audio-talk

Here's a thing...

I'm taking apart a cheap 'sub-zero' push pull guitar amp. Basically it's a copy of an original Fender blues junior. Ruggedly built, it would have survived being thrown down the stairs ( ref, the Who or the Kinks and a Vox ac30)

But amongst other things two of the HT smoothing caps have swollen and burst. Rated at 450 volts..... no matter.

The pp output transformer is marked for 0-220-440 volts as primary, the secondary 13vac. I find this interesting and actually helpful in that it says more about the spec ( the ratio) than I expected. But that it was made primarily for mains voltage makes me smile!

Who needs a gap?

The cabinet and excellent 12 inch celestion speaker will become a variant of the fender Champ that I can more or less build sitting upside down in a bucket of water.

in those days output transformers were passed to the winder specified in voltage. every volt is a multiple of turns, whatever the turns per volt is depending on core/laminations and the area A which on ei transformers is the area of the central protrusion.

for mains and push pull there is no gap.

torroidal mains transformers give excellent hf bandwidth , and enough bass for lead or rythm guitar, but not for hifi bass or a Fender Bass. But if used single ended parafeed: for hifi you make the resonant tank frequency such that the bass is boosted but neither do you want a bloom at midbass, so keep the boom boom added down at below low bass and benefit from the correction of the roll off from lacking inductance. Hence a smaller parafeed cap than otherwise, net result better bass, though the phase is wrong its in my experience not noticeable. Phase goes wrong at bass in standard se transformers speaker drivers room conditions et al, anyway! its not the end of the world. Phase was the cause of endless arguments between Dave Slagle and Mike Lefevre on audio asylum last century. The reason for the long extended arguments was Mike's vested interest in Parafeed, and the strong presence of Bottlehead diyers in that forum.

guitar output transformers wont be interleaved much if at all. Might get 3 primary, 2 secondary. But can sound very good, because when you interleave to extremes you have more and more problems which is counter productive. Per Fritz best arrangement is 1/2 primary as first and last winding, full primary in between all secondary layers. Then all turns are equidistant from each other. 3 primary sections 2 secondaries is fairly normal for guitar opts and for such as WAD diy amps etc. And it sounds pretty good as non of the degrading effects of excessive sections come in force. Worst result of 3/2 is if they also compromise on inuctance, bass will be poor. HF would end up -3db at 20khz, but its not like super tweeters which might come down to -3db at 20khz, from the other direction which seems pointless, but is actually much more audible. However in budget amp domain the slight hf roll off isnt damaging. said person building a kit amp probably doesnt have speakers to cover the full range either.


Bit of a digression, so back on youre push pull output transformers. Keep them for guitar amps. For scratch building valve amps youd get better results pse with voltage torroids i.e. 230v : 9v is in the right ball park. You’d be amaised how good that sounds with the tank circuit tip, and the hf of a torroid beats budget EI output transformers or converted EI voltage devices.

I remember the thing about toroids, all thought provoking, ta much.

I'm going SE however; always my first choice for hifi and guitar amps, so the interesting pp trafo can wait patiently for other things.

andrew Ivimey wrote: ↑Sat Mar 12, 2022 3:45 pm I remember the thing about toroids, all thought provoking, ta much.

I'm going SE however; always my first choice for hifi and guitar amps, so the interesting pp trafo can wait patiently for other things.

Steve restocked (edit spelling Apple don’t know the word restacked) his with a gap for SE. All I’m saying is that’s a massive undertaking!

Respect.

Non no no no no....
I have s couple of genuine Fender champ SE output transformers from America. I am basically lazy!

cool, what is the primary resistance, because if its about 250 ohm you could use them also for a Raymond Bates which I too am supposed to be building! But with neuralgia Ive degenerated into suffering can’t bearseditis!

I can measure that and 250 ohms sounds about right. El84/6v6 like around ' 8k ' for primary I believe. But this is a guitar amp and as I aim to swap out output valves, 6w6, 6v6, el34, 5881 and 6l6 I'll be changing the cathode resistor, HT and possibly output transformer too, just for cursiosity, as my old mum said about cheap wines.

The front end is to begin with 6SL7 as the original Champ model used one. Everyone now uses ecc83 for obvious reasons; the circuit remains the same. I'd like to add tone circuitry too and extra gain and that will need another valve stage. I have room but the mains transformer can't supply enough heater current.

I'll be using 6ax5 rectifier but a bridge of diodes is much easier ( as in all Champs and many cheapo modern spawns) I like valves !

Yes, I like the centre tap secondary dual plate rectifier. Its a problem finding transformers for that these days.I should really do listening tests on the hybrid bridge, might be its not much different sounding. I was reading on tube diy I think it was that modern production 5AR4’s are failing and the USA guys are putting silicon diodes to protect the shoddy modern production 5AR4’s from PIV they are supposed to handle, but can’t handle! In same post it was claimed that the nice valve rectifier sound is unaffected by these additional silicone diodes. That possibly infers the hybrid bridge might be ok. But that is only the say so of someone, until Ive proved to myself hybrid isn't detracting I’ll not advise it. But it would make modern situation a bloody sight simpler and cheaper as loads of good value transformers for sale not centre tapped.

It's fine uf4007 are cheap.

As for 5ar4s or gz34s, I have a few original Mullards! ( and some Chinese shuagangs too). The only poor gz34s I've used so far have been two JJs - they flashed over after a couple of weeks. Yuk!

5 volts though so gz34 and 5y3 are out as all I have is 6.3 winding so 80s and 83s ( which would be unusual, are out too!)