PP OT used for PSE (one tube each side B+ on CT) - Is it possible ?

[kagliostro]

Is possible to use a PP OT for a PSE amp (2 tubes in parallel)


connecting each tube at one extremity of the OT and B+ on the CT ?


Thanks


K

[sluckey]

The signals at each end of the OT primary would be in phase. I would expect them to cancel each other and result in no signal coupled to the secondary.

[kagliostro]

Thanks Steve

and if the two half windings can be phase reversed, swapping one of the CT connections ?


Franco

[shooter]

I'm going to set up a test for kinda the same thing, I have 2 SE-OT and want to make a PP tranny outta them, so I want to get the phasing right.  I was gonna try and put a sig-gen signal in the OTand check phase relations out for each.

[sluckey]

and if the two half windings can be phase reversed, swapping one of the CT connections ?

It would work on paper. Probably work for real, but I don't know how well.

But, the OT you drew only shows a single wire for a CT. Does your actual OT really have two separate primary windings that you must connect two wires together externally to form the CT?

[kagliostro]

Does your actual OT really have two separate primary windings that you must connect two wires together externally to form the CT?

No, no out connection, I was investigating about the feasibility in an hypothetical case

---

I'm going to set up a test for kinda the same thing, I have 2 SE-OT and want to make a PP tranny outta them, so I want to get the phasing right.  I was gonna try and put a sig-gen signal in the OT and check phase relations out for each.

A friend got an old italian Corland amp that used a pair of SE ot as a PP OT





he had under hand a regoular PP transformer and tried it on the amp ................... the PP transformer is still on the amp and the two SE OT are on the junk box to be used for a small stereo SE amp


Franco

[jjasilli]

I get that pushing the design envelope might be fun.  But:


If biased in SE (Class A), an EL-84 "puts out" 5.7W @ 57mA max current draw; so 2 of them put out 10.4W total into 2 speakers.  In PP Class A - Class AB(1), both tubes put out 11W, @ 91mA max current draw.  No real difference in output, but lots more current draw through the OT primaries.  And, with SE you have the option to run stereo preamps into 2 speakers.


If biased for higher voltage PP Class AB(2), then max current draw rises to 114mA.  I'm not fully sure of these numbers, but you get the idea.  The SE OT's should be spec'd for your current draw, which they might not be expecting.

[shooter]

Not to side track things but...., the trem circuit, footswitch.  Does the 22k cathode R keep the osc off till the FS shorts It out?

[sluckey]

Not to side track things but...., the trem circuit, footswitch.  Does the 22k cathode R keep the osc off till the FS shorts It out?

Surely you meant to post this in another thread???

But since you asked, which amp are you referring to?

[kagliostro]

I think he refers to the Corland schematic I posted


Franco

[shooter]

Corland schematic

That's the one, it's nbd, just sorta stood out

[jazbo8]

Try seaching for SEPP or single-ended push pull for some examples, but there does not appear to be much benefit for such topology, espically for guitar amps...

[sluckey]

Does the 22k cathode R keep the osc off till the FS shorts It out?

Yes. The oscillator operates with grid leak bias. The cathode must be connected to ground to oscillate. I've never seen a footswitch implemented like this before.

Sorry 'bout my other post. I was using IE. For some reason I cannot see most of K's pics when using IE. He's the only one. Don't know why. Works fine with Chrome.

[shooter]

Thanks Sluckey, it did seem odd but I only got a few trem ciruits in my head.

Franco, I did some web fishing and there seems to be a fair amount on using PP trannies for SE, the main "complaint" is they distort early and just don't have the Punch you get with a true SE OT.  I can't find anything much going the other way, 2 SE trannies configed as PP.  Does your setup have wire colors on the SE OT input side?, specifically does the same wire from each tranny go to a tube?

Thanks again to you both

dave

[HotBluePlates]

I get that pushing the design envelope might be fun.  But:


If biased in SE (Class A), an EL-84 "puts out" 5.7W @ 57mA max current draw; so 2 of them put out 10.4W total into 2 speakers.  In PP Class A - Class AB(1), both tubes put out 11W, @ 91mA max current draw.  No real difference in output, but lots more current draw through the OT primaries.

Don't forget that to get 2x EL84's to output 2x5.7w = 11.4w, they also draw 2x57mA = 114mA through the OT; doubling power by doubling tubes also doubles the current sucked from the power supply.

