I've been reading as many of the other El34/MA based threads and I'm just as, if not more, confused as when I started.
I picked up a Magic eye version of the quad EL34B PP XD-SE Music angel amp and have since discovered there are many variants.
Mine is red plating on one of the Valves, so I thought I'd start out by checking and adjusting the Bias.
The chap I bought the amp off supplied a handy bias measuring tool for an additional few quid which is indicating 30-31 on 3 of the EL's and 34-35 on the one thats red plating.
Trouble is, it's a bit confusing underneath as to which of the 6 pots adjust the bias, and I was hoping someone on here could point me in the right direction.
I've attached a few images, hope they come out.
Here is the red plating Valve doing its stuff:
And a picture of the underside, with some labels in case someone can match up the numbers and letters to explain which pot does which valve:
I've also noticed a few other troubling things...such as no earth connection to the chassis... although it seems that the Ground on both Input and Output have continuity to the chassis....
I guess I need to deal with one issue at a time.
Any help greatfully received!
I picked up a Magic eye version of the quad EL34B PP XD-SE Music angel amp and have since discovered there are many variants.
Mine is red plating on one of the Valves, so I thought I'd start out by checking and adjusting the Bias.
The chap I bought the amp off supplied a handy bias measuring tool for an additional few quid which is indicating 30-31 on 3 of the EL's and 34-35 on the one thats red plating.
Trouble is, it's a bit confusing underneath as to which of the 6 pots adjust the bias, and I was hoping someone on here could point me in the right direction.
I've attached a few images, hope they come out.
Here is the red plating Valve doing its stuff:
An externally hosted image should be here but it was not working when we last tested it.
And a picture of the underside, with some labels in case someone can match up the numbers and letters to explain which pot does which valve:
An externally hosted image should be here but it was not working when we last tested it.
I've also noticed a few other troubling things...such as no earth connection to the chassis... although it seems that the Ground on both Input and Output have continuity to the chassis....
I guess I need to deal with one issue at a time.
Any help greatfully received!
Last edited:
That's usually not enough of a difference to make a tube redplate, unless it's just worn out. If you swap the two EL34's, does the other one redplate as well?
Also without a schematic, it's going to be a tough go figuring out what the problem is. Your version seems to be different than the one's that come up in a search.
Maybe post a pic of the topside.
jeff
Cheers for the reply Jeff. I've attached a couple more pics... I also swapped valves 3&4 (Viewed topside...3 was red plating prior to the swap) and as you can see... no red plating on either now!
The bias numbers are still out a little (34 on number 3 viewed topside, 30/31 on the rest So I'd still like to get that sorted... if anyone can advise on which bits to twiddle...?
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
I have the same amplifier, it has fixed bias which is set by the four carbon resistors on the PCB above the valves in your picture. The PCB is marked a,b,c,d. I replaced them with pots to adjust the bias. The bias in my amp was 70ma, needless to say the tube life was very short.
I added a cl60 between circuit ground and chassis ground along with chassis ground to IEC ground.
I added a cl60 between circuit ground and chassis ground along with chassis ground to IEC ground.
I would suggest replacing the EL34's with a new matched quad, as your tubes are probably at or near the end of their life.
So after some internet trolling, and looking at several schematics, which of course are all different, I found something that may be of some help. Have a look at the Lampizator link, and you'll see that the PS board on the right is the same as yours. Scroll down near the bottom and you'll see the "bias the output tubes" section with pics of the blue adjustment pots, marked 1,2,3,4. There appear to be two extra blue pots on your PCB near the center left. I think those are for the heater balance, and should be left alone. He shows the cathode resistors (A,B,C,D) and how to measure the voltage drop across them. Looks like he changed them from 22 ohms to 10 ohms for convenience. As you have the plug-in bias adaptors, you shouldn't need to do this. Ok, good luck and report back on your findings.
Music Angel XD-800MKIII reworking
jeff
Hi, Further to Jeff's response above.
1 - Note that the KT88 schematics he links to at the Lampizator site has a topology more similar to a Williamson, while the El34 version has a topology more similar to a Mullard. Attached is a schematics I found on my harddrive of a EL34. Unfortunately I no longer know the origin of the drawing, but it is likely to be a reverse engineered drawing. Note that as with most other MA amplifiers it seems that circuit details and component values can vary between each production run, so use it as a guideline not as gospel! Furthermore it appears that your amp has already seen some DIY action. For example bleeder resistors on the PSU capasitors seem to be retro fitted.
