OT with 64-ohm secondary ... any use for it?

[krash]

I have a pair of old military theater amps that ran 6550s for 30W of power each, into 64 ohm output. I figure the primary is something like 4-6K. is there any use for these trannies? They are enormo.

I figure if you hook them to a regular 16 ohm speaker that'd be reflecting way too small an impedance back to the tubes. I guess you could run a series 4x12

[mhuss]

I like the 'four 4x12 cabs in series' solution.

There's no easy way to make those useful that I can think of, short of rewiring a 4x12 cab with all four speakers in series.

--mark h

[Bob-I]

I'm not a fan of series wiring. If a speaker or a connection opens, you lose all the load and possibly the OT.

[unklmickey]

hi Josh,

i can think of something, but it's totally perverse. you'll probably think me nuts for even thinking it, but, hey that happens a lot.

without doing the math, the primary impedance you would use with an EL-84 is probably about double what you would use with a 6550.
(i know there are other factors to consider, but this probably won't be going anywhere, anyway.)

if you put 8 ohms on the secondary, your primary impedance will be 1/4 of what you originally had. so you could use 8 EL-84s instead of 1 6550.
you didn't say how many 6550s were on each transformers, but whatever that is multiply that by 8.

yeah, i know it would look strange to see all those little tubes with that big iron, but i warned you it was perverse, didn't i?


well it was good for a laugh anyway.


cheers,

unk

[krash]

well, that's one of the wilder ideas I have heard. Yeah this was a push-pull amp so that'd be 16 EL84s. Try retubing the amp for $200!

anyway.

Some other ideas I had was to wire up a 100 or so ohm power resistor in series with the speaker output and run a pair of 6V6's, getting a total of like 1.5 watts actually into the speaker and the other 20W into the resistor.

Or the other way around ... put a 7.2K or so resistor in series with the OT primary between the primary and the tube socket so the impedance (resistance plus impedance) would look like 8K even though it was actually only reflecting back like 1K.

Neither is a very good idea though.

Probably best to make them into a matched pair of big paper weights.

[unklmickey]

i agree that the idea of using resistors to try to match the impedance is not so good.


if the transformers are as big as you made them sound, your papers will stay put in the next hurricane.

[VacuumVoodoo]

You could use a dedicated impedance matching transformer between the speaker and the transformer you have. IIRC Edcor still make these.

[cobalt]

Just thinking, but, if the tranny was push-pull for 6550s at 30W, primary is likely 3k(class A around 300V).
How about push-pull par EL86?
That would make the primary around 1.5k and secondary 32, at 50W.
Not perfect but doable.