Will this work Se bridged type

[dstrat]

nevermind.

[Dave H]

I want to see what it sounds like, a SE amp or a PP amp, do i get crossover distortion like a PP 6bq5 amps get, is the output wattage the same as PP or the same as PSE.

I'll take a guess at some of those. It will sound like an SE amp as it's two SE amps in series. Both amps are single ended class A so there's no cross over distortion. It's not a push pull amp. I think the OT primary halves would have to be wound on the same core for it to be push pull. It's two SE amps with their outputs in series. The output will be twice the power of a single amp into double the load impedance i.e. if the single amps were 4W into 8 ohms the series amps will be 8W into 16 ohms. They could also be wired in parallel for 8W into 4 ohms.

[nickb]

I'll take a guess at some of those. It will sound like an SE amp as it's two SE amps in series. Both amps are single ended class A so there's no cross over distortion. It's not a push pull amp. I think the OT primary halves would have to be wound on the same core for it to be push pull. It's two SE amps with their outputs in series. The output will be twice the power of a single amp into double the load impedance i.e. if the single amps were 4W into 8 ohms the series amps will be 8W into 16 ohms. They could also be wired in parallel for 8W into 4 ohms.

I'm not so sure. I think that as drawn the two SE sides are working in complement i.e. as the current rises in one it falls in the other. That is push-pull by any definition.

Now, you could drive the each SE stage with the same phase and change the phase of one of the transformers. In this case it will sound like an SE stage.

[nickb]

You may want to look at the BiValve for inspiration - it was made by THD and has 2 SE stages feeding separate primaries of the OT.
You're using 2 OTs but the idea is similar.
I don't think it has a PI.

http://www.thdelectronics.com/BiValve_Manual_080105.pdf

Here's the schematic.

I've seen this arrangement before and never understood why the two primary windings. Why not just connect the tubes in parallel and have a single winding?

[Dave H]

I'm not so sure. I think that as drawn the two SE sides are working in complement i.e. as the current rises in one it falls in the other. That is push-pull by any definition.

I'm not sure either
In a push pull amp don't the two tubes have to be pushing and pulling on the same load? They are coupled by the transformer core in a normal pp amp but there's no coupling via the core with two separate transformers.

[Dave H]

The heater supply doesn't look right on the schematic in #19. It has both the centre tap of the winding and bridge rectifier -ve grounded, that shorts the left side diodes of the bridge across both halves of the winding.

[mozz]

Thanks for posting that schematic. I'll have to spend some time trying to see what is going on. Another question. How can i see, on a scope, if it is acting like a SE amp or PP? I have a Syscomp CGR202 so i can bode plot the freq response and the phase shift. The current draw from the mains should remain the same at whatever level the amp is driven to. Maybe. Or is the plate current gonna change due to the secondary load will be changing(if it even is gonna do that?). What is it gonna look like when it does distort? I have pretty much wired up a Princeton reverb tonight (without the reverb) for a front end. Concertina into 2 amps, will be 6GK6 (13.2w diss tube) at about 310v plate & screen , individual cathode biased with 150r/100uf. Just have to mount the pots and wire to the sockets, another day or 2 before i fire it up.

[nickb]

I'm not sure either
In a push pull amp don't the two tubes have to be pushing and pulling on the same load? They are coupled by the transformer core in a normal pp amp but there's no coupling via the core with two separate transformers.

They are coupled as the same current runs through both secondaries.

[nickb]

Thanks for posting that schematic. I'll have to spend some time trying to see what is going on. Another question. How can i see, on a scope, if it is acting like a SE amp or PP? I have a Syscomp CGR202 so i can bode plot the freq response and the phase shift. The current draw from the mains should remain the same at whatever level the amp is driven to. Maybe. Or is the plate current gonna change due to the secondary load will be changing(if it even is gonna do that?). What is it gonna look like when it does distort? I have pretty much wired up a Princeton reverb tonight (without the reverb) for a front end. Concertina into 2 amps, will be 6GK6 (13.2w diss tube) at about 310v plate & screen , individual cathode biased with 150r/100uf. Just have to mount the pots and wire to the sockets, another day or 2 before i fire it up.[ATTACH=CONFIG]47734[/ATTACH]

I would look at how it clips. If it's symmetrical it's push-pull.

[Dave H]

I would look at how it clips. If it's symmetrical it's push-pull.

