Mesa Subway Blues Power Tubes Daughter Board Rebuild

[Pyramid Head]

Hello everyone!

I've got a Mesa Subway Blues that broke down a while ago. While I'm repairing it, I thought that as I'm already working in the Chassis I could seize the opportunity to make some improvements.

It's a late model with Bright/Fat Switch, 1/2 Power Speaker Jack & unpotted PCB (Mesa used to encase parts of the PCB in huge blocks of Epoxy in many Amp Models in that era, to hide their "Mojo Module").

These amps run INSANELY hot! I wouldn't have it any other way as the cranked tone is great, but the way the amp was designed make it particularly vulnerable to heat, mainly at the Daughter Board where the EL84 Sockets are located along with a few components. It's a common point of failure and damage...

• The issue is that all the Amp's Tubes are PCB mounted...

Since the Power Tubes run so hot and that I've discovered that a resistor had already been replaced on the daughter board (Blue 220k Resistor) AND that the 2.2M Resistor on the right is faulty (see note attached to picture), I have decided to just remove that board.

~> I will install Chassis Mounted EL84 Sockets (Belton Micalex) to get rid of the PCB overheating and potential (intermittent or invisible) damage to the PCB & the Trace eventually caused by pushing in the EL84's again and again...

~> I'll rewire the leads and Resistors around the Sockets either on a small Turret Board or using Solder Lugs, Terminal Strips, to keep everything away from the heat.*

*Here's where I would need your help!

~> I would like your input on how you would layout the components, how you would go about this.

~> I'm thinking about replacing all the Resistors with ones with a higher Power Rating.
What would you suggest for Wattage?

~> Lastly, I'd like your input on the lead dressing. I'm new to this, I'm learning fast, reading a lot, studying hard, but I haven't covered everything yet.

I know that AC Heater wires are usually supposed to be twisted, but the 2 Green wires aren't at all. Would twisting them be a good idea?

Also, along with your layout suggestions, I'd like you to include the lead dress please, with any tips you can give me!

Thanks so much!

~> Daughter Board with all Traces highlighted & all components identified.


*Note: It's hard to see, but the 2.2M Resistor on the Right is faulty: it's missing a small chunk in the lower right end. Ohm measurements keep wildly fluctuating until the DVM goes crazy and loses track.


~> Board without notations: You can see better how much space I have to work with.









Thanks!

[goldenGeek]

I've fighted a few of these Mesas, but I haven't rebuilt the tube board. However, there is a guy at YouTube that did this with a DC3, which actually is built pretty similar. Check that out, you might get some ideas from that how to do it

https://www.youtube.com/watch?v=jQZbvFQ_Jn4

[Stevem]

There looks to be plenty of room there to go with chassis mounted tube sockets and like three 4 terminal terminal strips as used in this picture of a 1960s Sears Silvertone amp.

No need to fret about the heater wire runs for the output stage in terms of twisting them or not, output tubes have very little gain and they are getting driven by a high level of signal so 60 HZ hum will not be a issue.

[Phil_S]

Probably 1/2W resistors are more than adequate and I wouldn't be so quick to replace all of them. I'm doubtful that doing so will achieve what you may imagine. If you have specific resistors that you know run too hot, then, yes, replace them with 1W rated. You really should measure operating temperature with a thermal probe or infra red thermometer and not just guess. When replacing components, there is risk of damaging something else with the heat or lifting a trace. The wattage rating is determined by the voltage drop across the resistor, not the absolute voltage.

For tube socket connections, you might consider getting these from long time TAG-er Lou Rossi: https://luigiretro.com/LRC-VT9A I think, in this situation, the advantages are self-evident.

Generally, I suggest that you slow up. Make very few or only one change at a time. IMO, otherwise you risk losing the mojo of the amp.

[Stevem]

Another think to keep in mind is that most 1/2 watt rated resistor's are also rated for no more then 350 volts.

[ChopSauce]

So what are you trying to achieve: prevent possible failures for the future?

[Pyramid Head]

Didn't want to be warned for multi-posting so I answered everyone at once.

