MK 2C+ Class A question

[sheeba]

I have a MK 2C+. (100w simuclass) I've just recapped the amp. Using a bias probe I was measuring the plate currents of the outer pair of 6l6gc's. I put the rear switch is in the "class A" position. With no signal on the input the idle current is about 48ma with 460vdc on the plates. As the input signal is increased the plate current also increases. It can exceed 100ma at full tilt. This is the first time I've seen a mk2 simuclass. This one operates the outer tubes in triode mode. Does this behavior sound normal? I was surprised as the plate voltage drop wasn't significant. When I received the amp one of the outer tubes was smoked. I'd like to avoid that on the new set. In the simuclass mode the current in the outer tubes actually goes down as the input signal increases. I'm now questioning my sanity.
Any thoughts or insights would be greatly appreciated.

Thanks

 

[user 41014]

Simul-class (75W) was designed for EL-34's in the outer pair. If you're going to run 6L6GC as many do you need to find tubes of a lower mA at idle to suit your taste. 460V is on the lower end for a IIC+ but they all vary.
Maybe a mesa yellow color 6L6?

When servicing it is good to see if the .1uF coupling caps at the PI have been upgraded to 630V from the 400V.

 

[sheeba]

Thank you for the tip regarding the coupling caps. I checked them for leakage but left them in. Regarding the EL34's, I spoke to Mesa Boogie and Euro tubes about a set of tubes with two different ratings. They both told me that they don't recommend a mixed set. I was surprised they didn't. Do you think the issue I'm seeing is "normal" except for the current rating? I was thinking that the triode wiring is making the tube more sensitive?? Thank you for taking the time to reply. I won't have much hair left after this one.

 

[GJgo]

While Mesas aren't as sensative to power tube bias as something like a Marshall, since Mesa's gain comes from the preamp section, in the Class A slots you still want to put a ceiling of 45-50 mA at idle and on the A/B slots say around 35-40 mA. Of course the bias % will vary a bit with wall voltage & amp tolerance but that's a ballpark.

EM is right, when using 6L6 in the Class A slots you're going to need a fairly cold tube pair. In my testing RED is always safe, and sometimes YEL will fly in Class A for 6L6. For the A/B pairs, I usually see GRN is as hot as I'd like to go in MKIIs and GRY in MKIIIs, MKIVs and Rectos.

 

[sheeba]

The tubes I'm currently testing with are yellow grade. The class A tubes are idling at 46-48. The a/b tubes are 18-20ma.

 

[user 41014]

My personal testing of yellows was a wide tolerance -some matching greens perfectly. I'd use it more as a guide but test the mA to see the actual.

If you swap the inner for outer what readings do you get?

 

[GJgo]

The tubes I'm currently testing with are yellow grade. The class A tubes are idling at 46-48. The a/b tubes are 18-20ma.

That sounds about normal-ish. I'd save the YEL for the A slots & grab some GRY or maybe even BLU for the A/B slots. On average I see about a 6mA shift per color code.

That said your A/B slots are colder than average for a C+. Component drift is a thing!

EM is right about the pragmatic reality, and for guys that don't have a way to test them in the amp, I've tested hundreds of them and report on the averages.

 

[sheeba]

I swapped the pairs and measured 14ma per tube for the a/b spots. The Class A sockets are again 46ma.

 

[GJgo]

On average I've seen YEL in the A/B in the low-mid 20s so it looks like your A/B bias resistor may have drifted cold. No big deal as long as you have hotter pairs to compensate, so then BLU might actually be the ticket in your case.

 

[sheeba]

The resistors on the power tube board are all within 1% or very close. I took some measurements on the power tube board without the power tubes installed.
Bias Supply at (E) is -72.5
after 220k resistors -60.5
@ #1 socket (class A triode) -49.0
@ #2 socket (class A/B) -60.3
@ #3 socket (class A/B) -60.4
@ #4 socket (class A triode) -49.0

Does this all look correct? The factory bias resistor is 33K the second one appears to have been cut off.

 

[user 41014]

If the tap is about -60, you are supplying the full negative to the grid. Lol- are you sure there is a bias R there? That does not seem right.

Pics of the PS board?

That -60 could explain the cold.
-51 is what I might think from the IIB but that could have changed.

 

[sheeba]

Keep in mind these voltages were measured without power tubes installed. I believe they come down to -55 (A/B sockets) with tubes installed. I'll get a few pictures up later tonight

 

[sheeba]

 

[Henz]

The resistors on the power tube board are all within 1% or very close. I took some measurements on the power tube board without the power tubes installed.
Bias Supply at (E) is -72.5
after 220k resistors -60.5
@ #1 socket (class A triode) -49.0
@ #2 socket (class A/B) -60.3
@ #3 socket (class A/B) -60.4
@ #4 socket (class A triode) -49.0

Does this all look correct? The factory bias resistor is 33K the second one appears to have been cut off.

I have a factory DRG C+ and my measurements are identical to yours , with power tubes IN .And component wise. My plate voltage hover around 500vdc. 105 PT. Wall voltage around 120 - 122 VAC.

 

[sheeba]

I have a factory DRG C+ and my measurements are identical to yours , with power tubes IN .And component wise. My plate voltage hover around 500vdc. 105 PT. Wall voltage around 120 - 122 VAC.

