Fender Twin bias weirdness, advice sought...

[HTH]

I've got a channel switching Fender Twin here, one of the recent ones (not the red-knob 80s ones). Its lost a lot of volume, 'was' taking your face off on '2' (clean channel), but now needs cranked up to at least 5 or 6 for any decent volume.

The bias is acting strange in that the two sides (i.e. the left or right pair) bias up around 20mA apart from each-other irrespective of whether you swap the 6L6s round, or even put a 'known good' matched quad in that match in other amps.

I'm at a loss with this one - anyone care to have a look at the schematic and suggest where to start? - would be very grateful.

http://support.fender.com/schematics..._schematic.pdf

[HTH]

anyone? - Bueller?

[Jazz P Bass]

Well, what have you checked on the amp?
Did you measure the voltage at 6L6 pin #5 of the tubes. (measure at the socket/ tubes out, then tubes in)
Here is a list of possibilities:
Faulty soldering on the output tube board
C106-C107 phase inverter coupling caps/ may be leaking voltage to the grid
R115-R129/ R114-R128 1.5K bias resistors/ out of tolerance
R118-R119 470 5 W screen resistors/ out of tolerance-open
D100-D101 failed & are leaking to ground
Break out the DVM.

[HTH]

Well, what have you checked on the amp?
Did you measure the voltage at 6L6 pin #5 of the tubes. (measure at the socket/ tubes out, then tubes in)
Here is a list of possibilities:
Faulty soldering on the output tube board
C106-C107 phase inverter coupling caps/ may be leaking voltage to the grid
R115-R129/ R114-R128 1.5K bias resistors/ out of tolerance
R118-R119 470 5 W screen resistors/ out of tolerance-open
D100-D101 failed & are leaking to ground
Break out the DVM.

Good point, I should really post my findings so far.

I'll get out my notes in the morning and post them here.

Thanks for your post and suggestions, much appreciated.

The one thing I'd thought is that the trem pot that the bias goes through is a dual-gang pot and this could be causing problems if they're quite different in value, right?

[pdf64]

Turn the trem intensity control to minimum and then re-check the bias. Pete.

[Chuck H]

Not sure about this, but I don't think Fender ever made a channel switching Twin with a bias vary trem. It must be a later Twin and therefore has a preamp grounding trem that doesn't affect any bias conditions.

[HTH]

Not sure about this, but I don't think Fender ever made a channel switching Twin with a bias vary trem. It must be a later Twin and therefore has a preamp grounding trem that doesn't affect any bias conditions.

I've attached the schematic on my first post if you fancy having a look - the bias goes through the intensity pot, then to the bias splitter resistors, looks like the old bias-varying trem circuit, but I could be wrong. Thanks for responding either way, Cheers!

[Steve Conner]

The bias is acting strange in that the two sides (i.e. the left or right pair) bias up around 20mA apart from each-other irrespective of whether you swap the 6L6s round, or even put a 'known good' matched quad in that match in other amps.

Check the output transformer and screen resistors.

[J M Fahey]

Its lost a lot of volume, 'was' taking your face off on '2' (clean channel), but now needs cranked up to at least 5 or 6 for any decent volume.

The bias is acting strange in that the two sides (i.e. the left or right pair) bias up around 20mA apart from each-other

1) I do not think both facts are related, at all.
2) To get over the bias obsession:
How do you measure idle current?
Did you try to use the R149/R150 "bias/balance" trimmers? (which are there just for that purpose)
3) Now on to the real problem: this Fender schematic shows very useful signal voltage levels .
Inject 2.5 or 5 mV AC @ 1 kHz on J1 as suggested and trace the signal along its path, checking that everywhere you have the approximate values.
Good luck.

[Chuck H]

Yup. That's a bias vary trem. Please to ignore above post and check bias with trem off.

[HTH]

ok, so I've just got round to looking at this again and I'm a little embarrassed to admit this, but the effects loop wasn't switched in - there is a big difference in volume if you don't have the loop switched in (even with nothing plugged into the loop). Obviously its got a knock when the owner was last moving it around (doh!!!!).

[Chuck H]

Ha... I had a little Mesa Subway amp that had a mute switch in the back on the UNDER side of the chassis. I was going to sell the amp to someone but when I moved the amp into the living room for demonstration I must have triggered the switch. Since I never used the switch (and you can't see it) I didn't even consider that it might be the cause. No sale that day.