Mark IV version - internal indicators?

[cho]

I have my 1993 Mark IV open right now. From the outside, it appears to be a "B" version, but I would like to verify the version on the inside. Does anyone know the internal indicators of an A or B version? Perhaps a marking, or particular circuit configuration or component?

Thanks!

 

[Rob Lockwood]

As far as I know the effects loop is a reliable indicator. If you have 4 jacks in a square pattern labelled normal and switchable loop then its an a. If you have the 3 jacks in a triangle with one send and two returns then its a b.

 

[igfraso]

cho,
what serial number is it?
The no-A/no-B versions are approx. in the s/n 6,000s. Do a search here and you will find a couple of threads about the no A/no B.

The back of the chassis looks like a B version (3 jacks in the effects loop) but the power cable is not detachable.

 

[cho]

The serial number is IV00614X. I know all the indicators, visible from the outside, and I think I have read most of the threads here. In one thread, I think I remember reading that there may have been some amps made with the B inputs/outputs (i.e. a B version from the outside), but actually having an A version inside the amp, in terms of the changes to the sound. I am just curious more than anything...

Is there a distinctive A circuit card vs. the B circuit card?

 

[igfraso]

Mine is s/n 617X (March 93).
I sent it to Mike Bendinelli for clean up/check up and he told me it was a no-A/no-B (also known as early B). That was the first time I have ever heard about this "in between" version. He also said that it was the best sounding version but I never understood if he meant the best of all the Mark IV or the best of the B versions.

My early B has all 12AX7 tubes while the A has a 12AT7 in the reverb. Output tranny is 562004R-1 and power tranny 561136. Chassis is black (A versions are silver?).

I don't know how all this compare to the B versions. For what I know, early B go between approx. s/n 5900 (Jan 93?) and 6800 (Nov 93?).

 

[sduck]

There are differences in the circuit boards. I don't have the pictures handy, but did some research on this at one point while trying to restore a broken one. Perhaps a dedicated google search will turn up these pictures.

And I've never really understood the power connector indicator - I've seen it mentioned more than once that the B versions have detachable power cables, but both of the ones I've had have been B's and both had completely connected, non-detachable power cords. Both serial numbers are in the 63xx range.

 

[igfraso]

Serial numbers 63xx are precisely those no-A/no-B versions. They are (as told by Mike Bendinelli) different from the A and from the B versions.

 

[cho]

This is all interesting, but I still do not know what makes the amp a noA/noB.... Just by serial number alone? There must be a way to determine it. We will probably need pictures... I have two Mark IV's from 1993. ( S/N: 614X and 664X). They both have the "B" effects loop/satellite sends. I will try to get some internal pictures and specs on both of them. If anyone has an internal picture from an A or a later B model, that would be useful, for comparison. Maybe we can finally get to the bottom of this mystery....

 

[Rhoads]

So here a picture of my Mark IVa ! i don't have the one for the MarK IVb ! i hope it can help you !

 

[sduck]

That's interesting that I may have a no-A/no-B version. I've been meaning to send it off to the factory for a tune up - maybe I can get the details then.

 

[cho]

Thanks, Rhoads.

Here is my Mark IVb 614X, dated on the inside as 7/16/93:

On mine, right about in the middle of the circuit board, it says "Mesa IV I"

I can't quite make out what is says on yours. Can you read it?

One thing I notice is that your amp has 3 silver "can" components on the mid-right side of the image, whereas my amp has only two. Are these transistors?.... I am sure there are some folks here that can see lot of differences...

Also, it would be interesting to see a later B version, to see if the early B's really were different.

 

[sduck]

Thanks for posting these! I can spot many differences. Those large electrolytic caps are the big obvious difference, but there are lots of other smaller things.

Now you've done it. I'm going to call mesa tomorrow and arrange to have mine sent in for servicing. I'll take some pictures of the guts, considering it may be one of the no-a/no-b ones. I've never been able to get a great sound out of the lead channel without using the eq - I'm pretty sure the filter caps need replacing.

 

[cho]

I think we have verified the "no-A" part of the mix, but not yet, the "no-B" part. We still need to compare these to a later B model....

 

[Rhoads]

No problem Cho ! Glad to help you

It's write on circuit board Mesa IV-E ???

But i have seen a post on the board about the difference between the A and the B and some one put some nice picture about it ! I have this picture on my other computer but i don't have it right here !!!

10 min later ...

I found it !!http://www.grailtone.com/forum/viewtopic.php?p=91961&sid=ed0f5ad0700621dab2ed069d1d45d4bb

scroll down at the 12 page.

that what you looking for !!!

Patrick

 

[cho]

It's write on circuit board Mesa IV-E ???
Patrick

Interesting. I guess you have a Rev. E of that board, and mine is probably a Rev. I.

I still don't see any evidence of an early B being different from a later B...

 

[igfraso]

I am not the best person to analyze differences in electronic circuits. There are lots of very knowledgable people in this forum so I would differ to them.

Just a suggestion.
In the Mark IV thread, I read that the A version has a smaller power transformer than the B.
But in the pictures I see here above, both power transformers seem to be identical (in the A picture and in the early B picture).

Only thing I could guess is that the "early MkIVB" name might imply that they have the main transformer from the MkIVA series which were leftover during MarkIVA production.

Yet, a "no-A/no-B" name would suggest a circuit that is different from both A and B versions.

 

[cho]

I think you may have something there...My two Mark IV's have different transformers. The 614X has a smaller power transformer than the 664X. Now I really need to put the same tubes in them and A/B them to see if I can hear a difference.... Alas, I've decided to sell them, as I am just plum happy with my Mark III.