5 watt unequal wear??

[UKBoogieboy]

Hi, I was wondering, as in norm,al use both valves are being used, but in the 5 watt mode only one is, so does it mean that one valve will wear quicker than the other?

And is it normal to bang really loudly when switching the 50 watt/5 watt modes, and will it bust the speaker.

 

[Don]

It seems like one would wear out quicker if you used the 5 watt mode a lot, though when I was working on my Super Reverb, I temporarily installed a set of 6L6 tubes that I had used in my 5:50 for two years and they measured almost exactly the same as each other when I set the bias.

I put the amp on standby when I switch up from 5 watts to 50.

 

[EtherealWidow]

It's fine switching from 5 to 45 to 90 watts on a Mark V, so I would sort of assume that it would be cool for all Mesa amps. And both power tubes are being used in the 5 watt mode. 1 is producing sound and the other is balancing out the output transformer.

 

[tmac]

Check out post #4 on the following page (very informative):

http://music-electronics-forum.com/t15200/

He suggest the following method for operating a push-pull OT in class A with only one tube actually amplifiying the signal:

"Yank out the signal drive to one side of the output tube pairs. Now the OT still sits at nearly zero magnetic field offset because there's the non-signal tube running an offsetting DC bias to keep things balanced magnetically. And you can start dinking with the bias because you need to up the standing DC current to the working output tube to get signal swing in both directions, not just one. That requires upping the DC bias to both tubes to keep the transformer magnetically balanced, and so you eventually work your way to a DC bias current that would otherwise halfway saturate the OT, but is balanced by the current in the inactive tube. You have to do this because of the output tube needing to be able to turn off as much as it turns on to get signal swing. So the tube is working as hard as it is in a SE setup. So is the inactive tube, which is resetting the transformer iron to near zero."

If this is the way the express works, then both tubes should wear approximately the same.

Actually, I wanted to know myself if this is how it worked, so I'm going to check the tube voltages when the amp is set to class A.

Torquil

 

[J.J]

Cool, let us know what you find.

 

[tmac]

Ok, here are the quiescent DC voltages on the power tubes. I have some JJ power tubes in right now, and I don't know of they are "approved" by Mesa or not. The tube closest to the power transformer is tube L (left when seen from the rear of the amp), the other is tube R:

30W mode:     Tube L         Tube R
Cathode:       0               0
Control Grid:  -11            -11
Screen Grid:   300             300
Plate:         360             360

5W mode:       Tube L         Tube R
Cathode:       4               0
Control Grid:  0               -11
Screen Grid:   200             200
Plate:         360             380

So I would guess that in 5W mode/class A, tube L is used cathode biased. The other tube still has fixed bias, and would have no audio signal on its control grid. It is activated just to cancel any large static magnetic field created by the primary windings of the OT.

Although, it doesn't seem like tube L and R will pass the same amount of current, from the above numbers. Both have around 200V difference between cathode and screen grid, but tube L has only a 4V bias, while the other has an 11V bias.

It would be interesting to check the actual current through both tubes. I one were to convert the amp to adjustable bias in class AB/30W mode, maybe some problems could arise when running in class A mode. E.g. assume that I bias my amp hotter than default. If I don't also reduce the cathode bias resistor that is used in class A, the current through each tube would not be equal anymore in class A mode because tube L would pass more current... It might not be important with a slight discrepancy, though.

Torquil

 

[soundchaser59]

And is it normal to bang really loudly when switching the 50 watt/5 watt modes, and will it bust the speaker.

The STANDBY switch is your friend.....

Oddly, the manual doesn't say to use standby mode to switch power levels. It should say, but it doesn't. The sales hype says ".......at the flick of a switch...." So I guess I can see where the confusion arises.

But I figure if you would not deliberately make that sound while playing, then dont let the amp make that sound while switching. The thugs at the retail dealer all told me to put it in standby when switching power levels. So I always do. Must be a reason why it isn't footswitchable.....

 

[EtherealWidow]

And is it normal to bang really loudly when switching the 50 watt/5 watt modes, and will it bust the speaker.

The STANDBY switch is your friend.....

Oddly, the manual doesn't say to use standby mode to switch power levels. It should say, but it doesn't. The sales hype says ".......at the flick of a switch...." So I guess I can see where the confusion arises.

But I figure if you would not deliberately make that sound while playing, then dont let the amp make that sound while switching. The thugs at the retail dealer all told me to put it in standby when switching power levels. So I always do. Must be a reason why it isn't footswitchable.....

I know that the Express and the Mark are 2 completely different amps, but Mark's do it just fine and the popping noise is actually very common, if not, inevitable. You hear people whine about it all the time when going from 90 to 5 watts.

 

[J.J]

tmac, something sounds wrong about the values for the left side tube in 5 watt mode, or there may be something else we don't know yet. With 4v bias, 360 on the plate and 200 the screen, that tube should melt ! It's cathode current and plate dissipation would be sky high.

 

[soundchaser59]

I know that the Express and the Mark are 2 completely different amps, but Mark's do it just fine and the popping noise is actually very common, if not, inevitable. You hear people whine about it all the time when going from 90 to 5 watts.

