MKIII, Exposed!

[RussB]

Hello!

I bought myself a PURPLE stripe MKIII Simul-Class head this past summer. I added a bias adjustment pot, and changed all the electrolytic caps, as well as a few ceramic tone caps to silver mica's. I also added 2.7k 2watt screen grid resistors on the "Class A" tube sockets. This was recommended by none-other than Mike B. himself.

Here are a series of pictures of what was done...

Here's the chassis, as I received it

The factory bias supply, note how R117 is "sniped" out of the circuit.

Here's some shots of the chassis, with all the new stuff...

Bias pot, with new bias supply caps

A shot of the sockets, with the 2.7k resistors. I changed the factory-installed 220k bias supply resistors going to pin 5 of the outer sockets to 150k, in order to get a better "balance" between the inner and outer set of tubes. There is also a 2.2k 1/2 watt resistor going to pin 5 of the 6L6's.

Here's a 220k resistor I added to the reverb circuit, as part of a Mesa-sanctioned "reverb upgrade"

I am current using SED EL34's and 6L6's in my amp...

EL34's current draw, 41.5mA in Class A, 38mA in Simul-Class. -40.8v negative voltage to pin 5
6L6GC's current draw, 34mA. -47.3v negative voltage to pin 5

 

[ned]

Sweet. I haven't been brave enough to take my soldering iron to a amp... yet.

What are the differences in sound with the mods?

Ned

 

[RussB]

Sweet. I haven't been brave enough to take my soldering iron to a amp... yet.

What are the differences in sound with the mods?

Ned

Well, all the electrolytic caps that were replaced aren't really a "Mod", they're just maintenence. The reverb "Mod" was done to all later MKIII's. It eliminated an oscillation at high volumes and extreme reverb settings. Same with the 2.7k resistors on the sockets...alll later MKIII's had this. There are a few other component swaps in there as well that were part of the later stripes's circuitry.

The only real modification is the adjustable bias pot...that can have a profound effect on the amp's overall sound and character, and it lets me try many different tubes, at different bias settings.

All in all, it's a lot of hassle to keep changing tubes and settings, then try to evaluate all the data. I've found a spot I like with the SED's, and I'm happy just to leave it there for now

 

[Joey707]

What kind of pot did you use for the bias, and where did you get it? Adding an adjustable bias is something I've been considering doing for a long time.

 

[RussB]

What kind of pot did you use for the bias, and where did you get it? Adding an adjustable bias is something I've been considering doing for a long time.

It's a 50k carbon trim pot. I got it from HoffmanAmps.com

If I was to do it over, I'd get a 50k cermet pot from Mouser.com

 

[Joey707]

Thanks Russ. I'm an old friend of Mike Bendinelli as well. There are a couple of other mods that can be done to the reverb to increase the amount and depth. The easy one is as follows:

Place a 3.3K resistor in series with the black wire at the effects return jack. So remove the black wire from the effects jack. Solder one end of the 3.3K resistor where the wire was. Solder the black wire to other end of the resistor. Place some shrink tubing on over the resistor. This will increase the amount of reverb.
There is a more extensive mod to get even more reverb, but you'd have to ask Mike about that one.
Just curious, have you added a RHY 2 master volume?

 

[RussB]

Great info Joey, Thanks! I see a 3.3k resistor in my MKIII's future!

I can't say I'm a friend of Mike B, but when I called and spoke with him, we went on for about 45 minutes! He's a great guy, for sure. Actually, all the people at Mesa are real helpful and friendly. Their customer support is second to none

 

[Shai`tan]

I really appreciate this post. Thx loads.

 

[908ssp]

Very nice post. Interesting that Mesa still takes an interest in their long out of production amps. Most companies act like they never existed.

 

[RussB]

just figured I'd bring this up to the top, with all the talk of re-capping as of late... 8)

Here's the power supply filter caps I ended up with, They're made by United Chem-Con (UCC) I got them from Triode Electronics. There are 4 in my MKIII. Resistors and diodes are replacements as well.

 

[scottcrud]

How hard is it to replace the caps underneath the pc board, directly under the two you have in the pic?

 

[Dasein]

Can someone show me what a cap looks like? Can you provide a picture with one circled?

 

[zeppman]

Dasein-

Caps are the cylndrical blue objects in the picture posted above by RussB. Don't go poking around in there though unless you know what you are doing.

 

[RussB]

How hard is it to replace the caps underneath the pc board, directly under the two you have in the pic?

Not hard at all, if you know what you're doing 8)

The fun is soldering to the PC board...you must work quickly, use enough heat, but not too much...and you can still raise a trace :?

 

[Dasein]

Thanks guys... I just could never visualize them.

 

[lshred365]

So what's the Varo diode on pin 3>8 for transit spike? Its not on my MK3 RED Stripe Simulclass.

 

[silentrage]

Hi Russ, I have a blue stripe Simul, I'd like to play around with tubes by adding a bias mod, especially a pair of KT88s with 6L6s.
Do you know if I can just throw in a pot where R117 is or do I have to change a few resistors and caps in the bias circuitry?