Artisan 15 Redplating tubes
Posted: Fri Nov 08, 2013 5:38 am
I bought a used Artisan 15 from Sam Ash about a week ago and am loving it so far. Great tones, loud as all sin, easy to play and really does inspire me to play better
Now the bad:
I took the amp out of the cabinet and with the speaker connected, light out, I watched the power tubes. After about a minute or so, they developed a small spot of red-plating along the seams of one of the power tubes. I thought that since these were the original equipment, they might be a little worn out. Since this is Cathode Biased, I thought I'd try another set.
The JJs I tried redplate, the NP Mullards I tried also red-plate. (Note this is a very slight red-plate, the JJs more so than any others, I dont think you'd be able to see it with the lights on though) (At this point, Im not willing to trust my NOS Mullards in it).
So I looked into the amp and found 2 minor issues.
1. The Cathode resistor on the power tubes is 100r (seems very low for a pair of EL84s)
2. the first filter cap is 100uf (6ca4 datasheet indicates this shouldnt be over 50uf otherwise you cause heavy rec tube wear)
Now I read that in very early releases of this amp, this was stock but in later releases of this amp, the Cathode Resistor was changed to 120r and the first and 2nd filter stage was changed to 47uf.
Since I bought the amp used and its obviously older than the warrenty period, I decided to attempt some maintenance on this thing.
I took some reading with the amp as it was and was getting around 23w of plate dissipation which is WAY high. Here were the values:
Plate Voltage: 379v (between pins 7 and 3)
Cathode Voltage: 12.6v
Cathode Resistor: 100r (measured)
I changed the Cathode resistor to 120R 10w and the first filter cap to 47uf 450v. I re did the calculations and this was around 16w plate dissipation. Still too high.
Here were the values:
Plate Voltage: 374v (between pins 7 and 3)
Cathode Voltage: 13.6v
Cathode Resistor: 120r (measured)
I changed it out once again to a 250r 10w and now the plate dissipation is around 11w, which is within specs now. The stock tubes no longer red-plate but the JJs and Mullards still do.
Here are the reading from the last change:
Plate Voltage: 384v (between pins 7 and 3)
Cathode Voltage: 14.6v
Cathode Resistor: 250r (measured)
I checked for DC leakage from the PI and didnt see any.
So here is are the questions.
What is the plate dissipation designed to be at? (Datasheet says 12w, the amp config suggest 15w)
Should the 2nd filter stage also be changed to a 47uf 450v?
Anything else to check that could be causing red-plating
Now the bad:
I took the amp out of the cabinet and with the speaker connected, light out, I watched the power tubes. After about a minute or so, they developed a small spot of red-plating along the seams of one of the power tubes. I thought that since these were the original equipment, they might be a little worn out. Since this is Cathode Biased, I thought I'd try another set.
The JJs I tried redplate, the NP Mullards I tried also red-plate. (Note this is a very slight red-plate, the JJs more so than any others, I dont think you'd be able to see it with the lights on though) (At this point, Im not willing to trust my NOS Mullards in it).
So I looked into the amp and found 2 minor issues.
1. The Cathode resistor on the power tubes is 100r (seems very low for a pair of EL84s)
2. the first filter cap is 100uf (6ca4 datasheet indicates this shouldnt be over 50uf otherwise you cause heavy rec tube wear)
Now I read that in very early releases of this amp, this was stock but in later releases of this amp, the Cathode Resistor was changed to 120r and the first and 2nd filter stage was changed to 47uf.
Since I bought the amp used and its obviously older than the warrenty period, I decided to attempt some maintenance on this thing.
I took some reading with the amp as it was and was getting around 23w of plate dissipation which is WAY high. Here were the values:
Plate Voltage: 379v (between pins 7 and 3)
Cathode Voltage: 12.6v
Cathode Resistor: 100r (measured)
I changed the Cathode resistor to 120R 10w and the first filter cap to 47uf 450v. I re did the calculations and this was around 16w plate dissipation. Still too high.
Here were the values:
Plate Voltage: 374v (between pins 7 and 3)
Cathode Voltage: 13.6v
Cathode Resistor: 120r (measured)
I changed it out once again to a 250r 10w and now the plate dissipation is around 11w, which is within specs now. The stock tubes no longer red-plate but the JJs and Mullards still do.
Here are the reading from the last change:
Plate Voltage: 384v (between pins 7 and 3)
Cathode Voltage: 14.6v
Cathode Resistor: 250r (measured)
I checked for DC leakage from the PI and didnt see any.
So here is are the questions.
What is the plate dissipation designed to be at? (Datasheet says 12w, the amp config suggest 15w)
Should the 2nd filter stage also be changed to a 47uf 450v?
Anything else to check that could be causing red-plating