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K50-2 bad Hiss (Help) [message #7303] Sat, 12 August 2006 23:59   tweed112 Messages: 34 Registered: January 2006 Location: Mass Member I have many Kustoms, There is one I have with a bad hiss K50-2 I have read all the forums and looked at everything on this website...everyone seems to say this is normal with old kustoms? Its hard for me to believe that my amp was sold new with this problem? you could never mic up the amp? I am a amp tech and Ive spent 3 days with this amp everything checks out good, I have the schematic all voltages and componets check out good, I put on a 3 prong ac plug and there is nothing wrong with this amp ( that I can see) still this hiss is pretty bad! So....heres the question.... Are you guys saying this is just the way old kustoms are? I have 8 different kustoms all are old some have a slight hiss and some have almost no hiss at all....I love these amps and am addicted to collecting them but this hiss thing is about to stop me putting money into buying anymore.. Any comments would be helpful......thanks Report message to a moderator

Re: K50-2 bad Hiss (Help) [message #7309 is a reply to message #7303] Mon, 14 August 2006 07:43   stevem Messages: 4736 Registered: June 2004 Location: NY Senior Member Some of these early transistors are much better than others, and any gain hiss from the first couple of gain stages will be ampilfide greatly by the the time it hits the outputs. These transistors now a days are like 40 cents each and are far more silent than the originals so many times its faster to just replace them while you have them out for testing. If you have a scope you can go thru the amp stage by stage and see where the noise is getting dumped in the auido chain. Also make sure the brite switch is working, if its on they add alot of hiss and I have also had a amp where the shell connection on one of the reverb jacks was not making good connection and was adding a masive hiss yet the reverb was still working. Report message to a moderator

Re: K50-2 bad Hiss (Help) [message #7338 is a reply to message #7309] Wed, 16 August 2006 22:59   tweed112 Messages: 34 Registered: January 2006 Location: Mass Member Hello, Yes My guess also was in the preamp stages.. are you saying with a cross reference just replace those transistors with new ones? I dont mind at all replaceing the originals if it helps..also yes the bright switch works and I keep it off but still its to much high hiss for me. Report message to a moderator

Re: K50-2 bad Hiss (Help) [message #7339 is a reply to message #7303] Thu, 17 August 2006 07:20   stevem Messages: 4736 Registered: June 2004 Location: NY Senior Member I would concentrate your search in the first 2 to 3 gain stages,most all kustoms made after the frank models and before the metal face amps use the same 2 to 3 gain stages layout wise. Does it seem like you have way more gain than your other kustoms or just more hiss?It may also be a componet tha has changed value and is biasing just one transitor too hot.The tantalum caps used in these amp are known to fail and or change value. Report message to a moderator

Re: K50-2 bad Hiss (Help) [message #7348 is a reply to message #7339] Thu, 17 August 2006 21:57   tweed112 Messages: 34 Registered: January 2006 Location: Mass Member Hello, No the gain doesnt seem any different than the others, I will go back and check the tantalums... also this weekend Ill put the scope on the gain stages ...I only used the DMM on the voltages and values and grounds. I would love to get rid of this hiss as the amp is Dead mint and the amp has the greatest sound and the REVERB is soooo vintage! Ill let you know the outcome.....Thanks again Report message to a moderator

Re: K50-2 bad Hiss (Help) [message #7351 is a reply to message #7303] Fri, 18 August 2006 07:32   stevem Messages: 4736 Registered: June 2004 Location: NY Senior Member Just take your time and go thru it, and you`ll get fixed. Report message to a moderator

Re: K50-2 bad Hiss (Help) [message #7695 is a reply to message #7351] Tue, 10 October 2006 22:53   tweed112 Messages: 34 Registered: January 2006 Location: Mass Member Hello stevem, Thanks for the advice on my K50-2 you were Right!!!! I replaced Q1 the 1st transistor after the input .. also i was looking at the schematic and noticed that the resistor at the input of the transistor was marked as a 22k 1/2watt and the resistor that was in the amp was a 12k 1/2 watt and the 12k was the original...the schematic says 22k so i put a 22k in also replace the filter caps and put a new heavey duty 3 prong power cord.. this amp that i love so much was so bad with high hiss that i could never gig with it..now there is (NO) hiss at all even at full volume!!!!!! thanks again......oh ya the best advice you gave me was GO SLOW!! through the circuit Report message to a moderator

Re: K50-2 bad Hiss (Help) [message #7699 is a reply to message #7303] Wed, 11 October 2006 07:15   stevem Messages: 4736 Registered: June 2004 Location: NY Senior Member Cool deal!Play on my friend. Even though kustom sound great with pedals I have found that some prevous owners may have beat them up with too much drive signal from them, and these original transistors can`t take it. Report message to a moderator

