Ampeg SVT 6146 Cap Job

[DrGonz78]

So I am about to order the caps for this Ampeg 6146 amp. It has been recapped at one point but it must have been 30yrs ago. At least one cap in there is an original from 1970. In comparison to what is in the amp and the schematic shows me that someone used the best values they had on hand to recap. However, they got rid of C12b 40uf 450v. Also, the cap job looks real hodge podgy.

For example, the can cap on the schematic (C12) is 70x40x40uf @ 450v but in this amp it was replaced by 100x40uf @ 450v. So the 100uf stage has a 80uf 450v (C10)capacitor in series to the plate supply. So I could either order a 100x40uf can cap and keep it wired the same way that it is or order the 70x40x40uf and get it back to stock. Also there are two 80uf @ 450v ran parallel which they replaced the 30uf 600v cap. I am planning on getting it back to stock unless there are better recommendations to go by.

There is this original 1970 fire cracker 20x20uf @ 450v cap ran parallel that supplies filter to H nodes and ties R52 1k to the screen supply. That cap was not referenced on the 1969 schematic that I had been studying and thought it might be the missing C12b cap. Just today I realized that this one is the 1970 version C of the schematic and it suddenly made sense.

Now when I look on Fliptops for their cap kit for this amp they have this listed... Where is the 100uf 450v C10 cap in their kit? Also, Why do they show 3 70x40x40 cap cans??
https://www.fliptops.net/catalog/p-1...vt-w6146-tubes

1pc 70x40x40/450v -- Makes sense
2pc. 70x40X40/450v -- ??? confusing ??? Typo on the site?
1pc 40/600v (replaces 30/600v) -- I have 30/600v cap already
1pc 100/350v -- bias cap
1pc 220/300v -- screen
1pc 47/500v (preamp) -- I have this cap too

So my plan is:
1pc 70x40x40/450v -- Just like the schematic C12[a][b][c] This gives me back the 12b that is missing from the amp
1pc 100uf 450v -- For C10
1pc 30/600v -- For C11
1pc 100/350v -- C8
1pc 220/300v -- C9
1pc 47/500v -- For (H) supply
1pc 47/500v (preamp)-- C23

I plan on replacing R47 and R48 on the power amp for good measure and R47 is cooked good and hard on the preamp. Death cap is still in there so I will be cutting that sucker out too.
Any advice? Any thoughts?

[The Dude]

Two of the C12 caps are paralleled- a 70uF and a 40uF. I imagine they didn't have a 3 element can cap, so they used a 100uF instead, which is close enough to the 70+40=110. It won't matter which way you go, if I'm reading you right.

[g1]

Yes, paralleled caps add, so 100uF would be close enough to a 70 paralleled with a 40.
Not sure what you meant when you said "two 80uf @ 450v ran parallel which they replaced the 30uf 600v cap", did you mean series?

On the fliptops kit, I think one of the lines should have been 70x40 rather than 70x40x40 ? Otherwise they would have just said qty.3 of 70x40x40.

[DrGonz78]

Not sure what you meant when you said "two 80uf @ 450v ran parallel which they replaced the 30uf 600v cap", did you mean series.

Yup my bad I meant series.

[DrGonz78]

Two of the C12 caps are paralleled- a 70uF and a 40uF. I imagine they didn't have a 3 element can cap, so they used a 100uF instead, which is close enough to the 70+40=110. It won't matter which way you go, if I'm reading you right.

I guess I don't like the choice of 80uf in the c10 position. Since 100uf in series with 80uf is 44uf. The original filtering according to the schematic would have been 52uf. So I think 100uf for c10 will give me 50uf. So I think the 100x40 450v can cap is what I will order. Also I plan on using a 47uf 500v cap to replace the 40uf 450v that feeds the H node. That increase in capacitance there shouldn't be worry right?

[g1]

Close enough, no worries.

[DrGonz78]

Nice I appreciate the feedback very much. This is my second Ampeg of this era and I just want to get it right the first time. Thanks again everyone!!

[DrGonz78]

Not sure what you meant when you said "two 80uf @ 450v ran parallel which they replaced the 30uf 600v cap", did you mean series?

Just to revisit this question again... Whoever did the cap job on this amp DID parallel two 80uf 450v caps. So instead of having 40uf at that stage there was a whopping 160uf. Good thing I am setting this amp straight again. Not many of these 6146 SVT's left out there.

[g1]

Whoever did the cap job on this amp DID parallel two 80uf 450v caps. So instead of having 40uf at that stage there was a whopping 160uf. Good thing I am setting this amp straight again. Not many of these 6146 SVT's left out there.

If that was the C11 position, it's shown as 480V. So likely there was more than 480V sitting on those 450V caps.

[DrGonz78]

So in parallel it adds the uf's together but the voltage rating is doesn't add together. What in gods name was that tech thinking?

[g1]

Probably wasn't thinking at all. Just got lucky.
As Enzo often points out, the caps are usually rated at a max. 'working' voltage, and there is usually some leeway built in. I'd be interested to know what that voltage actually measures though, once it's back up and running.

[DrGonz78]

I will post a bunch of stuff like that at the end for sure. I was told "no rush" on this one so I am taking real slow and thorough. I love when you get clients like that.