Parallel single ended valve amps, any experience?

[Avinunca]

Originally Posted by Gabe001

Nice amp! There's one for sale on Facebook marketplace at the moment for £2k. Thanks for the link

This has been for sale for a while. It is a very early one and I think overpriced.
The later models have a separate choke input PSU.
Mine is a one off and was made by someone who was assembling amplifiers for the owner. He allowed the guy to built it for his own use with genuine parts. It used a repurposed metal chassis and has an enormous psu in a separate hammond case. I plan to rebuild it into a plinth like the real mcCoy I had made.
It has an issue at the moment but it sounded amazing when I last had it running. It was if the singers were really in the room. Something to do with 2nd order harmonic distortion I believe.

[Valvepower]

Hello,

The subject of single ended transformers has obviously come up in this thread and here are a few pictures of the stripped-down SE output transformer from one of the ‘Convertible’ Tripletone EL84 amplifiers I bagged at Tonbridge.

The transformer is being stripping down so I can fit a bracket to the frame, to hold the speaker socket,, which had been broken off.

The ‘gapping’ paper can be seen in one of the photos and measures 0.002”

Terry

[Gabe001]

Never seen one taken apart before. Are the laminations and paper glued?

[Valvepower]

No gluing or suchlike…

Basically, when I removed the transformer from the frame it all fell out in a pile on the bench… ‘I’ laminations ‘an all...

On reassembly, I’ve got to make sure the top ‘I’ laminations are firmly against the ‘E’ laminations…

Terry

[G6Tanuki]

The issue of DC through the single-ended output-transformer is sometimes handled by replacing the usual output-transformer-in-the-anode-to-HT-path by a suitably rated choke, which only has to handle DC.

Then an output-transformer is coupled to the abode via a capacitor, the other side of the output-transformer being grounded.

So there is no DC going through the transformer, only the AC component.

You can therefore optimise the two components more easily because they're each only having o do one thing. Downside is that you end up with two big, heavy and expensive components!

It's sometimes called a "Parafeed" amplifier/


I've used the choke-in-the-anode approach in a quick-and-dirty transmitter modulator - do a search on "Heising" modulation for historic examples]

[Gabe001]

Originally Posted by Valvepower

No gluing or suchlike…

Basically, when I removed the transformer from the frame it all fell out in a pile on the bench… ‘I’ laminations ‘an all...

On reassembly, I’ve got to make sure the top ‘I’ laminations are firmly against the ‘E’ laminations…

Terry

So this is why they're called EI - core transformers!

[Gabe001]

Originally Posted by G6Tanuki

The issue of DC through the single-ended output-transformer is sometimes handled by replacing the usual output-transformer-in-the-anode-to-HT-path by a suitably rated choke, which only has to handle DC.

Then an output-transformer is coupled to the abode via a capacitor, the other side of the output-transformer being grounded.

So there is no DC going through the transformer, only the AC component.

You can therefore optimise the two components more easily because they're each only having o do one thing. Downside is that you end up with two big, heavy and expensive components!

It's sometimes called a "Parafeed" amplifier/


I've used the choke-in-the-anode approach in a quick-and-dirty transmitter modulator - do a search on "Heising" modulation for historic examples]

What a splendid idea g6tanuki. I like that. Lots more info here.
https://www.tubecad.com/2014/09/blog0308.htm

If the transformers misbehave in my PSE because of the large-ish current , its certainly something to consider as a plan b.

[kalee20]

Originally Posted by Valvepower

On reassembly, I’ve got to make sure the top ‘I’ laminations are firmly against the ‘E’ laminations…

Easy way to do that is make sure they're a close fit (but not a tight fit) in the clamp or whatever.

Then, pass full DC - or more - through the primary winding. They'll all pull together. Clamp-up with the current on.

If you dip, or impregnate, similarly keep the current on till everything is finished. It might be difficult inside a vacuum chamber, but on removal you can reconnect, with long leads, to your power supply so that you can bake it and cool it with the electromagnetic force present. Then everything will be locked together just as you want it!

[GrimJosef]

Originally Posted by Gabe001

So this is why they're called EI - core transformers!

