Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shooter on April 23, 2015, 03:34:56 pm

Title: totally crazy build
Post by: shooter on April 23, 2015, 03:34:56 pm

 have an old build that needs repurposed, was a stereo KT88 5c1-ish build, too many issues soooo
I was thinking of doing a PSE KT88 with plexi style pre.  my question, can I parallel the SE-OT's together.  One is an Edcor CXSE25-8-5K and the other is a Hammond 1628SEA.  Both are 5K to 8ohm 25W.  I'm pretty sure Mr. Ohm is ok with it, but, not totally sure.  the KTs would want 1/2 the impedance for PSE and the OT's in || would be 1/2 the 5K?.  would the speaker also now be 4ohms?  which is good for a 2X12.  Thanks for any input, ya, it'll be a pig :icon_biggrin:

Title: Re: totally crazy build
Post by: PRR on April 23, 2015, 08:56:43 pm

Paralleling two *different* audio transformers is unlikely to work. If the turns ratios are not *exactly* the same, they fight each other and throw a too-low impedance at the tubes.

Title: Re: totally crazy build
Post by: shooter on April 23, 2015, 09:41:56 pm

Thanks PRR, I knew something didn't smell right.  The 5k impedance is on the high side so I have some margin, but I don't wanna mess up close to $200 in trannys.

Title: Re: totally crazy build
Post by: PRR on April 24, 2015, 10:38:06 pm

The OTs would not be "messed up"; the amp could be way down on power (less than if you only used one tube and OT).

I realized: leave the primaries un-paralleled, each to its own plate. Parallel the 16 Ohm secondaries to get an 8 Ohm output. This way, any 1%-10% difference of turns-ratio is a small (maybe inaudible) change in clipping action.

Title: Re: totally crazy build
Post by: shooter on April 25, 2015, 11:22:14 am

 

Quote

leave the primaries un-paralleled

Thanks PRR, I went web fishing and found a link to a 140 page pdf written in Italian!  but the pic's showed a similar config,  Also there was a P-P config, wiring the 2 SE OTs "like" a P-P OT and running the output side in series.  There were a lot of complex math and complex words though :dontknow:

I was going to build the PA section separate from the pre so I might get all the pieces parts mounted then experiment with both types, SE 2 tube 2 tranny separate primary and PP config with 2 SE trannies.  I'll draw up a working schematic 1st and hopefully use the brain trust here to get close.

Thanks again for the help
dave