Mark iii Blue Stripe 60w gain and volume loss on lead channel.

[BoogieJim]

Hi all.
As the title says… and it’s not tube related. I’ve swapped and cycled.

I’ve been poking around with this thing lately: first of all doing a cap job, which was successful (but am unlikely to do that myself again due to how hard these things are to work on) then a bias pot mod which was also somewhat successful. This I’ve covered in another thread.
Initially the amp sounded great, but sometime during my messing around with the bias mod issues the amp started sounding poor on the lead channel only. It is lacking volume and gain. When you switch back to clean it increases significantly in volume.

Anyone have any idea what it could be?
I’ve bought a red stripe recently, which is off its head good, so I’m selling the blue stripe. I’d like to get this sorted out before I find a buyer.

Cheers

 

[woodbutcher65]

If you think a Mark III is hard to work on, I'm confident you've never worked as an electronic technician in most fields of electronics in the past several decades. By comparison with MANY devices I've had to repair, it's incredibly easy to do ANYTHING to any Mark series amp.

If guitar amp techs think Mesas are hard to work on, you guys have lived very easy technician's lives to date.

Ever had to put two labor hours into disassembling something just to replace a faulty FAN, and another two hours reassembling it?

Device: CRT based video projector, one of eleven mounted in a flight simulator. Project: Replace the three CRT assemblies in ONE projector with pre-aligned replacement assemblies. "By the book", it's eight labor hours PER projector, WORKING ON A LADDER. Cost of dropping one of the new assemblies: Eight thousand dollars. Don't drop one! Actual time to complete the task is typically a few hours longer than the book says.

Yeah, Marks are easy.

Your problem may be something as simple as a bad preamp tube. Roll them. If the problem persists, map it out by going through all functions and controls and make notes. I know you say it's not tube related but...try again. I've had factory fresh bad tubes sabotage a troubleshooting effort before. Fortunately I'm very well stocked on tubes and can be certain of every tube by pretesting it in another amp.

Variable bias mod: I never recommend doing this to a Mark amp. Every one I've seen that has had this done becomes problematic eventually. Stick with the factory bias setup and stick with Mesa selected color graded tubes. I've found this contributes greatly to overall reliability.

I reverted my own Blue Stripe to factory preset bias.

 

[BoogieJim]

You obviously didn’t read the first sentence of the post. It is not the tubes

 

[BoogieJim]

No one has got an input?

 

[BoogieJim]

???

 

[user 41014]

You mean it's not the tubes? lol

I hate to sound like a skipping record but you seem extremely resistant to putting your bias where it should be. If your car was idling at 3000 rpm's you wouldn't try to fix something that doesn't sound right until you brought the idle down to 600. From what I've read you still haven't biased your amp for 30mA to check how it performs under normal conditions.

You certainly could have done damage with 46mA, and unless someone has changed the output driver coupling caps from 400V to 630V you're dancing in the fire not jumping over it.

None of which of course should affect the preamp but you are not trouble shooting from a baseline.

All joking aside and side stepping the "technical" diatribe, if it was me I'd make sure there is 630V couplers just because you're in there, put the bias back and even tho it may be beating a dead horse I would take 2 tubes from your red stripe and "roll" the 2 tubes (not just one) in the V2/V3, V2/V4 and V3/V4.

If you don't like 30mA you can take it back. But since your red stripe is kicking @ss, what is its bias V? I am very interested in knowing what you think is the bee's knees bias point.

One of the first things I would do is probe all the couplers on the board for shorts. Especially those surrounding V2-V4. Maybe DC rail volts are going where they shouldn't?

 

[VisaL]

If you think a Mark III is hard to work on, I'm confident you've never worked as an electronic technician in most fields of electronics in the past several decades. By comparison with MANY devices I've had to repair, it's incredibly easy to do ANYTHING to any Mark series amp.

If guitar amp techs think Mesas are hard to work on, you guys have lived very easy technician's lives to date.

Ever had to put two labor hours into disassembling something just to replace a faulty FAN, and another two hours reassembling it?

Device: CRT based video projector, one of eleven mounted in a flight simulator. Project: Replace the three CRT assemblies in ONE projector with pre-aligned replacement assemblies. "By the book", it's eight labor hours PER projector, WORKING ON A LADDER. Cost of dropping one of the new assemblies: Eight thousand dollars. Don't drop one! Actual time to complete the task is typically a few hours longer than the book says.

Yeah, Marks are easy.

