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|4th April 2008, 03:08 PM||#1|
Join Date: Oct 2007
music angel 845 SET
A call for some advice please. I have a chinese music angel 845 single ended valve amp which sounds surprisingly clear and pure. However switch-on with 240volts here in the uk produced a loud thump from the amp (not from the speakers). I overcame this by fitting a switched box between the amp's power plug and the wall socket. The box swiches in resistors until the caps are charged then switches in the full 240v, which has stopped the loud thump. Recently I made up another box with 5 x 100ohm ww resistors which lowered the voltage to 217, more in line with the voltage of 220 which the amp is designed for. All sounded fine for a while. Now the tonality of the amp has suddenly changed. The treble is greatly reduced, bass is reduced, and the upper mid is prominent, giving the sound a megaphonic quality. Transparency is greatly reduced. I can't see any connection between the boxes and the resultant sound. Returning the amp to 240v supply does not cure the problem. Methinks signal capacitors may have failed but the incorrect tonality is on both channels. I intend shortly to measure all the capacitors and resistors to see if any are out of spec.
Before I begin examination I thought I would invite thoughts and observations from you. Have you come across this problem? Can defective valves bring about such altered tonality? I shall be most grateful for guidance and advice.
Peter Parsons. Cornwall UK.
|4th April 2008, 08:34 PM||#2|
diyAudio Moderator Emeritus
Join Date: Jan 2003
Location: Near London. UK
I have edited your post by removing your email address before the spambots inundate you. I can put it back if you wish...
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
|9th June 2008, 04:27 PM||#4|
Join Date: Nov 2004
Location: Saint Louis, MO
I appreciated that last link about the Music Angel 845 amps. Do you have any other links about people modifying them, or common problems with the amplifiers? Rather than doing a scratch build for a friend, I was thinking of starting with a MA and making some improvements.
|10th June 2008, 06:03 AM||#5|
Join Date: Oct 2004
Location: Adelaide South Oz
My Music Angel 845 is now "Cactus" and is for the time being in the "too hard to fix" basket.
In OZ our power line is 250V AC. That means I had +880V for B+, that across series connected +450V rated electrolytics. The electrolytics protrude through the case and the top one in the series connction had it's can at half rail, depending upon its plastic film wrap to prevent short to case. I found that out the painfull way.
Despite that, mine failed I think as result of a bias failure or output tube (845) short. One output transformer now has an open circuit between primary ends BUT has a short between one primary end and the secondary.
I have a friend who still has a good unit which he drags around to my place to have me trim the bias each year BUT he runs his off a mains conditioner set for 220V AC output.
On balance I think I would recommend against the purchase of one of these units. Its cheap, in fact its cheaper than you could buy all the parts and build it from scratch, but that cheapness is reflected in its design and construction.
Just my opinion, and possibly coloured by the fact that my unit is now "Cactus" and requiring a replacement output transformer.
Some minor mods I did early on.
|11th June 2008, 11:01 PM||#6|
Join Date: Jun 2008
Music Angel 845 upgrade
I own such a music angel 845 and have modified it like this
have shown here on the link applied.
1. The Music Angel amplifiers are Cheap.
2. The Music Angel are never built the same way, even if it is suppose to be.
3. If you are thinking of upgrading it, take everything from scratch.
4. Even the power supply card are NOT the same on each amplifier.
5. You have to know almost as much about tube amps, as you would have to know to be able to build it from scratch, to be able to modify it properly.
6. The only thing I found to be usable to me was the Output trans and the chassis. (Maybe the the bias pot's are OK, the 10volt Filament output, and the Grid bias voltage from the power card.)
There is 6.3volts AC Filament for the Preamp-tubes as well.
in mine this gives Hum.(maybe this is not the reason.)
7. The power supply for the Pre-amp-tubes and the driver tubes was taken directly from the B+ 900Volts.(855v) Only with 3x33K resistors in series.
Meaning if you change anything in the circuit, voltage will jump up or down depending on load. AND the voltage takes Forever to stabilize.
8. it could be a nice amplifier to play a little around with, since it is not so extremely expensive. Back side is the lethal voltages.
9. My friends amp, same 845 amp is not singing anymore.
Something is shorting from time to time inside it. Traces on power PCB is all of a sudden vaporised, we fix and it happens again??
Don't know what the problem is really.
I am pretty satisfied with the result, if you don't care all the mess.
I changed Pre tubes to Russian types 6N9S, working good.
Also the Driver tubes was changed to Russian type 6N8S.