I think it will clip symmetrically if a PI is used so it will be about 8W push pull in class A then? I was getting confused with my standard push pull class AB 6bq5 amp being 16W with the same B+ voltage

[nickb]

Just threw this together from Fender schematics, all wiring is not shown or complete. Just looking to see if the idea is correct.

[ATTACH=CONFIG]47684[/ATTACH]

Moz - I given this some thought and there is a fundamental problem that I just didn't spot earlier.

The secondaries are in series so the load current flows through each and so drives the corresponding tube's plate. Large transformer destroying sized voltage can appear on the plates. The biggest problem occurs where the signal has the highest rate of change.

This is what I see in simulation using EL34's and a 400V supply with a 100% overdrive on the output and plates:



If order for this to work reliably you need to limit the input so the tubes never get to cut off. This naturally changes the fundamental tone so you will never hear the actual sonic character of the tubes being over driven. All things considered, I don't think this is a good arrangement.

There are plenty other dual tube configurations you can use which don't have this problem. Using two EL34's in class A the output is around 20W for all of them.

1) Current arrangement is push pull with transformers in series, other's are
2) Push Pull with secondaries in parallel
3) Single ended (both tubes are driven from the same signal phase) with secondaries in parallel
4) Single ended with a single transformer with two primaries
5) Single ended with a single transformer with tube in parallel and a single primary.

Here are the output voltage, frequency spectrum and plate waveforms for these operating in class A with 96% of max drive signal.

Dual Transfo Results.pdf

PS: Right now I don't have a clear understanding of why these voltage spikes happen but it has something to do with the non-linearity of the tube . With real transformers, as opposed to the ideal ones I used, the leakage inductances will make the voltages even bigger.

[Dave H]

Thanks, I was hoping you'd do a simulation

[mozz]

Well, after fixing a few bugs in my fast wiring job, you are right on the money. It does work at low levels, after the volume is turned up a bit, it clips very nasty, much like the simulation and does some wired stuff like a square wave in between another square wave. Also some nasty oscillation like i have never seen on a scope. Meter monitoring my plate voltage starts to go crazy.

I actually later put a 12au7 as V2a&b and it helped to give more range on the vol control but still the same problems. I thought i had a wiring problem but scoped throughout it was textbook clean, preamp voltages are easily within fender spec'd, the PI does distort a little but driving the tubes hard it just can not come close to taking it. I tried all possible combinations on wiring the output transformers as i thought my problem was there but going back to + and + to speaker and both gnd's to gnd was the only way to get any power out, maybe 3v rms to dummy load 16ohms.

Mains current draw was about 74 watts idle, when i turned it up the draw actually went down to 65 watts.
.
Another problem was a bad motorboating at 1hz or less Tubelab over on the DIYAUDIO.com website had also mentioned this. If i fire it back up will take some scope screen shots. Looks like my idea does not work, this time, so maybe going to convert it to parallel and try again.


EDIT: Before i tear it down, i want to try 8 ohms , just to see it, and i want to put a old analog meter on the plate voltage to see what was going on there also. I forgot to measure the output trannys ratios, i thought they were 8k but maybe it was 5k, just for reference. Plates were only 265, screens 260, individual cathode resistor 150r was about 8v. Just put that in the bias calc and for 13w tube it is 100%, that should not have made any difference though.

[mozz]

Well, finished it up for now. I haven't totally given up, next try will be diving 2 speakers,one with leads swapped, so that might work. I ended up getting rid of the phase splitter and just running 2 single ended with the outputs in series. Transformers were reading 12.2k pri 8 ohms sec. Measured the speakers and they were 4 ohms each, so that leaves me with 6.1k with 4 ohms, no problem. I get about 10 watts output on both 4 and 8 ohms dummy loads(16w clipping). Barely stuffing a 12" speaker into this cabinet over the weekend with my new parts express baffle circle cutter and router.

[J M Fahey]

It is a push pull, just poorly made.
It will neither sound like SE any more, because by mixing both outputs out of phase inherent assymmetry gets applied both ways, not exactly cancelling but present on both sides, so output waveform (at least at clipping) will be funky, nor a true PushPull because both cores are separate.

As of

The reason to do it is because it is not being done.

there´s a zillion ways to do such things.
Just 2 examples:
* you may light the cigarette filter and smoke it, filtered through the tobacco section.
* you may fill your cup with sugar and add 2 teaspoonfuls of coffee to it.
and so on and on