Restacking An Output Transformer.

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Cressy Snr
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#46 Re: Restacking An Output Transformer.

Post by Cressy Snr »

Interesting all this talk of parafeed. So let’s take a look at some hard facts:

Cheap option keeping the Danbury OPTs as push pull:

Sowter 8982 plate choke 100H 50mA 2 off @ £156.64 ea.

Using Paul Joppa’s formula for calculating parafeed cap size gives a cap value of 8uF.
Now we don’t want to be compromising our sound by using a crap capacitor in such a vital position in the circuit. So let’s take an 8.2uF polypropylene cap from the middle of the range ‘MR’ series of Clarity Caps sold by Hi-Fi Collective. These are £93.53 ea. for a 630V job.

So 2 high quality plate chokes plus a pair of decent quality parafeed caps gives a grand total of £500.64. (cheap option)

But what you really want if you are going to do a proper job, is a parafeed transformer with permalloy in its laminations, so let’s add in a pair of Sowter 8983 5K parafeed output transformers with 30% mumetal laminations and ditch those Danbury transformers with their M6 GOSS laminations. The Sowters are £182.37 each

Add these to the cheap option and you have £865.08. plus p+p so let’s round it all up to £900 for the sake of argument.

So cheap option with existing TXs would be approx £550 with p+p
Expensive option would be £900 with p+p.

Doing it my way ZERO. Nuff said.

No doubt parafeed is very nice, but I don’t have the sort of dosh detailed above to throw about. So I did what I could, with the help of what I had, and a few generous people who long term loaned valves and good PSU caps. The sound is way beyond what I expected.
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Wolfgang
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#47 Re: Restacking An Output Transformer.

Post by Wolfgang »

Why not?
I was referring to some Lundahl OPT information, maybe I misinterpreted it regarding what I called compromises. But it seems that one has to make decisions -which looks like a compromise to me - regarding output wattage,max inductance,lowest frequency and loss (doesn't that refer to the core material used?) when using an air gap instead of parafeed.

"Single-end output stages:
The core of Single End output transformers have an airgap. The purpose of the airgap is to accept the DC current of the output tube without saturating the core, leaving enough headroom for the sound signal. As a result of the airgap, the primary inductance is lower for SE output transformers compared to P-P dittos. In addition, the inductance tends to vary with DC current. For our high quality C- cores with carefully ground surfaces, the variation is within +7% of rated value.
Step 1 We recommend that, given your secondary load impedance (4, 8 or 16 ohms), you select a secondary connection alternative with 0.5 dB loss. This will give you a power limit of 25 W at 30 Hz. If you find that you require more bass headroom, select a secondary connection alternative with 0.8 dB loss.

Step 4 We define Power Low Frequency Limit, FPL, as the frequency where LP = RLOAD. (The reactive impedance of the transformer equals the primary load impedance). At FPL, the output power is reduced to 50%. For the LL1623 / 90 mA in a 0.5 dB loss connection, FPL = 16 Hz (RPRIMARY = 3.0 kohms and LP = 30H).
Step 5 We define Response Low Frequency Limit, FRL as the frequency where a (small) output signal is reduced with -1 dB due to finite primary inductance. FRL =  / , if you solve  in LP = (RLOAD in parallell with RANODE). For the LL1623 / 90 mA and a 300B triode, FRL = 7 Hz. (RANODE = 650 ohms, RPRIMARY = 3.0 kohms and LP = 30H)"
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Nick
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#48 Re: Restacking An Output Transformer.

Post by Nick »

I may be confused about what core and when you are talking about. The only core material in general use that would have a problem with DC current AFAIK is nickel. This will be the case both for the single ended transformer and the anode choke.

You could of course use a CCS instead of a anode choke for parafeed.

I may be getting confused, are you comparing [ SE and PP ], or [ Parafeed or not ]. I used brackets to make clear which two options I am asking about.
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Wolfgang
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#49 Re: Restacking An Output Transformer.

Post by Wolfgang »

Yes, I wasn't clear enough. I am comparing parafeed or not considering that NO core material is NOT affected by DC. Maybe I simply misinterpreted this :
"The purpose of the airgap is to accept the DC current of the output tube without saturating the core, leaving enough headroom for the sound signal." and :" ..the inductance tends to vary with DC current. For our high quality C- cores with carefully ground surfaces, the variation is within +7% of rated value." So why would anyone spend a ton of money for an expensive OPT if despite air-gap the inductance can still be affected by DC? If that's the case then better getting a cheaper OPT and use it in a parafeed circuit with CCS.

So let's start here. Monolith Magnetics for instance offers 3 different transformer types at very different prices. The most expensive is the "nanocrystalline" . That's what they say: "Nanocrystalline materials start their life as an amorphous ribbon and are heat treated to form crystals at the nanoscale (10 nm to be precise). These cores exhibit very high permeability, very low hysteresis levels up to high frequencies, resulting in a pristine sound experience."

How much of all this is true I cannot say and if I look at other OPT Tech Blogs like the Electra-print I find that M19 lamination is good enough and the benefit of exotic core materials are bogus.

