|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
98.14.44.2
Retubed my amps to glorious effect but one channel has a DC offset of 2v while the other is still at zero. Turning the pot near the meter all the way down gets me to 2v. As the tubes are set as two rows of six tubes next to each other, is the offset a discontinuity in transconductance of the output tubes from one row to the other? Will playing musical chairs with the output tubes possibly correct this issue?
Thanks in advance.
Charles
Follow Ups:
On a 12-tube model the driver tube is the last one of the row in the rear, away from the input connectors.
If one section is weak, this can cause a DC Offset that can't be corrected.
Another symptom of a bad driver tube is the DC Offset mysteriously changing without any other reaction from the amp (such as an obvious output tube arc).
Hi Ralph
Are you talking about the last 6sn7 or the last pair of 6as7 tubes?
Charles
If you have a 12-tube MA-1 and it has 6SN7s, then it will be the last one in the row. The earlier driver used a 6CG7.
Mine are MA-1 deluxe that were brought up to 3.1 so they have 6sn7 drivers. I did a recent retubing and thought I tested each tube but replacing the last 6sn7 in the line brought the offset down to zero. Thanks Ralph!
Charles
to have the resources we have here? We are lucky.
ET
This is for the MA50 and 60, but is applicable. What you want is two from the positive phase and two from the negative.
"Statistical Swap: This procedure is the remedy for a DC offset which cannot be zeroed
(see DC offset procedure). Using a potholder or similar object, randomly remove two
tubes from the front bank of output tubes and exchange them for two tubes from the
rear output tube bank. Perform the DC offset adjustment and note the improvement. If
the situation worsened, you may have to perform several random swaps before the proper
balance is achieved. This condition occurs occasionally as the result of variances in
tube production and should in no way be considered a defect of either the amplifier or
the output tube compliment."
Dan Santoni
Nt
"Will playing musical chairs with the output tubes possibly correct this issue?"
Yes. Take two from one side and swap with two from the other until you get it to zero. Often there will be one or two tubes drawing heavy current and those need to be balanced in the amp.
Dan Santoni
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: