Fender Twin, modern

[plook]

I've got a Fender Twin, the modern type with the LEDs on the back for the power tubes. One was red because a fuse had blown. I've fixed that with a new fuse.
I have a few questions.
Do the LEDs stay on in yellow as normal?
The mA readings using bias sockets is 60mA for the V9&10 and 110mA for V11&12.

Has anyone out there worked on this amp and can offer tips.

[dcribbs1412]

Hi plook
not sure if this will help...
from a twin manual

Output Tube Bias Adjustment Instructions —
1. Turn on the amplifier and let it warm up for at least two minutes
with the STANDBY SWITCH (item C) in the ON position and the
OUTPUT SWITCH (item D) in the OUTPUT HIGH position.
2. With a digital voltmeter set to its most sensitive DC voltage scale,
measure the voltage between the two BIAS TEST POINTS (item K)
labeled “BIAS” and adjust the OUTPUT BIAS ADJUST control (item J)
for a reading of .08 VDC (80 mVDC).
3. Connect the digital voltmeter to the two BIAS TEST POINTS (item
K) labeled “BALANCE” and adjust the OUTPUT BALANCE ADJUST
control (item L) for 0 VDC.
Notes:
1. Make bias and balance adjustments only when necessary. If you
are unsure about setting bias and balance, it is best to LEAVE THIS
FEATURE ALONE, as it is pre-set at the factory.
2. For optimum sonic performance, set bias to .08 VDC, then set bal-
ance.
3. For optimum sonic performance with increased tube life, set bias
to .06 VDC, then set balance.
4. For optimum tube life, set bias to .04 VDC, then set balance. If a
bias measurement of .04 VDC cannot be obtained, replace the
6L6GC output tubes.
5. If the output tubes can’t be balanced, check the bias setting
and/or replace the 6L6GC output tubes.

looking like from 40ma cooler to - 80ma hot...
might look into a new set of tubes...just a thought

Darin

[plook]

That is fantastic, exactly what I needed. Thanks do much. I'll let you know how I get on.

[dcribbs1412]

That is fantastic, exactly what I needed. Thanks do much. I'll let you know how I get on.

Plook
here's a link to a manual...page 6 has some bias settings also

http://support.fender.com/manuals/guita ... manual.pdf

Darin

[plook]

That link to the pdf represents the exact bias control of the amp i am fixing, the other instructions are for another model which shows 0.8v rather than 60mA. Unfortunately that pdf is corrupted and has all sorts of weird symbols in it, so it's hard to follow. I'll try to find another version of that.

the amp serial number is: CR-294982

[Firestorm]

I think the LEDs are supposed to be green, the red ones come on if there is a tube "failure" likely a screen short. So the blown fuse was not a cause, but an effect. Your current readings suggest that at least one tube in the high reading pair is suspect. This is a Pro - Tube model. I'm sure the proper manual and schematic can be found at Fender.com

[plook]

Thanks, Fender mailed me the manual and schematic today for my model.I appreciate that tip about the screen and will check it out before adding new tubes.

The schematic is a complete nightmare. Could anyone tell me where the screen resistors are in this amp? I looked inside and i can't see anything burnt out.

[Structo]

Sure, page 2, 6L6, V9 and V11, R118 and R119

And 6L6, V10 and V12, R130 and R131

[dcribbs1412]

The schematic is a complete nightmare. Could anyone tell me where the screen resistors are in this amp? I looked inside and i can't see anything burnt out.

Plook
while you have it open
if you have a camera, some pics might help out.


Darin

[plook]

Thanks a lot for that. I was rushing and panicking a bit when i wrote my last message, but i since took the time to look up the pins on the 6L6 to find the screens and measured the 470 ohm 5w resistors - all good. I had to remove the 'upper deck' circuit board to get to the resistors - to physically check them out. I just don't like tube bases being connected to boards like that. All tests perfect. Got the bias set at 60mV with a fresh set of tubes and the amp sounds great.

Thanks for the help. I'm just assuming that the bad tubes may have caused that fuse to blow. One of the tubes has a broken off keypin, so who knows what happened before it came to me for fixing.

I put the chassis back in before i could think of taking a detailed picture.