Re: K50-2 bad Hiss (Help) [message #7711 is a reply to message #7699] Thu, 12 October 2006 14:11   tweed112 Messages: 34 Registered: January 2006 Location: Mass Member Hello Stevem, I just bought (5) original kustom service manuals each manual is a couple hundred pages, one of these manuals is full of original Kustom Service Bulletins!! and one service bulletin has to do with bad noise and hiss! I thought you might find this interesting so here it is Word for Word..its also sighed at the bottom by the natioal service manager This Service Bulletin comes from the desk of Robert Brinkman(Bob) National Service MGR on original Kustom letterhead Dated August 1970...... 1) Tips on eliminating noise and hiss on old amplifiers Since the noise comes from the preamp and there are yet some gremlins that will cause this, we should look for any 100K or higher resistance, because they are breaking down. Look for these resistors mainly in the basecircuits of the transistors. this sounds like a grumbling noise and may be intermittent. the breaking down of the resistor is usually a internal failure, and cannot be blamed on the desigh or transistor loads since there is not much current to feed a base bias of .6v from the emitter side or from the collector dividing net work. Report message to a moderator

Re: K50-2 bad Hiss (Help) [message #7718 is a reply to message #7303] Fri, 13 October 2006 07:40   stevem Messages: 4736 Registered: June 2004 Location: NY Senior Member Thanks for the info!Its funny, this is the same problem, and the same value resistor that goes bad in the preamp plate circuit of many tube amps, only due to the much higher voltage in the plate circuit the noise is more a a dull cracking sound.The carbon comp resistors in Fenders are famous for this. Report message to a moderator

Re: K50-2 bad Hiss (Help) [message #15260 is a reply to message #7718] Wed, 16 February 2011 03:46   El Músico Loco Messages: 12 Registered: February 2011 Junior Member I am resurrecting this old thread in hopes someone can give me a few good P channel Jfet substitutes that can be used as Q1 in the Kustom K50. The original is a 2N5033. I tried a 2N5460 but it does not work. Thanks again for the help. Chris Report message to a moderator

Re: K50-2 bad Hiss (Help) [message #15261 is a reply to message #7303] Wed, 16 February 2011 12:52 chicagobill Messages: 2005 Registered: April 2003 Senior Member I don't think that I've ever needed to replace one. What's wrong with yours? What didn't work with the 2N5460? What is the purpose in the circuit? Report message to a moderator

Re: K50-2 bad Hiss (Help) [message #15262 is a reply to message #15261] Wed, 16 February 2011 16:02   El Músico Loco Messages: 12 Registered: February 2011 Junior Member Hi bill Here is the schematic. The above thread describes solving the hiss on this model by replacing the Jfet. I just re-read the thread and it talks about an update to check all 100k or resistors to see if they have drifted high and this may be causing the hiss problem. Let me know what you think. Thanks Report message to a moderator

Re: K50-2 bad Hiss (Help) [message #15263 is a reply to message #7303] Wed, 16 February 2011 16:49 chicagobill Messages: 2005 Registered: April 2003 Senior Member OK, it's the first input stage. Ooh, the volume control is at the head of the circuit, just like a Frankie circuit. Based on the circuit, there is almost no voltage there, and it isn't being stressed at all, so I would think that nearly any p-channel fet would work there. Did you check the pinout for the new one before you installed it? The gate is most important, the drain and the source can be interchanged in most cases. As for the noise, do any of the controls change its' sound? Try to isolate it by sections and then try heating or cooling or parts to see if any react in a good or a bad way. The service note talked about high value carbon resistors failing and causing noise. The only high value resistor I can see there is R22. Report message to a moderator

Re: K50-2 bad Hiss (Help) [message #15264 is a reply to message #7303] Wed, 16 February 2011 17:14   stevem Messages: 4736 Registered: June 2004 Location: NY Senior Member I can not see why the 2N5460 did not work for you as it crosses over to a NTE326 replacement just like the original 2N5033 does. Be sure to check how you installed it like Bill said to do. I bet someone blasted that first gain stage with a cranked stomp box! Report message to a moderator

Re: K50-2 bad Hiss (Help) [message #15265 is a reply to message #15264] Wed, 16 February 2011 18:24   El Músico Loco Messages: 12 Registered: February 2011 Junior Member I swapped the 2N5033 back in to check that I didn't mess something up in swapping in the 2N5460. So the amp still works. I did try the 5460 in two positions checking it against the pin out and it didn't work. I am going to put a socket in tomorrow and try again. The hiss is definitely in the treble control area. Report message to a moderator

Re: K50-2 bad Hiss (Help) [message #15542 is a reply to message #15265] Wed, 20 April 2011 12:28   El Músico Loco Messages: 12 Registered: February 2011 Junior Member Hi, I finally have gotten back to the K50 and want to report that after it was pointed out above the marking of Q1 is incorrect having the source and drain pins swapped I did substitute the P-channel Jfet Q1 with a P-channel 2N5460 and it solved the hiss. C4 is listed on the schematic as a .01uf and there was a .0082 so I put in a .01. Here is a picture of the transistor orientation for anyone who needs to do this fix in the future Thanks for the help here. I will start another thread now to get the tremolo working. Chris Report message to a moderator

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