Power transformers

[passfan]

I came across a couple of transformers I got out of old organ chassis and I would like to run them by you guys to see if you think they would work. The first one is 300-0-300 unloaded. It has a 6.9v heater winding center tapped a 5 volt winding and a 45 volt winding center tapped. It weighs right at 9 lbs on the button.
The second one is 350-0-350 unloaded with a 13 volt winding center tapped and a 5 volt winding it weighs in at just a hair under 10 lbs.

The three output transformers as best I can tell were a

6600K running a quad of EL84's
6000K running a pair of EL84's
7600K running a pair of 6L6's

I shot 100 volts through the primary of each and measured the output , squared the turns ratio and multiplied by a secondary impedance of 8 ohms to come up with these primary impedances so they may be a little off. All of these transformers look like they were rarely used. Both chassis seemed original and the fusing was intact and unblown. I got all this and a couple of chassis for around 100.00 bucks. I haven't gotten to the multicaps and resistors yet.
Any advice on what to build with which transformer would be greatly appreciated. I had my heart set on an express and possibly a liverpool.

[RJ Guitars]

Passfan,

300 volts unloaded @ 250-300mA is just right for an Express build. All the transformers in Trainwreck amps are big beefy units so you might have something about right as far as the specifications go.

Ken Fisher also seemed to prefer output transformers that were rated somewhat above the normal impedance match for a given tube set. In his Liverpool (4 EL-84's) he used a 5.2K input impedance and in the Express (two EL34's) he used a 6.6K input impedance. It's not entirely clear why he chose transformers with those values but his reputation for making great sounding amps needs no defense.

keep us posted on what you build,

rj

[guitardude57]

Wow, nice score. Chances are the Pt's have ample current to drive about any amp circuit you would want. Depending on the type of organ, some of them had anywhere from 30-40 tubes for preamps, power amps and oscillators. If they had electronic oscillators, the current would be much lower, and you would need to find more info on the Manufacturer to know the Ma's available.

Obviously, the one with 5 volt is a Tube recto feed, and the 45 volt is a bias tap.
The other one has a 12.6 volt for heaters.....instead of 6.3

Either PT could power large or small bottle amps.

The OT's..... same.

Chances are the 2 X EL84 is an 8K, but can be anywhere from 6-10K.
"" "" "" "" 4 X EL84 is a 4K, but again has breathing room.
6L6's anywhere from 2.5 to 8K depending on design

[rfgordon]

It's important to have some idea of the current capacities of the different windings on the PT. If you know the organ amp model they came from, take a look at the schematic and note the types/numbers of tubes. Look up all the plate and heater currents on datasheets. That number is a good reference.

In my experience, however, vintage PTs are built to a better standard than most modern ones, and many commercial applications were nowhere near their true capacity.

One exception to that is the 5V rectifier filament winding. My practice, when using vintage iron, is not to use a 5U4 unless the original application called for it, as the 5U4 draws 3A filament current. Many other recto tubes draw 2A or less.

[passfan]

Many thanks for the good advice and suggestions guys. Rich , both chassis came with 5u4 rectifier tubes installed. Cool thing about the deal is I got all the tubes:

6-EL84 baldwins with green lettering
2-12ax7a Sylvanias with yellow lettering
2-12ax7 baldwins with green lettering
1-5u4g baldwin with green lettering
1-5u4g unknown with yellow lettering and shiny black plates
1-12au7 unknown with long shiny black plates
1-12at7 unknown with short shiny black plates
2-6l6gc unknown with long shiny black plates.
1-oa3 RCA

The unknowns came out of a Conn organ and had Conn in yellow lettering on the bases. All of them work and some of them are in my deluxe right now. Since I have absolutely no idea what chassis numbers these were I thought I would restrict myself to only running what was on the chassis. One transformer was running all 6 84's and 4 12's through two output transformers with a 5u4g for power. The other one was running the 6l6's and the 12au and 12at through one output transformer with a 5u4g. I think I will be alright if I stay under those parameters.

[passfan]

Passfan,

300 volts unloaded @ 250-300mA is just right for an Express build. All the transformers in Trainwreck amps are big beefy units so you might have something about right as far as the specifications go.



keep us posted on what you build,

rj

Just curious RJ , I see references on dropping a power trannys voltage by using a zener as a reference on the center tap. How does that work and could it be possible to drop the 350 volt down to 300 and the 300 down to Liverpool voltage ? I'd really like to build both amps as I always overbuy parts to have some for the bin afterwards.

[RJ Guitars]

Passfan,

300 volts unloaded @ 250-300mA is just right for an Express build. All the transformers in Trainwreck amps are big beefy units so you might have something about right as far as the specifications go.



keep us posted on what you build,

rj

Just curious RJ , I see references on dropping a power trannys voltage by using a zener as a reference on the center tap. How does that work and could it be possible to drop the 350 volt down to 300 and the 300 down to Liverpool voltage ? I'd really like to build both amps as I always overbuy parts to have some for the bin afterwards.

I've done it and I can attest it works pretty darn slick... but that was a one shot experience that went well. I suspect there are others out there that could add more technical detail and experience.

I needed to lose 13 volts off of a big stancor transformer and this is what I used... http://www.mouser.com/ProductDetail/NTE ... ybyw%3d%3d . I know of one other guy that I'll ask to respond, he is doing this very thing right now on an ODS style build.

I based my efforts on page 222 of Gerald Webers "Desktop Reference of Hip Vintage Guitar Amps". I did deviate from his recommendation in that I chose a threaded cathode to ground iteration of the diode rather than use the threaded anode part number that he gave.

rj