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On one of my pair of M60 MkII.2 monoblocks, I have to turn the DC offset knob/dial all the way counterclockwise to get the needle to remain reasonably steady (i.e at zero) when I dpress the test toggle switch. Even then it isn't quite perfect in the sense that I feel I need to twist the knob the just a little bit more to zero it perfectly (of course there isn't any travel left. While I don't denote any sonic anomalies, I curious as to why I have to go all the way to the end of the knob on one amp whereas on the other, the zero position is somewhere in the middle. Any clues?Also, I've noticed that on the amp in question, the needle moves in a direction opposite to that in which the knob is twisted, whereas on the 'good' amp needle movement and knob twisting direction are the same. What gives.
I've conducted a visual inspection of the 6as7Gs and there don't appear to be any shorts. Could there be an issue with one or more of the tubes that doesn't show up in the visual inspection? Is the problem perhaps at the driver tube end of things? Or should I not be concerned especially since both amps sound equally fine, even tough I hve to teist the offset knob in one of them all the way and still end up a little short of the zero position?
Follow Ups:
Following the suggestions of several, I swapped the rightmost front tube for its leftmost analogue, and hey presto, I can now zero in the dc offset such that I still have perhaps 15% of travel left on the dial/knob. Is this good enough, or should I try swapping another tube (or two) to see if I can get the amp to zero in somewhere around the middle of the knob's range of movement? If there is a suboptimal tube (or two) lurking in the amp, should I be concerned that the remaining good ones are being overdriven?
The fact that you had success on your first try suggests to me that either you were very lucky to pick the one "bad" tube to exchange with a "good" one from the other side OR that all tubes are aged about the same on average, so that swapping one of the quartet allowed you to achieve balance at a slightly different point on the bias potentiometer. If anal, you can probably identify a truly dead or failed tube (which might be the basic problem) by pulling all tubes and balancing the amp with just one tube on each side, swapping tubes until you have individually tested each one. (You could even put a known good NOS tube on one side and test the 8 others against it, one at a time.) I suggest that before you do that you should label the tubes and take note of their present locations on the chassis, so that you can return them to those locations once you are done.
Good enough.
I alway preform adjustments with (speaker) load.
Revels @ 6ohm and Silverlines @ 8 behaved differently,
At times will also use a multi-meter instead of
built-in meter . . . if in doubt.I on ALs' (or perhaps Ralphs') recommendation
switched (while on - YES B+ too) and HOT swapped
with kitchen mitts to balance the (4 tube) halves
to point where meters reflect closer to (12 oclock)
positions. It really works for a stubborn tubes,
INVARIABLY there IS some freakish balance.I struggled with this approach (initially), but
the AMPS ARE SOLID - they are OK with this.
I (at times) had to discard an offending tube
that skewed the measurements wildly . . .I spoke with Ralph recently . . .
I believe that Ralph IS (now) out west for
a show - Rocky Mountain Audio Show (?name?)
Sorry guys, I reread the original post. [Up too late listening to music I guess.] I did not read the post correctly the first time around. I now see that the problem with DC offset is only with one amp. I would certainly look for tube problems.
Regarding the speaker issue, I had the problem of not being able to adjust DC offset with my esl's attached. Spoke with Ralph who told me to disconnect the cables. I had the difficulty with and without zero's in line. I noticed Bob received the same advise. Sorry if I misled you. Just trying to help. I will stay on the sidelines as usual.
Doc
I am sure that Ralph will pipe in here sooner than later, anyway, I have the Zero's in line with my speakers and Ralph did "tell" me to do all the biasing and the DC off set with the speakers not attached to the amp. Somthing about the Zero's giving the amp's metering circut false readings, and that some speakers will have the same reaction. He told me it is much more accurate with the speakers dis-connected than with speakers connected even with the Zero's out of the equation. This does not dirctly ad to the problem solving but I just wanted to tell all that is what Ralph told me.............Bob
I was one of the very early adopters of the ZEROS and while I never heard anything from Ralph on this, Paul Speltz (who mfgs/sells the ZEROs) did advise that bias and offset be set WITH the speakers hooked up to the amp, BUT WITHOUT the ZEROs in line. i.e. finialize the settings wiuth speakers hooked up, and thn insert the ZEROS into the mix, knowing that meter reading with the ZEROS inserted will be off.
