| Kustom studio 4 valve, valve replacement [message #18064] |
Sat, 19 January 2013 10:07  |
jorisg
Messages: 17
Registered: January 2013
Location: Netherlands
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Junior Member |
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Hi to all,
I just purchased a nice looking Kustom studio 4 valve amp, sounds is not that great though. Checked the valves and these seem quite a mess, they are all 6L6 GC valves but of different brands....... Can somenody advice me which brand of valves and which type of 6L6 to use as replacement?
Thanks a lot for the help!
Joris
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| Re: Kustom studio 4 valve, valve replacement [message #18065 is a reply to message #18064] |
Sat, 19 January 2013 12:25  |
Kustom_Bart
Messages: 601
Registered: October 2010
Location: Greenville, MichiGUN
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Senior Member |
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Get a matched set of JJ or Ruby tubes. Get on E-bay and find them is your cheapest bet. Matching them is important and then biasing them once they are in is important as well. Biasing them is important as the new tube may run hotter or colder than the old ones. Biasing them correctly will make your amp sound the best as well.
[Updated on: Sat, 19 January 2013 12:27] Report message to a moderator |
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| Re: Kustom studio 4 valve, valve replacement [message #18074 is a reply to message #18066] |
Mon, 21 January 2013 12:06 |
PxDxBx
Messages: 11
Registered: December 2012
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Junior Member |
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I completely rebuilt my 4 valve (Custom Kustom). I recapped the tube driver board and upped the current value on the cathode resistors (which had burned open). I strongly suggest checking/replacing (at the very least) the 4 cathode resistors and the 2 resistors on the output of the driver board with higher current value flameproof resistors for protection.
I pulled the single bias adjustment pot off of the driver board and mounted it in a box on the back of the amp. I also mounted 5 bannana jacks in the box tapped from the 4 cathode resisors & ground. With this setup, I can adjust the bias without disassembling the amp; I just have to jack-in with my voltmeter and dial it in. It takes a little fumbling around with Ohm's Law to get the cathode current right. You'll need an accurate resistance measurement of the cathode resistors. I also measured the plate voltage, but this is not for the faint of heart, as it idles @ ~480v
I installed a matched quad of JJs, which makes it easier to bias, because it only has a single adjustment pot. They're worth the extra $$$. I biased conservatively @ ~38ma. I use a processor, so I'm not trying to run the tubes blazing hot (and unnecessarily shorten their life). It sounds great and has enough crank to drown out a drummer. Pd
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| Re: Kustom studio 4 valve, valve replacement [message #18083 is a reply to message #18081] |
Wed, 23 January 2013 12:37 |
PxDxBx
Messages: 11
Registered: December 2012
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Junior Member |
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If you remove the rear wood panel of cabinet (if it still has one; mine didn't) and loosen the 4 screws on the top of the cab, the whole preamp will slide out of the back. I had a similar issue on ch2. I took it apart to clean the pot(s), which were pristine. There was no wear at all on the pots, which is impressive for a 30+yr old amp. It turned out to be a cracked solder joint. My suggestion is to re-flow the solder joints on the offending pot and then test it while you have it apart. Its easier than opening up the pot to clean it. The pre is completely enclosed, so there isn't much dust that gets in.
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| Re: Kustom studio 4 valve, valve replacement [message #18093 is a reply to message #18064] |
Thu, 24 January 2013 15:59 |
PxDxBx
Messages: 11
Registered: December 2012
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Junior Member |
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As far as not being handy with a solder iron, here's the trick:
1) Buy a solder iron
2) Buy some solder
3) Get some wire (both stranded & solid core)
4) Get an old circuit board or project board
5) Heat up the iron
6) Make blobs till you get good at it
It's really that simple. Everyone who plays with electronics should learn how to solder. Practice tinning wires, soldering wires to the board, and soldering wires to each other (end to end). Most repairs come down to fixing a loose connection, which is easily solved by re-soldering the joint. Once you get the tools, it will only cost you the electricity to heat up the iron and penny's worth of solder... or you can pay someone $75 to do the same thing.
Just my $0.02
pd
Other things to collect as your financial situation allows:
Wire strippers
Needle nose pliers
Hemostats
Diagonal & flush cut pliers
Desoldering pump
Third hand (the thing with the alligator clips & magnifying glass, arguably the most time saving tool you can own)
[Updated on: Thu, 24 January 2013 16:14] Report message to a moderator |
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| Re: Kustom studio 4 valve, valve replacement [message #18096 is a reply to message #18094] |
Thu, 24 January 2013 23:29 |
PxDxBx
Messages: 11
Registered: December 2012
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Junior Member |
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Just re-flow (melt) the 3 solder joints on the pot, and add a tiny bit of solder if needed. Sometimes fresh solder will help it melt faster. Make sure that you use a pencil-type iron, not a gun (it's overkill). You really can't hurt the pot. The pots are old-school heavy duty ones. Just keep the solder where it belongs an you won't have any issues. Avoid big blobs; less is more. A desoldering pump or bulb helps if you get too much solder on the joint. You can just melt the solder and suck it up.
