I've just recapped my old mark IIb

[Tommy]

What a difference!

It's sounds big and tight now, maybe a little brighter and definitely more dynamic.
Huuuuuge improvement in tone!

Orginal filter caps: 2x220uF/285V, 4x30uF/500V, bias supply - 2x 50uF/75V

...I've replaced them with
220uF/350V, 33uF/500V and 50uF/100V TAD-caps (which are relabeled F&T)


I also put some preamp tubes and now all the microphonics are gone


...done the one wire reverb mod (which actually is 2 wires mod, because i've also connected reverb tank to sleeve)


also installed ON-OFF switch for turning off the fan without messing around inside amp.

[gonzo]

nice pics!

did this cap mod just last year, totally worth it.

[Elpelotero]

wow. you totally went for the gold with this! Look incredible...And an excellent choice of preamp tubes if I must say...

[acorkos]

i can see from the pic what the "one wire reverb mod" is doing....did mesa specify the wrong tanks when they designed these amps? seems accutronics makes tanks that ground the input and outputs....?

what audible difference does the one-wire reverb mod make?

[asmith9509]

Wow- great job. You must have either very small hands, or a lot more patience than I do. I've worked on a lot of amps, but one look inside the chassis of my IIa and I decided to take it to my amp tech to get the cap job done. Those things are packed so tightly I don't know how anyone can work in there!
Interested to learn more about the one-wire reverb mod- what is it trying to accomplish? How does it improve the amp?

[Bshizzle]

Wow did you do that yourself?

I wouldn't mind having that done to my Mark IV because I feel like with it being so old, it is missing something.

How much would it cost to pay someone to do this?

[Tommy]

Thank you guys. It took me nearly all day but yes I've done it on my own. I also cleaned all pots, conectors and tube sockets.
Thats normal maintenance job. I've done cap job just to make sure that this amp will be okay for the next 27 years.



This reverb mod is very simple and all what it does is, it makes better ground connection between chassis and reverb tank.
Shield in the reverb return cable is the one which is making ground connection (reverb send cable isn't shielded).
Sometimes reverb return connector is corroded and it makes resistance. It's hard to replace it because it's located under circut board. Anyway reverb send connector is usually clean, so the simplest way for better contact is to connect two RCA sleeves. The same thing may be done at the other end of cable by connecting sleeve to reverb tank.
Since doing this mod, I've noticed that there is more reverb and it seems less noisy.

(...)depending on the resistance of the corrosion of the ground, it will hold the reverb circuit slightly above ground and even a few ohms can make the reverb circuit in itself and amplifier within an amplifier by adding two gain stages and an output transformer as well as it's own preamp voicing.

Don't believe it? I have a IIC+ here in my shop that has so much gain it's almost unusable with the reverb know above 3. Oddly, the reverb now acts as a drive pot, so the higher the setting, the more clipping it produced. The clean channel is fine.

The reason, the reverb ground. One wire and the problem was solved.

In my country NOS RFT tubes are cheaper than JJs so I always keep a bunch of them.

[Bshizzle]

so how much would it cost to have someone do this for you?

[merendina]

Thank you guys. It took me nearly all day but yes I've done it on my own. I also cleaned all pots, conectors and tube sockets.
Thats normal maintenance job. I've done cap job just to make sure that this amp will be okay for the next 27 years.



This reverb mod is very simple and all what it does is, it makes better ground connection between chassis and reverb tank.
Shield in the reverb return cable is the one which is making ground connection (reverb send cable isn't shielded).
Sometimes reverb return connector is corroded and it makes resistance. It's hard to replace it because it's located under circut board. Anyway reverb send connector is usually clean, so the simplest way for better contact is to connect two RCA sleeves. The same thing may be done at the other end of cable by connecting sleeve to reverb tank.
Since doing this mod, I've noticed that there is more reverb and it seems less noisy.

(...)depending on the resistance of the corrosion of the ground, it will hold the reverb circuit slightly above ground and even a few ohms can make the reverb circuit in itself and amplifier within an amplifier by adding two gain stages and an output transformer as well as it's own preamp voicing.

