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Got one of these on the bench. The clean channel seems to be ok. Dirty channel, volume tops out at around 4 or 5 and then no sound at all after about 30 minutes. Power tubes all have that wunnerful blue glow. They dont seem to be red plating. Power tubes are 6L6's. The bias is set to 6L6, and the guy said he installed fresh Mesa (rebranded Sovtek) tubes in it about 2 weeks ago. Any suggestions? I "fixed" the amp about a week ago and now it is back. He has the correct impedence when he is running it. No fuses are blown in the thing. I have checked all the plate resistors and they are within spec. Thanks for looking.
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Sooooo..... On the board and on the schematic, it says bias: 100ma across standby with 4 pwr tubes on. So, I measure 3.5V AC across the 350 ohm resistor which gives me 10ma. WTF? What would cause the bias to be off by a factor of 10? The plot thickens... The pot will not let me adjust it past say, 7 volts or so... -
The 350 ohm 10w resistor is in the screen supply. You can't read what you want off that. They mean set your meter to measure *current*, and connect the leads to the tabs of the standy switch. You do this with the power off, and you leave the standby switch off. When you turn on the amp, your meter will provide the connection across the standby switch and you can read the current thru it. Be sure to move the leads around to measure current as well, and not just the meter switch setting. It won't work any other way....The farmer takes a wife, the barber takes a pole....Comment
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Oh...be sure to use clips to connect leads to the stby sw. Don't try to hold probes there with your hands. One slip and you have fireworks when the lead hits the chassis (or something else HV shouldn't touch).The farmer takes a wife, the barber takes a pole....Comment
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Ok. sweet. I knew you had to be in series to measure current but I've never checked bias that way. Now that I have done this, it measured 156ma. I knocked it down to 100. The clean channel is cool. The dirty channel only goes to about 4 or 5 on the volume and that is it. After that, around 8 or so, I get a squealing. The thing plays for about 30 minutes and over heats and then no sound. I let it cool off, play with it again and the same. 6L6 in the power section. New Mesa 12ax7's in the pre section. Bad pre amp tube??Comment
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I'd check plate load resistors in that channel first. Carvin is notorious for try'n to get away with 1/4w plate loads and they don't last.The farmer takes a wife, the barber takes a pole....Comment
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Yeah, the schematic says they are half watt but indeed they are not. Also, the schematic I have is wrong. Its not a big problem, as I am an electronic tech and I have to follow circuits with no schematics all the time. It also says I have a pull pot mid boost. Nope. The main board in it says Rev. B. The schematic I have has Rev. C forward. Alot of the R and C numbers are incorrect. But yes, The plate load resistors are 1/4 watt. I will remedy that soon enough. Would that cause my symptoms?Comment
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Also, I just tried to crank the clean channel. It tops out at around 6 or 7.Comment
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