You also can use the 6N8S in preamp stage, if you don't need all the gain. Only thing,,, You have to solder a lot of resistors paralleling the
33K's to get higher voltage due to more current draw.
Actually I had to do this to get the correct currents after rebuilding it too.
Also I put in a bunch of PP-47uF/500v Caps, Changed all the coupling caps to Paper in oil's,paralleled with Teflons, changed all the resistors to Dale brand, put on some 1.2uF/1200V caps in parallel on the 900Volt, and some PP-500v/47uF in series.
Adjusted the bias to 65mA on the 845's.
Had to adjust also the Bias voltage/Grid voltage to get this right.
Extra caps on the 10v filament.
All the other things this guy Lukasz Fikus recommended.
This amplifier sounds pretty good on my Coral's Flat8 II, but we have been testing the ZEN variations 9 now, running it with the Bride of Son of Zen preamp, connected in NON balanced mode.
I must say this is much better. ZEN9 with NO feedback, maybe more similar, Zen9 with just a little feedback 47K + 33K, I liked it much better than the Angel. (Maybe do that on the Angel too ?? ))
Maybe it is even cheaper to make the ZEN amps?? NO Probably not.
Certainly much more work, but less dangerous though.
|21st December 2009, 06:52 PM||#7|
Join Date: Mar 2008
MUSIC ULYANOV mod by adam2a3
Hello, My MA amps have a separate dc supply for the driver tubes, not resistors to drop from the 800V ps. The first thing to do with this amp is to connect the IEC ground post to chassis (missing wire) Second, to remove the plastic insulation rings from the black "zero" speaker posts. You don't want some day your speaker floating on +800 Volts! Third, add fuses at the power transformer secondaries! Forth, replace all interstage caps with others of better quality. And finally use better 6sn/l7 tubes. 845 tubes are ok. My amps produced only 10 Watts sinus continuous @8 Ohms/ch sounding like very good transistor (!) amps. I usually drive stacked Quad ESL57's.
My latest mod is to remove the 845's and install GM70's with jumbo4 male (!) sockets (taken from broken tubes) attached on them without removing the amp's 845 sockets. A dual floating external dc stabilized supply provides 20 Vdc @ 3A per channel having PTC's in series with filaments for soft start. Inside the amp I used the dual 6SN7 driver as described in "lampizator..." designed by our friend from Austria (by the way I can't reach him, his address looks invalid).
I took one more step towards the disaster (don't try it) by adding on one channel's +800 Volts the second channels FLOATING supply, but without doubling it, so I get a B+ of 1050 Volts under load (95mA's per GM70). Also I added both windings of the driver's p supplies to get whithout doubling the single driver B+ required of +400 Volts. Final result is a clean 24 W sine cont @ 8 Ohms/ch with super bass response and a total excellent musicality.
It is not a Music Angel any more but a MUSIC ULYANOV! After 2-3 hours the power transformers are getting hot to be touched but nothing exploded yet, and the electrolytics with 1KV+ close to chassis holes are still working. Meanwhile, a descent external stabilized-current sourced-shunt regulated HV power supply is on the way to be built and replace the hot one. But you may keep the 800 V per channel whithout any mods and have less output power from the GM70's.
|11th November 2013, 03:46 AM||#8|
Join Date: Jan 2006
Your help please! Your instruction and/or advice on how to adjust Voltage/mA for a Dared Vp-845 amp as I replaced a few damaged caps and blown resistors(I have a Fluke 77 Voltmeter).
Your help is truly appreciated.
|11th November 2013, 04:12 AM||#9|
Join Date: Sep 2006
Location: Mar del Plata, a BIG seasonal getaway city, can see the Ocean from our residence.
Sounds like that .33 uF/630V cap between the drivers & the 845s.......Why is that cap rated at only 630V? Isn't one suppose to design that cap to hold the full B+?
|18th August 2015, 10:31 AM||#10|
Join Date: Aug 2013
I know this topic is old, however I finally modified my MA 845 following Dietmar's Circuit 4.2.
The only differences from this circuit are:
1) 150k@2w "to Bias" resistor (instead of 220k)
2) Coupling Capacitor 1uF Mundorf Silver Oil Gold (instead of3x Capacitor Mix)
3) 845 F1 / F2 Pot is 100k instead of 250k.
I checked the circuit a couple of times, nothing different than the schematics.
All I hear is a strong 50hz noise (is probably putting more than 1w to the speakers) I can even see the cones moving.
Any further ideas on how I could start troubleshooting?
My initial idea:
1) disconnect the grid of the 845 and verify if the hum persists.
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