"M19 & M6 LAMINATIONS

Core material made of M19 and M6 has been used for many years. The metal alloy is made of metals to offering uniform increase in inductance as flat as possible for power transfer over a given bandwidth. These materials were developed to sell product to audio transformer companies for the high fidelity amplifier industry back in the 1950's. Designers had the other materials (except amorphous core) and did not use them for these products. Nickel had its place in low level audio for the recording industry.

These standard materials, when used for SE types, would offer the flattest, most efficient power transfer and overall acceptable measurement. M19 is used for SE types due to no zero crossing as in push-pull types M6 is more costly and its attributes not used in SE service. M6 can be used for push-pull as well as M19.

WHAT ARE THE ATTRIBUTES OF M19 AND M6?

When someone states that the sound created with exotic core materials is far superior to standard lamination materials, you must ask yourself what happened to this reviewer's amplifiers bandwidth and power? This sound is just the new setting of the bass and treble controls. Our reviewer will probably just say its "It sounds better" having no idea what happened. We know that if there was an improvement then it should be measurable.

To date there are many materials available but with only two overall differences in the peak permeability minimum (Gauss/Oersted) with an average of 2000 for M15 to M56 lamination materials and 6800 for M6 (75% Grain). This indicates that realistically only two types are any good for a full bandwidth flat response and these are M19 and M6 laminations."
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Nick
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#50 Re: Restacking An Output Transformer.

Post by Nick »

If you say so, I like amorphous core transformers.

The advantage of parafeed is the output transformer does not see DC. The disadvantage is that you either need an anode choke or CCS, and a extra (large) cap in circuit and (IMHO most importantly) the output transformer passes through the 0 flux point.

As I said when questioning your post, none of that affects the choice oif core material.
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Cressy Snr
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#51 Re: Restacking An Output Transformer.

Post by Cressy Snr »

Two days down the line with the newly rebuilt output transformers and nothing has come up sound-wise, to make me question, in any way, the wisdom of doing the conversion.

We've had BBC Radio 3 on for quite a large proportion of the day and to be frank, we've never heard classical music sound as..well..musical. It's delightful. Loads of repertoire has been added to my Apple Music library, and the vinyl will be getting an airing this week, once I've dug some out and cleaned it.

The sound was great with the fudged Hammond 1627SE OPTs but these 5K conversions are on a different level. The Hammonds sound thick in the bass and coloured in the lower mids in comparison.
The conversions sound even handed across the frequency range and complement the 45s very well.
The exercise was well worth doing.

I'll see if I can get some scope traces done at some point. It looks like a dirty selector switch on my sig gen that was causing the problem with intermittent signal dropouts.
“The sound you love today may not be the sound you love tomorrow, and with tube circuits you can continually retune your gear to what reflects your current vibe...so you can get even down deeper into your truth.” (Harvey Rosenburg.)
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Paul Barker
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#52 Re: Restacking An Output Transformer.

Post by Paul Barker »

Exceptionally unusual to achieve what you did with those transformers without failing. Many lamination styles don’t lend themselves to deconstruct, being soaked for days and baked. You wreck them trying.

It’s a very hard won achievement. But aswell you’ve built knowledge of air gaps from that great resource and first attempt you’ve got a better result than a professionally made SE output transformer. I’d take that. Well done.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
Cressy Snr
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#53 Re: Restacking An Output Transformer.

Post by Cressy Snr »

Yep! I'll definitely take it. :D
Thanks Paul.
“The sound you love today may not be the sound you love tomorrow, and with tube circuits you can continually retune your gear to what reflects your current vibe...so you can get even down deeper into your truth.” (Harvey Rosenburg.)
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andrew Ivimey
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#54 Re: Restacking An Output Transformer.

Post by andrew Ivimey »

Special irony (!) here. This is anything like not 'beginners'. It's really interesting to me even though, basic theory aside, I don't understand very much at all of what you are talking. Fascinating, amazing!
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Wolfgang
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#55 Re: Restacking An Output Transformer.

Post by Wolfgang »

none of that affects the choice of core material
Correct. Obviously a misunderstanding on my side!
I tried to sort it out and this is where I am at right now. Hope this is correct.

1. High permeability and low hysteresis (amorphous cores) would be good for better linearity and lower harmonic distortion for low frequencies at small signal levels. This would be of some advantage over other core materials.
2. The magnetic operating point (zero signal point) shifts with DC in one direction (unsymmetrical) and larger AC signals will consequently drive the core into saturation earlier, adding more distortion. This is true for all core materials with or without air gap. An air gap only adds more headroom before that can happen but reduces the inductance which on the other side will create more distortion at higher signal levels if we don’t use a bigger OPT (customized for the amp power)when we are adding an air gap.
3. Symmetrical distortion produces 3rd and 5th order distortion. Unsymmetrical distortion produces mainly 2nd and 4th order distortion. For a tube amp I would prefer those. That’s why I also prefer SE over PP. So in this case better core materials (amorphous )and parafeed circuit might not be my first choice. I also don’t want too much of those 2nd order distortions either because I know already how bad that can sound with an SE OPT amp . Maybe some 3rd and 5th harmonics are in the end better sounding in the case of an SE OPT amp than “too much” of 2nd or 4th? Practical testing and listening is required to figure that out.
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Nick
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#56 Re: Restacking An Output Transformer.

