Mark iii blue stripe 60w redplating

[user 41014]

From general experience and reading posts, anything near 70% in a mesa class AB is just not heard of.

There are service notes someone has on a boogie for sale stating that the bias was at 50mA and it was described as too hot. This is from the person that did the final ok on these amps new and who does the current service for these old Marks.

The notes said something to the effect that he brought the bias in line with normal. The inference was that it was a screaming hot boogie on fire! fire! fire! fire!

 

[BoogieJim]

I’m just wondering if that means actually “dangerous” or just not as per factory spec

 

[user 41014]

I’m just wondering if that means actually “dangerous” or just not as per factory spec

I think you answered that with the title to your thread

 

[BoogieJim]

I think you answered that with the title to your thread

Fair point. But to be fair, the red plating must have been caused by a bad connection or the first trim pot I used being bad.
Once I put the resistor back in, the mA didn’t change but the red plating stopped and is not present with the current trim pot.
So to my way of thinking, the amp is biased below the maximum dissipation of the tubes and is not red plating, so in my mind it should be good to go.

I was more wondering if you knew something about the design of the amp that I don’t that would make the current operating conditions “dangerous.”

 

[BoogieJim]

Just put a 250k trim pot in and it still won’t go below 40mA

 

[user 41014]

I finally pulled up a schematic but it just shows "factory select" for the 2 bias resistors.

Are we sure R102 is the target?

The schematic says -43V bias for 60W.

 

[BoogieJim]

I finally pulled up a schematic but it just shows "factory select" for the 2 bias resistors.

Are we sure R102 is the target?

The schematic says -43V bias for 60W.

Yeah r102. But some amps only have the one resistor, instead of 2 in parallel.
Just seems odd I need such a high R trim pot

 

[mark1406]

From general experience and reading posts, anything near 70% in a mesa class AB is just not heard of.

There are service notes someone has on a boogie for sale stating that the bias was at 50mA and it was described as too hot. This is from the person that did the final ok on these amps new and who does the current service for these old Marks.

The notes said something to the effect that he brought the bias in line with normal. The inference was that it was a screaming hot boogie on fire! fire! fire! fire!

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

Has your issue been resolved?

Regards

Mark

 

[BoogieJim]

Has your issue been resolved?

Regards

Mark

Yes and no.
The replaying is gone, but with a 250k trim pot installed I still can’t get the bias below 40ma.
It’s functional, but it’s sounding like sh*t at the moment. It seems the lower the bias current goes the worse it sounds. It just seems quite loose and flubby, not how it used to sound or how you would expect a mark to sound.
I’m holding off doing anything major atm due to a potential issue with the PMEQ pedal I use in the loop.

 

[mark1406]

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?

The bias control should be mounted where the factory select resistors are. The pot should be a ten turn pot where possible as this allows finer adjustment.

Regards

Mark

 

[BoogieJim]

The bias control should be mounted where the factory select resistors are. The pot should be a ten turn pot where possible as this allows finer adjustment.

Regards

Mark
View attachment 4179

Thanks. That’s where it’s installed. In place of one of the resistors. But even with a 250k pot I can’t get the bias below 40mA. And the lower the bias goes, the worse the amp sounds

 

[Jay Omega]

The bias control should be mounted where the factory select resistors are. The pot should be a ten turn pot where possible as this allows finer adjustment.

Regards

Mark
View attachment 4179

I bet if you trace the circuit you’ll find the schematic to be incorrect

 

[BoogieJim]

I bet if you trace the circuit you’ll find the schematic to be incorrect

Im not sure what to hope to achieve by doing that. The bias pot works, it’s just that it seems to want a 500k pot to get it down to 35mA. But with it sounding worse the lower I go I’m not sure if I could be bothered doing that.
I’m going to check out a red stripe today. If I get it, I will put the blue stripe back to stock and sell it.

 

[BoogieJim]

Well… I endured up buying the red stripe and I can tell you, it blows the blue stripe away, BIG TIME.
Original owner, original cover, original paperwork, serviced about 3.5 years ago and just AMAZING.
So good, in fact, I’m wondering if the blue stripe was ever working properly. It seems to have so much less gain at the same settings, less presence too.
Anyway, I’m returning it to stock and selling it. This red dude is where it’s at.

 

[mark1406]

Yes and no.
The replaying is gone, but with a 250k trim pot installed I still can’t get the bias below 40ma.
It’s functional, but it’s sounding like **** at the moment. It seems the lower the bias current goes the worse it sounds. It just seems quite loose and flubby, not how it used to sound or how you would expect a mark to sound.
I’m holding off doing anything major atm due to a potential issue with the PMEQ pedal I use in the loop.

I don’t understand how pedal in the effects loop affects or could be affected by the bias of the output stage.

40mA isn’t a bad bias point. I assume the valves are 6L6’s, but what brand are they?

In my SOB amp (encountered a decade ago) I found Sovtek valves were lasting 18 months and would start red plating.

That is something to bear in mind.

Regards

Mark

 

[mark1406]

I bet if you trace the circuit you’ll find the schematic to be incorrect

As far as the bias circuit goes, it very simple and the same circuit as used by Tweed Fender amps. So it’s likely to be correct in its topology.

Power supply circuits tend to be pretty basic.

Regards

Mark

 

[mark1406]

Thanks. That’s where it’s installed. In place of one of the resistors. But even with a 250k pot I can’t get the bias below 40mA. And the lower the bias goes, the worse the amp sounds

When you say the worse the worse the amp sounds. Under what conditions does the amp sound worse?

I can’t imagine the amp sounding worse when clean. Do you mean when the power stage is overdriven?

Regards

Mark

 

[BoogieJim]

I don’t understand how pedal in the effects loop affects or could be affected by the bias of the output stage.

40mA isn’t a bad bias point. I assume the valves are 6L6’s, but what brand are they?

In my SOB amp (encountered a decade ago) I found Sovtek valves were lasting 18 months and would start red plating.

That is something to bear in mind.

Regards

Mark

I wasn’t suggesting the pedal and the bias are related.
I was saying that the pedal fault could be the cause of the “sounding worse.”

But now I have a red stripe in front of me, I wonder if the blue stripe has other issues as well.
We aren’t talking about a slight difference here, we are talking about the difference between a face melting boogie and a slightly over driven fender.

 

[BoogieJim]

When you say the worse the worse the amp sounds. Under what conditions does the amp sound worse?

I can’t imagine the amp sounding worse when clean. Do you mean when the power stage is overdriven?

Regards

Mark

Over drive sounds weaker and the amp has less presence

 

[Jay Omega]

As far as the bias circuit goes, it very simple and the same circuit as used by Tweed Fender amps. So it’s likely to be correct in its topology.

Power supply circuits tend to be pretty basic.

Regards

Mark

It’s basic indeed, but not the same as a fender