What to do with these old transformers??

[BiBi]

Hello,
I recently picked up an amp that was an old Italian guitar head. Ridiculously large cabinet, and it used a pair of EL503s which are far out of my price range to replace. The transformers appear to be of very high quality at least. The power transformer secondary has a 290-0-290 winding, a bias supply for the El503s which is around 25V, and ss diode rectifiers, so just a 6.3(around 4amps) winding. The output transformer is a bit on the strange side. I've got the standard three wires on the primary, which when used as I got it showed 2k5@16ohm on the secondary. The secondary has a total of five wires. Two wires are a separate secondary winding which reflect around 6k@8ohm. The third unused wire is on the same winding as those used originally and one connection has 8k@4ohm, while the other has 5k@16ohm.
I'm open to ideas at the moment but my preference is probably something that's cathode biased (I'm not so keen on putting a new bias trafo in). If anyone has a suggestion on an output tube pair that would fit these ratios/voltages it would be much appreciated.
My first inclination is to try something with a pair of 6L6s at 380VDC into 5k winding. I would think they are better suited to the lowish voltages than say an EL34 but that may just be my lack of experience...

[Enzo]

Did you already strip the transformers from the old amp? I wonder if the extra windings on the OT might be dedicated NFB windings. Not all that common, but you see it from time to time. By tracing the wiring of the old amp, you should findout what the extra wires are used for one way or another.

Nothing wrong with cathode biased power stages, but if your existing bias winding is too low in voltage, a voltage doubler is a very simple way to get all the voltage you need.

[BiBi]

Thanks for the reply. I needed to clean the chassis as it shows a lot of rust so yes, I took the transformers off the frame. The three "extra" wires on the output secondary were actually wrapped and tucked away in what looks to be the original factory "setting". The amp itself didn't use any negative feedback at least not on the one I got, nor a later model which I found a schematic for.
With that separate winding on the secondary unconnected, I would have thought it would somehow damage the output transformer.
Good tip on the voltage doubling, I'll check to see if it can supply the current necessary for that.

[Kagliostro]

Which is the amp

is it a FBT ............

http://www.chambonino.com/modify/mod1a.html

FBT 1200R Amplifier Tube Conversion

anyway you can convert it to use other final tubes

hope this can be of some interest

and curious, please post the name of the amp

I've FBT 500-R2 and FBT 1200R schematics that use EL503, so, if you have interest ask

Kagliostro

[Enzo]

Current demand on a bias supply is minimal. Typically the largest user of current is the voltage divider resistors themselves. That ain't much.

[BiBi]

Hi Kagliostro,
Yes in fact it's an FBT 500R so without the 2. I also found the schematic for the second version and there doesn't seem to be much of a difference except for the output impedance-- my guess is there was originally a cabinet with two 8ohm speakers. Thanks for the link to the Champ electronics site-- the EL36 is a very interesting alternative although I'd be a little afraid of anode caps in an open cabinet!
Enzo,
I was under the assumption that anytime one used a voltage doubler the current was cut in half? Was that just a rough estimate or am I actually wrong?
Thanks for the replies!

[borislavgajic]

please can enyone post schematic of FBT1200R tube amplifier.......?

Ireally need it

thanks
Boris