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In Reply to: RE: Excessive DC offset on MP1 posted by Lew on December 22, 2023 at 09:57:21
The LED begins to light up at about 13mV. In normal operation the DC Offset tends to be about 1mV or less.
Its rare to have an IC failure to cause the servo to malfunction. If the preamp does not have a vacuum tube high voltage regulator, then there are no known instances of the servo opamps failing. They have been damaged in older units (Mk1 and some Mk2s) that have the tube regulator due to arcing of the regulator.
Usually the DC Offset LED lights up due to a tube (6SN7) problem. It might be a bad connection on a tube socket too. If the socket is worn, moving the tube while the preamp is on might have the DC Offset LED going on and off due to poor connections in the socket itself.
There have been some updates to the servo system over the years, all with the goal of extending its dynamic range and limiting its bandwidth so as to promote stability of the DC level at the output of the preamp.
Follow Ups:
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Forgot to mention the most obvious cause: one of the 6SN7s is not up to snuff. I will look into that. And yes, my MP1 does have solid state voltage regulation in conjunction with constant current sources. Thanks for your response.
Hi Lew:
If it helps, I very recently had a similar issue with my MP-1. With Ralph's help, I traced the issue to a pin in the PSU connector of the amp side being pushed in thus losing connection. Pulling the pin back in position fixed the issue. In this case though, the DC offset light on that channel stayed lit and the 3 6SN7 tubes (CCS and the gain tube in the line stage) did not lit at all since the pushed-in pin was for filament supply.
Thanks for the tip. As Ralph knows and hinted at in his response, my MP1 is very old, probably mid-1990s vintage. I have owned it for more than 20 years myself. Along the way I have upgraded it in parallel with the factory but sometimes to satisfy my own whims. The fact remains that the servo is very old school. On the other hand, it has never failed to work well enough to at least shut down the DC Offset warning lights. (Typically, I have seen 4-5 mV of DC Offset per channel, which has been acceptable.) I am now working on the theory that maybe the high value resistors in series with the output of the op amp are now too high to work with the way I just recently re-set the bias voltages on the circlotron output tubes. Perhaps the resistors are dropping the V from the op amp such that it cannot fully correct the DC Offset, on the side that connects to the circlotron tube grids. (I realize that the current draw is miniscule, but there must be at least a tiny drop in V from the op amp across a 1Meg resistor. I am going to test my theory tonight (Xmas Eve). I don't expect to see Santa Claus, but I would rather that my MP1 would lose its resemblance to Rudolph, the Red LED Preamplifier. I have already proven to myself that there is nothing wrong with the op amp per se, and if it is a tube, then there would need to be at least one of the two 6SN7s failing per channel, because I also see 44mV DC Offset on the L channel.
My own MP-1 is around the same vintage. Bought it from Ralph in the 90's as well. It uses 12AU7 for the voltage amp in the line stage. I had it upgraded to solid-state power supply a few years later it's otherwise stock. I recently measure its DC offset after it's fully warmed up and it's around 5mV as well.
The MP-1 Mk iii's DC offset measured only .1 mV once the servo lights go off on the other hand.
I think I did the tube regulator to solid state regulator cum CCS, very soon after I purchased my MP1. I have also removed the 12AU7 socket and replaced it with an octal, so I have been using 6SN7s for probably 20 years. Even though the two tube types look very similar on paper (mu, plate resistance, transconductance, etc, are near identical if not completely identical), the 6SN7 sounds far better. Highly recommended.
Now you perked my interest.
Are you saying that, if I figure out a way to adapt the filamen supply and the tube socket, I can drop in a 6SN7 in for the 12AU7 in my vintage MP-1 without making other changes?
"Ask Ralph", but the circuit should work for a 6SN7, as is. I just also noticed you can buy an adapter socket that plugs into a 12AU7 socket and provides pin holes on the top side for a 6SN7, for only $10. Search "12AU7 to 6SN7 adapter", and you will see them for sale. The MP1 provides 6.3V filaments as is and that's fine for the 6SN7. I cannot recall whether the 6SN7 needs more current for its filaments, compared to 12AU7, but even if it does, the PS should tolerate it. But again, ask Ralph.
Thanks Lew. I'll check with Ralph.
I was also thinking a socket adapter would be the way to go. I believe the MP-1 employees a +6.3V and -6.3V regulated filament supply for the 12AT7/12AU7 and 6SN7 tubes. 12AU7 filament spec is 12.6v/0.3a or 6.3v/0.6a and 6SN7 is 6.3v/0.6a. I need to check and see if the 12AU7 in the MP is running 6.3V or 12.6V filament and see how the off-the-shelf adaptor is wired.
No need to check; I am sure the filament supplies are +/-6.3V, and the 12AU7 receives 6.3V.
Got it. Thanks!
Also found 6SN7 - 12AU7 adaptors are available in both 6.3v and 12.6v versions.
You want the 6.3V version.
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