Gary Mattis
Active Member
I posted it a few month's ago and I had a few problems with one of the the amplifiers after a couple of weeks. And I changed V1 in both to a 12AU7 instead of the 12AX7 that I first used to replace the 6EU7 that it comes with originally. The tape player needed more gain than the aux 1 or 2 had and it was playing OK but at a lower volume. On mic 2 it had more gain and was louder but was over gain with the 12AX7s in V1. I decided to try two good 12AU7s a Mullard and Siemens that I was saving. The 12AU7s solved the problem and sound fantastic. I have a tube type FM tuner and I ran it into aux 2 and that worked out fine. So I have CD on aux 1 , Tuner on aux 2 and tape on mic 2 , I still have Mag input and mic 1 left over for a turntable later on. I had a problem with one of the amplifiers after playing more than a hour and it was the two cathode 15 ohm resistors that I used to replace the 30 ohm balance pot and the Two 100k ohm feed resistors were going up to 150k ohms after they got hot when the 15 ohm cathode resistors started failing hot. I put a center tap ohmite ceramic resistor on each cathode and grounded the center tap. That seems to be good sounding better than the normal cathode resistors did. I did not have any balance pots that size. I also replaced the feed power resistors also. The other amplifier with the stock balance pot has worked perfectly fine with no problem at all. I burned up 3 vintage 7868 tubes using the 2 watt cathode resistors instead of a balance pot, they worked but couldn't handle the heat and showed no signs of burn marks warning me something was wrong, they just crapped out and looked perfect like new. I almost thought that I made a mistake buying 7868 amplifiers because of the tubes blowing on me because the cathode resistors were fooling me into thinking that it was a tube problem when it was a hot cathode resistor problem, they would work again after it cooled off and I replaced the tubes. Now I couldn't be more pleased with the amplifiers both working good. The tubes I used in each amp are as follows, Left Amp has a Mullard long plate 12AU7 in V1 and a Amperex 12AX7 long plate in V2 and a Mullard box plate 6C4 phase inverter and 4 vintage 7868 tubes and a KLH 10 speaker. The Right amp has a Siemens 12AU7 long plate in V1 , V2 had a Telefunken 12AX7 smooth long plate and a Mullard 6C4 box plate for the phase inverter and 4 vintage 7868 tubes and a KLH 10 speaker. I know that I could blow up the speaker's pretty easily if I wanted to but I don't play my stereo that loud and I like the sound of KLH speaker's being slightly pushed . I'm not hurting the speaker's just putting them on the edge slightly . It gives me what I wanted out of it and I can upgrade my speaker's later if a good deal comes my way. So after some trouble the project is working out good now. The tubes and transformers are running pretty cool after a few hours of operation at half volume. 500 volts on the plates and has negative 24 volts on grid 1 . All the plates look good in the darkness also. It is amazing how quiet the idle is and how clear the reproduction is. Been thinking about making custom wood cabinets, face plates and vintage knobs , just not quite sure if I want to stack them up to save space or to go side by side yet . How ever I do it I want to see the tubes glowing. Clapton After Midnight just came on the radio and it sounds great !
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