Why the relationship between the direction of the bias adjustment and the direction of the needle travel are opposite for the two amplifiers is not easy to say, because there are several possibilities but none of them are a major cause for concern.The fact that you have to turn the bias adjustment to one extreme in order to balance out one amplifier suggests that one or more output tubes on one side of that monoblock are suboptimal with respect to the tubes on the opposite side, i.e., the two groups of tubes are collectively no longer well-matched (hope that's not too opaque, but it's the simplest way to put it that I can think of right now). Try swapping tubes from one side of that monoblock to the other side of the same monoblock. (The M60 has two groups of four output tubes each on opposite sides of the chassis as you face the unit from the front; for example, take any two tubes from one side and swap them with any two tubes on the other side, etc.) Eventually, you should find a configuration that permits easier biasing. Of course also in the process you may find that one or more tubes needs replacement. Why do you not hear a problem? Because the circlotron design is incredibly tolerant of bad output tubes or mismatching of output tubes. However, I bet when you straighten things out, you will notice a tad of improvement in the sound, just a tad.
As to the question of whether you should balance the amplifiers with or without the speakers in the circuit, I think that is spelled out in the owner's manual, and there is a definite preference. (I just cannot remember what Ralph wants you to do.) I personally ALWAYS balance my amplifiers with the speakers in the circuit. Also, contrary to what doc says, I cannot imagine why having your Quads hooked up should make one amplifier act funny and not the other; presumably each speaker in the stereo pair represents a nearly identical load unless one of them is defective. In other words, biasing with the speakers in circuit may give you a different bias point vs biasing with speakers out of the circuit, but the difference should be the same for both channels.
Greetings from KatRita country. Looks like the proverbial bomb went off here but I got power back 4 days ago and the service into the house had to be replaced and the system sounds better than ever. I guess this is what you call a silver lining. Phones and cable are still two months away so audio will be even more prominent in my life now than ever.
On to the problem. I think Lew nailed it. It's virtually always failed/declining 6AS7s that are the culprits in this situation. Swapping normally restores a level of balance between the sides unless tubes have totally crashed.
Ditto on the leaving speaker wires connected. Of course, I defer to Ralph but I cannot imagine a scenario where this wouldn't be the correct approach. Good luck!
Thanks very much for the detailed explanation Lew. I suspect you're spot on with the issue of a not easily detectable suboptimal tube. I'll try out what you and other have suggested...hopefully later tonight.
I have to disconnect the speaker cables to make offset or bias adjustments.
Ralph advised that it is sometimes required depending on the speakers used. Otherwise I would definitely check for tube problems. Also, if there is a switch for class A operation make sure it is in the A mode for setting offset.
This model only operates in Class A. I'm surprised to hear you advise disconnecting the amps from the speakers (and cables of course), since I was sure that the bia and offset setting were speaker dependent. I'll look into it. BTW, I use the atmas to drive a pair of Quad ESL 57s...a very copacetic match.
The problem is definitely the speakers. I drive esl's also and they create a problem biasing with the cables connected. Please try it without the cables connected and let me know how this works out. Also if you have a meter it may be more accurate. Attach it to the disconnected speaker terminal and set bias using the most sensitive setting.
Doc,The issue is definitely not with trhe speakers. I have switched the suspect amp from one Quad to the other with identical results. Also, as Lew points out, if the speaker type were at fault, the issue should also be reflected in the trouble free amp - which it is not. I used to own M-50s before the current M60 MkII.2s, and have the manual for that model lying around somewhere, and I could almost swear that the instructions call for biasing with the speaker in circuit. Anyway, haven't had any issues driving ESLs (63s or the 57s) with the Quads. I suspect the issue is very much related to a sub-optimal output tube. I will check and confirm later tonight.
Try swapping a couple tubes from one side of the same amp to the other side of the same amp. Dont know why because I am not in any way technical in electronics but this has worked for me more than once in the past.
Thanks for the tip. I seem to vaguely remember hearing something about that before. I'll give it a try.
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