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Author Topic: KMD GV60 Guitar Amp....RIP?  (Read 8959 times)

Lamf77

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KMD GV60 Guitar Amp....RIP?
« on: June 10, 2012, 04:56:25 PM »
Hi,
I have  a KMD GV60 guitar amp I bought nearly 25 years ago.  I believe it has a solid state preamp and tube amp.  It has Sovtek  5881/6L6WGC Tubes.
I am a casual "hack" player and the amp has never really been used heavily.  The last time I used it was about 1 1/2 years ago for about a half an hour with out any issues at all.
I decided to attempt to start playing so I plugged it my Strat and fired up the KMD and got nothing.....no crackle, no hiss, nothing from the speaker.  The power light goes on as well as the light on the footswitch works.  When I look at the tubes there is a very, very slight orange glow from where they attach to the amp and after awhile they did get warm.  I tried a different cord and guitar to the same consequence.  I took out the tubes and the connections are clean and the tubes do no look fried.  However, it may be close to 10-12 years since they have been changed.  I have only used the amp a handful of  times over the last few years.
The speaker connections are sound and even unplugging the speaker input does not elicit as much as a whisper.
So.....my question is where do I go next?  Could the tubes go bad just by sitting?  Since the amp is worth about $200 in working condition is it worth fixing?
I am no expert so I thought this would be a great place to start.  Any advice or comments would really be helpful.
Thank!
Mike

polo16mi

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Re: KMD GV60 Guitar Amp....RIP?
« Reply #1 on: June 10, 2012, 07:26:33 PM »
May i suggest you to check your guitar and cable in other amp, to make sure they are ok?

Check your amp input jack, maybe rusty or dirty, and no good contact.

If not, stay around, there are amp repair masters here.

Hope help.

Lamf77

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Re: KMD GV60 Guitar Amp....RIP?
« Reply #2 on: June 10, 2012, 08:39:21 PM »
Yep....plugged into my small Peavey practice amp and all was good.
I was wondering...even if the input jack for the guitar was dirty, if I tap the tubes I would get a sound...I get nothing.  Thanks for you advice!

Roly

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Re: KMD GV60 Guitar Amp....RIP?
« Reply #3 on: June 10, 2012, 10:41:00 PM »
Nah, valves (toobes) don't just go silent like that, and the fact that they are lighting up and getting warm are both good signs.

*Utterly* silent suggests to me that the speaker circuit is open somewhere because you normally get *something*, a little residual hum or hiss.  And this is a worry because if you drive the amp with the load open circuit you can easily destroy the output transformer, and in this case that would mean the amp is a write-off.

So I would try testing the speaker, either by driving it with another amp, a multimeter, or even flashing a 1.5V or 9V battery across it should produce some sort of "blats".

The majority of amp problems (in fact all electronics problems) are due to dirty or corroded connections, and since this amp has been in storage for a long time, this is most likely what is wrong (unless of course rodents have eaten the speaker); it's a matter of finding what should be connected that now isn't, and the speaker plug and socket are a natural suspect.

I can't find a circuit/schematic so I'm flying blind, but a rear photo shows only one fuse and that will be for the mains, however there may well be another fuse internally for the High Tension (HT or B+) supply for the output valves.

This may have blown because the electrolytic filter capacitors in the power supply tend to "dry out" and go low resistance if not used for a long period, as here, and this protective fuse will blow at first power up, and very effectively silence the amp.

You can disconnect the amp from the mains and remove the chassis (after first removing the output valves), and see if there is an internal fuse to be seen.  If so and it has blown you can try replacing it with an identical one, but if that too blows then post and we can take it from there.

Taking a few crisp and well lit internal photographs and posting them to this thread will also be of assistance, as would a circuit/schematic if you can obtain one.

Replacing old filter caps is something that all amps need after about 20-30 years and done frequently by techs, but is also normally within the scope of anyone who is a bit handy and can solder properly.

HTH
If you say theory and practice don't agree you haven't applied enough theory.

Lamf77

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Re: KMD GV60 Guitar Amp....RIP?
« Reply #4 on: June 11, 2012, 10:54:29 AM »
I took my meter set it on ohm.  Attached red to red and black to black on the speaker and it read about 7.8?  Then I put one to the tip of the plug and the other on the other end and got a similar reading?  Do not know if this is the right way or shows the speaker plug works.  I will take everything apart when I get a chance and see if I spot any other fuses.  Will also upload photos...thanks!
« Last Edit: June 11, 2012, 11:33:28 AM by Lamf77 »

bassetrox

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Re: KMD GV60 Guitar Amp....RIP?
« Reply #5 on: June 11, 2012, 11:33:16 AM »



The majority of amp problems (in fact all electronics problems) are due to dirty or corroded connections, and since this amp has been in storage for a long time, this is most likely what is wrong (unless of course rodents have eaten the speaker); it's a matter of finding what should be connected that now isn't, and the speaker plug and socket are a natural suspect.

