Mark iii blue stripe 60w redplating

[BoogieJim]

Hi everyone.
Been a member on there for a bit but this is my first post.

I have a 60w blue stripe mark iii that I have just done a cap job on as well as installed a bias trimpot.
When I fired it up afterwards the tubes appear to be redplating, but the strange thing is that I was able to bias it to the correct mA.
Is was reading 454v on the plates and I have it biased to 44.6ma, yet the tubes slowly start glowing.

What should I be looking at here?
Thanks

 

[woodbutcher65]

What's the voltage on the cathodes? Plate to cathode voltage is the starting point for calculating your tube dissipation and thus proper bias voltage.

It's not necessary, and is in fact probably a bad idea, to install adjustable bias on a Mark III. Stick with Mesa output tubes and their selected ranges and don't worry about it. As long as you stick to Mesa tubes it makes it pretty foolproof.

My Mark III had adjustable bias when I got it and it was problematic. When I reverted to stock fixed bias it became ultimately reliable.

 

[BoogieJim]

What's the voltage on the cathodes? Plate to cathode voltage is the starting point for calculating your tube dissipation and thus proper bias voltage.

It's not necessary, and is in fact probably a bad idea, to install adjustable bias on a Mark III. Stick with Mesa output tubes and their selected ranges and don't worry about it. As long as you stick to Mesa tubes it makes it pretty foolproof.

My Mark III had adjustable bias when I got it and it was problematic. When I reverted to stock fixed bias it became ultimately reliable.

Thanks for the reply.
The 454v was measured pin 3 to pin 8

 

[user 41014]

You are bumping up on 70% dissipation while most Marks will not even be close to that for class AB. Drop it to 40mA and you'll prob be ok but lower 30's would seem more typical on the "high end" for these amps.

 

[BoogieJim]

You are bumping up on 70% dissipation while most Marks will not even be close to that for class AB. Drop it to 40mA and you'll prob be ok but lower 30's would seem more typical on the "high end" for these amps.

Isn’t it the tubes that determine plate % dissipation?

 

[GJgo]

I believe the PV determines it. With class a/b in Mesas I shoot for 25-35mv, as mentioned try this.

https://www.amplifiedparts.com/tech-articles/tube-amplifier-bias-calculator

 

[BoogieJim]

I believe the PV determines it. With class a/b in Mesas I shoot for 25-35mv, as mentioned try this.

https://www.amplifiedparts.com/tech-articles/tube-amplifier-bias-calculator

Thanks for that.
I’m not sure I understand why a tube with a known 30w rating would need to be biased colder in a particular amp.
70% is the accepted bias point for everything as far as I’m aware.

 

[BoogieJim]

I believe the PV determines it. With class a/b in Mesas I shoot for 25-35mv, as mentioned try this.

https://www.amplifiedparts.com/tech-articles/tube-amplifier-bias-calculator

I think another part of the issue may be the resistor that I replaced.
My searching didn’t give me a clear indication whether I should replace R117 or R102, due to some amps having one resistor or the other, some having both. I replaced R102 with a 50k cermet pot.

Did I replace the wrong one?

 

[BoogieJim]

This really is a strange one…
I just removed the trim pot I installed and put the factory 68k resistor back in.
The bias is now reading higher (49mA) yet the tubes are no longer red plating!

What the..?

 

[BoogieJim]

Anyone?

 

[user 41014]

Thanks for that.
I’m not sure I understand why a tube with a known 30w rating would need to be biased colder in a particular amp.
70% is the accepted bias point for everything as far as I’m aware.

What was the mA before the service? Before / after pics?

70% is the accepted standard for max safe dissipation just like running your car at the red line for RPM's. For that to be safe, you need porting/polishing, high end race parts that can put up with the abuse and you accept regular tear down/rebuild of the motor to maintain the ability to ride on the edge.

For a tube amp that means you are going to blow through power tubes, and ideally you up the voltage rating of your components. Or you just drop the mA to less than 50% as the Mark's are intended to be.

49mA is beyond what any Mesa running 6L6GC should be put through red plating or not. When you had the trim pot in did you dial the mA back to 40 or less to try it?

Since you have the ability to test voltages it might help to capture the negative bias voltage along with the actual tested resistance used and resultant mA. The resistor/rheostat values might not be true and did you test what R value you used on the 50k rheostat (assuming you only inserted the trim pot into the path at 2 points)?

 

[user 41014]

To be sure, and I'm not looking at a schematic or amp, is your R 68k or 6.8k? If it was a IIB the bias R can be a 6.8K || 10K and then series into a 15k.

So are you blue-grey-red as I would think on the older Marks or blue-grey-orange as you have stated the value of?

 

[BoogieJim]

What was the mA before the service? Before / after pics?

70% is the accepted standard for max safe dissipation just like running your car at the red line for RPM's. For that to be safe, you need porting/polishing, high end race parts that can put up with the abuse and you accept regular tear down/rebuild of the motor to maintain the ability to ride on the edge.

For a tube amp that means you are going to blow through power tubes, and ideally you up the voltage rating of your components. Or you just drop the mA to less than 50% as the Mark's are intended to be.

49mA is beyond what any Mesa running 6L6GC should be put through red plating or not. When you had the trim pot in did you dial the mA back to 40 or less to try it?

Since you have the ability to test voltages it might help to capture the negative bias voltage along with the actual tested resistance used and resultant mA. The resistor/rheostat values might not be true and did you test what R value you used on the 50k rheostat (assuming you only inserted the trim pot into the path at 2 points)?

