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]

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?

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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..

 

[richwalters]

Thanks guy's..

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

Transformer, Choke, Manufacturer & Supplier :Majestic Transformer Co. and SOWTER PUSH-PULL OUTPUT TRANSFORMERS a modified UO71.
Beware...... these aren't cheap and customed' spec.
The Majestic is rated for 175W at 15Hz to 55Khz.....customized Williamson 18 sectioned whereas Sowter is generally 14 sect 20Hz to 70Khz both (-3dB)
Soundwise both are well balanced trannies. The Majestic is a little harsher and the bass on both is destructive but I prefer the MAjestic for classic as it's more precise.
The 20Hz Sowter which is slightly smaller, strangely sounds bit softer.
Generally design for 20dB global nfb at 1Khz.
I also have a 15Hz Sowter equivalent and this tranny has given me a hard time trying to sort out the HF response. That slightly larger core exponentially increases the leakage parasitics that the HF b/w has to be curtailed to 30Khz whereas the Majestic I get 55Khz for same size.

I will sort some closed loop graphs out of both used in identical circuits.

richy

 

[kegger]

Thank you Rich..

 

[kegger]

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..

Thanks, actually all the leads are on screw terminals, so I can tickle the primary
with small AC from my variac an measure the secondary, I'll do that in due time.

 

[kegger]

I'm really more interested in what the specs are on the choke an the power tranny
to see what can actually be gotten out of the amp, then decide on output tranny's.

The current was ran so low on these output tubes, with 8 EL34 in UL 140w is really
low, I don't know if they did that because of the outputs or maybe other tranny's.

Either way I was still curious what people in the know would suggest for decent iron.
(I've just never ventured into "big" tube power really) 😉

Thanks guy's..

 

[richwalters]

The Bug with big amps is designing them properly; one will spend alot of dosh with big tubes, power supplies, watchdog circuits etc but hit and miss methods will show up as smoke.The higher B+ with plenty of joules is no game. I don't want to be a deterrent, but tube amps in this power class is up against lightweight SS competition, so it might as well be properly designed to compete and deliver proper sound. There's no point making a BIG amp which cannot deliver.
Here's some performance graphs of what one can expect from various output transformers in that 150/200W cat. The obvious notice is the E&I designs is increased leakage capacitance (due to large size) resulting in rapid curtailment of the HF response which limits the useful upper response. Examine the differences around 10Khz on the OLG curves...the transformer design differences show here and shows how good the winding design has been. There is no further solution (other than toroid designs) so with parallel p-p,the primary windings have already half the turns compared to single p-p; so the frequency response due lower parasitics is improved.
The open loop curves of amps with identical drive circuits except for the gnfb (~20dB) loop components ......squarewave optimised with overall amps having +16dB stability margin which is good. Ironically, Sowter design shows a better squarewave quality which one would expect better cound quality, but I preferred the slightly smoother Majestic 18 sectioned design.
The main issue is trying to extend the HF response using the available amount of topend loop gain without risking instability..which can give disastrous results at this power level.
The design cue is to avoid the o/p stage being loaded with any more R-C's than necessary; this having an influence on high end distortion. The interwinding leakage capacitance between primary and secs (with all relevant winding sections connected together) will be between 1nF & 2.5nF; this is quite high and will sap top end energy and the complex phase errors will result in higher distortion. For circuit optimisation, a slightly different approach to these designs is needed by optimising the transient response at both LF&HF ends, which inter-relates with the phase response, i.e stability. One has to get this right.
A phaseshift graph is shown of the amp using the Sowter 150W transformer, this will prove the stability margins. Those wanting to do phaseshift measurements; use maths, Lissajous or the two screen phase method.

Just as a hint; I use a 600W switching PFC power supply for the B+: 300V for modest listening at reduced quies current and the full 600V for instrumentation. The tubes; the EH KT90; however some graph plots were used with 6550 New Ed and KT88's, which were found to have short lives under the conditions. The EH KT90 provides the lowest distortion...a tribute to the EH engineers.

More anon.

richy.

 

[kegger]

Those are some nice looking square waves there Rich, and I believe I have seen
your driver circuit in the past and liked it, how do you normally run your outputs?
(do you tend to run pentode, ul, or triode) ?

I tend to run UL, then use additional 12db of feedback or less on most of my builds.
(again for me and my personal builds that means 60wpc or less amps)

-----------

I don't want to put manufacturers on the spot with these particular amps till I know
all of the going ons inside of them is why I haven't posted what they are, as of yet.
(if anyone does want to know you can PM me though)

But they do run 8 EL34 per mono block, in UL at 390v at the plate in fixed bias here.

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The manufacturer wants $900 shipped for a pair of outputs is why I was thinking of
looking at "alternatives", with that much cash, I wanted to see what was out there.
(and I can't buy a single from them as the older model is no longer made)

I was thinking of maybe having it rewound, but for the cost, and not sure how good
they were or there rating that doesn't look all to appealing to me either really.

 

[richwalters]

Those are some nice looking square waves there Rich, and I believe I have seen
your driver circuit in the past and liked it, how do you normally run your outputs?
(do you tend to run pentode, ul, or triode) ?

I tend to run UL, then use additional 12db of feedback or less on most of my builds.
(again for me and my personal builds that means 60wpc or less amps)

I design UL 40% tet with roughly 20dB gnfb, optimum thd point at LF cutoff around 1.5% this corresponds to optimum transient response, and HF end thd roughly 0.5% 10Khz both at full power depending on design. The top end HF power handling stuff is mighty tough stance but thd follows "bathtub" curves. Couple enclosed.
There is also enclosed curves for a ECL82 7W amp, may seem simplistic with exactly the same design procedure as I give larger designs. This amp is a modified original with different pre-tubes and that curve doesn't represent the original Mullard version.
Just that with larger designs issues become more important as real watts are being dissipated.
The thd v.s frequency shows two curves, the 20W output is with B+ 300V. Also available graph:- power with 2K A-A with minimum B+.
This illustrates that excellent quality is available with low B+ and high Iquies, pertaining to nearish (2/3) class A operating conditions hence low thd.
There are numbers of problems: There is a relationship between Iquies and THD at high frequencies as the transformer parasitics create phase delay errors, so all tube amps will show this as increased thd with rising frequency. A high Iquies will force the o/p stage into controlling the transformer; the other way round isn't desirable, but unfortunately effects tube life..(can't have it all).
However some 50 yrs ago, KT88's were being run in power amps in brutal conditions; 600V B+ at 60mA quiescent..(36W)....a tribute to some of the low thd figures that some designs were capable of. The famed veteran GE6550A could handle this, but other modern KT88 clones I found lasted on average less than 2000 hrs under such conditions. So my dual voltage B+ concept arrived to preserve some sanity to tube life.
I copy Morgan Jones comment about power amps stuck in racks stewing at 60°C often forgotton to be switched off longer than 24 hrs, only a note from the studio cleaner to reminding that fact. These days that wouldn't happen.

Graph enclosed of THD at 100W level using Majestic transformer. This Thd is clean....power supply switching graze just visible on waveform, but this is at a very low level, lower than a CD player.
One thing at a time but the SS power supply presented the greatest challenge.. and watchdog complexity coupled in with a bias checker switch..It makes opportunic sense to have a low and high power configuration. More anon

richy

 

[kegger]

Thank you Rich, did not mean not to respond, just some issues on my end here.

Looks like I may have found the manufacturer of the original output transformers.
(still hammering out details)

Plus the choke and power tranny.

Got the specs on the PT and Choke so far..
The power transformer high voltage winding is rated for 335V @ 700 mA.
The choke is 2 henry @ 800 mA.