BF Showman speaker outs? (impedances)

[hawkeyeinexile]

so, IIRC my '64 Showman was designed for a 1x12 or 1x15 cab at 8 ohms. it has the original iron & stuff throughout.

is the Ext. Speaker wired to add a cab in series or parallel? i.e., if i run an 8 ohm cab from the Speaker jack and add an 8 ohm cab from Ext. Speaker, am i halving or doubling the impedance?

i don't know if the OPT can handle 4 ohms or what it was designed to handle...

two maestro amp pros have given me different advisements: one says don't go under 8 ohms, the other says it'll handle 4-16 ohms

(thanks in advance)
(yeahman)

[pdf64]

The 'ext speaker' socket is in parallel with the main speaker output.
So if you plug 2 x 8 ohm cab to your amp, it will be driving into 4 ohms.
It won't operate as efficiently, so the power output will reduce a little, but it shouldn't cause a problem / damage to your amp.

'two maestro amp pros have given me different advisements: one says don't go under 8 ohms'
That's the rule for solid state amps. Tube amps are different (because they are connected to the speaker via an output transformer).

'the other says it'll handle 4-16 ohms'
That's the generally accepted rule of thumb for tube amps, though going over the nominal load impedance (eg 16 ohm cab on a 8 ohm rated amp), and overdriving the amp, can put additional stress on to the output tubes.

[gldtp99]

Yes, a BF Showman should have an 8 ohm OT and a BF Dual Showman should have a 4 ohm OT.
But it doesn't always work that way in the real world---- for whatever reason, i've seen 125A29A (4 ohm OT) and 125A30A (8 ohm OT) in both BF Showman and Dual Showman heads.
The best way is to check the p/n on the OT in your amp (in a BF Showman this is the # that starts with "125....." not "606.....")---- then you'l know for sure which OT you have and since the two speaker jacks are wired in parallel---- you'll also know how to calculate total speaker loads of various cab combinations.
PS---- if the p/n of the OT in your BF Showman amp starts with "022......."-- then you have an OT pulled from a later amp or a "factory replacement".............gldtp99

[hawkeyeinexile]

thanks, fellas. i'll open 'er up later when i get a chance.

for now, have a Mather cab, slant-front 2x12 with a switch to go upper speaker only (EVM-12L) for 8 ohms or both (add Cele G12-65) for 16 ohms. sounds pretty great (thanks to Val King)