I'm looking for help in choosing a layout, placement and type of terminals/turrets/eyelets for my Circuit Rebuild using discrete components.

https://www.youtube.com/watch?v=jQZbvFQ_Jn4

Yeah! A CLASSIC Video from Brad the Guitologist!
Love this guy! I'm an avid fan of his!

And what a legendary 4 parter Video Series!
The Saga of Ahab slaying his Whale!

I had seen it previously, but probably because I was so focused on the mystery behind the Mesa's malfunction, I didn't register in my long term memory his rebuilding of the EL84's Power Tubes Daughter Board with discrete components!

If anyone needs proof of how and why this Board with PCB mounted Sockets with critical components located very close by and the Hi-Voltages + Extremely high Temperatures
of Mesa's Dyna-Watt EL84's amps that most often run at/or over 100% Dissipation Rate (I think the Subway Blues was 120% ) is such a huge Potential Failure Point, just tell them to watch Brad's video!

He doesn't go into the 100%+ Plate Dissipation Rate issue, but mentions a great many times the fact that he had changed Components Values to significantly lower the Bias to bring it down to the Goldilocks Zone of ~70%.

While helpful seeing Brad's rebuilt of the Power Tubes Circuit, sadly he just skipped ahead so we didn't get to see him build it nor make his usual comments explaining what he's doing + giving a ton of helpful info...

The end result is not so hard to see, but it's also not super clear. Add the fact that he was missing some Resistors so he used some additional parts to get in the ballpark of the original value, adding some cluttering, and also that he barely used Soldering Terminals and really went classic PTP makes this much less useful to me regarding my questions about how to make a good layout...

I won't make a Rat's Nest or Spaghettis... I'll make it clean, tidy, clear visually so identifying & servicing will be a breeze while still being efficient by not using unnecessary lengthy wires as is often the case when someone builds a circuit with the intention of doing it clean, using Terminals (Lugs, Dual Terminals), Eyelets, Turrets, etc..

I'd still like your input on how you would build the thing while keeping in mind the elements I mentioned. What type of "hubs" would you use (Lugs Terminal, Turrets, etc.)? What about the supporting structure and where would you locate the components?

So what are you trying to achieve: prevent possible failures for the future?

EXACTLY.

Mesa has built many amp models in that era that used the same design flaw, using a PCB Daughter Board on which the EL84 tube sockets are mounted, along with a few critical components.

And since Mesa runs those Dyna-Watts amps insanely hot (usually over 100% plate dissipation rating, some as high as 120% !!!), meaning that the Power Tubes run extremely hot (and since heat rises & the Sockets are located directly onto the PCB, the Board is heated up too) and that there are High Voltages running on the PCB...

It's no mystery why this is a common failure point on those amps, and very often the source of other damages...

On the Daughter Board:
~> I measured again and found out that BOTH 2.2M Resistors are bad. They measure around 450-500k for one and 500-550k for the other...
~> The 220k Grid Resistor (the Blue one) on the right has previously been replaced too...

When the amp "blew up" a while ago, it took 6 High Voltage Power Supply Resistors with it! They are all bulged up and look cooked, burnt and black.
(On the Schematic: The chain of 6 Resistors (3x820 & 3x1k) in the Hi-V Supplies section, right above this title.)

I'd stored my amp at my mom's for a few years.One day she came to me and said that she got my amp fixed as a Christmas gift. I learned that she had brought it to an Electric Appliances Repair Store which also dabbles in Electronics...

When I learned of this, the 30 days warranty on the work had expired, the 160-180$ bill didn't give any details except "time worked" and the price charged for a new pair of Electro-Harmonix EL84's from Thetubestore (60$ !), and the amp still didn't work properly.

The tubes, of course, were & still are fine. Until recently, I thought that the only thing that they did was scam my mom for a pair of EL84's, but I saw that they had replaced the 220k Grid Resistor on the PCB.