Your problem may be something as simple as a bad preamp tube. Roll them. If the problem persists, map it out by going through all functions and controls and make notes. I know you say it's not tube related but...try again. I've had factory fresh bad tubes sabotage a troubleshooting effort before. Fortunately I'm very well stocked on tubes and can be certain of every tube by pretesting it in another amp.

Variable bias mod: I never recommend doing this to a Mark amp. Every one I've seen that has had this done becomes problematic eventually. Stick with the factory bias setup and stick with Mesa selected color graded tubes. I've found this contributes greatly to overall reliability.

I reverted my own Blue Stripe to factory preset bias.

Is there a technical reason behind why you dont recommend adjustable bias in Mark amps? Atleast simulclass amps run the outer pair really hot while inner pair is really cold. I think for tube longevity and tone it makes sense to bias marks..

Op, Check (clean&retension) also tube sockets.

 

[BoogieJim]

You mean it's not the tubes? lol

I hate to sound like a skipping record but you seem extremely resistant to putting your bias where it should be. If your car was idling at 3000 rpm's you wouldn't try to fix something that doesn't sound right until you brought the idle down to 600. From what I've read you still haven't biased your amp for 30mA to check how it performs under normal conditions.

You certainly could have done damage with 46mA, and unless someone has changed the output driver coupling caps from 400V to 630V you're dancing in the fire not jumping over it.

None of which of course should affect the preamp but you are not trouble shooting from a baseline.

All joking aside and side stepping the "technical" diatribe, if it was me I'd make sure there is 630V couplers just because you're in there, put the bias back and even tho it may be beating a dead horse I would take 2 tubes from your red stripe and "roll" the 2 tubes (not just one) in the V2/V3, V2/V4 and V3/V4.

If you don't like 30mA you can take it back. But since your red stripe is kicking @ss, what is its bias V? I am very interested in knowing what you think is the bee's knees bias point.

One of the first things I would do is probe all the couplers on the board for shorts. Especially those surrounding V2-V4. Maybe DC rail volts are going where they shouldn't?

The amp sounded great before any bias fiddling was done. This was with a bias of 46mA.
Since I recapped and did the bias mod, somewhere along the line it started sounding low in gain and volume, but I’m not sure when this happened.

I have rolled every tube in every position.

I don’t really care what the bias point is as long as the amp sound right.

Would the reverb circuit be responsible for these symptoms?

 

[BoogieJim]

Is there a technical reason behind why you dont recommend adjustable bias in Mark amps? Atleast simulclass amps run the outer pair really hot while inner pair is really cold. I think for tube longevity and tone it makes sense to bias marks..

Op, Check (clean&retension) also tube sockets.

I will check the tube socket tension.
Cheers

 

[VisaL]

The amp sounded great before any bias fiddling was done. This was with a bias of 46mA.
Since I recapped and did the bias mod, somewhere along the line it started sounding low in gain and volume, but I’m not sure when this happened.

I have rolled every tube in every position.

I don’t really care what the bias point is as long as the amp sound right.

Would the reverb circuit be responsible for these symptoms?

so 45ma means something like 20w plate dissipation. Pretty high for el34 but not alarming for 6L6GC. Dont know what you’re using.

The fact that there is a loss of both volume and saturation is why I cant see it being the reverb. Reverb comes along later in the circuit. Check wiring off the board to pots and etc around the area which is only used for the lead channel as clean works ok so you should be able to narrow it down.

 

[BoogieJim]

so 45ma means something like 20w plate dissipation. Pretty high for el34 but not alarming for 6L6GC. Dont know what you’re using.

The fact that there is a loss of both volume and saturation is why I cant see it being the reverb. Reverb comes along later in the circuit. Check wiring off the board to pots and etc around the area which is only used for the lead channel as clean works ok so you should be able to narrow it down.

I’ve got 415’s in it at the moment. Which I’m lead to believe are one of the most robust 6l6 tubes.
I did have an issue with one of the very fine, white wires going to one of the pots breaking free from the board. It was fixed immediately. But it was was either the treble or bass pot, not gain or volume or one that does channel switching.

Anyway, thanks for your input. I’m sure it’s something simple but these things aren’t easy to work on and I don’t have a lot of spare time. Hence hoping for someone to narrow it down for me a bit.

 

[woodbutcher65]

One technical reason I don't like adjustable bias in Marks is because it allows people to adjust the bias too hot which can result in damage to the bias circuit in addition to leading to tube meltdown. I've found that modern manufactured power tubes are just more likely to develop grid shorts and nuke themselves than "OG" tubes so I'm opposed to altering the amp in a way that is likely to almost literally add fuel to that fire.