Indeed.

If you imagine two Es placed open side to open side then you can see that two Is could be stamped out of the gaps between the Es' 'fingers' without any metal being wasted. If you impose the condition that the different sections of each E should all be the same width (let's call it 'one unit' wide), except for the middle finger, which should be twice that width, then you find that each E must be six units high by four units wide, and when you lay an I across the open side of the E the resulting transformer lamination stack becomes six units by five units. This six-to-five ratio is very common in modern EI transformers.

Cheers,

GJ

[Radio Wrangler]

Originally Posted by Valvepower

The ‘gapping’ paper can be seen in one of the photos and measures 0.002”

Funny how two subject areas come together and quite ludicrous information becomes useful.

When I was formally taught machining (Rolls-Royce, Filton) I was showna technique for gently touching a rotating milling cutter onto a surface to get a reference level. Wet the metal surface with some cutting fluid and plaster on a bit of.... wait for it!...... "Izal Medicated" loo roll. It stays on by surface tension, its absolutely waterproof and oil proof. AND it's exactly two thou thick! so you can bring your cutter in quickly until it just rucks up the paper, and you know you have 0.002" to go to actual contact.

I never thought of it for gapping cores.

Who'd have thought it, the stuff does have a use after all!

David

[Ed_Dinning]

Yes, standard construction of output transformers for most SE 1940's and later sets such as DAC90a.
When rewound it is possible to use 2 thou Mylar tape in the gap.

Variation of gap from 0 (not quite as all lams do not touch) to 4 thou will producea wide range of frequency response/ distortion effects

Ed

[Valvepower]

Hello,

Wet the metal surface with some cutting fluid and plaster on a bit of.... wait for it!...... "Izal Medicated" loo roll.

Well, I didn’t expect a thread on single ended amplifiers lead to Izal Medicated" loo roll… Anyway, I can clearly remember when were kid my ‘ole Grandparents using Izal loo paper back in the 1960’s… it’s just one of those lasting memories of my Grandparents along with their Dansette Dorchester in the kitchen and a Bush TR130 in bedroom.

As from now, I won’t be able to think about singe ended output transformers without thinking of Izal loo paper

Will Izal loo paper have better 'audiophile' properties than the original paper or even Mylar?... No, Terry! - stop-it - and be sensible

Then, pass full DC - or more - through the primary winding. They'll all pull together. Clamp-up with the current on.

kalee20, just tried what you said, and it worked great… many thanks for that as its going to really help.

The DC resistance of the primary is 390Ω and taking the typical SE EL84 Ia as 50mA applying 19.5V gives me the required current. To make sure it going to pull in the lams I increased this volage by 10% to 22V.

I found it interesting the amount of ‘pull’ of the coil has with 56mA, thus giving a practical example of the effect of the standing anode current has on the magnetic properties of the transformer.

When rewound it is possible to use 2 thou Mylar tape in the gap.

I have some 0.003” mylar in the workshop.

Ah speaking of Filton, we visited the Concord Museum at Filton about a month ago, when I went to Bristol to attend the hi-fi show, and I have to say it’s definitely worth a visit.

Terry.

[Pamphonica]

The Pamphonic 606/A amplifier used in the 606 radiogram from about 1946 used parallelled push-pull EL32s. The main reason was to get more oomph from a rather low HT, courtesy of a direct-from-mains power supply! I restored one of these for the son of the Pamphonic founder, installing an isolating transformer to avoid nasty shocks later on. The output transformer was tiny, nasty, and open-circuit, and had to be replaced. No mains transformer, but a huge choke! Not a large saving.

Not single-ended, and not HiFi! But may be of interest.

-Jeremy

[Radio Wrangler]

Something lurking in that diagram which is rarely noticed is that the DC current in the amplifier HT not only passes through valves and output transformer, it also passes through rectifiers, fusebox and substation transformer. So in the days of zillions of tellies and radios all with half wave mains rectification, substation transformers had to be designed to survive the DC component and hopefully balance those on three phases against each other. Two into three doesn't go, so that's where six-phase comes from.

