Help with caps!!!! *warning!! nekkid amp pics inside*

krankguitarist

krankguitarist

Krankitupologist
Allright, I've got a couple of questions for all ya'll amp gurus out there :). I'm replacin some of the caps on my '68 fender showman reverb, and could use some advice:

*note: I know pretty much next to squat about electronics. What I do know I either learned from my dad or "taught" myself. I do know how to safely discharge the filter caps though, so its unlikely that I'm gonna kill myself with this. Please excuse any facutal errors and/or gross incompitence*

First off, First pic:

252525small.jpg


That mallory capacitor in there says 25 25 MFD 25 VDC. Does this mean that its basically two 25µF 25V caps in one can? If so, can I just wire up a pair of 25µF 25V caps in its place?

Those orange drop caps definitely dont look stock.

Next pic (filter caps):

filtersmall.jpg


There's three 20 MFD 500 VDC caps in here, which are easy enough to find. There's also two 70 MFD 350 VDC caps in here, which I haven't been able to find. What I have been able to find is some 100µF 350V caps, would these be an acceptable replacement, or do I need to find somthing of the original value?

You can also see the damaged resistor in the upper left hand corner on this one...that thing looks like it got really hot at one point. Can't quite tell what value it is at the moment, gonna have to look it up on a schematic w/ my dad...cause I can't read schematics for ****.

I'm planning on replacing all the Mallorys with Sprauge Atoms. After this I'm gonna re-tube her, and see if I cant figure out whats wrong with the reverb and vibrato.
 
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Re: Help with caps!!!! *warning!! nekkid amp pics inside*

On the filter caps you can go higher on the uF (i.e.) 70uF to 100uF. The VDC is VERY IMPORTANT! If it says 350 DVC it MUST Be that value or higher. I would find the proper values of ALL resister on the cap board and replace them with metal film resistors. Also take note that All electrolytic caps are polarized + and - and have to be oriented that way.(Draw a diagram B4 removing them) The caps on your main board are decoupling caps. I don't believe the dual 25-25uF caps are available anymore? But single 25uF 25VDC caps are readily available. Rememeber... they are electrolyte to. Same rules of polarity apply. Hope this helps.:)
BTW, You know how to discharge the caps right? If Not DON'T TOUCH ANYTHING ! Wait until you know what your doing or bring it to someone who knows.
 
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Re: Help with caps!!!! *warning!! nekkid amp pics inside*

Do not change the coupling caps on the circuit board unless you really have to. Is there a reason why you want to do that? Are they bad in some way? I'm not a vintage amp nazi. If they need to be changed, change them. It's like needing new tires on a car. But, if they're still good, don't mess with them. That falls under the "if it ain't broke, dont' fix it" approach.

On the filter caps, that's a no brainer. These are more equivalent to an oil change on a car. You need to do this every 10 years at least. These look original. You can easily check the ends of filter caps for bubbling. As old as they are , it's a good idea to make the swap now regardless of the look.

Going with two 100uf's in the place of the two 70uf's will yield 50uf of total capacitance versus the 35 uf that the dual 70's yielded. It's a tad improvement in filtering.

I've attached a piece of a layout from one of the weber blackface layouts so you can see this a bit easier.

Notice how one of the 70 uf's is sitting in there backwards? This is critical. Also, when you pop the 70's up, you'll notice two 220k resistors. Make sure you get them in there right. They distribuite the load across the two caps.

Notice the 350v values on the 70uf's? (Weber uses smaller chinese e-caps, so he lists the values as 450v) When they are linked up + to - as a series pair, the total voltage handling double to 700v. Be sure not to get 450v Sprague Atom blue replacements for those two spots as they will be too big to fit in the little dog house. BUT, make sure on the 20uf's you get the right voltage (at least 450)

I also see that they have used some 1 watt carbon comp resistors between the 20uf's. If it was me, I'd replace all four of those resistors with metal oxide flameproof ones. Not only are they flameproof, but they wont' change in value as they heat up.
 
Re: Help with caps!!!! *warning!! nekkid amp pics inside*

Whaddayamean "discharge filter caps"?

*ZAP*

:amish:

no worries bro, learned bout that back when I first started learning how to bias my old marshall JCM800.

As for the coupling caps...I guess I don't really have a reason for changing em. I'll leave em be for now.

You guys kinda pre-empted my next question though :p. I'll replace the resistors as well. Though I'm having a little trouble determining the value of the "damaged" one...the color bars aren't visable anymore. Any suggestions?
 
Re: Help with caps!!!! *warning!! nekkid amp pics inside*

Scott F Quote: "These are more equivalent to an oil change on a car. You need to do this every 10 years at least.":smack: ( quoted out of context):laugh2: :laugh2:

Remind me never to buy a used car from you....amps. fine, but not cars.