Though not all of his designs are "perfect", when it comes to Marks, I trust Randall Smith's notion of bias circuitry and usage of pretested, preselected ranges of tubes.

 

[user 41014]

The amp sounded great before any bias fiddling was done. This was with a bias of 46mA.
Since I recapped and did the bias mod, somewhere along the line it started sounding low in gain and volume, but I’m not sure when this happened.

I have rolled every tube in every position.

I don’t really care what the bias point is as long as the amp sound right.

Would the reverb circuit be responsible for these symptoms?

Is this an assumption or did you check it before you did the work? Are you not willing to report what the redstripe bias volts are for fear of proving 46mA is just stoopid? I could check bias V's on one of mine but if you're not going to help yourself you might as well put this in the hands of someone that isn't hung up on the most basic part of a bias circuit or that can fix whatever you did.

20W of a 30W tube is 66%. On a data sheet that is ok. A boogie was not designed for that dissipation but this horse is long dead. One version of the 387/7581A story is the higher testing ones made the 35W cut. When you put a tube that was not a rugged 415 it slowly started to redplate so the 415 is the only reason you are not redplating from what you laid out.

How could this be more clear that you choose to ignore a problem biting you in the face?

46mA is not original to that amp despite your resolute inference that it was. To think it's ok because you tested it at that value after you did some work and then to resist normalizing it while asking for help is beyond what I should have stepped into. None of the pots you mentioned operate in isolation of the others either.

You haven't once bothered to run your blues in a safe zone. Good luck chasing your tail.

 

[Jay Omega]

I’ve got 415’s in it at the moment. Which I’m lead to believe are one of the most robust 6l6 tubes.
I did have an issue with one of the very fine, white wires going to one of the pots breaking free from the board. It was fixed immediately. But it was was either the treble or bass pot, not gain or volume or one that does channel switching.

Anyway, thanks for your input. I’m sure it’s something simple but these things aren’t easy to work on and I don’t have a lot of spare time. Hence hoping for someone to narrow it down for me a bit.

It's hard to help with the limited information that's been provided unfortunately, my best advice would be to go through the amp and check your DC voltages to make sure everything looks correct. If that's all good, fire up your function generator and give it a sine wave input and trace the signal thru the amp with your scope to identify where the fault is occurring. Once you've honed in a little bit we can hopefully point you in the right direction

 

[VisaL]

One technical reason I don't like adjustable bias in Marks is because it allows people to adjust the bias too hot which can result in damage to the bias circuit in addition to leading to tube meltdown. I've found that modern manufactured power tubes are just more likely to develop grid shorts and nuke themselves than "OG" tubes so I'm opposed to altering the amp in a way that is likely to almost literally add fuel to that fire.

Though not all of his designs are "perfect", when it comes to Marks, I trust Randall Smith's notion of bias circuitry and usage of pretested, preselected ranges of tubes.

My mark simul class was running the outer pair around 90%. I adjusted the bias to cool down the outer pair. So like quite the opposite from your argument people biasing too hot. Inner pair was something stupidly low, that i raised 45-50%. I have old Mesa labelled sylvania tubes and for me i does not make sense to unnessarily cook those, so in goes the bias trimmer.

I’ve heard many stories about how well Mesa branded tubes actually are matched and selected, but since i personally havent done those measurements, I cant vouch for that. I would still advocate some critical thinking instead of relying on marketing material. Early on Mesa was very clever, you can sell an amp only once, but valves are recurring business, hence the story ’you should only use Mesa tubes’. It is clever business practice.

 

[Jay Omega]

My mark simul class was running the outer pair around 90%. I adjusted the bias to cool down the outer pair. So like quite the opposite from your argument people biasing too hot. Inner pair was something stupidly low, that i raised 45-50%. I have old Mesa labelled sylvania tubes and for me i does not make sense to unnessarily cook those, so in goes the bias trimmer.

I’ve heard many stories about how well Mesa branded tubes actually are matched and selected, but since i personally havent done those measurements, I cant vouch for that. I would still advocate some critical thinking instead of relying on marketing material. Early on Mesa was very clever, you can sell an amp only once, but valves are recurring business, hence the story ’you should only use Mesa tubes’. It is clever business practice.

Interesting, why did you decide to drop the bias on the outers? For Class A, tubes are typically biased around 100% (or even slightly over 100%) of the plate dissipation and rely on the operation to drop below on average. Dropping the idle dissipation gets you closer to, or in, Class AB.

The Marks I’ve checked had the inner 6L6s biased around 50% of the max plate dissipation of the design basis power tubes, which is great for reliability / extending tube life. I noticed that when I bias Marks hotter I also get more noise with no improvement in sound.

