Tweet
Good day all!
A friend of mine gave me his SF Deluxe Reverb with Master Volume to take a look at. Apparently he had just finished the black-facing proceedure (leaving the MV) and decided to fire it up with no success.
His account:
Once it had time for the filaments to heat up, he took it out of standby. Immediately one of the tubes began to get red hot, a few sparks...then he turned off the power.
My guess..probably not in time to save the power tube(s)
(The value of the 470 and 1.5K resistors on each 6V6 socket checked out.)
I decided to carefully check the chassis vs. the schematic (both AB763 and the later MV version layout, with all his work looking pretty good.
To be sure I heated up the solder joints and checked connection continuity, resistor value, and cap value with nothing out of the ordinary. Grounds also seemed to be in order. After this, I put in a known working 5U4G and checked voltage - looked fine. Then I installed known good 6V6GT's. Fired it up with standby on...filiments were lit in all tubes, voltages before the standby checked out.
Took it off standby and hoped for the best. Everything looked good. Checked voltages in various places again relative to schematic...still good. There was proper voltage to the preamp supply as well.
All good signs considering my friends account. Plugged in a guitar and here is what happened.
Result:
VERY LOW OUTPUT VOLUME. No strange noises just really low volume. Cranked and volume control pulled, it was louder/distorted but still VERY LOW. Vibrato works and Reverb works.
I shut the amp down and tried the external speaker jack - No SOUND AT ALL. Is that interesting? I should also point out a strange smell, not seeming to be the result of something in my investigation, as much as perhaps something that had burnt out from before. Hard to describe the smell...not powerful, but there and electrical.
Tonight I intend to try replaceall the preamp tubes with know "good ones" and see if that doesn't make a difference. Do the old Tap Test as well.
Questions:
1. Does this situation speak of a blown OT? I don't want to jump to this conclussion as is quite easy - like one would self-diagnose a problem with a car, but I can't help thinking my friend would have run the amp without the speaker plugged in at some point and may have set the amp up for failure. Not to mention its 30 years old. When checking the voltage oof teh OT primary at each tube, not only was the voltage within range, but I did here a good pop throught the speaker. If not misteken this is a good sign for teh OT. Thoughts?
(He is a fine luthier and makes great acoustic instruments but electronics are not his strong suit.)
2. Would it be OK to use a SF Princeton OT to test and see if the currnet OT in the SFDR is bad? Maybe a quick in and quick out, because I am sure it would be a huge change in the volume level.
Any help would be apreciated. Spent about an hour and a half so far.
Thanks in advance for all your expertise and advice.
A friend of mine gave me his SF Deluxe Reverb with Master Volume to take a look at. Apparently he had just finished the black-facing proceedure (leaving the MV) and decided to fire it up with no success.
His account:
Once it had time for the filaments to heat up, he took it out of standby. Immediately one of the tubes began to get red hot, a few sparks...then he turned off the power.
My guess..probably not in time to save the power tube(s)
(The value of the 470 and 1.5K resistors on each 6V6 socket checked out.)
I decided to carefully check the chassis vs. the schematic (both AB763 and the later MV version layout, with all his work looking pretty good.
To be sure I heated up the solder joints and checked connection continuity, resistor value, and cap value with nothing out of the ordinary. Grounds also seemed to be in order. After this, I put in a known working 5U4G and checked voltage - looked fine. Then I installed known good 6V6GT's. Fired it up with standby on...filiments were lit in all tubes, voltages before the standby checked out.
Took it off standby and hoped for the best. Everything looked good. Checked voltages in various places again relative to schematic...still good. There was proper voltage to the preamp supply as well.
All good signs considering my friends account. Plugged in a guitar and here is what happened.
Result:
VERY LOW OUTPUT VOLUME. No strange noises just really low volume. Cranked and volume control pulled, it was louder/distorted but still VERY LOW. Vibrato works and Reverb works.
I shut the amp down and tried the external speaker jack - No SOUND AT ALL. Is that interesting? I should also point out a strange smell, not seeming to be the result of something in my investigation, as much as perhaps something that had burnt out from before. Hard to describe the smell...not powerful, but there and electrical.
Tonight I intend to try replaceall the preamp tubes with know "good ones" and see if that doesn't make a difference. Do the old Tap Test as well.
Questions:
1. Does this situation speak of a blown OT? I don't want to jump to this conclussion as is quite easy - like one would self-diagnose a problem with a car, but I can't help thinking my friend would have run the amp without the speaker plugged in at some point and may have set the amp up for failure. Not to mention its 30 years old. When checking the voltage oof teh OT primary at each tube, not only was the voltage within range, but I did here a good pop throught the speaker. If not misteken this is a good sign for teh OT. Thoughts?
(He is a fine luthier and makes great acoustic instruments but electronics are not his strong suit.)
2. Would it be OK to use a SF Princeton OT to test and see if the currnet OT in the SFDR is bad? Maybe a quick in and quick out, because I am sure it would be a huge change in the volume level.
Any help would be apreciated. Spent about an hour and a half so far.
Thanks in advance for all your expertise and advice.
Comment