All kidding aside, I'm enjoying this thread and learning a lot.

Dave
 
Re: Help with caps!!!! *warning!! nekkid amp pics inside*

All kidding aside, my point was they are required maintenance every so many miles (or years in amp terms). Whereas the coupling caps are like tires, only when you need them.

On the mysterious value, measure it with with a meter! Also, if you take it out, you might find that there are some markings on the other side.
 
Re: Help with caps!!!! *warning!! nekkid amp pics inside*

Scott_F said:
On the mysterious value, measure it with with a meter! Also, if you take it out, you might find that there are some markings on the other side.
Click to expand...

Damn, I shoulda thought of that :smack: .

I'll check that out...gotta wait till I get my next paycheck before I can order the caps/resistors anyways. *grumbles*

Wayyy excited bout gettin this thing back to a ass-kicking, non-humming state though. Thinking about pairing it up with this thing:

http://www.avatarspeakers.com/G410.htm

Thanks for all the help guys!
 
Re: Help with caps!!!! *warning!! nekkid amp pics inside*

Got another little question...I've got carbon comp resistors on my power tube sockets as well, I'm thinking it'd be a good idea to swap those out with some of the metal film/oxide variety. Sound like a good idea?
 
Re: Help with caps!!!! *warning!! nekkid amp pics inside*

Just yanked up one of the leads on that resistor, it measures out at about 14.2K. I'm assuming a 15K resistor would work in its place?
 
Re: Help with caps!!!! *warning!! nekkid amp pics inside*

krankguitarist said:
Just yanked up one of the leads on that resistor, it measures out at about 14.2K. I'm assuming a 15K resistor would work in its place?
Click to expand...

Look at the schematic B4 you assume ANYTHING ! Go to www.ampwares.com and look for the proper value. I took a quick look at the schematic. I could see a 10k resistor in the power supply? Double check 2x's solder Once.:)
 
Re: Help with caps!!!! *warning!! nekkid amp pics inside*

kmcguitars said:
Look at the schematic B4 you assume ANYTHING ! Go to www.ampwares.com and look for the proper value. I took a quick look at the schematic. I could see a 10k resistor in the power supply? Double check 2x's solder Once.:)
Click to expand...

Just asked my pops about it, he said that he thinks the resistor might have opened up a bit. Checking out this schematic (I'm trying to learn how to read em):

http://www.ampwares.com/ffg/schem/twin_reverb_sf_100_schem.jpg

I think I can see a 10k resistor in there right next to the 20uF 500V caps. I'll point it out:

http://www.effulgencemusic.com/sdpictures/10k.JPG

Only problem is I'm still having trouble figuring out where the 2.2k 1 watt resistor (red/red/red/silver in the pic) is in there. Its got the wrong values for a couple of the filter caps too (100uF 350v versus 70uF 350v). So...I'm not entirely sure if I've got the right schematic or not. I'll keep looking.
 
Re: Help with caps!!!! *warning!! nekkid amp pics inside*

krankguitarist said:
Only problem is I'm still having trouble figuring out where the 2.2k 1 watt resistor (red/red/red/silver in the pic) is in there. Its got the wrong values for a couple of the filter caps too (100uF 350v versus 70uF 350v). So...I'm not entirely sure if I've got the right schematic or not. I'll keep looking.
Click to expand...

I saw that to? You sould meter that one. There is a 3.3k in on the schematic.
Could it maybe be Orange,Orange,Red. 3.3k ? maybe the orange darken over the years? It certainly looks like 3 red strips in the pic? But the meter will tell the truth.
 
Re: Help with caps!!!! *warning!! nekkid amp pics inside*

Sounds like you're due for a 10,000 song tune-up.
 
Re: Help with caps!!!! *warning!! nekkid amp pics inside*

Looks like they're all twin schematics...ya get that same schematic when ya pull up the "100 watt twin reverb schematic" on the silverface twin reverb page.

That 3.3k resistor isn't anywhere near the 10k resistor in that schematic though...plus that one isn't marked as a 1 watt resistor.

I think...hold up, here's somthing:

http://www.effulgencemusic.com/sdpictures/10k.JPG

BAM! Just updated the pic. Think I might have found it :).
 
Re: Help with caps!!!! *warning!! nekkid amp pics inside*

I think that we've gotten as close as we can get. This makes me feel pretty comfortable about going with the 10k resistor.

Now I just wait for my paycheck to arrive :).
 
Re: Help with caps!!!! *warning!! nekkid amp pics inside*

Looks like you've gotten a lot of good advice, here. Pay heed to Scott's comments about how the caps are connected. Some are in parallel (same ends connected together) and some are in series (daisy chained). This is very critical, unless you want next July 4th to come a lot sooner, if you get my drift (you don't want them blowing up).

Make diagrams, check and double check your work, and if possible have someone else check it also.

Noth
 
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