The Basics.\
MFA Lumi phono stage
Tube 6EM7
A half of the tube is an ampliifer, capacitor coupled to previous stage, B half is a cathode follower to out
Plate Voltage should be 150 on A, power supply voltage 330
A side is fixed bias with a regulator
Problem: Plate voltage is 300, as is voltage on cathode follower resisto, Unit plays but affected side is louder than the other in phono. CD off the line stage is not affected.
Measures taken.
Replaced these resistors: drop, plate, cathode follower, grid between stages (no cathode resistor on A--goes straight to ground.
Resoldered all grounds.
Checked bias regulator--okay
Switched tubes between channels
New socket
I haven’t changed above coupling cap, decoupling cap (both measure well and are not old), nor the polystyrenes between the grid and plate on A. If they were shorted, I would think the grid would also read 300, and it doesn’t. I can’t believe that if they just went bad, 300 volts would suddenly appear, but at this point, in fact, I can believe anything.
Over the years I’ve had problems with this unit, I think because it runs hot, and is shelved in a way that does not dissipate heat well. That has to change. I mention this because, if there is a problem in a remote location due to a weakened solder joint, I wouldn’t be surprised, and it wouldn't be the first time.
I’m stumped.
MFA Lumi phono stage
Tube 6EM7
A half of the tube is an ampliifer, capacitor coupled to previous stage, B half is a cathode follower to out
Plate Voltage should be 150 on A, power supply voltage 330
A side is fixed bias with a regulator
Problem: Plate voltage is 300, as is voltage on cathode follower resisto, Unit plays but affected side is louder than the other in phono. CD off the line stage is not affected.
Measures taken.
Replaced these resistors: drop, plate, cathode follower, grid between stages (no cathode resistor on A--goes straight to ground.
Resoldered all grounds.
Checked bias regulator--okay
Switched tubes between channels
New socket
I haven’t changed above coupling cap, decoupling cap (both measure well and are not old), nor the polystyrenes between the grid and plate on A. If they were shorted, I would think the grid would also read 300, and it doesn’t. I can’t believe that if they just went bad, 300 volts would suddenly appear, but at this point, in fact, I can believe anything.
Over the years I’ve had problems with this unit, I think because it runs hot, and is shelved in a way that does not dissipate heat well. That has to change. I mention this because, if there is a problem in a remote location due to a weakened solder joint, I wouldn’t be surprised, and it wouldn't be the first time.
I’m stumped.
That's the schematic of the line stage. I never figured out how to post drawings.
I have the bias set to be -1.3 at the grid of the voltage amplifier, and that's what it is. It comes off a regulator.
Let's see: on A side I referred to: plate resistor 221K, Drop before that 11.5K
.1 cap through a 1k grid resistor goes to the follower side. The follower resistor is 10.7K. There is feedback from there back to the previous tube.
One thing I did notice. It appears that the drop resistor failed somewhere in all this. I was getting 300 v at the decoupler cap. It went to 330, which is the supply voltage, so I replaced the drop resistor, thinking my problems were over. Nope, except the voltage at the cap is back at 300. It stands to some reason that what took out the drop resistor is the culprit at the plate, but damned if I know what that would be.
Is that to the 6SN7? Or is that to the 6EM7 (pin 4?)?
Again 6SN7 or 6EM7? If 6EM7 than that's pin 5? According to the schematic 6EM7 plate resistor is 43k.
From the schematic there should be no cap going to the follower side, it's DC coupled. The cap is from the plate of the parallel 6SN7 sections to pin 4 of 6EM7.
Please post voltage readings in regards to pinout of the tube, makes it easier for me to picture.
Example:
Pin 1) 212v
Pin 2) 300v
Pin 3) 222v
Pin 4) -1.3v
Pin 5) 212v
Pin 6) .8v
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There are two grounds. One from the cathode on A the other from the cathode follower. Both are connected.
In fact, the ground to pin resistance readings with the amp off match those of the other channel.
Ok then post the voltage readings of both circuits on all tube pins. Have you tried swapping the tubes?
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Will do on the voltages, as noted. Yes, I've swapped the tubes. The problem does not travel with them.
You've got an interesting problem. Maybe a bad tube socket not making contact on one of the pins?
Some progress. In checking the voltages, I noticed -5 on the grid, pin 4. It's supposed to be about -1.3. I suspect that extra bias is the problem. What is weird is that the bias had been reading -1.3 while I've been having the problem. I'm guessing a meter is not fast enough to catch variances. I need to learn to use a scope. Anyway, now it's reading -5, and nothing has changed at the plate. Still 300.
So now, why the bias change on the grid? What is putting it there?
The regulator for the bias has a test pin, at the output. The reading there is -1.5 as always and hasn't wavered. From there it goes through a 2m and a 20K resistor. I replaced both resistors and the coupling cap from the previous tube with no effect. Still -5 at the grid, 300 at the plate. Something is putting that bias there, but I have no idea why.
I'll check the other voltages tomorrow, though I know that pin 3 where the cathode follower is also reads 300. It should be around 150, I think. I suspect it is affected by the problem plate voltage on pin 5.
This is a doozy. I've got to learn to use a scope.
Thanks everyone.
The question is how to get the bias back down. I can't figure out where the negative voltage is coming from. I can't find a short anywhere. I thought it might be coming from the raw DC from the regulator, but no short there, and the resistance readings are similar to the other regulator. I have a different cap from the previous stage. No change. Tried changing tube. Nope.
I looked for a short from the feedback coming off the cathode follower, but no.
The extra negative voltage is applied between the 2 meg and 20K resistors (both new). So the question is, is there some way that extra negative bias voltage can appear on a tube other than through a short? Or is there a certain short I should be looking at? Very strange.
about the only thing I can think of to put 300 on the gain stage plate is to pull almost no current at all - as in a bad/open cathode bias resistor (assuming one was used as grid leak was sometimes employed by Bruce Moore) - have you measured the bad channel's cathode bias resistor value vs the good channel's resistor?
He has checked the cathode resistor and connections, besides if there was an open connection on the cathode that still doesn't account for the -5v at the grid.
As stated he has -5 on the control grid which will give him close to 300v at the plate even with the cathode resistor connected. Current is about .1mA.
ok - if there's no cathode R on the gain stage then this is one of Bruce's fixed bias or grid leak schemes - I had some of Bruce and Scott's custom preamps which ran bias supplies
"Replaced these resistors: drop, plate, cathode follower, grid between stages (no cathode resistor on A--goes straight to ground."
"Replaced these resistors: drop, plate, cathode follower, grid between stages (no cathode resistor on A--goes straight to ground."
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