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My trials and tribulations with this amp are well documented in this forum. This thing still stumps me. I drive Bozak Concert Grands through a McIntosh C22 reissue and Heart 6000CDP. The amp performs great until you start to crank it a little [about 11 o'clock on the volume dial]. At this point, the power tube in position 4 starts to glow cherry red and the right channel begins to distort slightly. You can only see this in a dark room. I have sent this back to McIntosh twice and they swear it "tests" within specs each time. They are beginning to think I'm a little nuts! As soon as you back down on the volume to about the 10 o'clock position, the power tube stops glowing and the quality of the sound gets back to normal. Could my speakers or something else be causing this? My next step is to buy a new main circuit board from Mc and see if there is something that is drifting out of specs when the amp is pushed. As stated McIntosh cant seem to find the problem. Any ideas??
Thanks again,
John
Follow Ups:
John,I use these amps and this does not sound right. Since both outputs do not glow we can assume the problem is with one side of that channel. Your no load DC voltages should be Pin #;
2-0v, 3-444V, 4-444, 5- -53V,7-0V, 8- .6v. I suspect one of the 220 ohm sand block resistors are out of spec. or damaged. Check them carefully.
The cherry red condition is caused by incorrect bias. You may want to check the 10uf 160V caps or the 200V 1.5 watt zener diodes. If any of these parts leak it looks like the bias could be upset. Your AC voltage on pin #5 should be 133V at rated output.
I assume you have swaped out tubes and the condition stays on the same tube socket?
Good luck.
Ron-C
I had an original MC275 (and other Mac gear) so I can share some experiences based solely on their 1950s-60s gear. The bifilar transformer will allow you to pull V4 right out while the amp is playing. (No, the B+ doesn't rise and you will not open Pandora's box-- did it dozens of times for minutes at a time with NO problems) With V4 pulled, does the distortion increase or decrease? What is the impedance and efficiency of the Bozaks? If I had to make a guess, I would say that the phase inverter tube (12AU7) on that side, or a resistor or capacitor associated with that tube, is defective. It sounds like one half of the PP pair of KT88s is receiving disproportionate drive levels from the phase splitter. Don't discount parasitic oscillation in the phase inverter which is then magnified in the finaldrive stage. A decent tech with a signal generator and a dual-trace scope should be able to wrap this up in under an hour. I'm assuming you have switched tubes, cables, speakers, speaker cables etc from left to right and substituted known good tubes... Chris
it is dc coupled to outputs. the previous tubes are all cap coupled, so a bad one wouldn't carry forward. I am making assumption that new circuit is similar to old one. I've done small balancing of outputs with slight mismatches in the 12AT7( final Cathode follower )which give a slight variation in grid voltage for the outputs...
regards,
Douglas
Yeah, that "bootstrapped" driver can be problematic. I would still split the difference of suspicion b/w the inverter and the final drive; while DC coupling can wreak havoc in terms of swamping the control grids of the output, I have found that a small phase inverter imbalance (bad tube, out of spec resistor, leaky coupler) can be greatly magnified by the DC coupling. I wouldn't rule out a bad microchoke in between the KT88s either (if the new 275 uses them!) I damaged one with a thermal runaway 6550 and had a hell of a time tracking down the source of distortion and reduced power. Chris
After looking all over the place in an old 275, I finally started poking at her with scope and signal gen, found the bad cathode resistor bypass cap, and an open coupling cap. the bypass sounded like water dripping and about as loud, almost enough to allow me to ignore but, decided," I'd better find this before I reduce world population of 275's by one". I will be replacing all of the 'lytic caps in the newest arrival, and go with Schottky diodes in PS. Of all amps this one will be worth it, there's only five of 'em for $15 each for 600 V ones @ 10 amps. Any starting place you can point me for adding resistor in negative supply to reduce voltage to Selenium levels?
you can probably get in there yourself. First things first, determining the actual bias current. Check DC resistance of the cathode winding, both halves, on both transformers, and record them. This is between ( i think ) pin 8 and ground. should be about 12-16 ohm. Turn it back on and measure votage( w.r.t. ground ) at these pins and use Ohm's law to get current. There should be a voltage spec for this pin in the manual but they say it is within....anyway, how close it it to plate dissipation limits? If you need to drop it( increasing, the negative voltage at the grid, that's a bit more than I can tell you without schematic in front. I have adjusted the old 275's, but not without a bit of small surgery. Careful of the high volatge under there and don't turn it on without some load across its speakers.
regards,
Dougals
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