using a PP output tranny on a SE amp

[smackoj]

I have heard that a normal push pull OT with a CT and 2 leads from two output tubes can be used on a SE amp with only one output tube. Has anyone ever done that? If it does work I would like to try it but not sure if one lead gets left unattached while the other lead and the CT hook up as a normal SE tube would?

Any advice would be appreciated.

Jack D

[John]

Somewhere there is a thread on that  very thing, or maybe the thread morphed into that. Anyway, IIRC it's possible, but way way more trouble that it's worth. I did a quick search and couldn't find the thread.

I found this doing a google. Hope it helps!
http://music-electronics-forum.com/t25816/

[stratele52]

Few years ago I read that does'nt work properly.. If I remember the asnwer was from hammond Transformer.

[smackoj]

great article john, thanks. I can see now that this idea is not even 'half baked' 

muchas gracias amigos

Jack D

[Colas LeGrippa]

It  DOES work properly. Hammond suggests a PP tranny for SE application in one case, see Hammond website you might find the thread ( 125 vs 125SE ). It might not be suitable for HI-FI because of the reduced frequency range, but for guitar it is excellent. I hooked up a HAMMOND 1650N push-pull output transformer in my own SE build to see how it would react, and the tone is just great. Those who say it doesn't work right don't know what they're talking about.

Colas

[HotBluePlates]

... I hooked up a HAMMOND 1650N push-pull output transformer in my own SE build to see how it would react, and the tone is just great. ...

Be fair, that's comparing apples to lugnuts.

The 1650N is a 60w hi-fi PP transformer weighing 8 lbs for response down to 30Hz. By comparison, Doug's 100w Twin Reverb replacement tranformer weighs 5.3 lbs.

So it will very likely work well in a SE circuit, even if it is too-big and too-good. You won't readily notice the effects of reduced bass or core saturation that would otherwise occur if you used a smaller PP transformer, or one designed for less power and less bandwidth.

Short answer to the original question:

You can do it, and the amp will work. You will likely get less power output to the speaker and/or reduced bandwidth, possibly some added distortion if the core is small enough for the power passed through the transformer such that core saturation occurs. Depending on how you connect the primary to the tube and B+, and what load you apply to whatever tap(s) available, you may get "improper" loading giving less power output than otherwise obtainable.

Depending on your desired outcome and incentive to use the transformer at hand, all of the above may be perfectly acceptable.

[smackoj]

Well, I did try it and it was worth the minor effort of hooking it up. I used a very old 1950's something PP OT out of a floor radio/record player. It did make a very nice edgy tone but alas it quickly became unusable. It started making a high pitched screech along with each note. I tried several things to tame it down, but decided to install a regular SE OT and the screeching ended. I must admit, I would have liked to have used that old piece but I guess that I was in that kind of Twilight Zone where the amplifier sounds like a million dollars right before it blows apart! A fun attempt anyway and I learned a lot along the way, so thanks for all the good input fellas.  buenos noche amigos. 

[stratele52]

I send a e-mail to Mercury Magnetic and I just receive an answer from Patrick Selfridge.

This is his answer;

Hi
 
By not using the center tap on a push pull, it will function, but it is not true single ended.
It also stresses the output transformer over time, and doing so with our transformer voids the warranty.
Thanks.
 
Patrick
 

[Colas LeGrippa]

Why Hammond would recommend to use  a PP OT for SE use if it didn't work right ? Ok one thing I agree about is that we could obtain the same tone with a smaller SE OT, since there is one winding unused in a PP tranny if used as a SE. My SE build using a PP tranny sounds very good, ask my friends and my public !

Colas

[sluckey]

Guitar amp builders have been abusing manufacturers' specs for over 50 years. That abuse contributes to the charm and wow factor as much as, if not more than, innovative circuitry. It would probably be a ho-hum world if guitar amps all operated on 300V rather than the bleeding edge of molten glass, copper, and iron.

[stratele52]

Where I could read what hammond said about that ? I can't find it. I want to learn.

Probably their transformer are build for that ? It look its a matter of air gap ....
____________________________________________________________________________
+1 for Sluckey

[HotBluePlates]

... Hammond suggests a PP tranny for SE application in one case, see Hammond website you might find the thread ( 125 vs 125SE ). ...

Where I could read what hammond said about that ? I can't find it. I want to learn.

I can't find it either.

What I do find is on the push-pull 125 page, Hammond directs you to their 125SE page if you need a single-ended transformer.

On the single-ended 125SE page, Hammond directs you back to their push-pull 125 page if you need a push-pull transformer.

Maybe you read through their pages too fast and didn't notice the ending on the (otherwise same) part number?

Comparing similar parts between those products, the 15w 125E weighs 1.5 lbs, where the 15w 125ESE weighs 3 lbs.

Double the core to get the same power handling at the same frequency response. That highlights what I keep saying, which is that the unbalanced d.c. in the single-ended circuit requires a bigger core (and an air gap) for the same power output and frequency response. Because the push-pull circuit does away with the unbalanced d.c. and thereby reduces the standing magnetic flux in the transformer, a smaller core can be used for the push-pull circuit.

So if you go backwards and use a push-pull transformer for a single-ended circuit, you will be able to pass less usable power through the core, because the unbalanced d.c. and its magnetic flux is taking up some of the core's capacity. It's not that you can't do it, but you won't get 50w of SE power through a 50w push-pull transformer.

... unless you use parallel feed, but that wasn't the point you were making ...