So, while I'll be replacing the Power Supply Resistors and testing every single component in the amp, I'll rebuild the Power Tubes Sockets Circuit with discrete components to replace the bad 2.2M Resistors and greatly mitigate the overheating issues.
~★~ ~★~ ~★~
I'd like to state that I have the necessary skills, knowledge and experience to solder components neatly both PTP and on a PCB...
~★~ ~★~ ~★~
Thanks guys for your help so far, you are all wonderful!

~> Phil_S has given some warranted constructive feedback.
I thank him for his contribution, his concern and the fact that he advocates caution.

There is a great deal I can learn from him and you all.
~★~ ~★~ ~★~
In my next post, I'll show some pictures of layouts and wiring I have found that look interesting, and I'll ask your opinion on those!

I'm really looking forward to hearing from you all and go back to having a normal discussion with you!

Take care!

[sluckey]

It's two sockets and six resistors. How difficult can it be? In the time it took to write that colorful, long winded post you could have had it done!

[pompeiisneaks]

I've not read most of your reply because I caught what seemed to be the tone of it pretty quickly.

You've posted a single post on the forums, nobody here has any concept of who you are, or what your skillset is. We regularly have people come here with 0 knowledge of amps and are doing dangerous things. People here tend to assume that it's best to educate and assume less knowledge, to then find out later the person knew more.

I find it odd you've seemingly taken offense to people that know absolutely nothing about you, somehow didn't know anything about you.

I hope people continue to want to help you here, but remember they are doing it for free, out of the goodness of their hearts, and sharing with you their well earned knowledge.

Nobody intended to insult your intelligence because they'd have to know what level of intelligence you had, to be able to.

Please assume that people here are doing their best to help, and giving of their knowledge freely, and things may be a lot smoother.

I don't like it when people are negative and I work hard here to keep the tone civil. Phil_S has posted a LOT here and his simple misunderstanding that you weren't retaining the PCB wasn't intentional, nor intended as some kind of slight.

Your denigration of a product that professional amp builders use to speed up amp building is odd, but you've got every right to talk ill of something you've never used, but I don't see what we gain from that.

I felt his comments came from a perspective of someone new to amps that may end up making things worse and he was trying to help.

Again, since he knows 0.000000 information about you, he has to make assumptions based on your 1 post.

Please be more accepting of the help, or, you'll find less of it.

~Phil

[ChopSauce]

I didn't caught that the previous posters didn't manage to answer your questions ...

Please, assume that, being facing the problem, you might as well be the most competent person on the forum, with respect to this subject. Your comments about the video enforces this assumption: you noted several point of details, which obviously were missed by other members.

Note, also: there might be someone in the world - maybe even several persons - able to deal with this problem as nicely as possible, but we are not responsible for the fact that they don't post on this forum to reply your question ...

That being exposed :

So what are you trying to achieve: prevent possible failures for the future?

EXACTLY.

Mesa has built many amp models in that era that used the same design flaw, using a PCB Daughter Board on which the EL84 tube sockets are mounted, along with a few critical components.

And since Mesa runs those Dyna-Watts amps insanely hot (usually over 100% plate dissipation rating, some as high as 120% !!!), meaning that the Power Tubes run extremely hot (and since heat rises & the Sockets are located directly onto the PCB, the Board is heated up too) and that there are High Voltages running on the PCB...

It's no mystery why this is a common failure point on those amps, and very often the source of other damages...

So, I'll second Phil_S' comments and insist that yet PCB mounted sockets aren't the best option, it doesn't explain the amp overheating, does it?
(which would therefore imply that changing the PCB may not be needed at all - the professional amp repair tech solution, I guess)

Finally - if you do want to run the amp that hot, anyway - which solution you'll end up with as an implementation is a matter of taste, education, room, material, skills ...

... & I (roughly) guess that I would be done with a few tagboards and soldering to sockets, but I haven't the time to sketch a layout - even for myself, today - sorry,

[Pyramid Head]

Hi.

Thanks for the post pompeiisneaks.
It has helped clarify things a lot for me.

I can now understand where the tone was coming from, and I must say that I totally understand why forum members advocate caution and careful thinking with newcomers as by experience they must know that a good part of new members don't know what they're doing so they're trying to do good and prevent injuries, people damaging their amp and doing work/mods that they won't like or don't need, often because they haven't thought things through.

