PP output trannie from 2 SE trannies?

[Greg Smith]

Just to clarify my intent, I'm wondering if two SE output transformers could be wired with the primaries in series to make a PP output transformers.

I have a number of pairs of SE transformers kicking around, all scavenged from 60s era hi-fi units, and I'd like to wire two of them together in a PP output stage for a guitar amp. They are designed for 6BQ5 (EL84) tubes, so I'm thinking of putting together something along the lines of a Marshall 18 amp.

My thinking is that you could wire the plate leads to the plates (as normal), the B+ leads go to B+ (as normal), and the secondaries could either be wired in series or parallel, depending on the desired output impedance.

A bit of a cheapskate effort I'll admit, but who wants to blow $40 on an output transformer if they don't have to?

Greg

[azazello]

Connect parallel it's like "short" the outputs, if You connect serial /in case, par ex.,
8 ohm for every output/ You must connect 12-16 ohm speaker. If You connect 8 ohm, it will decrease Ra for tubes. Sound will be with more distortions.
In both cases it's not normally! I tried, in serial, sound was better, You can try!!

[Greg Smith]

My thinking is that, if the secondaries are connected in parallel and IN PHASE, then the secondaries would not be short-circuited, but the output impedance would be half the impedance of either of the transformer secondaries.

For example, two 8 ohm secondaries connected in parallel (and in phase) would match to a 4 ohm speaker load.

This is all hypothesizing, though. I need to try it out (when I have time).

[azazello]

Connect parallel output coils/half impedance/ is not the same like output coil for 4 ohm. If We count output coil according primary and alfa, number
of turns for 4 ohm doesn't give half impedance..../if nomber of coils for 8 ohm is, par.ex., 120, number for 4 ohm is 80-90, but not 120/2=60!! for half impedance/. In our case parallel conn. is for aprox. 2-3 ohm.
Good luck!

[BudP]

Should work fine if you can work out the impedances and keep the phasing correct.

I know this works for power trannies; I built a dual-voltage high voltage supply by doing the same thing with a pair of similar microwave power transformers - all of which I've ever seen having one side of their secondaries grounded to the frame. Since a full-wave-bridge rectifier circuit has the secondary center-tap grounded anyway, I figured all I had to do was to get the primaries phased right, and feed the "high" end of each secondary to its own 3B28 Xenon rectifier. Using a DPDT switch to either series or parallel the primaries, I ended up with a monster 1.4kv - 2.8kv supply.

Bud

[mesherm]

What you would have is the equivalent of two SE amps strapped together with the inputs 180 degrees out of phase. Not the same as true push-pull because the magnetic field produced from each tube's idle current doesn't cancel out. But since the OTs are gapped for SE use it will still work. Configure the driver circuit like that for a PP amp feeding each SE tube with opposite phase signals and connect the OT speaker outputs in series.

[Greg Smith]

Thanks for your responses on this. I'm not sure when I'll actually get around to trying this, but it's good to have this sort of information at hand.

If I connect the secondaries in series, am I going to wind up with an output impedance that's the sum to the two secondaries (8+8=16 ohms), or do the out-of-phase primaries leave me with the output impedance of only one of the transformers (impedance stays at 8 ohms)?