 

[VisaL]

Isnt the outlet pair just your typical push-pull configuration with bias set high (well, too high)? If this is not the case, why Mesa recommends to use el34 in that spot and/or recommend tubes rated for lower bias current? So essentially they recommend the very same thing I did, bring the max dissipation closer to what tube can withstand.

 

[Jay Omega]

Isnt the outlet pair just your typical push-pull configuration with bias set high (well, too high)? If this is not the case, why Mesa recommends to use el34 in that spot and/or recommend tubes rated for lower bias current? So essentially they recommend the very same thing I did, bring the max dissipation closer to what tube can withstand.

Correct that the output section is push pull, the difference between the inner and outer pairs in simul amps is that the inner pair is setup for pentode Class AB, and the outer is setup for triode Class A. In a push-pull Class A output section, tubes are biased at max plate dissipation at idle and the bias is half way between cutoff and saturation. In a push-pull Class AB output section, tubes are typically biased around 60-70% of max plate dissipation at idle with the bias closer to cutoff.

Part of the challenge with non-adjustable fixed bias is that the designer has to settle on circuit values that would ideally work with a wide range of tubes. There is an inherent tradeoff here, where you may decide to optimize for a particular tube type. Back in the Mark II days, the values were such that the amp would work with four 6L6s, or 6L6s for the inner pair and either EL34 or 6CA7 in the outer. So that's three tube types in the outer position Mesa accounted for, and there are performance ranges for each of these tube types.

By the Mark III, Boogie seemed to have recommended that EL34s should be used in the outer. This is likely because they decided to optimize the circuit to support the tubes they were using at the time. 6L6s will run too hot in a circuit optimized for EL34s, which is especially a problem in the Class A sockets because you're already at max plate dissipation. So I would agree that if you want to use 6L6s in the outer, you will want ones that are lower bias current as you mentioned, but in general you will want tubes that won't be much beyond 100% dissipation at idle and ideally are biased half way between cutoff and saturation.

It sounds like you changed the bias to protect your tubes because you had longevity concerns with the high idle dissipation. The tradeoff here is that you may end up in Class AB territory, so instead of having a simul class (simultaneously running Class A and Class AB) you will just have a Class AB amp where the inner pair are likely to be running differently than the outer.

At the end of the day, if it sounds good, and you're not blowing up tubes, and you don't fry your output transformer, then all is well.

 

[BoogieJim]

Is this an assumption or did you check it before you did the work? Are you not willing to report what the redstripe bias volts are for fear of proving 46mA is just stoopid? I could check bias V's on one of mine but if you're not going to help yourself you might as well put this in the hands of someone that isn't hung up on the most basic part of a bias circuit or that can fix whatever you did.

20W of a 30W tube is 66%. On a data sheet that is ok. A boogie was not designed for that dissipation but this horse is long dead. One version of the 387/7581A story is the higher testing ones made the 35W cut. When you put a tube that was not a rugged 415 it slowly started to redplate so the 415 is the only reason you are not redplating from what you laid out.

How could this be more clear that you choose to ignore a problem biting you in the face?

46mA is not original to that amp despite your resolute inference that it was. To think it's ok because you tested it at that value after you did some work and then to resist normalizing it while asking for help is beyond what I should have stepped into. None of the pots you mentioned operate in isolation of the others either.

You haven't once bothered to run your blues in a safe zone. Good luck chasing your tail.

Why is it not clear to you that the bias is not the problem relating to the sound? Why do you feel the need to sit there on your high horse and talk down to me.

1. The amp was fine in all aspects running your so called stupidly high bias.

2. The sound problem crept in at some point while I was trying to REDUCE the bias current.

3. You clearly haven’t even read the thread title. It’s a 60w amp so there is no “inner and outer” tubes.

4. Stop making assumptions as to why I did the bias mod. I had zero concerns about tube life. I wanted to be able to bias the amp to what I understand is an accepted idle dissipation for whichever tubes I chose to put in there.

5. I am not “unwilling” to post the red stripe bias (also a 60w). I haven’t taken a bias reading yet. I am too busy enjoying the amp to unplug it.

6. The bias circuit WAS original with no component changes. If as your highness suggest the bias mA is not correct, it clearly indicated some other sort of issue in the amp

7. I did get the bias into a “safe zone” which is when I notice how crap is was sounding.

So either pull your head in and help, or FO.

 

[modder]

Also suffering from a very similar issue, blue stripe 60w. Lead mode has no gain or volume. Swapped out all the tubes, no bias mods, no improvement.