First Post - Bias voltage dropping until red-plating - follows the tube

[Chuck H]

Yes. But unless you have a way to easily monitor current on each individual tube with both installed then installing them one or the other is a solution.

[ThermionicEmissions]

Yes. But unless you have a way to easily monitor current on each individual tube with both installed then installing them one or the other is a solution.

Thanks, I'll give it a shot tonight. I guess having another bias probe would be handy.

[Leo_Gnardo]

But...but...they're so pretty

I'm impressed - you really run the filaments whilst the tubes are on the tree!

Saw your photos on your gallery, also kool.

By coincidence I just had a 6550A go kapoot in a Marshall JCM800, purple sparks & all. Must be the day for it.

Mesa's "matching" leaves something to be desired. Sometimes I wonder what the heck they're doing out there in Petaluma between roller skating festivals and surfing. Lately I've been getting EL84's well matched - and burned in if you want - from Antique/CE. JJ seems to be making the toughest ones these days. Don't hesitate to swap in a higher value cathode resistor to let your output tubes run a bit cooler.

[Jazz P Bass]

"7ma difference doesn't seem all that well matched to me. "

That difference is the idle current draw of the tube.
No, they are not an exact match. But what is idle current a measure of other than where the tube idles?

Next to look at is the output amplitude of each tube, one at a time. (assuming that you have a parallel heater circuit)

That info is typically more important than idle current.

You are actually measuring the 'gain' of the tube.

[ThermionicEmissions]

I'm impressed - you really run the filaments whilst the tubes are on the tree!.

LoL, no, just an LED in the "socket" (which is just a 1/2" copper pipe end-cap). I (briefly) considered using the actual filaments, but then realized burning the house down would kinda suck...

JJ seems to be making the toughest ones these days. Don't hesitate to swap in a higher value cathode resistor to let your output tubes run a bit cooler.

Thinking of doing this, or an adjustable bias pot. Hopefully the cathode resistor is on the little board where the power tube sockets are mounted, and not the main board. Pulling the main board on this amp looks like a major PITA.

[ThermionicEmissions]

"7ma difference doesn't seem all that well matched to me. "

That difference is the idle current draw of the tube.
No, they are not an exact match. But what is idle current a measure of other than where the tube idles?

Next to look at is the output amplitude of each tube, one at a time. (assuming that you have a parallel heater circuit)

That info is typically more important than idle current.

You are actually measuring the 'gain' of the tube.

Interesting, thank you! TIL

[Enzo]

SO many people worry WAY too much over things like tube specs. You see a few milliamps different idle DC current and worry. Depending upon who matched the tubes, they may be matched for transconductance. How can we compare that? Even idle current: they can test tubes on their tester and under those conditions the numbers match up fine. That does not mean in your amp that the same thing will happen.

SOme matched tubes are batched into only three groups (Groove Tube say). The entire range into high, medium, and low. How close can we expect them? A lot of OTs are simple wound and so the two sides of the primary have different DC resistance for the same winding count, and so will have different current on each side. Nothing else in a guitar amp is all spec'd out for balance. There is nothing precision in a guitar amp.

[ThermionicEmissions]

SO many people worry WAY too much over things like tube specs. You see a few milliamps different idle DC current and worry. Depending upon who matched the tubes, they may be matched for transconductance. How can we compare that? Even idle current: they can test tubes on their tester and under those conditions the numbers match up fine. That does not mean in your amp that the same thing will happen.

Maybe I should just give up and stick with SS. The more I get into it, the more it seems like black magic sorcery

[Pedro Vecino]

I put an email in to Mesa about it to see what they say. Their amps are cathode-biased, so there's no way to adjust the idle.
Edit: no "simple" way to adjust the idle.

Thinking of doing this, or an adjustable bias pot. Hopefully the cathode resistor is on the little board where the power tube sockets are mounted, and not the main board. Pulling the main board on this amp looks like a major PITA.

Mesa 5:25 is not a cathode bias amp. It's fixed bias in high power mode. That is the explanation for the lack of symmetry in the idle current. With cathode bias that difference with those tubes should be much smaller or non-existent.
Itīs with power reduction mode activated where it is left with only one active EL84 and uses a cathode bias system.

[ThermionicEmissions]

Mesa 5:25 is not a cathode bias amp. It's fixed bias in high power mode. That is the explanation for the lack of symmetry in the idle current. With cathode bias that difference with those tubes should be much smaller or non-existent.
Itīs with power reduction mode activated where it is left with only one active EL84 and uses a cathode bias system.

Ooooooooooooooh, that explains it, thank-you! This amp is so fricken complex. I'm seriously thinking of selling it and buying a kit so I can really learn how these things work.

[Leo_Gnardo]

Thinking of doing this, or an adjustable bias pot.

Mesa may get back to you with their boss's three page explanation of why he doesn't allow bias pots in his amps. It boils down to mostly covering his butt. Early days - mid 70's - 1st generation Boogies did have a bias pot. And there was a movement on amongst amp tweakers of the time, that the right thing to do was dial the bias current up to the point output tube plates glowed red and had very short service lives. Mesa tended to go the opposite direction, run the outputs cold so output tubes would last past the warranty period, also so the user could experience distortion - guitarists want distortion don't they? - at all volume levels.

Now Pedro's reminded me of the low/high power operating modes, it's no wonder the tube left on in low power mode takes a thrashing. Small wonder it goes out of match with its mate, assuming they were matched in the first place.

Building a simple straightforward amp kit can be an instructive experience. Plus you have thousands of boost, overdrive & fuzz pedals - including a wide variety of kits - to choose from for your distortion tones.

[mikepukmel]

Mesa may get back to you with their boss's three page explanation of why he doesn't allow bias pots in his amps. It boils down to mostly covering his butt. Early days - mid 70's - 1st generation Boogies did have a bias pot. And there was a movement on amongst amp tweakers of the time, that the right thing to do was dial the bias current up to the point output tube plates glowed red and had very short service lives. Mesa tended to go the opposite direction, run the outputs cold so output tubes would last past the warranty period, also so the user could experience distortion - guitarists want distortion don't they? - at all volume levels.

Now Pedro's reminded me of the low/high power operating modes, it's no wonder the tube left on in low power mode takes a thrashing. Small wonder it goes out of match with its mate, assuming they were matched in the first place.

Building a simple straightforward amp kit can be an instructive experience. Plus you have thousands of boost, overdrive & fuzz pedals - including a wide variety of kits - to choose from for your distortion tones.

But Leo, evybody wants the panache of Tube distortion! I wanna sound Just Like AC/DC. (Dirty Deeds, Done Dirt Cheap and all that)

[Leo_Gnardo]

But Leo, evybody wants the panache of Tube distortion! I wanna sound Just Like AC/DC. (Dirty Deeds, Done Dirt Cheap and all that)

I wuz gonna mention "start simple, like Mike Pukmel and his legendary Deluxe." So much for good intentions.

You want AC/DC? Have a look/listen at what G1 just posted: http://music-electronics-forum.com/t46677/