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Old 20th July 2014, 08:05 PM   #1
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Location: Boston, MA
Default Plate voltage on preamp tube too high

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 havenít 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 doesnít. I canít 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 Iíve 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 wouldnít be surprised, and it wouldn't be the first time.

Iím stumped.
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Old 20th July 2014, 08:29 PM   #2
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Hello fellow Bostonian,
If you could post a schematic with circuit values that would be a better way for us to see what is happening. You checked the bias regulator but what is the actual voltage your getting at the grid?
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Old 20th July 2014, 08:41 PM   #3
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Is this the schematic?

From what I read on another thread the bias is -2v from a voltage divider off the filament supply?
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File Type: gif mfa line amp.gif (5.9 KB, 184 views)
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Old 20th July 2014, 09:38 PM   #4
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Quote:
Originally Posted by famousmockingbird View Post
Is this the schematic?

From what I read on another thread the bias is -2v from a voltage divider off the filament supply?
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.
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Old 20th July 2014, 10:58 PM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by trombone View Post
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.
Sounds like the ground wire for this circuit could be disconnected.
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Old 20th July 2014, 11:03 PM   #6
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Quote:
Originally Posted by trombone View Post
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.
Is that to the 6SN7? Or is that to the 6EM7 (pin 4?)?



Quote:
Originally Posted by trombone View Post
Let's see: on A side I referred to: plate resistor 221K, Drop before that 11.5K
Again 6SN7 or 6EM7? If 6EM7 than that's pin 5? According to the schematic 6EM7 plate resistor is 43k.


Quote:
Originally Posted by trombone View Post
.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.
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

Last edited by famousmockingbird; 20th July 2014 at 11:05 PM.
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Old 20th July 2014, 11:04 PM   #7
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Quote:
Originally Posted by rayma View Post
Sounds like the ground wire for this circuit could be disconnected.
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.
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Old 20th July 2014, 11:18 PM   #8
rayma is offline rayma  United States
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Quote:
Originally Posted by trombone View Post
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?

Last edited by rayma; 20th July 2014 at 11:19 PM. Reason: mistype
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Old 20th July 2014, 11:59 PM   #9
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Quote:
Originally Posted by famousmockingbird View Post
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.

--Whoops. Right, pin 4. The circuit may have changed some over the years. The plate resistor is 221K. That 43K is actually 4 43K resistors in parallel on the cathode of the follower, 10.7K

The two sides are connected by a 1K resistor. The coupling cap I mentioned comes from a pair of 5691s coupled to the grid of the 6EM7.

This is just the phono stage. 6SN7s don't come in until the line stage which is not a problem.

I'll get on the voltage readings. I could do it from memory, but I'll run them again to be sure. Thanks very much for you attention to this.



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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Old 21st July 2014, 12:00 AM   #10
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Quote:
Originally Posted by rayma View Post
Ok then post the voltage readings of both circuits on all tube pins. Have you tried swapping the tubes?
Will do on the voltages, as noted. Yes, I've swapped the tubes. The problem does not travel with them.
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