filter cap replacement

[rwooley]

Hello all, I'm new here. I'm trying to nurse my KMD GV60 back to health. It was producing a loud buzzing sound--but only when something (anything) was plugged into it. I suspect there is some kind of internal kill if nothing is plugged in. Given that the amp is about 25 years old, I decided that it couldn't hurt to change out the filter caps. I had trouble finding equivalent caps, but finally found what I thought would work. I tried attaching photos of them, but the post kept timing out.  I modified the old clamps to fit the new caps and soldered them in, being careful that positive and negagive were correct (I thought). When I turned it on, I got a lower hum right away and within about 10 seconds, one of the capacitors exploded. The line that connects the two capacitors together I thought should be connected to negative on both capacitors. However, looking closer at my photo from before removing the originals, it looks like the connecting line between the two goes to negative on the left and positive on the right. That is not how I wired. I wired negatives together. So, here are my questions. Would it ever make sense for the positive of one capacitor to be connected to the negative of another, as it seems is the case with the originals?  Assuming that I reversed the polarity and can replace and reinstall a new one, is there any likely damage to other components based on reversing on my first attempt? Finally, am I right to assume my replacements should work? The specs are matching--2200uf and 63V. Temp rating on new one is double.The originals had four leads, but only two were used, so I though replacing with a 2-lead capacitor would be acceptable. Am I off-track here? 

PS, I've also attached the schematic, but I couldn't even identify the filter caps in it. I'll try to post photos of the caps if I can get this thing to actually post this time.
Thanks for any help.

[Enzo]

It exploded because you wired it backwards.

Your amp is solid state other than the power tubes and has both positive and negative power supplies.  In a negative supply, the positive terminal would be grounded.  Note the bare wire between the two caps is connected to chassis, which would be ground.


In two caps side by side that does result in the positive of one being connected to the negative of the other, but look at the whole picture, and it becomes simply that both have one terminal grounded, the other being hot for either the positive or negative supplies.

I don't see 2200uf filter caps in the schematic either.  Where did you get that value?

[Roly]

If you look at the circuit diagramme you will see that all the polarised caps in the power supply are (for different reasons) connected +ve to -ve (the open or white box end being +ve, the black of filled box end being -ve).



C29 & C30, C44 & C45, C42 & C43, and the unnumbered pair in the high voltage supply, are all wired this way, +ve upwards, -ve downwards.


I'm just a little bit confuselated.

2200uF/63V is what I would expect to find in the power supply of a fairly hunky fully solid-state amp, but this one has a valve (toob) output stage and shouldn't need anything like that in either value or voltage for the preamps.  So where exactly are these new caps wired into the circuit?  Which caps do they replace?  C? and C?

We do have the right circuit for the right amp here I hope; this does have a couple of large valves (toobs), and not a big heatsink with transistors on it?

I also have to say that I'm mildly astonished a) by the manufacturers' claim of 100 watts out of a pair of 6L6's, then b) by the HT voltages shown on the circuit, 585V for the 60 watt version, and 715V for the "100 watt" version.

My RCA databook gives a "design-maximum" plate voltage for the 6L6-GC of 500 volts; and 55-60 watts from a pair is what is normally expected.

Even allowing for the fact that they are cathode-driven, and even deep in Class-B with loads of crossover distortion, this seems to be off the datasheet.  Run on the high power position I would expect this amp to eat output valves for breakfast. {but it's far from the first such I've seen}

[rwooley]

I don't see 2200uf filter caps in the schematic either.  Where did you get that value?

I've tried posting pics, but I keep getting an error message. On the sides of the original capacitors, the print reads: "1=2200uF (-10+50%), 5= -, Un 63V, -40/085/56 TYPE 1, IEFF, (100HZ) 5,3A (50 degrees C)"

Since numbers 1 and 5 terminals on the cap are the only ones used (and why they were designated in the printing), my assumption was that if I replaced this with a two-lead  capacitor with 2200uF and 63V, that I'd be fine. My new ones read "2200uF, 63VDC, HU3 105 degrees C" I suspect I would have been fine if I had not reversed the one replacement polarity first time.

[rwooley]

2200uF/63V is what I would expect to find in the power supply of a fairly hunky fully solid-state amp, but this one has a valve (toob) output stage and shouldn't need anything like that in either value or voltage for the preamps.  So where exactly are these new caps wired into the circuit?  Which caps do they replace?  C? and C?

We do have the right circuit for the right amp here I hope; this does have a couple of large valves (toobs), and not a big heatsink with transistors on it?

I've read people refer to tubes on this amp, but I don't see any. I'm going to try to post a pic of the bottom of the circuit board. To me this looks like what you describe above--a big heatsink with two transistors on it. The other side is just the circuit board with all solid state components. I'm squeamish about posting more than one pic here, or I would include images of both sides.

[g1]

  That's a GS100 chassis.  Did someone stick it in a GV60 cabinet?  Where does it say GV60 on it?

[Enzo]

Aha!

We mention tubes because they are in the schematic you posted for the model name you gave us.


2200uf caps make a lot more sense in a solid state amp.

