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Bias adjust on '65 Twin Reverb Reissue AB763 - 2004?
Bias adjust on '65 Twin Reverb Reissue AB763 - 2004?
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Old 4th May 2010, 07:48 AM   #1
gogogomez is offline gogogomez
diyAudio Member
Join Date: May 2010
Question Bias adjust on '65 Twin Reverb Reissue AB763 - 2004?

Okay I have a couple of questions as I am newer to servicing amps myself.
Yeh, these may be 'common sense' questions to a seasoned vet of amp repair, but I'm basically the 'limited/dangerous knowledged' serviceguy

I replaced the 6L6GC tubes a while back and had no issues with them for about two years.
A couple of days ago I blew a fuse. Replaced the fuse and blew that one too.
Replaced the fuse and a couple of days later turned it on.
The amp came on but with lowered volume.
tubes bad likely...
visually inspected and saw two tubes had darkened glass on the sides.
Replaced those tubes and noticed the first position 6L6GC was glowing abnormally purpleish on the side. switched two more to verify this was a position issue and not a tube issue.
I don't have an ohm meter but adjust the bias to stop the abnormal glowing on the first position 6L6GC.
success. BUT the first position tube still feels hotter to the touch after running a few minutes than the others.

My questions:
1st Is it normal for the first 6L6 to run a bit warmer as it's the first poweramp tube or is something esle potentially wrong resulting in this first position tube to be running hot and possibly overheating/cooking?

2nd: Is adjusting the bias control pot left lowering the voltage and right adjustment raising the voltage thus why the tube started glowing normal when I adjusted the pot left (looking from the bottom of the amp next to the power transformer)?

I would prefer running the amp a bit cooler myself with a lower voltage as I do run some overdrive and delay effects. Also don't mind trying to extend my tubes lives a bit...

I know it's ideal to adjust the bias with an ohmmeter, but is my thinking/guessing wrong.
I've seen lots of info on how to adjust the bias, but don't recall anyone stating the exact effect of left or right adjusting that bias pot.

thanks for the help.
I will be investing in an ohmmeter and doing this right soon, but need a 'quick fix' to last me two weeks til payday

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Old 4th May 2010, 01:43 PM   #2
OneyedK is offline OneyedK  Belgium
diyAudio Member
Join Date: Oct 2007
Both 6L6's should be equally hot, If it's a matched pair.
Since you confirmed that the problem is "position related",
I'm suspecting a leaking coupling capacitor. If this is true,
replace them both. Fender RI amps are known to have lesser
quality components than the original...

Verify this with a voltmeter. There's no quick fix, if
bias is wrong, it can destroy a tube and in rare cases
even the output transformer...
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Old 14th May 2010, 01:46 AM   #3
gogogomez is offline gogogomez
diyAudio Member
Join Date: May 2010
Question checking 6L6GC with ohm meter advice on readings...

Okay.. My buddy has one of those sockets you plug into the 6L6 socket then hook to an ohm meter.

I did everything I thought I was supposed to do.

Since I am new to amp servicing, I was seeking some answers to my encounters.

Now when I plug the test socket in the first (looking at the back of the amp and to the far left) I'd get my mA readings, but no readings on the second, third, or fourth sockets. seems strange to me. Is that normal or is a resistor bad?

Now the other strange thing to me...
I purchased a ''matched set'' of 6L6GC JJ tubes.

I measured each one in the first socket and got varied readings.
I set the bias adjustment low to see what readings I'd get.
Now without adjusting the bias control any further at this point, testing the various NEW out of the box matched set of JJ tubes I got 23.3, 13, 11.1, 9, and on older tubes Fender Groove tube 17.5, a Sovtek 6L6WXT rated 8.2, and one sovtek rated 9.5. I know the readings are because I have the bias adjusted low, but didn't expect that wide of range. I was expecting a difference of 0.2 mA, not 2 to 4 mA... or between 9.5 and 23.3 like the hottest tube read!?

oddly, three of the sovtek 6L6WXT tubes that WERE in the amp when it quit working ended up NOW reading over 70, over 60, and over 40 mA without changing anything except which tube was in the (first socket) test socket.

now evidently those hi reading tubes are no good, I'm guessing, but not sure.

The amp sounds fine now that I matched four 6L6GC with similar (within 2 mA of each other) tubes. I'm running it cold with the mA set to 25mA for the highest reading tube and it's working without crunching, or crackling and the tubes are glowing just nicely.

Is there something else I should be concerned with or checking here before using this amp for any length of time? Seems to me to now be in 'normal working' condition.

thanks for any input...

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