I really owe everyone an apology for the way I reacted. I didn't have any idea of the context the members were acting from, which lead to a serious misunderstanding on my part.

People who know me have always told me that I'm one of the most open minded, flexible and Zen, imperturbable person they know...

I really don't understand why I took that post so personally and even worse: lost my cool and acted upon it! It's so out of character for me! And I'm very sorry.

One thing to note about me is that I have a very expressive, intense and sometimes dramatic writing style, but it doesn't mean that I'm remotely emotionally involved to the same degree & that I'm not a very down to earth and rational person!

English is not my native language, and while I learned it at a very young age and am fully fluent in it, since I only did a small part of my scholarship in English, I know that I'll never be as fluid and able to articulate my thoughts with as much nuances and precision as when using my native language, French.

So please be forgiving for my numerous mistakes.

~> I will soon drastically edit my previous post to remove all the... Bad parts.

I'll take this opportunity to tell you all a bit about where I come from. It'll help those who are so kind as to care enough, take the time and effort to provide people like me with help, support and guidance.

I've always been an extremely intellectually curious person. During all my education, I have been lucky enough to be in programs for gifted students, which allowed me to study a WIDE range of subjects and gain a solid scientific background.

In College, I chose to study Music! Jazz Electric Guitar with a double specialty in (1) Interpretation and (2) Song composition & arrangements. Still, I had very varied and strong interests in sciences. To further pursue these interests, I decided to take extra classes in Human Biology, Advanced Statistical Mathematics, Psychology, but sadly my class choices were very limited (and I already had 12-13 different classes only for the Music program, PER SEMESTER, so the workload was utterly insane), as was time, so that's all I could get. If I had the choice, I wouldn't have completed 2 Stats Maths courses! I probably would've even ditched Psychology too in order to get classes on subjects closer to my passions like Physics, Chemistry or other hard sciences.

I've been a Music/Guitar Teacher since I was 18, it's a great passion of mine, but was forced to stop a few years ago due to falling ill with crippling, extreme Chronic Pain... Had to stop practicing Martial Arts too...

I'm a DIYer, never shying away from learning how to make, build, repair things, no matter what amount of material and learning difficulty I have to face.

I have bases in Electronics, and I have decided, ~3 months ago, after years spent contemplating the idea, that I will learn how Tube Amps work, are built, what each component does, etc., in order to first repair my broken little Mesa, then learn how to Build & Mod Amps, Pedals, to finally get to the point where I will Design my own Amps.

I'm an extremely quick learner, I'm very passionate and an almost pathological perfectionist. A trifecta that helps me delve deep into any project and pursue it relentlessly!

While it's true that I'm fairly new to Tube Amps Building and Repairing, I'm reading, studying a lot and I've got some people in my entourage that are very knowledgeable and experienced and who are guiding me in my learnings.

There are many things I can do, but there's so much more I have to learn.

The reason why I am asking about such trivial things as how I should go about with the layout of the components & wires and what type of physical support along with wether to use terminals, turrets, eyelets, etc., is because I want to know what would be the best way to achieve my goal & why (like lead dressing) and also because I don't possess these parts yet: there's no local electronics store that carries old tech such as Turrets nor high quality components so I'll need to order the parts.

And since I'm going to be placing a huge order to build myself a kit of the most commonly needed essential parts for future projects, along with some specialized tools, I want to know what I'll need to add before completing my order as I've got a good discount coupon that I can use only once...

That's why I haven't already done this simple work! I've thought about some layouts, but I'm far from satisfied with what I came up with and I can't just take some components and position them in my amp to easily see what would be best since I don't have them yet!

I also want to avoid rookie mistakes too!

Your help is greatly appreciated and I hope we can get past this unpleasant event.

Thanks!

[sluckey]

Self-impressed blabbermouth.

[Phil_S]

Well, I guess I've been told.

[sluckey]

Well, I guess I've been told.

Yes, and under 5000 words!

[pompeiisneaks]

sluckey.... I don't know that you're helping the situation here...

~Phil