To address your points:
You MIGHT be getting error messages if your photo file is too large for the file size limit of this forum board.  I don't know for sure.  I myself have found lately I need to click a button twice for it to really work on this board.  If yours keeps timing, try clickcing the button a second time.  Also there are numerous photo sharing web sites like Photobucket where you can put up your photos and then give us links to them.  These services are free.  Someone else may suggest certain ones as better than others, I don;t have occasion to use them much.

The number of legs doesn;t matter if only two are in use.  Larger can caps are sometimes made with extra dummy legs just to give them more points to solder, and that makes them more sturdy on the board.  Electrically they are not important.

The value is what matters, and getting enough voltage.   SO if the originals were 2200uf at 63v, getting the same is right.  You could also use any higher voltage, so if you couldn;lt find 63v, then 100v would do, as long as they physically fit the space.

Yes, chances are if you had installed it right the first time, it would have worked.

You can always use a higher temperature rated cap.  I never really looked, but I have to say I can't think of a place to buy 50 degree caps, most are 85 or 105 degree.  Higher temp ones will tend to last longer.

[rwooley]

Geez. You are right. I had found some paperwork that told me it was a GV60, and I never actually double-checked the model number on the unit. Now the inconsistencies are making sense. (eg. references to tubes that didn't seem to exist). I actually have a different amplifier--the GS100D. Sorry to have posted the wrong schematic/model number. Honestly, I was pretty focused on simply replacing the two capacitors, so the schematic wasn't that critical to me, and I guess I wasn't looking closely enough at those details. I'm looking online for the right schematic. Again, sorry to have wasted folks' time looking at the wrong diagram. I should have double-checked.

[joecool85]

Geez. You are right. I had found some paperwork that told me it was a GV60, and I never actually double-checked the model number on the unit. Now the inconsistencies are making sense. (eg. references to tubes that didn't seem to exist). I actually have a different amplifier--the GS100D. Sorry to have posted the wrong schematic/model number. Honestly, I was pretty focused on simply replacing the two capacitors, so the schematic wasn't that critical to me, and I guess I wasn't looking closely enough at those details. I'm looking online for the right schematic. Again, sorry to have wasted folks' time looking at the wrong diagram. I should have double-checked.

It's ok, it happens.  Let us know when you find the new schematic.

[DrGonz78]

After reading this last night I was searching all over for any solid state schematics that I could find... That will be tough to find...

But I did come across this cool image file of all the KMD type amps...

[g1]

  Yep, schematic does not seem available on the web... for free.  Did find it here for $15 (gs-100D, download):
http://www.schematicconnection.com/store/asp/product.asp?recorprod=1&product=355&cat=127&ph=&keywords=&recor=&SearchFor=&PT_ID=
  Otherwise, they were apparently bought out by Fender, so you could contact Fender & see if they have them.

[g1]

I also have to say that I'm mildly astonished a) by the manufacturers' claim of 100 watts out of a pair of 6L6's, then b) by the HT voltages shown on the circuit, 585V for the 60 watt version, and 715V for the "100 watt" version.

Just wanted to comment on this even though we are now dealing with a different model of amp.  The gv60 or gv100 power amps are basically copies of later (cathode driven) Musicman amps.  The cathodes are not at ground due to the transistors, so the full B+ is not across the tube.  Also, the screens are run at around half the plate voltage.  I believe this is how these circuits get away with running 6L6's at such high voltages (see Musicman schematics).  I believe the higher power Musicman amps claimed 150W from a quad of 6L6.

[Roly]

I actually have a different amplifier

Ahhh-HA!


@g1 - I've worked on cathode driven MusicMan amps, and I was none too impressed by them either.  The fact remains that the plate diss on a 6L6-GC is 30 watts, and my memories of the MusicMen was that you couldn't get the plate diss down to something reasonable without getting excessive crossover distortion, seriously Class-B.  Red-plating at idle or crossover when driven is not a happy choice.

I've seen a few amps both valve and transistor that were well off the data sheet and those experiences have made me very conservative when it comes to running devices harder than the manufacturer recommends.  The 6L6 is a tough old bottle, but some, particularly the 6CA7/EL34 can give up horribly just pushed to its limits.

I've seen quite a few like this...

[rwooley]

Just an update here. I received and installed my filter caps, and all worked well on my second attempt (when I actually soldered them in correctly). I never received the schematic I ordered, but a moot point now. The amp is working good again. The really loud amp buzz is gone.

Though my amp buzz is now gone, I notice that I still get quite a bit of buzz out of one of my guitars--telecaster. It mostly goes away when I touch the metal control knobs or the tip of the cord--so it is obviously related to grounding. I guess before I had two sources of buzz--the old filter caps and the guitar. Now, after swtiching out the caps, I just have guitar buzz on this particular guitar. It is not noticeable on my other guitar. I'll have to figure out the guitar issue, but just wanted to close the books on the amp capacitors. Thanks for the comments and suggestions. I really appreciate forums like this where people help each other work through problems.

[DrGonz78]

Let me guess the other guitar has humbucker pickups? Telecasters have single coil pickups and that leads to humming issues with those guitars. So, don't worry about the amp on this issue and there are ways to get the Tele to quiet down a bit. Look it up... Congrats on fixing the amp!!