Thanks for the reply.
I didn’t actually measure anything before I did the service and installed the trim pot. My error there.
But once I removed the trim pot and put the resistor back in (68k, not 6.8k. Measured) and also the mesa tubes that were previously in it (415’s) the red plating stopped and the mA went up slightly. I then tried my other set of tubes (tungsol) the mA dropped slightly to about 47-48 mA which is closer to the 70%.
I did try dialling the trim pot to both extremes and the change was minimal, certainly not enough to bring it down to 50%.
I did actually measure the bias voltage as well, but I can’t remember what it was off hand. I’ll do that again.
And yes, I only removed r102 to install the trim pot, with the wiper connected to the + side, not the ground side.

 

[BoogieJim]

To be sure, and I'm not looking at a schematic or amp, is your R 68k or 6.8k? If it was a IIB the bias R can be a 6.8K || 10K and then series into a 15k.

So are you blue-grey-red as I would think on the older Marks or blue-grey-orange as you have stated the value of?

 

[Jay Omega]

Thanks for the reply.
I didn’t actually measure anything before I did the service and installed the trim pot. My error there.
But once I removed the trim pot and put the resistor back in (68k, not 6.8k. Measured) and also the mesa tubes that were previously in it (415’s) the red plating stopped and the mA went up slightly. I then tried my other set of tubes (tungsol) the mA dropped slightly to about 47-48 mA which is closer to the 70%.
I did try dialling the trim pot to both extremes and the change was minimal, certainly not enough to bring it down to 50%.
I did actually measure the bias voltage as well, but I can’t remember what it was off hand. I’ll do that again.
And yes, I only removed r102 to install the trim pot, with the wiper connected to the + side, not the ground side.

Use a larger value pot if you want to increase the bias voltage, which will reduce idle dissipation. Hopefully there is enough range with a 100k-250k for your Tung Sols. Looks like you’re starting to see that all power tubes aren’t created equally. You might also check min resistance case to make sure your trimmer won’t hurt something

My best suggestion is to put back the 68k and call Boogie and tell them you want the right power tubes for your amp

 

[BoogieJim]

Use a larger value pot if you want to increase the bias voltage, which will reduce idle dissipation. Hopefully there is enough range with a 100k-250k for your Tung Sols. Looks like you’re starting to see that all power tubes aren’t created equally. You might also check min resistance case to make sure your trimmer won’t hurt something

My best suggestion is to put back the 68k and call Boogie and tell them you want the right power tubes for your amp

Thanks for the input.
I had a 50k pot in there but I have now removed it and put the resistor back in.
I have since got a 100k pot to try.
The fact that all tubes aren’t created equal is the reason why I wanted a trim pot installed. I want the freedom to get whatever tubes I like (or that I can obtain) I have the amp biased correctly.
Obviously the 415’s that were in it were not the correct tubes for it. The TungSol’s seem slightly more appropriate, bias wise.

 

[user 41014]

I think the take away on the tubes was the 415 can take more abuse than just about any other 6L6GC on the planet. They are the correct tube if they are good and you are lucky enough to have good ones. The old Marks were designed for the 415 and it is on a very short list of tubes that can handle high PV, not that it would matter in your case if you ran your amp how it was built for- using high teens to 30's mA's.

If you want to ride the red line put the 415's away and kill some cheap tubes. JJ tubes will take abuse and while they are not cheap you can replace them cuz if you're running that hot you will be replacing them. The other take away here is despite anything you know about tube dissipation targeting 70% is dangerous on your boogie.

Here is what I would do with what you have but this assumes you have the correct bias resistor which I haven't checked:
A) Get your 68k bias voltage (and mA)- I assume it is lower neg than it should be
B) Ditto for your 100k so you have a reference point to triangulate a target value range
C) Buy new trimmer and fixed R high enough that it will give you enough effective R in parallel.

For safety reasons, fixed R || pot gives you a fall back if the pot fails which on its own shuts down the grid suppression (very bad). The || R shuts down the power tubes which dampens the mood of playing guitar but you won't have to move the amp outside until the smoke clears.

 

[BoogieJim]

Ok, so before your last reply I managed to install a new trim pot (100k, not 50k) and now the bias sits the lowest I have managed to get it, 43mA, which with a plate voltage of 469v now, is about 68-69% so I am ok with that… for now. But it’s right at the end of its travel.
Next time I have the amp open, I think I will install a 250k pot which should give me enough travel to get the bias down around 35mA.

Having said that I have now played through the amp it sounds amazing. The cap job has really rejuvenated it.
It does seem to have a bit less gain on tap though, but bump the dial up slightly from where I normally have it and WOW. Sounds killer.

Anyway, thanks for your help. At least the amp is below 70% for now and sounds killer.

 

[user 41014]

Ok, so before your last reply I managed to install a new trim pot (100k, not 50k) and now the bias sits the lowest I have managed to get it, 43mA, which with a plate voltage of 469v now, is about 68-69% so I am ok with that… for now. But it’s right at the end of its travel.
Next time I have the amp open, I think I will install a 250k pot which should give me enough travel to get the bias down around 35mA.

Having said that I have now played through the amp it sounds amazing. The cap job has really rejuvenated it.
It does seem to have a bit less gain on tap though, but bump the dial up slightly from where I normally have it and WOW. Sounds killer.

Anyway, thanks for your help. At least the amp is below 70% for now and sounds killer.

Another fail safe approach I didn't mention was a series R. It's really a user error fail safe in case someone dials out the trimmer not knowing what they are doing.

Roll one of the leads of your 100k R around a screwdriver to make it look pro and feed it into a trimmer leg in series. The low end of your mA then becomes the high end where it belongs, you have user error protection and you don't have to buy anything.

It would be interesting to see how low you can get and compare it cranked to what your WOW moment gave you.

 

[BoogieJim]

Yeah I thought of doing that. But space does not really allow.

You mentioned that you didn’t think I was “safe” to run a mark iii biased that high.
In what sense?