Don't believe it? I have a IIC+ here in my shop that has so much gain it's almost unusable with the reverb know above 3. Oddly, the reverb now acts as a drive pot, so the higher the setting, the more clipping it produced. The clean channel is fine.

The reason, the reverb ground. One wire and the problem was solved.

In my country NOS RFT tubes are cheaper than JJs so I always keep a bunch of them.

thanks very much,
i tried the mod and solved my reverb problem!!

[BStone]

Hi all. I see this is an old thread but it really helped. New to this forum and i feel silly. I have a Mark IIB I haven't used much over the years. Got it in trade for a Ampeg V4 around 10 or 12 years ago.

Anyhow the reverb barely seemed to work but I didn't care. Now I care so I bought a brand new tube yesterday to start troubleshooting. Didn't help. Googled everything. Finally I stumbled on this picture.

I could see the RCA connections were backwards. I flipped em around in the chassis and boom! REVERB! I'm guessing the previous owner had done that on accident and never noticed or something.

So now I have an extra AT7 tube and a revitalized amp.

[BlackBoxy]

i personally tried the genral purpose Tad chinese caps, barker's BMI are into my gear now, wouldn't swap any TAD or F&t with it !

[AimSnapFail]

Hi all. I see this is an old thread but it really helped. New to this forum and i feel silly. I have a Mark IIB I haven't used much over the years. Got it in trade for a Ampeg V4 around 10 or 12 years ago.

Anyhow the reverb barely seemed to work but I didn't care. Now I care so I bought a brand new tube yesterday to start troubleshooting. Didn't help. Googled everything. Finally I stumbled on this picture.

I could see the RCA connections were backwards. I flipped em around in the chassis and boom! REVERB! I'm guessing the previous owner had done that on accident and never noticed or something.

So now I have an extra AT7 tube and a revitalized amp.

Thanks for digging this up! I've got a IIA at home with minimal 'verb and I'll be doing this after work.

[dodger916]

A word on cap jobs: While it's true old, caps do wear out, rob tone and can cause damage if they leak or burst, it's not necessary to replace ALL the caps in an amp. Filter and coupling caps are the highest risk/greatest impact, but the other smaller caps don't always need to be replaced. For instance, I recently bought a 1966 Fender Pro Reverb that had the filter caps and coupling caps replaced, but the other smaller caps in the tone stack are within spec and original. A tech I spoke with (who does tons of work on old amps) said replacing all caps can rob the amp of its "mojo".

[McBarry]

Totally agree with Dodger, and would add,..
Cathode caps should be replaced too BUT the tantalum ones often seen in Mk 1, IIA, IIB and some III's are ridiculously robust and I leave them.
But electro's in cathode position can dry like any other electro.
And when failing, they will change to tone of that stage.

Agreed, tight real-estate in there but most components are manageable from topside.
Get a good pair of locking forceps and yr away..!!
Dave

[stokes]

A word on cap jobs: While it's true old, caps do wear out, rob tone and can cause damage if they leak or burst, it's not necessary to replace ALL the caps in an amp. Filter and coupling caps are the highest risk/greatest impact, but the other smaller caps don't always need to be replaced. For instance, I recently bought a 1966 Fender Pro Reverb that had the filter caps and coupling caps replaced, but the other smaller caps in the tone stack are within spec and original. A tech I spoke with (who does tons of work on old amps) said replacing all caps can rob the amp of its "mojo".

I wouldnt change coupling caps unless they are leaky,especially in a vintage amp.The value of the coupling cap shapes the signal as the it passes thru each stage.Bigger caps,i.e. .1mfd will pass more low frequencies and less highs than a .02mfd.If you change the coupling caps just because they are old and not leaking,you will absolutely change the tone and not neccessarilly for the better.In a vintage amp,if a cap or resistor value has drifted some,replacing it with an exact value will affect the sound of the amp.If the amp sounds good,leave the coupling caps alone,unless they are leaky.