Is it possible my Bassman 6G6-B wants 8 ohms?

[devbro]

I swear it's louder with an 8 ohm cabinet than with 4. I heard some Vibrolux heads from '63 had 8 ohm output tranny's. Could my Bassman have gotten one of these by mistake? Is there a way to test for what load the output tranny is looking for? Here's the code from the output tranny.
http://www.masterlynx.com/images/bassman/63_bassman4.jpg

[Old Tele man]

...according to the specs' for 125A13A, it's a 4-ohm output!

...with Leo Fender, DUAL speaker amps used 4-Ohm OT's while SINGLE speaker amps used 8-Ohm OT's because the SPEAKERS that he used were 8-Ohms each.

...so, if your Bassman had ONE speaker it probably used an 8-Ohm OT, but if it had TWO speakers (typical) then it used a 4-Ohm OT.

[devbro]

Thanks Tele. Actually, after trying the 4 ohm load live, I think it is louder. This may of course be due to switching from a Heritage 30 and Lead 80 (8 ohms) to a pair of EV12L's (4 ohms). Those 12L's are spanky clean:dude

[squeally dan]

Is it ok to run a bassman head with an 8ohm cab? WIll it damage anything if the amp is 4ohm?

[devbro]

I ran mine at 8 ohms for 5 gigs and it didn't hurt anything.

[Old Tele man]

...running 8-ohm speaker load on 4-ohm OT is OK, but anything much higher (16-ohms) is risking "possible" tube and OT damage when/if "fly-back" should accidently happen.

...FWIW -- the above is why many european amps have "reverse / overvoltage" clamping diodes across the primary windings of the OT.

[donnyjaguar]

OTM is probably right but you can test the turns ratio and calculate the impedance ratio if you'd like to confirm. This basically entails pumping 120Vac into the primary of your output tranny (tubes and speaker removed) and measuring the resultant voltage on the secondary. This can be a little dangerous for those not comfortable working with high-voltages.

[Old Tele man]

...the "transformer" equation is: Zs / Zp = (Ns / Np)^2

...or: Zp = Zs * (Np / Ns)^2

where:
Zp = Primary impedance, ohms
Zs = Secondary impednace, ohms
Np = Number primary turns, #
Ns = Number secondary turns, #

...however, Np/Ns-ratio is also called the "Turns-Ratio" (TR), so the equation is also seen as:

...or: Zp = Zs*( TR )^2

...so, if we assume Zp = 4K-ohms (for 2 x 6L6) and Zs = 4 ohms, then TR = 31.6:1, that is: there are 31.6 primary turns for every ONE secondary turn.

NOTE: turns and windings are synonymous here.

...now, if you put a 1VAC peak-to-peak (pk-pk) signal into the SECONDARY, you should read a 31.6VAC pk-pk signal across the PRIMARY.

...however, a 4K:8-ohm OT would have a TR = 22.4:1 and the same 1VAC signal into the secondary would produce only 22.4VAC pk-pk across the primary.

...unfortunately, this technique ONLY works with an AC-signal -- transformers do NOT "handle" DC-voltages -- so, this technique isn't something noobe's would normally tackle.

HTH!