2008, 06:06 PM   #8
OXAUDIO is offline OXAUDIO  United Kingdom
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On re-reading the post concerned, I realise that I had misinterpreted it to mean that the OPT in the amplifier had burnt out. (Didn't see or take in the reference to 'Electrostatics' - must have had what is sometimes called a 'senior moment'!)
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Old 19th June 2008, 07:10 PM   #9
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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..........
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Old 19th June 2008, 07:44 PM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by Valvenewbie
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?
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