Is it foot switchable? Or do you have to take your hand off the guitar and flip a switch manually on the back of the amp? I would see that as a clue, but in the end it may not do any harm at all, I dont know.....

 

[EtherealWidow]

I know that the Express and the Mark are 2 completely different amps, but Mark's do it just fine and the popping noise is actually very common, if not, inevitable. You hear people whine about it all the time when going from 90 to 5 watts.

Is it foot switchable? Or do you have to take your hand off the guitar and flip a switch manually on the back of the amp? I would see that as a clue, but in the end it may not do any harm at all, I dont know.....

It's footswitchable in a sense. If you can have channel 1 @ 90, channel 2 @ 45, and channel 3 @ 15 if you so choose. When you switch between the channels with different wattages there's often a popping noise that people ***** about all the time.

 

[Don]

That's interesting to know how the 5 watt SE mode works.

I've noticed that only one of the power tubes glows blue when in 5 watt mode rather than both of them in 50 watt class ab.

Also, when my PI/driver tube failed and the amp still worked in the 50 watt but not in the 5 watt mode!

BTW, that was a good reminder to replace the PI tube when replacing power tubes!

 

[MCK]

On this note, if I'm using the 5w setting predominantly and if that means only one of the EL84 is doing the grunt work... Does that mean putting in matched EL84s in there is not so important? I am toying with trying few NOS tubes in there and I have a number of good singles. I want to know if I might cause any damage by leaving the Mesa tube that only works on DynaWatt mode and change the one that is operational in 5W mode. Any thoughts? Thanks in advance for all your input.

 

[MCK]

Anyone has an idea? TIA

 

[Don]

I'd still use matched tubes.

 

[tmac]

On this note, if I'm using the 5w setting predominantly and if that means only one of the EL84 is doing the grunt work... Does that mean putting in matched EL84s in there is not so important? I am toying with trying few NOS tubes in there and I have a number of good singles. I want to know if I might cause any damage by leaving the Mesa tube that only works on DynaWatt mode and change the one that is operational in 5W mode. Any thoughts? Thanks in advance for all your input.

This is speculation, but if it works the way I think, it would be less important. It will influence the operation of the tubes/OT, but perhaps not enough to be important. As long as the quiescent currents are approximately the same, the current through the OT should be quite small (only the difference between the tube currents). I'm guessing that it is acceptable for most pairs of random tubes. But since this is speculation/guesswork, you could contact Mesa Boogie and maybe get a definitive answer.

- Torquil

 

[tmac]

tmac, something sounds wrong about the values for the left side tube in 5 watt mode, or there may be something else we don't know yet. With 4v bias, 360 on the plate and 200 the screen, that tube should melt ! It's cathode current and plate dissipation would be sky high.

Interesting observation, J.J. I have a couple of JJ tubes in it, from the previous owner. I will check the measurements again, and check it with Mesa tubes innstalled.

But anyway, I believe the cathode resistor is 120ohm, so 4V cathode voltage tells me that the cathode current is 33mA. With a plate voltage of 360V it gives approximately 12W power dissipation. This coincides with the upper limit on the JJ data sheet, so I guess I'm pushing it...

It seems that my tubes do not behave similarly to the JJ EL84 data sheet I found on the net, because it indicates a much larger current at a -4V bias, with comparable screen and anode voltages.

On the basis of the calculation of tube current using the known value of the cathode resistor, I think I am just pushing the limits a bit But I'll check if the figures change when using the Mesa tubes.

- Torquil

 

[tmac]

I think I made an error on the plate voltage of the left tube on the last measurement. It is 280V, not 360V. Also I didn't measure the cathode voltage accurately enough:

5W mode:       Tube L         Tube R
Cathode:       4.8               0
Control Grid:  0               -11
Screen Grid:   200             200
Plate:         280             380

So cathode current is 40mA in this case since the cathode resistor is 120ohms. Thus appr. 11W dissipation on tube L. I notice that the tube closest to the PT (tube L) is significantly hotter than tube R, after having left the amp in class A for a few minutes.

After putting in my Mesa tubes (label: 10 AC Red), I get:

5W mode:       Tube L         Tube R
Cathode:       4.6               0
Control Grid:  0               -11
Screen Grid:   200             200
Plate:         280             380

Cathode current: 38mA, and 10.5W dissipation.

- Torquil

 

[MCK]

Thanks much. How about the bias issue? In Class A mode is the amplifier using a Cathode Biased mode or Fixed Bias mode? Anyone know? Thanks much!

 

[tmac]

Thanks much. How about the bias issue? In Class A mode is the amplifier using a Cathode Biased mode or Fixed Bias mode? Anyone know? Thanks much!

The tube that actually amplifies the signal is cathode biased through the 120ohm resistor located on the circuit board that has the speaker outputs. The other tube which balances (at least partially) the current through the OT, is still fixed bias with the cathode at 0V and the grid at around -11V it seems.

At least this is my impression. It could be tested by checking the cathode and grid voltages of active tube socket, after taking the tube out. If those are both 0V, then it is cathode biased. I'm pretty sure it is, though.

Torquil