1973 Fender Twin Reverb Bias

[ca7922303]

Would like some help biasing '73 twin reverb. Thanks.

[Chuck H]

Model number? Schematic? Do we need to to look through the available info until we find the right model and then look up the schematic ourselves to help you?

[ca7922303]

Model number? Schematic? Do we need to to look through the available info until we find the right model and then look up the schematic ourselves to help you?

Not sure of the model/circuit number. Maybe photos will ID it.

[Chuck H]

My computer is too slow to load images right now. Besides, '73 was a VERY transitional year for this amp. It could be any of three different amps actually. Why am I looking at a spaghetti wired chassis photo? What model? How do you know it's a '73? Schematic? I'm a half hour in and I can't get definitive info to reply!?! How much time to invest in OPP? I care, don't get me wrong. But this is YOUR problem. Make it easier for us to help you.

[ca7922303]

My computer is too slow to load images right now. Besides, '73 was a VERY transitional year for this amp. It could be any of three different amps actually. Why am I looking at a spaghetti wired chassis photo? What model? How do you know it's a '73? Schematic? I'm a half hour in and I can't get definitive info to reply!?! How much time to invest in OPP? I care, don't get me wrong. But this is YOUR problem. Make it easier for us to help you.

73 is stamped on chassis in 1st photo and on bottom of transformer.

[Chuck H]

Ok. Sorry for being grumpy. But the AA270 and AC568 schems are the most likely matches to your amp. The AA270 seems to have a dual wiper for the balance pot unless I'm reading the schem incorrectly.?. And the voltages are illegible in the available AC568 schem anyway. Volume wars were going on at this time and the AA270 schem lists 405 Vp for this model. It's likely then that the AC568 would be higher in the spirit of change at that time. And this could be significant to the circumstances.

Just to ask... Why are you having trouble biasing the amp? Did you install new tubes? Is it not biased correctly then? Have you measured the bias as it is? Can you give more of a report about any trouble you're having?

[ca7922303]

Ok. Sorry for being grumpy. But the AA270 and AC568 schems are the most likely matches to your amp. The AA270 seems to have a dual wiper for the balance pot unless I'm reading the schem incorrectly.?. And the voltages are illegible in the available AC568 schem anyway. Volume wars were going on at this time and the AA270 schem lists 405 Vp for this model. It's likely then that the AC568 would be higher in the spirit of change at that time. And this could be significant to the circumstances.

Just to ask... Why are you having trouble biasing the amp? Did you install new tubes? Is it not biased correctly then? Have you measured the bias as it is? Can you give more of a report about any trouble you're having?

Just changed power tubes in it and haven't checked any voltages yet. Thanks for the help.

[Enzo]

Chuck, that is not a dual wiper, it is a wiper and a center tap.

[Chuck H]

Chuck, that is not a dual wiper, it is a wiper and a center tap.

Ah. That makes better sense. But the schem shows two "arrow heads". I would have expected a "dot" and an arrow head to represent a tap and wiper. At any rate, it's good to have clarity on the matter.

[Enzo]

Well, my copy is probably like yours, kinda fuzzy from copy of copy. I see it as an arrow up top and a dot underneath.

[Chuck H]

Ok. I can load images today (for the moment). I can see a tap on the bias balance pot, but it doesn't look to be used and the attachment of the components on the pot seems to match the AC568 schem. Unfortunately that schem on Schematic Heaven is awful and blurry so much so that you cannot even read most component values. Let alone voltages.

[ca7922303]

Ok. I can load images today (for the moment). I can see a tap on the bias balance pot, but it doesn't look to be used and the attachment of the components on the pot seems to match the AC568 schem. Unfortunately that schem on Schematic Heaven is awful and blurry so much so that you cannot even read most component values. Let alone voltages.

Can you zero me in on this bias balance pot. I'm not seeing it. Thanks.

[Chuck H]

...

[Chuck H]

There's a slot for a flat head screwdriver bit recessed in the pot case. This is NOT bias adjustment pot in the typical sense. It adjusts the balance between the tubes. If you find you need to raise or lower the overall bias for all tubes that would be done by changing the value of the 15k resistor soldered to the balance pot case.

[ca7922303]

There's a slot for a flat head screwdriver bit recessed in the pot case. This is NOT bias adjustment pot in the typical sense. It adjusts the balance between the tubes. If you find you need to raise or lower the overall bias for both tubes that would be done by changing the value of the 15k resistor soldered to the balance pot case.

All four(4) power tubes pin #3(442vdc).