Post by Nick »

Sounds ok to me.
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#57 Re: Restacking An Output Transformer.

Post by Mike H »

Nick wrote: Fri May 14, 2021 4:16 pm Also in use I think the flux will pull the I and E parts together, so leave the bolts loose until its first used.

This has reminded me of something - put the DC throurgh that you want it to run at - then tighten the screws

Nick wrote: Sat May 15, 2021 5:54 pm If you haven’t already, I would loosen the bolts and let the lams clamp together with the amp running then tighten up again (while running).
Yes, just like that ...

:lol:

Danbury transformers tend to be varnished so I'm surprised you managed to take them apart – Dave at Danbury used to use an oven to melt the varnish if taking one apart. Often it was a guitar amp mains from a Vox or Marshall that had released the magic smoke, due to the waxed paper giving up after one too many beers had been spilt onto it – and the customer wanted it rewound – or even if making a new copy, still needed to take the old one apart to find out what is inside.
 
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Cressy Snr
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#58 Re: Restacking An Output Transformer.

Post by Cressy Snr »

Mike H wrote: Fri May 21, 2021 6:42 pm
Danbury transformers tend to be varnished so I'm surprised you managed to take them apart…
Wasn’t too difficult as such. It was however tedious and tiring in the extreme.
With the aid of a magnifying light, carefully work Stanley knife blade edge into joint, by pulling backwards until a gap appears. Replace blade with spatula, twist and flick….Ping! off comes an I lam.
For an E lam, same procedure except that once you get the spatula in, slide it along each arm to break the varnish seal, then into the tongue down the middle of the coil former, twist, flick, ping!

Repeat many many times. There is no margin for error, so full concentration has to be maintained for EVERY lamination, and there are a lot.
“The sound you love today may not be the sound you love tomorrow, and with tube circuits you can continually retune your gear to what reflects your current vibe...so you can get even down deeper into your truth.” (Harvey Rosenburg.)
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Paul Barker
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#59 Re: Restacking An Output Transformer.

Post by Paul Barker »

Cressy Snr wrote: Fri May 21, 2021 10:30 pm
Mike H wrote: Fri May 21, 2021 6:42 pm
Danbury transformers tend to be varnished so I'm surprised you managed to take them apart…
Wasn’t too difficult as such. It was however tedious and tiring in the extreme.
With the aid of a magnifying light, carefully work Stanley knife blade edge into joint, by pulling backwards until a gap appears. Replace blade with spatula, twist and flick….Ping! off comes an I lam.
For an E lam, same procedure except that once you get the spatula in, slide it along each arm to break the varnish seal, then into the tongue down the middle of the coil former, twist, flick, ping!

Repeat many many times. There is no margin for error, so full concentration has to be maintained for EVERY lamination, and there are a lot.
Respect Steve, its not half as easy as you make it sound.

The laminations are not all that expensive, you just start a fresh. The money you pay Sowter et al for a finished transformer is for skill,labour,knowledge,experience. Per typical 30 watt audio transformer, £50 laminations, same for winding wire, and whatever bobbin and interleaving materials are used. maybe £10 shrouds/frames.

Its great what you did for yourself as a diyer retired who doesnt have to make sence of youre time spent. But buy new lams is a no brainer for repeatability.

At moment though in the UK lamination suppliers few and far between, I bought from Pennine Radio originally. Doubt they are still going. Google by all means. last time I bought lams and bobbins was Asco Components. Same sort of price. However during lock down I tried Asco for large scrapless EI lams to make choke input no capacitor supply for the 813 Matrix. But nothing came of my order. Im not finding companies whose staff are working from home are providing anything like the service we came to expect and know pre-Covid.

Im just waiting for everything to go back to normal.

A very good stratergy in absence of new lam supplies is to buy old Cut Core power transformers because they dont need the core mucked about with to just slip youre bobbins on in place of the original bobbins. One way to get Cut Cores is at the scrap yard. Just casually ask “can I see if you have any old transformers I can use” you pay by weight. Helps that Im known there due to my trade. But always wear PPE and keep a weather eye out for equipment on the move. Get the heavy equipment operators attention and he’ll stop operating anywhere near you. Its all pretty safety conscious, and there is no larking about like they do on them TV shows about scrap yards.
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Paul Barker
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#60 Re: Restacking An Output Transformer.

Post by Paul Barker »

Steve, youre very expensive costing on parafeed far above now, isnt how it works.

Buy a fluorescent lighting capacitor, 5 to 10 microfarrad. still available for £5 to £10. a nat,s forskin less good sounding than any audio polyprop with black plastic and gold writing by example, and who knows if anything internal is different to industrial use polyprop’s. Little in it Id say. I like the family sound of polyprops, industrial or snake oil whitewashed walls. YMMV.

The choke is simple to self wind.

Wheres the big expense now?

I guarantee you, on the cheap you will not be disappointed.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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