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k200 b [message #4033] Thu, 15 April 2004 07:47   stevem Messages: 4733 Registered: June 2004 Location: NY Senior Member Hi, Hey les I found something interesting in your amp while tracing out the problem. I have a 1970 revision 4 schematic (pc703) of the regulator/driver board, and the layout calls for 11 1% tolerence resistors on that board. 2 in the regulator section and the rest in the driver section. Your board only has the ones in the reg section, the ones in the driver are original 10%ers not 1%. I have gone thru my resistors that are 5% ones and found the ones that are what is called for with in 1% and put them in. It would be interesting to find out when this production change was made, If it was before or after what serial number kustom found that cost of those expensive 1% resistors was a improvement only techs with test equipment would see or hear. Or did they just run out of the 1% resistors while in producting and stuff the boards with the 10%ers. At any rate it is not the cause of you amps problem, I will get back to trouble shooting it over the week end. Report message to a moderator

Re: k200 b [message #4038 is a reply to message #4033] Thu, 15 April 2004 12:28   LesS Messages: 477 Registered: December 2002 Senior Member Hey Steve, This amp is one of the earliest K200B’s, which could have something to do with it having the 10% resistors. The production date coded on it is June 14, 1968. Production of the K200A had just ended, and I’m guessing K200B production could have started about June 1, 1968 (I know of another K200B that was made June 7). If you notice, mine has the large power switch nut like the K200 A’s had. The small diameter power cord could be from the A series too. -Les Report message to a moderator

Re: k200 b [message #4043 is a reply to message #4033] Thu, 15 April 2004 17:06 chicagobill Messages: 2005 Registered: April 2003 Senior Member Steven: I think the 1% resistors were added in the pre-amps and the driver stage, not for tolerance reasons, but because they were metal film resistors, and they produced less noise than the regular carbon composition resistors of the day. Recently, I was working on one of my Frankenstein heads, and replaced a few resistors in one channel pre-amp with modern carbon film resistors and I was pleasantly surprised to find a noticable reduction in the the background noise. When I have some time, I'm gonna replace all of the resistors in the other channel to see what happens. Bill Report message to a moderator

Re: k200 b [message #4047 is a reply to message #4043] Fri, 16 April 2004 07:53   stevem Messages: 4733 Registered: June 2004 Location: NY Senior Member HI. Yes you are right the metal film ones take alot of white noise out of an amp, they make a large improvement in signal flow gain stages. On alot of tube amps I rebuild for blues/rock players the feel and tone of those carbon resistors are what I have to keep in the amp to make them happy. the much higher voltages in tube amps make a hearable tone difference with carbon resistors. The tight tolerence ones in the kustom driver board, I think where put there to better balance the phase splitter section. Who knows Kustom may have even started matching the driver and output transistor gain before loading them in a board, or buying them matched. I think I remember a old RCA semiconductor catalog where they offered matched pairs of transistors. Maybe Kustom went this far when they started making amps with the 1% tolerence change.If I get the time next month I will scope one of my amps, then make the change and see if I find a waveform improvement/ and or better tone.The tip I have often stated here of putting in fast recovery diodes to change out the orignal ones in the power supply makes a sweet difference also.But its not just a bolt in change you have to mount 2, three terminal, terminal strips to load the diodes onto, which means drilling a extra hole next to the orignal rectifier bridges mounting hole but its worth the work! Report message to a moderator

Re: k200 b [message #4051 is a reply to message #4047] Fri, 16 April 2004 11:18 chicagobill Messages: 2005 Registered: April 2003 Senior Member Steven: I wouldn't be surprised if all of the transistors that Kustom purchased were specially selected, at least by the time the B series came out. I checked my files and I have two versions of the PC703 board schematic. The first one is marked Rev. 1 8-68. It has only two 1% resisitors in the regulator section, as you found in the amp you are working on for Les. The second is marked Rev.2, 5-69. It shows four 1% resistors, two in the regulator and two in the driver input section. My 200B is like the Rev.2 version with four 1% resistors, and according to the serial number list that Les posted, was made in the 2nd quarter of 1969. Bill Report message to a moderator

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