Class AB1 numbers you cited are for both tubes, so the current draw dropped from 114mA down to 91mA.

Actual signal current draw under any condition depends on supply voltage and OT primary impedance (within the limits of the tubes' capabilities).

Additionally, deep class AB idles the tubes cooler and uses a lower primary impedance to draw larger current swings from the power supply at full power, resulting in higher power output. However, the tubes must also be biased cooler to prevent overheating at maximum signal, so current limits of the OT are relaxed (because the critical current quantity in an OT is how much unbalanced d.c. it can handle before saturation, not a.c. signal current unless extraordinarily high).

[shooter]

connecting each tube at one extremity of the OT and B+ on the CT ?

I believe you would tape off the CT and wire it like SE, or PSE, one wire to the tube(s), one to B+

[jjasilli]

Don't forget that to get 2x EL84's to output 2x5.7w = 11.4w, they also draw 2x57mA = 114mA through the OT; doubling power by doubling tubes also doubles the current sucked from the power supply.


Yes.  To clarify: if ea SE EL-84 > it's own OT > it's own speaker; and you have 2 side-by-side circuits, then ea OT only gets 57mA current draw; AND you have 2X the Wattage Output total.  But, if the 2 tubes and 2 OT's are all wired together for PP, then ea OT must experience about 2X the current draw, which may harm them.

[jazbo8]

I believe you would tape off the CT and wire it like SE, or PSE, one wire to the tube(s), one to B+

No, that's precisely what you should NOT do, the core will saturate since the PP OPT has no airgap.

[trobbins]

If the CT is not used (winding from P-P used), then the inductance is 4x the inductance when each P-CT winding is connected in parallel.  For the same point on the B-H curve at idle, the idle current is 2x with the parallel configuration.

Because the core is not intentionally gapped, the BH curve is less linear, and a typical idle current for class A will push the operating point well into the non-linear saturation end.  The much higher inductance that is seen when current is close to zero, is not realised when a significant idle current is passing.

[kagliostro]

Does your setup have wire colors on the SE OT input side?, specifically does the same wire from each tranny go to a tube?

The wires color are the same, you can see it in the photo I posted

the connection of the wires is as in the schematic I posted with the photo

Franco

[jazbo8]

Why is the output loaded with a 4R speaker? It seems that would drop the power down to 5W! Did he try loading it with 16R? It should get closer to 18W, which is about right...

[kagliostro]

The amp was found with a 4ohm speaker and I think it can be the original speaker

with a load of 4ohm we calculated near 11200ohm a-a primary impedence

however now the amp has no more the two SE OT installed, my friend installed a standard PP OT on the amp

and he say now it sounds better

Franco

[jazbo8]

however now the amp has no more the two SE OT installed, my friend installed a standard PP OT on the amp

Good for him, that's a much better way to go.

[kagliostro]

My friend wrote to the founder of the brand and Mr Camillo Orlando, now octogenarian, told him that at the time the amp was build was not easy to find OT and so, for a few time, the amp were build with 2 SE transformers, later the brand builded his own PP transformers

Franco

[shooter]

that's precisely what you should NOT do

Thx Jazbo, that's why I like here, I got that info, "tape off the CT", from 2 different sites, both audiophile discussing this same topic.

Thx Franco, I thought the wire looked the same, but dusty wires and old eyes, I thought I'd verify it.

[HotBluePlates]

... To clarify: if ea SE EL-84 > it's own OT > it's own speaker; and you have 2 side-by-side circuits, then ea OT only gets 57mA current draw; AND you have 2X the Wattage Output total.  But, if the 2 tubes and 2 OT's are all wired together for PP, then ea OT must experience about 2X the current draw, which may harm them.

It don't work like that.

If you have 2x SE OT's arranged for push-pull, then each OT has a wire connected to the other OT, both of which are connected to the B+ supply. Now if each OT has a single tube plate attached to their free ends, you still have only single-tube current through each OT. Then stage in aggregate handles double the current, and the B+ supply does have double-current drawn.

The OT's would be unlikely to be damaged in any case; the worst-case would be more power drawn through the OT primary circuit than the core can handle and transfer to the secondary, in which case the OT saturates and you get a distorted output. It is unlikely that the wire of the OT is so fine that ~100mA would melt it. A 40ga. wire is rated to carry 137mA, and that's getting down to hair-fine.