Vibrochamp Filter Caps Bad?? -and more...

[2manynotes]

i have a silverface Fender Vibrochamp that was a project amp for someone before i bought it on ebay. they didnt know what they were doing, or encountered a problem they didnt know how to fix, and sold it as-is. the only thing is, i didnt know that when i bought it. they had stuck a piece of wire in the fuse holder to keep it running. i didnt know that either, until it just quit on me one day. the rectifier was glowing red hot, and crackling from the heat! i was sure i killed it, so i just let it sit in a corner for over a year.
then about a week ago, i got tired of looking at it and decided to do something about it. to make a long story short, the power tube socket was the culprit for it blowing fuses. it phelonic (sp?) material went bad, and was arching between the pins, and going to ground through the power transformer.
so i replaced both the power and recitifier sockets with ceramics. no more blowing fuses. so the amp "works" but then it had a nasty crackling sound whenever i played anything on the low E string. i read up on capacitors and decided to replace them all. i did that last night with good results, BUT the crackle isnt totaly gone. its still there, but nowhere near as bad as it was. i also seem to be getting a weird subtle oscilation on some notes, and a staticy type of noise on other notes. compared to it blowing fuses and being completely dead, this is a huge improvement, but i can tell it aint right.

i have everything i need to replace every component on the board, and im tempted to strip it all off, clean it up, and replace everything, leaving only the Filter Caps, and the transformers as original parts (assuming theyre original).
im -obviously- new to the whole tube amp thing, and im resigned to this amp being a project amp. something i can tinker with, and hopefull learn something from. im not the type to just turn a knob and be satisfied it works. i want to know what goes on behind the knob. ive been reading up on tube amps, and i have a very basic grasp of how they work, but barely.

my question is this: i know the filter caps supply very high voltage to the board. i know to drain them before working on the amp, as getting shocked can kill you. when the amp is running, tubes are in, speaker hooked up, i get voltages up over 400vdc. but when i shut the amp off, the voltages drop to 25, and then 10 within 30 seconds of shutting it down. -are the caps bad? shouldnt they be holding that voltage alot longer? or do i still have a problem elsewhere? how do you know when the filter caps are bad?
-also, ive seen replacement filter caps online -supposedly direct replacement for champs and vibrochamps- saying 20-20-20-20@475v -ive only got 3 leads comming out of the filter cap can. so that doesnt seem right to me.

can i take off the can, open it up and just swap out the caps inside?? -with the correct values of course... it'd save me money...

i hope someone can help. im told these are VERY simple circuits, and perfect for someone to learn about tube amps. i just need a place to start, and pointed in the right direction.

thanx
-
jim

[Amp Kat]

The caps you listed as replacements are four 20uf 450 volt caps in one can. The Vibrochamp only uses two and I'm showing 8uf's on the 5C1 schematic. The other connection is the ground that attaches to the can and goes to chassis ground. The caps discharge when power is turned off as there is a load pulling them down. If you disconnected everything and charged them up to 400 volts without bleeder resistors to ground they would hold a charge but maybe not the full 400 volts but pretty close. If they don't hold the charge I would then say they are bad. If they are the original caps I would change them anyway as they would be approaching 40 years and way over due. I would see if you can find someone who has a vibrochamp and copy the wiring exactly like an original because a lot of Leo's majic was the way he ran those wires. I'm sure some others have some excellent comments on these cool amps.

[bob p]

i have good VC porn shots somewhere on my PC if you need them.

[J Martin]

The high voltage is draining through the still hot power tube as the amp cools down. Without the tube in place you would have a much higher residual voltage on the filter caps.

There are several ways you can replace the can cap. The first it to buy an original type from Antque Electronic Supply. They have a 20/20/20/20 at 475 volts. You can wire the first two sections together to duplicate the original 40/20/20, or you can put a resistor or choke between the first two sections and connect the output transformer at the second section for better filtering. You can buy a 100 Ma. Hammond choke that will fit into the chassis. Another method would be to install a modern type can cap, which will require a clamp, say a 32/32 at 500 volt JJ or F&T. The third cap can then be installed inside the chassis and grounded near the preamp for possibly less hum.

I have used all these methods in my shop at one time or another. I think using the choke is the most refined.

[2manynotes]

i would VERY much like to see what the guts are supposed to look like. mine was modified before i got it, so i really dont know "the truth".

thanx!!

[2manynotes]

im sure these methods are great, but im not looking to modify anything. id like it to be as original as possible, and then play with it, if nessessary. so im really looking for a direct replacement, where i dont have to do anything weird to the chasis, or anything extra to the board. im on the Antique Electronincs website now, but im having a hard time finding a direct replacement. -its probably there, but unless it says "direct replacement for a fender champ / vibrochamp", im not confidant enough to buy it.
-got a part number?
and i still dont understand how the value of the filter caps is 20-20-20-20 --which sounds like 4 seperate caps-- when i only have 3 on the schematic??

thanx again...

[J Martin]

The first two sections of the 20/20/20/20 are wired together so you have 40/20/20. Its not considered a modification. I have seen them wired like this from the factory.

[MWJB]

"so im really looking for a direct replacement, where i dont have to do anything weird to the chasis, or anything extra to the board." A "direct replacement" will require a hefty soldering iron to free up the ground tabs on the old cap pan. If using the CE type you will again need a hefty soldering iron (175W?) to ground the new cap pan to the chassis or drill holes to mount/ground the metal mounting plate.

