Artisan 15 Redplating tubes

HAND-CRAFTED TONE. Artisan Series Amplifiers.
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Posts: 3
Joined: Sat Nov 02, 2013 4:10 am

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

Posts: 3
Joined: Sat Nov 02, 2013 4:10 am

Sun Nov 10, 2013 7:37 am

After taking a few more readings on this amp, I think I have it working as I would like it to. (IE where Im not blowing up power tubes that often, though I do plan on changing to 7189 tubes due to the high plate voltage in this amp). I would have really liked someone from Blackstar to chime in on this though. I do have a couple of other thoughts.

From website on this amp:

Class A 15W Combo
Switch for 5W triode mode / 15W pentode mode

Lets deal with the switch first off because that is the easiest to address.
Running a Pentode in Triode mode just means you strap the screen grids directly the the plate.
This switch does nothing like this. All it does is alter the signal from the Phase inverter to one of the power tube either dropping out completely or making the signal it gets very low so in essence, you have only 1 power tube conducting. You are effectively dropping the power output because only 1 power tube is doing all the work, but this has absolutely nothing to do with whether the power tubes are operating in Triode or Pentode mode.

Class A operation.
Im still working this out but my initial indications are, as I got the amp, it was Class A. Without getting too deeply into the math involved here You would have to run these EL84s at around 21w plate dissipation to get Class A operation out of them. Now, when I got the amp, they were running that hot. Unfortunately, the EL84 isn't designed to run plates that hot. As I noted in my other post, the stock tubes were redplating and any subsequent set of EL84s I put in also redplate. A definite sign of too much plate dissipation. In order the stop the destruction of any power tube I would put into this thing, I had to change the bias down to something a lot closer to spec (Which is 12w). This would put the amp into Class AB territory, with the amp operating largely in the class A range. In order for an amplifier to be "true" class A, the output must be biased in the center (the most linear portion) of the operating region of the output tubes. To get the amp down into the true Class A range, the plate voltage would need to come down to 250v in order to set the Bias correctly to get 12w out of the tube. This amp runs very high plate voltage, even exceeding the maxes listed in every EL84 datasheet I could find, though Blackstar isn't the first to do this and wont be the last. As an example, Fender ran their Deluxe reverbs at 420v+. The max rating for a 6V6GT is 315v (according to GE). They also ran some of the Bassmans on 5881s with 440v plate. 5881s are speced to have a 400v Plate max. So were left with this, I think the amp as I got it was running Class A with an absurd plate dissipation of 21w, causing very high tube wear and possibly damaging the amp when the power tubes fail or, push the amp into Class AB range by changing the bias point, put the power tubes closer to their spec'ed 12w dissipation and call it a day.
If you want to read more on this subject, see this post at
Its specifically talking about a VoxAC30 but the concepts are what we're taking away from that posting.

Now none of this actually addresses the most important aspect of this amp. How it sounds. It absolutely rocks. It sounds great, plays great and does inspire me to play better. My main concern with most of this has to do with the useful life of this amp. Stock, I don't see it living a very long life with as often as I gig. I do plan on putting a lot of wear on this amp and would prefer to not have to change tubes out so often or blow it up as these amps are fairly expensive.
Any comments would be greatly appreciated.

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