Bassman ohm question

[DieselMouse]

Hello, What is the speaker ohm rating for a 1967 50 watt bassman head, Is it 4 ohms? I have searched the net but could find little info.
Thanks, Mike Smith

[OrangeFlavorBeef]

You'd think they would have marked that SOMEWHERE on the friggin' amp, huh? But yes, it's 4.

Now, I've never run more than one cab, but I *think* that if you're running out of both jacks simultaneously, it's 8...but that's what I've been TOLD. Someone please confirm or correct this...
(that's the poop on the Fender Amp Field Guide anyhow...)


(we're talking about the AB165, right?)

[Casey4s]

Originally posted by OrangeFlavorBeef
You'd think they would have marked that SOMEWHERE on the friggin' amp, huh? But yes, it's 4.

Now, I've never run more than one cab, but I *think* that if you're running out of both jacks simultaneously, it's 8...but that's what I've been TOLD. Someone please confirm or correct this...
(that's the poop on the Fender Amp Field Guide anyhow...)


(we're talking about the AB165, right?)

The two jacks are wired in parallel in the amp. So if you used two cabinets with that head, at 4 ohms each, the result would be 2 ohms, not 8.

[DieselMouse]

Thanks for the answers guys, why they just couldn't print a 4 by the plug is a mystery.

OrangeFlavorBeef- Yes I believe it has the AB165 circuit, Ive read some info on them
but I just got it today and dont know too much about them.

[Tim C.]

Casey4's is right. It would be a 2 ohm load using both jacks as they would be wired in parellel.

Tim C. :)

[tom3k]

According to Gerald Webber, Fender trannys were designed to handle impedance mismatch up to a factor of 2 - in the case of a 4 ohm amp, you're supposedly safe as long as the load is no less than half (2 ohms) or double (8 ohms). This is presumably why Fenders could have parallel ext. speaker jacks without an ohm load selector.

My sister's BF bassman is the one model that doesn't even have an ohm load listed on the schematic (AA68?)! I've always assumed it was 4 ohms. Is there any way to measure the impedance load on an unmarked amp?

[Casey4s]

Originally posted by tom3k
According to Gerald Webber, Fender trannys were designed to handle impedance mismatch up to a factor of 2 - in the case of a 4 ohm amp, you're supposedly safe as long as the load is no less than half (2 ohms) or double (8 ohms). This is presumably why Fenders could have parallel ext. speaker jacks without an ohm load selector.

My sister's BF bassman is the one model that doesn't even have an ohm load listed on the schematic (AA68?)! I've always assumed it was 4 ohms. Is there any way to measure the impedance load on an unmarked amp?

I have a reference on Fender Transformers, and if they are original stock transformers, it is definatly 4 ohms.

But to actually measure the intended output impedance of a transformer, you will need to disconnect the OT and apply AC voltage to the secondary and measure the result on the primary to determine the turns ratio.

See if this link helps.....

Measureing an unknown OT