[Colas LeGrippa]

Stratele, if you want to learn, do what HBP says, he's a smart guy and knows what he's talking about.  Don't do what I do, I am an experimental twisted mind guru.

Colas.

[stratele52]

Colas I tried to keep my mind opened. I read every answer.

You tried PP on SE and its work . Is it by error you do ?  Sometimes this give great discoveries.

[HotBluePlates]

Don't do what I do, I am an experimental twisted mind guru.

B.S.!! Learning by doing is the only way you really know for sure. So you're way ahead of the game.

I might be able to help fill in detail about why you found what you did by experimenting. Like I've said before, I never argue with experience and results.

[stratele52]

Don't do what I do, I am an experimental twisted mind guru.

B.S.!! Learning by doing is the only way you really know for sure. So you're way ahead of the game.

I might be able to help fill in detail about why you found what you did by experimenting. Like I've said before, I never argue with experience and results.

+1

[Colas LeGrippa]

thanks for comments and help, bro !

Colas

[Geezer]

I can't post the schematic right now, being on my iPad, but David Allen had a plexi-type build several years ago that he used a push pull output tranny in single ended mode. I'm sure someone Has that schematic and will post it. Supposedly was a very nice sounding amp.

Class act......look for schem on this thread>>

http://www.thegearpage.net/board/showthread.php?t=691179

[Willabe]

Kevin O'Conner talks about using PP OT as SE OT in at least 1 of his TUT books and that it can be done.

He also talks about using a PP OT as a ULSE OT using the PP OT CT as the UL tap. I'll find it and re-read that section and post back some of the high-lights in a day or 2. (I'm in the middle of packing and moving at the time. Only a few miles away but it's still a major pain.)

HBP, you forgot this;     


                     Brad      

[HotBluePlates]

And we can take the discussion in a whole other direction...

You could use a PP transformer as the actual output transformer of a single-ended circuit using a parallel-feed arrangement.

Simply put, the B+ is fed to the output tube through a large plate choke. A coupling cap then connects from the tube plate to one end of the output transformer, whose other end is connected to ground. It's called "parallel-feed" because the plate choke runs to an effective a.c. ground (the B+ node) and the coupling cap places the output transformer primary in parallel, with its connection to d.c. ground.

The upshot of this method is you keep the unbalanced d.c. of the output tube off the output transformer primary, increasing its effective power handling and bandwidth (over the pure SE connection, not the original PP connection). The downside is you need a largish cap (usually film dielectric at a high voltage rating) and a big plate choke.

So this might be uneconomical over simply buying a purpose-built SE transformer, and as a result it has largely been relegated to occasional audiophile use.

[jazbo8]

"So this might be uneconomical over simply buying a purpose-built SE transformer, and as a result it has largely been relegated to occasional audiophile use."

I think it's generally the case now, but if you insist on using a PP OPT and run it safely for a long time, you can always un-stack the laminates and add a small air-gap to prevent the DC saturating the core. It's your money and time...

Jaz

[smackoj]

Am I understanding this correctly via Stratele's post about Merc Mag response: to run PP ot on a single ended output you simply attach the B+ to one primary side, and the power tube feed to the other primary side and ignore the center tap wire?

I have lots of old PP iron and don't normally run amps for more than an hour so I'm just interested in hearing the different tones without ordering, buying etc....

It's been fun reading the different views on the subject, thanks.

Jack D

[jazbo8]

Am I understanding this correctly via Stratele's post about Merc Mag response: to run PP ot on a single ended output you simply attach the B+ to one primary side, and the power tube feed to the other primary side and ignore the center tap wire?

I have lots of old PP iron and don't normally run amps for more than an hour so I'm just interested in hearing the different tones without ordering, buying etc....

It's been fun reading the different views on the subject, thanks.

Jack D

Generally not recommended. See http://www.diyaudio.com/forums/tubes-valves/171836-use-p-p-opt-se.html
Of course, no one can stop you from trying, just be aware there is chance that you can damage both the output tube and the transformer.

Jaz

[HotBluePlates]

Am I understanding this correctly via Stratele's post about Merc Mag response: to run PP ot on a single ended output you simply attach the B+ to one primary side, and the power tube feed to the other primary side and ignore the center tap wire?

Yes, you can do that.

Generally not recommended. See http://www.diyaudio.com/forums/tubes-valves/171836-use-p-p-opt-se.html
Of course, no one can stop you from trying, just be aware there is chance that you can damage both the output tube and the transformer.

We've outlined above what the realistic problems are with using a non-air-gapped PP transformer in a SE circuit.

I don't believe the arrangement is likely to result in real "damage". The most likely serious effect is partial permanent magnetization of the core due to unbalanced d.c. it was not designed to handle. But you'd probably need a lot of current or for a long time to realistically make that occur.

So as I said before, do it but know what the potential drawbacks are. In the end it might work out fine for you.

[jazbo8]

What happens if there is short (in the tube or circuit), would the transformer be damaged before the fuse pops?

Jaz

[HotBluePlates]

The arrangement of the OT won't change whether the fuse blows before there is damage to the amp. That's really gonna be determined by the size of the fuse and the ruggedness of the transformer (and/or power supply).

Another issue is whether there is even a HT fuse in addition to the mains fuse. Mains fuses generally protect the amp if there is a major fault (short to ground) in the power supply. You can be guaranteed the current draw will spike high enough and fast enough to pop that fuse, in that situation.

So it really depends on where in the circuit the short occurs and the exact nature of the short/fault.