Large power push pull output transformers, say 150W

[kegger]

If you were in the market for some 150w push pull outputs for multiple pairs of EL34/KT88
(this amp uses 8 EL34 per channel at relatively low voltage and current in UL at 140w)

What would be some of the choices you might suggest?
(any and all comments welcome)

Thanks,
Keg

This is new territory for me, so I'd be guessing..

[dcgillespie]

Depending on the power bandwidth you are working with, the 120 watt Hammond transformer will work, although would likely require fixed bias to reach your power goal. For say guitar amp frequency needs, that transformer should be able to handle 140 watts rather easily. From an ideal design standpoint I would rather like to see the primary impedance around 1500 ohms for a quad push-pull setup, as that would be a little easier on the screens. In any event, it's best to work on big suckers like this though in the winter time!!

Dave

[Wavebourn]

Unfortunately, big suckers would require high primary impedance that impacts on higher frequencies. Many small suckers in parallel are better, in terms of output transformer cost.

[kegger]

Thanks guy's, this is fixed bias.. (and HIFI use)

I have no specs of the output transformers in there, 1 is bad so I'm seeking alternatives.

[dcgillespie]

Can you measure the good one to at least get an idea of the turns ratio so the impedance can be calculated? For hifi use, the 280 watt Hammond unit would surely deliver the low end goods, but the high end performance would certainly be compromised. For that matter, I believe that virtually any transformer with an adequate core that can handle the lowest octave of the audio spectrum, with low distortion, that is rated for more than 100 watts, will have some degree of compromised HF performance -- although in this case, keeping the primary impedance low certainly helps in that regard.

Dave

[SY]

I'd look for something in the 1.2-1.5k:VC range. That's a decently low turns ratio and will match well to 4 pairs of EL34.

[richwalters]

Originally Posted by Wavebourn

Unfortunately, big suckers would require high primary impedance that impacts on higher frequencies. Many small suckers in parallel are better, in terms of output transformer cost.

Remember parallel p-p with Z halved needs fewer turns, hence lower leakage parasitics but a larger core for lower throughput power.
A well designed full b/w E&I o/p tranny should have an upper f cutoff
around -3dB 70Khz and for 150W, down to 20Hz expect a 21 Lbs weight.There are design advantages; copper wire OD can be increased without serious penalties further lowering the already low DC resistance. High DC primary resistance leads to higher THD.
I use 2K A-A....with 4xKT90 at 600V B+; easy 200W+.
If one is using a stereo or twin as I do and bridge o/ps, the power supply is going to need alot of attention.

richy

[Wavebourn]

Richy, I was responding about big sucker tubes. Indeed, paralleled tubes on lower B+ are more OT - friendly.

[kegger]

Thanks guy's..

Rich(Richy).. If I may ask, what output transformers are you using?

----------

As that was more my inquiry, I figured between 1.2-2.2k primary would
be where I'm looking but I was curious who makes a decent one or was
there 1 others have used with good success or thought was to be nice.

------------------

I can't get to the one in the amp as of yet(bunch of stuff to take apart)
It is a pair of mono blocks, running only 390v B+, and pretty low current.
(an yes I'd be looking to replace as a pair, weighing options as of now)


Appreciate all the info given, Thanks..

[hidnplayr]

You can measure the primary impedance while it's still inside the amp, just be very carefull
You'll need an o'scope with 2 probes, a dummy load and a signal generator..