using a 4 ohm extension cabinet with 1993 usa blues deville

[bluestom50]

From what i see on the schematicit calls for a 8 ohm extension cab. What will happen if i use a 4 ohm cab---will it work ok or will it harm the amp or tubes. When i get intothe ohms law etc it just confuses me. Any help will be appreciated, thanks, tom

[Mark Black]

When you only have two impedances to deal with you can use a more abbreviated formula rather than all the reciprocals needed for 3 or more impedances. I'll use "R" for resistance since it's the same formula:

R1 times R2, divided by R1 plus R2.

So 8 times 4 equals 32, divided by 8 plus 4 equals 12. 32 divided by 12 equals 2.67 ohms.

Plugging into the extension jack transfers amp output from the 8 ohm output transformer tap to the 4 ohm one (since the amp expects an 8 ohm extension cab, same math above gets you 4 ohms with the 8 ohm internal plus 8 ohm extension).

Will it hurt the amp? Probably not - I have done the same on my own early 90's Blues DeVille, but didn't care for the sound and the way the extension cab hogged more of the output.

[Steve A.]

When you only have two impedances to deal with you can use a more abbreviated formula rather than all the reciprocals needed for 3 or more impedances.

I use my Windows calculator all day long because it has a reciprocal key (1/x). To compute the total resistance of 2 or more resistors of unequal value in parallel (or 2 or more caps in series***) you take the reciprocal of the sum of reciprocals.
(*** With more than 2 caps in series I will do 2 caps at a time just to be safe. But I rarely run into a situation with 3+ caps in series.)

I usually use that formula with just two resistors, quite often to determine the value of a resistor to add to an existing resistor to create the resistance I want. In that case you take the reciprocal of the difference between the reciprocals of the existing resistor and the desired resistance. (Ignore the minus sign if you get a negative number.)
Yes, I know that sounds confusing but with a calculator with 1/x, M+ and M- keys it is much easier to do than to describe.

Steve Ahola

P.S. To answer the OP's question, if he disconnected and taped off one of the leads going to the internal speaker there would be no impedance mismatch when plugging in a 4 ohm ext cab.