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| Re: Kustom studio 4 valve, valve replacement [message #18101 is a reply to message #18064] |
Fri, 25 January 2013 21:21 |
PxDxBx
Messages: 11
Registered: December 2012
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Junior Member |
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It's a great amp, built with quality parts. I snap up old electronics whenever I get the chance, because they are easily repaired. You could spill a beer in this amp and just unplug it and let it dry; it would be fine. If new stuff breaks, most of the time, you just throw it over your shoulder and buy a new one. The traces and components are too small to repair with average tools. Old stuff rocks!
[Updated on: Fri, 25 January 2013 21:22] Report message to a moderator |
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| Re: Kustom studio 4 valve, valve replacement [message #18240 is a reply to message #18064] |
Tue, 19 February 2013 09:19 |
stevem
Messages: 4775
Registered: June 2004
Location: NY
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Senior Member |
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With the way those pots are mounted you will probibly have to unsolder that one least pesky one to get a good amount of cleaner in it to get it working as good as possible.
If cleaning it in this way will not get it up to snuff, than as a last ditch atempt you can take it apart by unfolding the 4 lock tabs with a diagnal cutter.
When you do this bend them back slow and try not to put a sharp fold in the metal as this can make that lock tab snap off when you go to fold it back closed.
Once you get the pot open and take out the shaft section( some pots will not let you do this without removing a C clip at the end of the shaft) and then take a look at the black resistive carbon trace on the front termial plate of the pot.
If you see deep worn spots anywhere( this happen mostly at eitheir end of the pots travel)then the pot is shot and will need to be replaced.
Note that when putting a pot back togethere the shaft needs to be set near what would be 12 noon, notice the stop tab on the back metal case of the pot.
You should clean and lube all the pots, even the ones that make no noise now to avoid waring groves in the carbon trace`s.
Try not to get too much cleaner/lube on the circuit board as sometimes a coating of such will make for small frying bacon type noises all on its own.
Shold this happen you can wash down the board with rubbing alky and a 1" paint brush, just make sure to let the board drian off in a way that will not let the stuff get into other pots,connectors or tube sockets.
[Updated on: Tue, 19 February 2013 09:24] Report message to a moderator |
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| Re: Kustom studio 4 valve, valve replacement [message #18252 is a reply to message #18064] |
Wed, 20 February 2013 09:42 |
stevem
Messages: 4775
Registered: June 2004
Location: NY
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Senior Member |
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You should be fine as JJ tubes are very close to spec in regards to pull matched idle current, like within 10 to 12% from what I have seen.
If you ever need/ start to play the amp louder then do this test.
Do this at night time, kill the lights in the room and play/crank the amp while looking at the power tube, play for 3 to 5 minutes while looking at the big gray steel plate structures inside the tubes(s).
As long as this plates do not start glowing red your OK bias wise for any condition that could hurt the tubes.
[Updated on: Wed, 20 February 2013 09:43] Report message to a moderator |
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| Re: Kustom studio 4 valve, valve replacement [message #18258 is a reply to message #18064] |
Thu, 21 February 2013 19:56 |
PxDxBx
Messages: 11
Registered: December 2012
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Junior Member |
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I run a matched set of JJs and they run within a couple of mA of one another. The existing bias setting *may* be okay IF the previous tubes were of a very similar current draw. If they run too hot, it'll shorten the life of the tubes (and/or fry the cathode resistors). If they 'red plate', they'll burn out very quickly. If they run too cold, it can foul the tubes. They run in the neighborhood of 250 deg F (121 C) when biased correctly. You certainly don't want them getting any hotter.
When it comes to bias, it doesn't really matter how loud the amp is turned up, because it is an adjustment of 'quiescent current', which is how much current flows through the tubes when they're at idle (when there is no sound being amplified). It's like the idle speed on a car. You don't want it to idle at 4000 rpm, nor do you want it at 100. Changing the tubes is like changing the carburetor. You might be OK without adjusting it, but it won't necessarily be at its peak performance.
If you don't feel comfortable doing it yourself, it might be worth having it done. If you don't mind the taking the gamble and it runs OK, then you might not have any problems with it as is.
Just my $0.02 (sorry for the length)
pd
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I will bring the amp to a repair shop, not that handy with a soldering iron 
I could easily make things worse! 
I will add them here.
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