2 - Get current flow at the position that is red-plating under control before buying a new quad. Although I have never worked on the EL34 version myself I will venture some advice for troubleshooting. Attached you will find a PDF with 4 items to check. I will address them one at the time.
a) Are you red-plating in both anode and pentode mode? The control board holds two relays that are activated with the single external switch. Theese are my first suspekt! It appears that it is not uncommon for these relays to get damaged when triode/pentode switch is toggled under power or without power capasitors being sufficiently discharged. If so the grid-2 may be "floating". This would do strange things to your bias current. Also check the integrity of the associated G2 resistors. Components to check is highlighted with a red circle in the pdf. My KT-88 amp use the same control board, and I found these had been removed by a previous owner and relay position hardwired for pentode by a previous (who had also replaced one transformer).
b) Bias checking resistors (yellow circle), not likely to be the issue on your amp at the moment since you actually have current flowing. Anyway, from experience I know that these can fail "open circuit". On the attached diagram they are shown as 2x10 ohm in parallell, on you control board they appear to be a single 22 ohm. Anyway, you may want to replace these with higher wattage 10 ohm resistors as per Jeff's advice above once you have the red-plating solved.
c) Grid bias resistors. (blue circle). These will likely be on the main amplifier PCB one end connected to the control grid (grid-1) and the other to the wiper on the biasing pot. You can use this to track what pot adjusts which valve position. Double check the values and soldering on all 4 four valves. Also note that you may experience a degree of interaction. So always re-check al four valves after any adjustment.
d) coupling capasitors (green circle). Probably ok, but check for leakage (dc-resistance). My experience is that MA have voltage ratings for "operating conditions". If the amp has been powered with tubes removed they may have seen over voltage. A leaky capacitor here would effectively increase bias voltage on the power valves thereby leading to increased bias current.
1 - Note that the KT88 schematics he links to at the Lampizator site has a topology more similar to a Williamson, while the El34 version has a topology more similar to a Mullard. Attached is a schematics I found on my harddrive of a EL34. Unfortunately I no longer know the origin of the drawing, but it is likely to be a reverse engineered drawing. Note that as with most other MA amplifiers it seems that circuit details and component values can vary between each production run, so use it as a guideline not as gospel! Furthermore it appears that your amp has already seen some DIY action. For example bleeder resistors on the PSU capasitors seem to be retro fitted.
2 - Get current flow at the position that is red-plating under control before buying a new quad. Although I have never worked on the EL34 version myself I will venture some advice for troubleshooting. Attached you will find a PDF with 4 items to check. I will address them one at the time.
a) Are you red-plating in both anode and pentode mode? The control board holds two relays that are activated with the single external switch. Theese are my first suspekt! It appears that it is not uncommon for these relays to get damaged when triode/pentode switch is toggled under power or without power capasitors being sufficiently discharged. If so the grid-2 may be "floating". This would do strange things to your bias current. Also check the integrity of the associated G2 resistors. Components to check is highlighted with a red circle in the pdf. My KT-88 amp use the same control board, and I found these had been removed by a previous owner and relay position hardwired for pentode by a previous (who had also replaced one transformer).
b) Bias checking resistors (yellow circle), not likely to be the issue on your amp at the moment since you actually have current flowing. Anyway, from experience I know that these can fail "open circuit". On the attached diagram they are shown as 2x10 ohm in parallell, on you control board they appear to be a single 22 ohm. Anyway, you may want to replace these with higher wattage 10 ohm resistors as per Jeff's advice above once you have the red-plating solved.
c) Grid bias resistors. (blue circle). These will likely be on the main amplifier PCB one end connected to the control grid (grid-1) and the other to the wiper on the biasing pot. You can use this to track what pot adjusts which valve position. Double check the values and soldering on all 4 four valves. Also note that you may experience a degree of interaction. So always re-check al four valves after any adjustment.
d) coupling capasitors (green circle). Probably ok, but check for leakage (dc-resistance). My experience is that MA have voltage ratings for "operating conditions". If the amp has been powered with tubes removed they may have seen over voltage. A leaky capacitor here would effectively increase bias voltage on the power valves thereby leading to increased bias current.
I don’t see how this scheme according to the music angel’s pdf could work. There is 680nF from G1 to ground, almost a short for AC. What am I missing?
Regards, Gerrit
Regards, Gerrit
Reviving this old thread (appears to be the least old 😀).
I just picked up one of these Music Angels - days XD500-mkIII on the front and XD-SE on the back. Anyway it is the 8 x EL34 version. I was hoping someone could point me towards the correct schematic. Directions for improvements would also be much appreciated...
I just picked up one of these Music Angels - days XD500-mkIII on the front and XD-SE on the back. Anyway it is the 8 x EL34 version. I was hoping someone could point me towards the correct schematic. Directions for improvements would also be much appreciated...