Thank you for letting me know! Seems like all is well.

 

[Henz]

Simul-class (75W) was designed for EL-34's in the outer pair. If you're going to run 6L6GC as many do you need to find tubes of a lower mA at idle to suit your taste. 460V is on the lower end for a IIC+ but they all vary.
Maybe a mesa yellow color 6L6?

When servicing it is good to see if the .1uF coupling caps at the PI have been upgraded to 630V from the 400V.

I often wonder why its recommended to put 600v coupling caps when the voltage is lucky to even reach 400. This is not a debate statement, more of opinions rather. I have 600's ready to put in but have been procrastinating. Also, what's your take on putting the outer foil lead to the lower impedance connection? Today's Orange drops when produced are not banded. And, lol, some say they can hear a difference between 715's and 225's.

 

[Jay Omega]

I often wonder why its recommended to put 600v coupling caps when the voltage is lucky to even reach 400. This is not a debate statement, more of opinions rather. I have 600's ready to put in but have been procrastinating. Also, what's your take on putting the outer foil lead to the lower impedance connection? Today's Orange drops when produced are not banded. And, lol, some say they can hear a difference between 715's and 225's.

Boogie swaps these with 600V because they tend to fail, no need to change them unless they’re leaking. At idle they do not exceed 400 VDC, but the transient voltage across them will exceed 400V. Remember, the voltage across them will be the anode voltage with respect to the power tube grid bias voltage

 

[Jay Omega]

I often wonder why its recommended to put 600v coupling caps when the voltage is lucky to even reach 400. This is not a debate statement, more of opinions rather. I have 600's ready to put in but have been procrastinating. Also, what's your take on putting the outer foil lead to the lower impedance connection? Today's Orange drops when produced are not banded. And, lol, some say they can hear a difference between 715's and 225's.

Boogie swaps these with 600V because they tend to fail, no need to change them unless they’re leaking. At idle they do not exceed 400 VDC, but the transient voltage across them will exceed 400V. Remember, the voltage across them will be the anode voltage with respect to the power tube grid bias voltage

 

[user 41014]

I often wonder why its recommended to put 600v coupling caps when the voltage is lucky to even reach 400. This is not a debate statement, more of opinions rather. I have 600's ready to put in but have been procrastinating. Also, what's your take on putting the outer foil lead to the lower impedance connection? Today's Orange drops when produced are not banded. And, lol, some say they can hear a difference between 715's and 225's.

I saw someone do the math once here explaining how these couplers can go go over-voltage to 400V when someone saw no reason to use more than 200/250V replacements and I called it out.

Jay has tube amp expertise so I will defer to his input. If the potential across is the anode (plate volts for the non-tech) minus the grid and the grid is -60, using 500 plate volts we are at 560V, or if subtracting the absolute value of 60V we are at 440V so I may not be getting the potential relationship he is pointing out.

I only know that the 2 most common fail points in all the old marks that go thru my hands, most of which remain, are these couplers and the 1st 30uF filter for which Mike started changing to 2# 40uF (with R) around 1985, also for the over-voltage failures that plague that particular filter of the 3 on the PS used up until then.

As for the outer foil connection in attempting to find the lowest possible noise floor in building/servicing an amp my take is it's a real thing for non-polar caps.

That said one would need a very sensitive meter to painstakingly work through lots of components the rest of the world scoffs at for a minuscule return on what I think is wasted time to even a hobbyist with nothing but time on their hands. I did try it. No one marks their non-polar caps.

It is my understanding that a long tail pair PI is in the layout of a differential amp. This is how you reduce noise from a books smart perspective in a meaningful way.

 

[Jay Omega]

I saw someone do the math once here explaining how these couplers can go go over-voltage to 400V when someone saw no reason to use more than 200/250V replacements and I called it out.

Jay has tube amp expertise so I will defer to his input. If the potential across is the anode (plate volts for the non-tech) minus the grid and the grid is -60, using 500 plate volts we are at 560V, or if subtracting the absolute value of 60V we are at 440V so I may not be getting the potential relationship he is pointing out.

I only know that the 2 most common fail points in all the old marks that go thru my hands, most of which remain, are these couplers and the 1st 30uF filter for which Mike started changing to 2# 40uF (with R) around 1985, also for the over-voltage failures that plague that particular filter of the 3 on the PS used up until then.

As for the outer foil connection in attempting to find the lowest possible noise floor in building/servicing an amp my take is it's a real thing for non-polar caps.

That said one would need a very sensitive meter to painstakingly work through lots of components the rest of the world scoffs at for a minuscule return on what I think is wasted time to even a hobbyist with nothing but time on their hands. I did try it. No one marks their non-polar caps.

It is my understanding that a long tail pair PI is in the layout of a differential amp. This is how you reduce noise from a books smart perspective in a meaningful way.

You got the idea. The plates may be sitting at 350V WRT ground at idle, and the bias might be -60V on the other side so the voltage across the cap would be 350 - (-60) = 410V at idle. Then during transients you’ll see an additional 60V across the cap if you’re running at full power.

The film foil technology doesn’t help you here either, once the electric field breaks down the dielectric in even a tiny place you’re hosed. Metallized film caps have a self healing property where they can tolerate small areas that were broke down by the electric field