HTH

Company i used to work for had a 'faulty' subwoofer returned. Turned out the escaped family hamster had made a nest in there. he'd dragged in some chocolate wrappers, bit of newspaper and left plenty of less savoury things before finding the wiring tasty. Then presumably absconded (alive) before or during the returns process.

http://ampworks.co.uk/ <- Not much to see here yet!!

J M Fahey

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Re: KMD GV60 Guitar Amp....RIP?
« Reply #6 on: June 11, 2012, 12:08:09 PM »
Quote
I took my meter set it on ohm.  Attached red to red and black to black on the speaker and it read about 7.8?
Did you read the speaker "alone"?
Meaning it disconnecxted from the amplifier?
Is it connected through a regular plug to plug or hardwired into the amp?
Post some pictures to avoid thousands of questions like this.
Show the speaker connection and the backpanel.

Lamf77

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Re: KMD GV60 Guitar Amp....RIP?
« Reply #7 on: June 11, 2012, 07:25:28 PM »
Here are some photos.  I unplugged the speaker from the amp when I tested it.  There is a photo here that show the plug removed from the back of the amp. Hope this helps...thanks!
« Last Edit: June 11, 2012, 07:27:13 PM by Lamf77 »

polo16mi

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Re: KMD GV60 Guitar Amp....RIP?
« Reply #8 on: June 11, 2012, 09:23:03 PM »
Maybe usefull for our gurus, post more pictures, of inside, components view. Need to dissasembly, but for fix it too.

J M Fahey

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Re: KMD GV60 Guitar Amp....RIP?
« Reply #9 on: June 11, 2012, 10:05:10 PM »
OK.
Keep the camera firm and push the button slowly and smoothly, the first 3 pictures are shaken.
Brace your arms (specially the wrists ) on a chair or table.
The 4th is fine.
With amp off:
1) confirm that you read around 7 ohms between tip and body of the red/black wire that goes to the speaker. Are you using the "200 ohms" scale?
2) plug a known good guitar cable into the jack labelled speaker output-8 ohms.
Nothing else plugged there.
What resistance do you read?
Still using the "200 ohms" scale.
Touch meter tips to each other, what do you read?
3) the front panel picture is unreadable, and guessing is dangerous, please repeat as suggested above.

Roly

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Re: KMD GV60 Guitar Amp....RIP?
« Reply #10 on: June 11, 2012, 10:30:56 PM »
@bassetrox - I practiced my craft for a time deep in the Australian bush and I have a heap of "sundry critters in the works" stories; metal mesh over speaker ports and the like were standard defensive modifications.


Well it sounds like the speaker itself is intact.  Take note: the amplifier must not be driven without a load (speaker) connected or it will be damaged.  Until we have a better idea of what is wrong be very moderate with any drive during power on tests - if you can't hear anything don't turn it way up and fang it.

We await crisp internal pix.

{My guess now is that it doesn't have HT on the output stage for some reason}
If you say theory and practice don't agree you haven't applied enough theory.

teemuk

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Re: KMD GV60 Guitar Amp....RIP?
« Reply #11 on: June 11, 2012, 11:04:53 PM »
As far as I know, those amps are practically almost like Music Man amps. The GV100 is almost like the MM GP3 so I guess the GV60 is almost like something else from Music Man. Circuit diagrams are poorly available (and the only ones I've seen are a sorry mess) but you can likely refer some key component values to certain Music Man schematics and thus get a very good overall idea of the internal circuitry.

Enzo

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Re: KMD GV60 Guitar Amp....RIP?
« Reply #12 on: June 12, 2012, 02:12:24 AM »
Just an opinion, but if I hear the amp is UTTERLY silent, and the speaker is not open, I suspect a missing power supply voltage - usually from a blown fuse inside the amp.  And if I had to guess further, it would be the high voltage fuse for the tubes.   And add to that, the most common reason for a high voltage fuse to blow is a failed/failing power tube.

Enzo

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Re: KMD GV60 Guitar Amp....RIP?
« Reply #13 on: June 12, 2012, 03:30:31 AM »
I used to be a KMD service center, way back when they were also Dean Markley and others.  I just looked, and the factory copy of the schematics for the GV60 and GV100 are indeed poor scans.  It is just readable, most of the voids don;t hit components on the drawing.  SO I suspect the poor copies you see around are probably the same thing the service centers got.

Nowhere near as bad as the stuff Deltalabs sent us for the Effectrons though.

I'll have to take my book apart to scan it, but I'll do so when I get a chance.  I'll post it somewhere in case it is a little better than some other copy.

Jack1962

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Re: KMD GV60 Guitar Amp....RIP?
« Reply #14 on: June 28, 2012, 02:38:24 PM »
As Enzo has said , I would check for voltage , I would start with voltage on the Center Tap of the output transformer. Seldom is a amp silent , most tube amps hum (amp hum) . check all fuses , clean all jacks and pots the pull out your metter and start poking around , USE CAUTION TUBE CIRCUITS USE HIGH VOLTAGE