Save yourself the hassle, buy 3x F&T 22uf/500v caps (I would use 47uf/500v for the main filter - 22uf might be "stock" but 47uf is better), ground the caps at main & screen nodes (either side of the 1K resistor in the power supply) to a PT bolt with solder tags, ground the remaining (preamp) cap to the #1 input jack.

Bend/cut the old filter pack tabs out of the way and you can leave it in place, keeping the amp looking externally stock (as well as functioning as stock).

[J Martin]

No you don't need a hefty soldering iron. You can mess up the amp with all that heat. After removing the wiring, a sharp blow with a medium flat blade screw driver will break the solder joints. Bending the new cap can tabs against the chassis will hold the can in place, so all you need to do is run a short piece of wire over to the transformer grounds to ground the cap shell. I have done this many many times and there is no damage to the chassis. I use a Klein 601 #4 flat blade with a hardened tip. Perfect for the job.

[2manynotes]

ive heard that whacking the solder joints with a small chisel (or screwdriver) was the easiest method. the solder joints look huge, and with all that metal, it would take a while to heat up, then it would be too hot to touch! so i can appreciate using a small sharp chisel.
im really impressed with all the info im getting here! i definately came to the right place. BUT, because im a newbie, im having a hard time following exactly what some of the instructons are. so i want to try and get it clear, using my own words...

SO... i can either get a can type direct replacement, and use the chisel, OR, mount 3 big fat caps inside, on the board. (or is it 4? im still confused. why have 4 in a can with 2 connected together? why not just have 1 40uf instead of 2 20's?? -and besides, my schematic still only shows 3 20uf's. )
as far as using a 47uf@500v for the "main filter"... are you talking about having all 3 @47uf? or just 1? -or2? what does using a 47uf do that a 20uf doesnt? store more energy? raise the plate voltage? do i need to "up" the voltage values on all my caps? (theyre all 400v caps now. some are 500v because i couldnt find the right value in a 400v)
and again, to be clear... one side of the 47uf goes to the 1K resistor (comming off pin8 of the rectifier) and the other to ground. then the next 47uf (or 20uf??) goes to the other side of the 1K resistor, (which feeds pin4 of the 6V6), and the last cap (what value?) goes on the "far" side of the 10K. the other side of the filter caps are soldered to ground.
am i missing anything? can i even FIT 3 huge caps on the edge of the turret board?

thanx again. i REALLY appreciate the info, and all your paitience. i ask alot of questions, i know...

- jim

[Fragger]

Jim, I`m new at this. I found some good advice on capacitors @ http://www.antiqueradio.org/recap.htm

[MWJB]

the F&T's are not particularly "big & fat" (.72" x 1.54")

Cap can - Yes you could use 1x40uf instead of 2x20uf, or wire 2 of the 4 caps in parellel to give you 40/20/20uf. The Champ filter pack was the same as used in Princetons that used 4 caps.

Individual, discrete caps - Use 1x 47uf at the main stage (first cap after the rectifier), 20uf for the other 2 (after the 1K & afer the 10K). 47uf main cap = better filtering, less hum, you might notice tighter response?

[2manynotes]

this is VERY cool. so am i to understand that there are 4 caps in there, but only 3 are being used? and/or to wire 2 in paralell (to get 40uf as the first filter "stage"), you simply run a lead from 2 of the contacts to the same point on the board, (comming off the rectifier) and the other 2 are the other 2 'stages" of the filter?

im seriously getting educated. i hope... hope im not 'dangerously educated"

-this site is also cool, because my wife bought an old OLD tube radio years ago 'cause she liked the way it looked. but of course it doesnt work. maybe with the info on that website, i could bring it back to life! that'd score me some points!! woohoo!

[2manynotes]

"The Champ filter pack was the same as used in Princetons that used 4 caps."


thats answers why the schematic only shows 3 caps. if it was what fender was using at the time, and had it on the shelf, then why not grab it, and only use 3 of the caps?! sure!

[2manynotes]

"so the amp "works" but then it had a nasty crackling sound whenever i played anything on the low E string. i read up on capacitors and decided to replace them all. i did that last night with good results, BUT the crackle isnt totaly gone. its still there, but nowhere near as bad as it was. i also seem to be getting a weird subtle oscilation on some notes, and a staticy type of noise on other notes. compared to it blowing fuses and being completely dead, this is a huge improvement, but i can tell it aint right.
"

i have recieved a ton of real world education on filter caps on this thread, and i will be looking into that very soon. i feel like i finaly understand what thay do, and how they work. i appreciate all the info.

but what about my other "problem" with this amp? if im not careful, i get a really nasty crackling sound - even at low volumes and a "clean" setting - when i play anything on my low E string. and it gets worse as i turn up the bass control. i was able to test the speaker by swapping it with another 4ohm, just to make sure the speaker wasnt blown (which is what it sounds like) and i get the same sound. so i can only assume that its somewhere in the amp. and on some notes i can hear high frequency overtones that sound like an oscilation of some sort, and a hissy static type noise on other notes. hhhmmm...

the amp itself is VERY quiet. even when cranked all the way. -you'd think there was a noise gate in there! im really amazed at how quiet it is. if only my Peavy Delta Blues was this quiet when cranked!!

any thoughts?