Servicing Mark IIb coliseum

[user 41014]

After wiring the PS board back up and locking it down the pots and jacks all got de-oxitized. Just waiting for some silicone to dry.

Almost time to Doug West this thing.

 

[user 41014]

I should have waited on the PS board as I finally upgraded it.

First things 1st tho and while I have a trad wl stock V30s ready for testing that just isn't good enough for a IIB. Rather than pull one of the halfbacks for a test run I decided to finish the homemade 4EB from a OS. I've only seen one stock 4EB. The guy agreed to sell it to me and then started selling off individual drivers, super rare 4ohm 12L's. What a you know what.

So finally even tho it's a non-rare 8ohm cab:

For anyone wanting to do this conversion four 8-32 bolts won't cut it for an EV. Between ups and fedex with/without people that know how to pack heavy items (or pack at all like gc sometimes) I've got many speaker and combo cabs in sawdust after losing the pitbull fight with EVM series drivers. EV wood screws on combos should always be tightened before shipping btw.

I used size 10 (10-24?) t-nuts into the front, with the 8-32 mesa nuts as additional support. You wouldn't be able to use bigger than 10 and don't want to go smaller.

 

[user 41014]

Back to PS.
I spent much time over-thinking this and sometimes it's better to just do it.

Why upgrade the PS?
All I can say is that while emailing Mike about another matter I mentioned if I see things go bad on the old Marks it tends to be the bias coupling caps that everyone knows should be upgraded to .1uF 630V and the 1st 30uF/500V coming in.
In the 2nd paragraph of this thread I stated this filter is pre-choke incorrectly as all 3 30uF/500V filters are post-choke.
Mike said these old PS's need upgrading this 1st 30uF 500V to double 47uF 350V filters in series, each parallel with a 1W 150K resistor. This is for the orange wire that feeds into the power screens (470ohm if not simul).

History of upgrade:
I never hear about this upgrade. GJgo has sent many amps petaluma way and when I see the pics of what he gets back I always noticed the changed PS board- only 2 30uF if 3 were there when the chassis was sent in.

It would seem the power tube screens (which is what this node feeds as the 1st B+ voltage) can go over-voltage hence the bulging 1st 30uF/500V and not the next two 30uF filters with their stepped down volts.

I saw the 30/30/40uF setup on both my late IIC+ and '95 KOB but Mike said look underneath the board and sure enough there are double 40uF caps including the one hiding under the PS board.

I can't tell you when the three 30's were changed, only that I know my early '82 still used that 30/30/30 supply and by some point in '84 for sure they had made the change.

For me that means at least 8 amps, many that have been serviced without upgrades, that came with three 30's. It also means thousands of amps would benefit from this upgrade to prevent the bulging 1st 30uF.

 

[user 41014]

There's any number of ways this can be implemented. I decided to build a module to install starting with the two new caps to replace the 30uF 500V closest to the bias caps and resistor. This puts everything on top of the board which in the end I liked.

I soldered the two in series and used shrink wrap on the connected leads. After being very happy with the Supertech 50uF/100V e-caps I decided to give these 47uF 450V caps a try.
These 47uF didn't wow me like the 50uFs I bought but they are consistent, very affordable, in line the BMI's test-wise but unlike the BMI's didn't come with some that should have been QA'd into the garbage before leaving the factory.

So overall having 105 degree and 450V rating when 85/350 is the standard called for these should hang in there like champs. But I always have the Mark IV brown caps in the back of my mind. Time will tell.

I put silicone on last night and got back to building the module this morning. In putting together some CAPI pre-amps that have been sitting around for some time I just came across Jeff Steiger's great DOA build suggestion of rolling leads over a screwdriver which I implemented for one resistor.

The module was intended to keep all + ends of the filters in the same direction and keep everything I do inside the board. The module + lead was ready to go into the PS board as the single cap would have.

The negative side of this module has the "side" resistor unsoldered. The lead of this 1W carbon comp was thicker than the cap and so was used as the board connect and then the cap leg soldered to the R leg.

You can see how the finished module would be attached after moving some things out of the way. I installed the module and am waiting for silicone to dry before getting back to the bias components I moved out of the way.

 

[Jay Omega]

Thanks for sharing! Where did you get the 1W carbon comps? I've never been able to find 150k.

I know you're down a path already, but Just Radios in Canada sells good quality 600V axial electrolytics at reasonable prices and I've had great luck with them. They have 30, 33, 40, and 47 uF axials. Thought I'd share in case you want to take a different approach with the next one!

 

[user 41014]

Thanks for sharing! Where did you get the 1W carbon comps? I've never been able to find 150k.

I know you're down a path already, but Just Radios in Canada sells good quality 600V axial electrolytics at reasonable prices and I've had great luck with them. They have 30, 33, 40, and 47 uF axials. Thought I'd share in case you want to take a different approach with the next one!

That is great info as I need to build a bunch of these modules. 600V is even better and I may need to make 10 of these but really is overkill. How long are the leads? The ST longer leads have been key for stretching all over the PS board.

There were 500V's available when I bought these 450V's but they were 2.5x the price of the 450V and 500V was even overkill. I think the 450V's reflect stock costs pre-pandemic.

I'm happy to share. I'm learning a lot working on my amps while using sources to learn amps 201 and know from my years of lurking many people are looking for info too often not being shared.

Most of my CC's have been from ebay. Resistor sellers are generally not like tube sellers. Prices vary greatly and I like to buy bulk if possible but only have a dozen of the 150K. Getting the 2W ones have been tighter to tolerance. I'm not willing to pay the "buy a pair" prices but they're out there. Smaller repair shops might have unopened packs of AB/Stackpole/Ohmite out there.

Luckily many people gave up on CC's after they stopped making them and many people wont touch them for their inferiority. Some of the prices now tho defeat one of the reasons of originally using them as they were cheaper than new fangled film caps. What- metal caps???

 

[user 41014]

I've been thinking about a bias mod. I've not been able to find a standard for implementing and I've seen videos of floating lug pots that are being used as rheostats and uncle doug putting the two lugs in between the supply and bias resistor and the wiper out. I liked that idea but lug to lug adds the pot value (10k) to the existing bias R in my mind so I'm missing something not even rusty could straighten out for me.

This is from the Mark IIB schematic that looks like it was originally the Mark IIA. It also looks pretty useless for the bias circuit. Is the "factory select" R value 6.8k, 10k, or both that calculates to 4.05k? Who draws a circuit like this with parallel R's for no reason?

My actual bias R is 5.6k like this top one below. I haven't figured out why C2 and R3 were added to the bias circuit vs the PS boards that have just one bias cap.

And while it seems natural to put a variable R in series I'm thinking about raising the bias R considerably and using a variable R in parallel with the high ohm mark desired just below the lower pot value. If anyone has a link to standard practices for trim pots please lmk.

It's just a guess but if I'm at 5.6k now I'm thinking a target range of 2.7k to 10k or 15k. If I'm getting this right higher R means a more restrictive grid and the ability to use hot tubes without fireworks.