Replacement for Wilma Oil Filled Caps in Vox AC50

[blackba]

I am working on my '66 Vox AC50 which has not been sounding right when I crank it up. I found all 3 Wima Oil filled 0.1uF film caps after the phase inverter to be leaking DC.
https://www.voxac50.org.uk/images/schematics/thumbs2/schematic_mark_03.jpg

I see that Mojo tone offers oil filled caps and wondering if that is a good option for replacement:
https://www.mojotone.com/amp-parts/Mojo-Mods/Mojotone-Vitamin-T-Oil-Filled-1uF-600V

Other options are the Mojotone Dijon, Sozo Yellows, or Venerable Mallory 150s. I usually use Mallory 150s, but since this is my favorite amp I wanted to do right by it.

 

[pdf64]

You are referring to the gold coloured Wima Tropyfol?
They look to be metalised polyester, same as 150s etc https://www.wima.de/en/wima-group/wima-company-history/
Note that the use of 400V caps between phase splitter and power tube control grids can be marginal, it’s good practice to use >500V types there.

 

[blackba]

Thanks for the help

The 0.1uF caps after the phase inverter are silver in color and 600V rated, they look very similar to the mojos I linked to. They look just like these: https://www.ebay.com/itm/2-Vintage-...437661?hash=item1c8e5b615d:g:-YwAAOSwYIxX2WoI

The gold colored Wimas in the tone stack (0.022uF, 400V) at least for the normal channel have tested good, still have to test the ones on the Brilliant channel.

 

[pdf64]

ebay listings may not be a reliable source of technical info.
I think Wima (not Wilma ) Durolit pictured in the linked ebay listing were a metalised paper type.
A chap here opened up a failed one, no mentioned of oil.
Here's a modern metalised paper cap from a high quality manufacturer https://eu.mouser.com/ProductDetail...CQz916VlHvkuy6NhsK41y18wx8cnS5zkzsC9FKSZzbMCW

While you're in there, check for drifted resistors. eg the 56k DCCF cathode loads overdissipate and drift up, which probably helps to preserve them.

 

[blackba]

ebay listings may not be a reliable source of technical info.
I think Wima (not Wilma ) Durolit pictured in the linked ebay listing were a metalised paper type.
A chap here opened up a failed one, no mentioned of oil.

While you're in there, check for drifted resistors. eg the 56k DCCF cathode loads overdissipate and drift up, which probably helps to preserve them.

I see my love for the Flintstones must be coming out as I keep referring to them as Wilmas

So sounds like I can just use Mallory 150s or the yellow Sozos for those 0.1uF 600V Wima caps. Not sure if I am going to hear a difference where its worth the money for the Sozos. Ironically on most every picture of vintage AC50's I see online, these three caps have been replaced, should have clued me in well before now.

I will check the 56k's on the cathode of the preamps, good comment. I found one of the 470k's after the phase inverter to be well out of spec. One thing that's not clear on these resistors is if they are mostly 1/2W with an occasional 1W or mostly 1W with an occasional 2W. For example the 470k's I mentioned look smaller than the 100k plate load resistors on the phase inverter tube. I SF Fender typically used a 1W for the 100k plate load resistors, I have been using these Vishay 2Watt metal films. Just not sure for the 470ks what wattage I should use, was thinking a 1W 470k Metal Oxide for those.

Thanks for the help, excited to get this AC50 back and going.

 

[pdf64]

I think that 1/2 watt would be plenty for the 470k resistors in the phase splitter (self balancing paraphase), there's nowhere near 480V across them, even instantaneously.
Good luck, they are great amps, very overspec'ed monster OTs

 

[blackba]

Good Call on the 56k's. One measured 75k, the other was 80k. Will get those swapped out.

 

[pdf64]

They need to be 1 watt at least, as there’s usually well over 200V across them.
To try and get the HT downstream of the screen grid node to more sensible levels, I insert a 4k7 - 10k dropper between the screen grid node and all the other dropper resistors.