Music Angel XD 800MKIII

PROCESS OF KT88 ANGEL TWEAKING




A small disclaimer note is in place here.
Before opening the amplifier like this, please ask yourself some questions. It may help you to avoid risk of accident, or damaging your beautiful amplifier.

1. Do you know and understand Ohm's law ?
2. Can you read the schematics of this amp?
3. Do you know, that DC voltage is 10 x more dangerous for humans than AC, and even 30 V DC can be lethal if the current flows via both hands? Do you know that there is 450 V DC inside this particular Angel?!!!
4. Are you aware, that capacitors can store the voltage for MONTHS AFTER DISCONNECTION?
5. Have you ever done any soldering project that actually worked ?

If one or more answers are negative, please think again if you want to go there. This is the last moment to stop...   ;-)

Music Angel KT88

At first I did not like it, I had low expectations and prejudice. It was SUPPOSED to be bad. Puhhleeze: push pull? Not SE? Not class A? No triode but pentode? Local feedback? Chinese tubes ???

But after minimal cleanup - it surprised me a lot.
It is very hard for me to admit, that perhaps this is the best Music Angel of them all. Powerful, well extended on both ends, clean, fast and energetic. All I can think of - is excellent. And the tubes are easy to change. I recommend it very much to everyone. Try it with the underestimated 6L6 tubes or even 6550's.








The actual job concerns ONLY the green input PCB, not touching the power section.

The power section has a switch for TRIODE operation and this should be kept permanently in TRIODE never pentode God forbid. Best idea is to CUT AND REMOVE the wires leading to this switch on the back side.

I think that the power stage of the angel is good, there is nothing critical to do there.
After doing successfully the stage-one modification as prescribed below, we can enjoy the music and forget all the tweaking.

The input stage is easy to work on, and the effort brings big improvement.

I suggest to get first the right parts and print the schematics which I provide. Get the tools you will need (just soldering iron, clippers, pincette and flux solder. )

Parts needed: 4 capacitors MKP or PIO - 0,47 uF or more by 400 V.
4 capacitors of 1 uF or more by 400 or 600 V MKP or PIO.
Thin wire - naked copper or tinned copper or silver plated copper or silver - approx. 0,5 m.

SMALL SIGNAL PCB TWEAKING

The underside of the input PCB. As you can see, not all parts were on bottom side, 3 resistors and 2 wire jumpers were hiding here.




Schematics:
First: I remove and short all series resistors from the circuit like R2, R9, R11, R13, R15. To be safer - keep them in place, just short them with a piece of wire.
To make the mod safer and to save time iI decided to keep the original tubes, generally to retain as much as possible from the stock amp.

Second, I replace all foil coupling capacitors (4 white cubes and 4 blue ones) with some decent, radial shaped, MKP, minimum 400 V DC, minimum 4 x the original size.

Third I removed the hated cathode capacitor C1
R1 and R3 get removed in both channels.
C2 and C5 can be doubled.
All is described in the scheme below.
Triode - pentode switch is cut off.
The feedback cable leading from the speaker terminals to green input PCB gets cut off.


Angel KT88 factory circuit

Angel KT88 real life circuit with suggestions

First - lift the input cable from potentiometer and it's grounded screen as well.







These three resistors as above get removed.  







Input divider resistors are removed as deemed unnecessary - the POT is serving that role.




Now it is time to deal with the cathode bypass caps. We remove them - the orange thingies.



MAGIC !!! The orange caps are gone !




We do the same with the second stage - driver tube cathode capacitor - the brown electrolytic standing between the white cubes as on the picture above. Out with it !!!








Next goes the resistor from first anode to second grid. We remove it and short it.





And on the right channel as well




The wire bypass.




Next we deal with the second stage - driver (outer) tubes. Resistors must go:




This one as well, but NOT the bottom-most one - green-blue-yellow-silver. This is grid grounder which MUST stay.




On the left hand side as well.







As well as on the right hand side.








Now we go underneath and do some damage there. The red wires are heaters supply (they steal the 6,3 V power from power tube heaters). If you want to convert the input tube to 6922, 6dj8, E88cc, or even 6H1P - just move one heater supply wire to pin 9 and cut the trace between 4 and 5 on top side.





Another resistor gets shorted ....

And the same in the other channel ...






We remove all white caps - 4 pieces, and all blue caps - 4 pieces. They are very low quality for audio application like this.




We can put two saved blue caps (not white as on the picture)  to the empty holes near the opposite corners of the PCB to bypass the biggest Lytics . We recycle two caps !



This is the second place for recycled capacitor. The Lytic-bypass.





Now time to short the output divider resistors: out with them ! Left hand side visible above.






And the same on the right hand side. 





And on the left again....




The last shorting wire underneath ....





The ready underside.





Last step is installation of coupling caps - I use standard no name MKP - 1 uF for the first stage and 2,2 for second stage.

Re-connect the left and right channel wires - ground to ground, and signal to first grid of the tube.




The PCB looks finished.






The other channel view.

Biasing the power tubes in the Music Angel KT88

Now its time to bias the tubes. All four power tubes individually.
We must install all tubes, fire up the amp, and get ready to pull the plug if something is smoking, or tubes glow too much, or the transformer starts buzzing.


We measure the voltage drop on 4 resistors on power supply PCB - these resistors are 10 Ohms and the correct reading will be 350 mV for 35 mA nominal bias current.  Some people bias KT88 higher, but I do not believe that these chinese tubes can take full spec 100 mA, and I think the transformers are inadequate. There will be too much heat and also there may appear buzzing. My conservative 35 mA sounds great and is very stable. Tube life is much longer this way and everybody is happy.
Be careful - the resistors do correspond to power tube positions - 1,2,3,4, but NOT NECESSARILY to potentiometers. (may be different order.)

In my angel - these two were correct





But these two - were the other way around .....


Turn the brass head of the blue pot until the reading is 350 mV. Do it SLOWLY and let the current stabilize for a while.

After setting all 4 tubes - use the amp for 5 minutes and re-bias. Maybe re-bias again after a week or more.

BUZZING