SOBR vs KOB limiter title match

[electric mayhem]

In one corner we have a new to me SOBR the Wood Rat taking on heavy weight champion KOB the Wood Ox.

As you can see the Sobra toothed Wood Rat has 24 kills as an underweight to Woody KOB's 16 as the reigning coliseum class heavyweight favorite.

 

[electric mayhem]

Cue the bikini clad and music for round 1:

 

[electric mayhem]

SOBR:
I've got in the habit of popping out the chassis of new to me amps to look for visual clues of problems and a quick but careful probe for cap shorts. I was told the amp sounds great but you never know what is lurking in the unknown swamps until you look.

No cap shorts but the varistor (MOV) is shorted and this makes at least 3 of about 10 (one 70's, others 80's) Boggies suggesting it is a good idea to check the old varistors for shorts if nothing else while you are in there.

The other thing it suggests is upping the rating since $1 extra per amp buys a lot more surge protection. I'm sure the US electrical grid was a great thing many decades ago.

The varistor won't stop the amp from functioning (short of an ill-timed V-spike) but I'm pretty sure I ordered some better than Mesa spec protection-wise so I'm going to look before plugging in.

I was psyched to see red ended (+) e-caps for the 30uF caps, but the silver 220uf ones should go even tho I don't see any issues.

 

[electric mayhem]

Someone before me had the presence in mind to remove the limiter. This is correctly called a mod since it makes a big change but it should be called child's play from a technical level.

A disc cap (sometimes on the board even if unused) is inserted from the now presence pot (which needs to be changed value-wise from the limiter) in a short wire run to the board and the brown wire from the transistor looking thing is removed. In this amp they tucked the brown wire under the PS board. I'm not a fan of open wires so I'll put some shrink wrap on it.

Tho not the silver top hat on some of my marks I believe this TC R508 8341 is a current limiting diode. I could not find info on that part but it is a 2 legged into a 3 legged socket just like the silver top hats. So prob a N-JFET with source and gate tied.

I am also not sure if this represents a functioning part of the limiter or is protecting something. I don't see any opto-couplers to protect in the circuit.

There is a jack on the back, ground railed with the speaker jacks that seems unneeded so it must be for stability. A lug used to be soldered to a wire making me wonder if the jack was once used as a slave out using a pot where the limiter pot was.

The old tape (what is that called?) used to mark the back of the original seventies Boogie marks the PRES on the face plate almost making it seem like PRES was done at the factory.

 

[electric mayhem]

No middle in the tone stack makes me scratch my head. Anyone add a Mid to a no-mid SOB?

And if so, any thoughts about it?

 

[electric mayhem]

KOB has both Presence and Limit.

The brown wire of the limit pot goes to one leg of the green socket for the limit.

The yellow wire from the presence pot meets the 8 ohm tap of the OT at a pot on the back I've never used but figure is a slave out to the jack next to the speaker outs. The wiper of the presence pot goes to the disc cap on the board via a 10k ohm resistor.

 

[GJgo]

You still have the only KOB I've ever seen. Would love to have some pics for my group.

 

[electric mayhem]

You still have the only KOB I've ever seen. Would love to have some pics for my group.

It is quite lonely knowing there may not be a mate out in the forest or a-high-plains-drifting. Alas, maybe they have all made the journey across the sea to Valinor and are awaiting their brother KOB the Wood Ox.

Pics- how many do you want? I will email them to you.

 

[mace]

Intriguing. I was unawares of such vintage gear and it’s esoteric existence. I looked at the photos trying to decipher SOBR and KOB but was not successful. What do the R in SOBR and what is KOB stand for?

Also, I think it is called label tape. A factory built product that made use of it is some truly vintage gear!

 

[electric mayhem]

Interesting thought about it coming from the factory with label tape (thank you!) if they were out of presence face plates at the time. That isn't what I was thinking but you just never know and no way of telling what has been done since.

The R is reverb and one of the pics shows the SOBR-A preamp board. Not that I was paying attention but I had never seen this board and I love nerding out on these 80s mesa's.

KOB is just SOB with a different power section so sadly there is likely no KOBRA board. K = coliseum = rob a bank to re-tube.

There is sales lit calling out the 100W/60W option as hob but I've never seen reference to the existence of this woolly mammoth.

I know there are simul-class ones in the woods (if you are brave and...) and tho I haven't seen the black marker there should be some DOBs out there. DOBR- prob, DOBRG? DOBG (dog of boogie gun)? HOBR-prob, HOBG the hobgoblin? HOBRG?

I wanted to snag the MS-12 from what looks like the same sus-4 cab the Mark IIC and III uses but SOBR doesn't describe me so I may have to pass this on to someone perpetually on the 1st step. Can you imagine if it had EQ too and someone accidentally used E for EQ and swapped the letters? SOBER????

Definitely the wrong label here as I live in a smoke cloud that doesn't smell all that bad. Cheers!

 

[mark1406]

Hi

I was wondering if you have a detailed picture of how the reverb is implemented in the SOB amps?

I have never seen an SOB with reverb.

Regards

Mark

 

[electric mayhem]

Hi

I was wondering if you have a detailed picture of how the reverb is implemented in the SOB amps?

I have never seen an SOB with reverb.

Regards

Mark

I can take more and here are some I already have of SOBR-A. The only thing I looked at so far was the anode resistors, 100k, to see what kind of gain they were going for.

The wires for the reverb pot look to be going to V3.

 

[electric mayhem]

The jack for the grey wire hides under the board.

 

[GJgo]

It is quite lonely knowing there may not be a mate out in the forest or a-high-plains-drifting. Alas, maybe they have all made the journey across the sea to Valinor and are awaiting their brother KOB the Wood Ox.

Pics- how many do you want? I will email them to you.

Indeed! A good front, back, and gut shot would be awesome.

 

[mark1406]

Thanks for the pictures electric mayhem. I appreciate that.

Regards

Mark

 

[electric mayhem]

Thanks for the pictures electric mayhem. I appreciate that.

Regards

Mark

Anytime. Any thoughts about the reverb, output or preamp are welcome.

 

[electric mayhem]

I measured the pots:
SOBR presence - 1M
KOB limit and presence - both 1M

For reference the IIC S presence measured 200k. Maybe the 1M is compensating for the output? I've seen reference to different or no PI stage and that the SOB does not use a long tail so there seems to be some significant differences in the output.

After dental exams I'm giving round one to KOB by decision. As you can see there is a second 40uF e-cap sitting under the PS board on KOB- not sure when Petaluma started using the new spec to prevent over-voltage but this 1985 KOB and my 85 plus both utilize this solution to the 3x30uF and resultant failed 1st cap.

And even tho you can see the KOB uses 2 varistors, one is failed. Add to that the IIC varistor which is shot.

I'm getting close to a 50% failed on varistor (MOV) checks. The good news is they were doing their job. The bad news is that is past tense and in an emergency transient spike the amps are no longer protected.

Still haven't found my bag of Bourns varistor upgrades.

Varistor Beware!!!!!!!!!!!!!!!!
Check them at least for short.