Fender Twin bias dilema

[paranoidpete]

Hi Folks, I have a 1996 Fender 'The Twin' (Evil Twin as it's known) and I see in the user manual that you can pull out the middle pair of 6L6's to reduce the total output on full power setting to 60 watts, (15 watts on the reduced power setting). It also informs that you have to alter the speaker impedance setting from 16 to 8 ohms (half the power = half the impedance.
With all 4 6L6's in place the output bias should be set optimally to 80 mVDC and the balance to 0 mVDC
My question is for you Guys ; Does anyone out there know if you have to reduce the bias reading to 40 Mv with a pair of 6L6's or is this setting unaffected ? (i.e. remains at 80 mVDC)

[guitician]

How the load is set will not require a bias re-adjustment since the plate voltage hasn't drifted that much. Check it to see where the plate voltage goes from 4-to-2 6L6 and if it's a large change then maybe re-check using a different method then a set current as 80mv.

[g1]

It should come down on it's own. The 80mV is for 2 tubes on each side (push and pull). When you remove 2 tubes it should drop to around 40mV, when you run at low power switch setting, the reduced plate voltage will cause the mV reading to drop even further.
So you shouldn't have to make any bias adjustments, but you may want to tweak the balance adjustment.

[paranoidpete]

Fender bias dilema

It should come down on it's own. The 80mV is for 2 tubes on each side (push and pull). When you remove 2 tubes it should drop to around 40mV, when you run at low power switch setting, the reduced plate voltage will cause the mV reading to drop even further.
So you shouldn't have to make any bias adjustments, but you may want to tweak the balance adjustment.

Hello gents , Thanks very much for your advice on this. Being a geriatric rocker I really don't know the in and outs of tube amps and I'm to old to start learning, ha-ha, but I can use a DMM and figure out the simple stuff.
With all 4 6L6's I biased to 80 mV and set the balance to 0 mV . I then removed the center pair and rechecked. The bias reads 35 mV and balance was still 0 mV. Testing the amp with a Strat plugged in, it sounds just great, so I presume that this will now do the trick !
Just for your interest my other later model Twin is fitted with a quad of 6V6's. That also works really well although on that occasion I had an amp tech do the conversion. Keep rockin, Pete (U.K.)