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In Reply to: RE: Got the DC-offset blues posted by ChasLieb on September 24, 2015 at 20:47:23
ask Ralph. Ralph knows his circuit better than any man alive, and he usually guesses correctly as to cause, even from long distance.
Meantime, you apparently have a volt meter. You don't mention what Atma-sphere amplifier you own, but it apparently uses 6AS7s, which means you've got a lot of 6AS7s up and running. Can you measure the plate voltage on each bank of output tubes? (There are separate supplies for each phase of the output stage. The supply for each phase is shared equally among all the output tubes on that half of the stage, so the plate voltage across one tube should be identical to that across any other, on that phase.) Having done that and if the plate voltages are equal on each phase, you could next measure the grid bias voltages; if one side is much more negative than the other, it indicates that that half of the amplifier is closer to shut-off than the other, not doing much work, which could account for that apparent low power without much distortion or any peculiar noise. The next question would be "why?"
Don't do any of this if you are not experienced working around high voltages and currents. Never touch the chassis with both hands at the same time; I keep one hand behind my back or in my pocket to remind me not to use it, when messing with a live circuit.
I have the MA-1 deluxe which is a 12 tube version of the MA-1 with a very robust power supply which I was given to understand was from the MA-2. The amp was brought up to 3.1 status and the voltage tubes were all changed to 6sn7s. I am assuming that one row of 6as7 tubes makes up one bank and when you say to measure voltage you are referring to the voltage in the tube socket of one anode to cathode in a socket on one row of 6 as compared to one anode to cathode of a socket in the second row of 6, in order to see if the PS has an imbalance between the two banks in B+ voltage?
6 tubes on the left and 6 on the right.
Is this the same as an MA-1 Deluxe? This amp has two rows of six power tubes running from right to left at the back followed by a row of five 6sn7 tubes. If split half right and half left then you are saying the left three of each row is balanced against the right three of each row?
Sort of- the tubes in each bank are in parallel, so its the bank of tubes that are to be considered, not the individuals in the bank.
The MA-1 Deluxe was built for a short time prior to the introduction of the MkII series. It had a sloping rear panel much like the MA-2.
The best way to do it would be to invert the amplifier on a safe and sturdy workbench. Then remove the bottom plate and then measure the voltages with the tubes all in place. Each 6AS7 has two discrete sections. However, before following this line of investigation, I would suggest first of all to follow Ralph's plan of attack.
This weekend I have been dealing with almost the same problem on my humble diy M60. I measured the voltages on the sockets of all tubes (tubes taken out). Everything was fine except the grid voltage on V4 (-285). I suspected there is a fault in 5K trim pot, replaced it, and the problem disappeared.
I fixed my neighbor's Novacron, which had a problem with "low power output" (much like that of the OP in this thread), and it also turned out to be due to what appeared to be an intermittent problem with a bias pot, which resulted in operating one bank of 6C33Cs in near shut-off conditions. (Bias voltage was around -80V, which will pretty much turn off a 6C33C that sees the typical plate voltage in a Novacron.) I gave the pot a few turns in both directions, then re-set the bias, and that alone seemed to cure the problem, which has not recurred. Either that or he had just erred in setting bias. He seemed to know how to do it, so I tend to think the pot was faulty, but I really cannot be sure.
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