David

[Valvepower]

I have a Selmer guitar amplifier using the KT55, which was effectively two valves, with their own cathodes and a common anode in parallel and sited in the same bottle.

This valve was designed for the AC/DC market - and that's not an Australian rock band fan base!

See the X-Ray in post #13

https://www.vintage-radio.net/forum/...d.php?t=190247

Originally Posted by Pamphonica

used parallelled push-pull EL32s. The main reason was to get more oomph from a rather low HT, courtesy of a direct-from-mains power supply! -Jeremy

[Gabe001]

Any red flags in the description below regarding the choice of output transformers for this project (apart from the fact that it's made in china)? I will need 2.5k (or 2k) primary impedance and about 10wpc power output. Each transformer will be carrying about 90-100ma of current. I tried to look for c cores which measure a bit better in terms of frequency response but they hit the import duty threshold and I'm also not fond of exposed connections or cages.

Thanks
Gabriel

"Single-Ended Output Power: 25wx2
Winding method: Golden Segmentation Cross
Match with: Single-Ended tube amplifier only, Not for Push-Pull tube amplifier!
Primary impedance: Raa =2k - 6k ohms
Iron steel: Japanes Z11 0.35mm,
Secondary Impedance: R2 = 0 - 4 - 8 ohms
Frequence response: 25HZ-33KHZ(-1db) , 10HZ-65KHZ(-3db)
Inductance: TH2811/1KHz 1v 50-60H
Primary max curent: 130mA -160mA
Resistance Tolerance: <1%
Inductance allowable error: <5%
Quantity : Good Matched 2 Pieces"

[Radio Wrangler]

Originally Posted by Gabe001

Primary max curent: 130mA -160mA

Well that tells you what to do with them. They don't give distortion or loss of inductance numbers for those currents, but it's a fair assumption that saturation is starting by 130mA and may be getting serious by 160mA.

So you want to keep your quiescent bias current plus the peak signal current total under those limits.

David

[kalee20]

It's a bit dubious how those spec's are derived!

I'm suspicious at "Raa =2k - 6k ohms" (anode-anode impedance) preceded by "Single-Ended tube amplifier only, Not for Push-Pull tube amplifier!" There's some inconsistency there...

Frequency response is meaningless without a test circuit (is it, for instance, with a stiff voltage source applied to primary, where finite primary inductance may be less important, and you'll have good LF response but leakage inductance will hurt HF response; or with a current-source driving it such as a pentode, where finite primary inductance will start to hurt LF response but leakage inductance will have less effect?)

And is the frequency response the small-signal response, where core flux swing is insignificant... or is it at full-power?

Winding method "Golden Segmentation Cross" doesn't mean much!

Primary impedance 2k - 6k with a single inductance figure, makes it impossible to comment!

Originally Posted by Radio Wrangler

...it's a fair assumption that saturation is starting by 130mA and may be getting serious by 160mA.

Trouble is, as well as the standing DC flux (caused by the steady quiescent anode current), there's also a flux swing caused by the signal.

The biggest flux swing occurs at maximum power at the lowest frequency. It can't be determined by the peak anode current, because the anode current rise is countered by an opposing current flowing in the secondary. You need the core area and the number of turns to calculate the flux swing, figures which you won't have. And this peak flux needs to be comfortably below the saturation limit for the core material - again, something you won't know (though I did a Google search for 'Z11 silicon steel' - no data sheets, but I found a couple of audio forums and the consensus is that it's a capable grade).

They're probably good transformers, but there just isn't the right sort of data to comment more. And it's an expensive to find out otherwise...

[Gabe001]

Thanks David, yes will do. It will be around 100mA so a good safety margin.

The range is because you can order the 18w, 20w and 25w version. I suspect 130ma is for the 18w and 160ma for the 25w. Not sure whether the inductance range follows the same logic.

I just ordered a pair of the 25w version, so I'll know soon enough. They won't be Sowter's, but I hope they do the job reasonably well. Total cost including delivery and VAT £160 for the pair.

[Radio Wrangler]

There also seems to be a 'wee discrepancy' between that peak current limit andthe power levels they're talking about, assuming class-A and something like 350v HT.

David