wagdog
Ack
I was a little dismayed at the early demise of the new el84's I put in my MESA express 5:25. Since I'm now waiting on a replacement set from Sweetwater, I figured it would be a good time to figure out what's going on in the amp and if I can use some JJ's I have in peavey classic.
Of course MESA doesn't have a spot to check the bias on their amps, nor do they offer a way to adjust it. The party line is "use our tubes, you'll be good". Well, OK, I'm good with that, but when their tubes die in under 4 hours of use, AND a buddy of mine just got a set of MESA tubes that are incredibly mismatched (the dude with the Jet City amp) - I felt I needed to dig a little further.
So, I built a little el84 bias probe.
My buddy Tim repairs stained glass for churches. He has all kinds of glass shaping devices. I asked him if he could cut open a tube for me cleanly; no problem. He did 3 tubes for me. I broke one of them them getting the guts out. The next, a 12AX7 came apart cleanly:
This is the 1 ohm cathode resistor. By measuring voltage across it, you can tell how much current is going from the cathode to the plate in mV, which corresponds exactly to mA because of ohms law (a 1 ohm resistor passes 1mA for every 1mV it drops).
This is the voltage divider for the B+. It's a 1M resistor off the plate and a 100ohm resistor to ground. Reading the voltage at the junction of the 1M/100r gives a reading in mV, which when multipled by 10000 is the actual B+. So 30mV is actually 300V of plate voltage.
All wired up:
and fully taped w/popsicle sticks for support:
After testing all the connections, and not wanting to risk the MESA right off, I popped the probe into my peavey classic with some JJ's. B+ read about 380V and 28ma current. Now, I didn't use 1% resistors, but I did measure them and they were pretty much spot on... and just how accurate is my $40 meter anyway? Those measurements give me an idle dissipation of just over 10w - which is high for an el84, but not unheard of.
Next the MESA with the new tubes they sent (one of which redplated and the set is being replaced): B+ 392V and 31mA of current - 12.1w(!). Yikes. I tested the JJ's in the mesa, they are about 30mA, so I'm comfortable running them knowing the MESA tubes are running that hot as well. The JJ's were very well matched; I got 4 from tube depot for the peavey and they were all within 1 mA of one another.
I wish they had a bias pot in the MESA, I'd have used it by now. I will probably install one soon - what good is a warranty when you don't have the amp you paid for 'cause it's blasting tubes? I'm also curious why MESA has their amps running the el84's that hot. They aren't the only ones, I know fender blues juniors run their el84's very hot as well (with no bias adjustment).
Anyway, it was fun building the bias probe - now I can tell how well matched the tubes are that I buy and get a better idea what's going on in my amps.
Of course MESA doesn't have a spot to check the bias on their amps, nor do they offer a way to adjust it. The party line is "use our tubes, you'll be good". Well, OK, I'm good with that, but when their tubes die in under 4 hours of use, AND a buddy of mine just got a set of MESA tubes that are incredibly mismatched (the dude with the Jet City amp) - I felt I needed to dig a little further.
So, I built a little el84 bias probe.
My buddy Tim repairs stained glass for churches. He has all kinds of glass shaping devices. I asked him if he could cut open a tube for me cleanly; no problem. He did 3 tubes for me. I broke one of them them getting the guts out. The next, a 12AX7 came apart cleanly:
This is the 1 ohm cathode resistor. By measuring voltage across it, you can tell how much current is going from the cathode to the plate in mV, which corresponds exactly to mA because of ohms law (a 1 ohm resistor passes 1mA for every 1mV it drops).
This is the voltage divider for the B+. It's a 1M resistor off the plate and a 100ohm resistor to ground. Reading the voltage at the junction of the 1M/100r gives a reading in mV, which when multipled by 10000 is the actual B+. So 30mV is actually 300V of plate voltage.
All wired up:
and fully taped w/popsicle sticks for support:
After testing all the connections, and not wanting to risk the MESA right off, I popped the probe into my peavey classic with some JJ's. B+ read about 380V and 28ma current. Now, I didn't use 1% resistors, but I did measure them and they were pretty much spot on... and just how accurate is my $40 meter anyway? Those measurements give me an idle dissipation of just over 10w - which is high for an el84, but not unheard of.
Next the MESA with the new tubes they sent (one of which redplated and the set is being replaced): B+ 392V and 31mA of current - 12.1w(!). Yikes. I tested the JJ's in the mesa, they are about 30mA, so I'm comfortable running them knowing the MESA tubes are running that hot as well. The JJ's were very well matched; I got 4 from tube depot for the peavey and they were all within 1 mA of one another.
I wish they had a bias pot in the MESA, I'd have used it by now. I will probably install one soon - what good is a warranty when you don't have the amp you paid for 'cause it's blasting tubes? I'm also curious why MESA has their amps running the el84's that hot. They aren't the only ones, I know fender blues juniors run their el84's very hot as well (with no bias adjustment).
Anyway, it was fun building the bias probe - now I can tell how well matched the tubes are that I buy and get a better idea what's going on in my amps.
392v ain't that far off. I guess I don't get why it isn't biased colder at that voltage. I've "read on the net" that MESA biases their power amps cold, but this one certainly isn't.