Cool PP OPT found in my junk bin. Trans OPT

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I found a pair of these HST output transformers in my junk bin. I normally build tube audio equipment, I am not as familiar with solid state designs from scratch.

Curious if anyone would know a good pair of transistors which would be a good consideration with the 48 ohm pp primary impedence of these transformers? I want to experiment with these in order to achieve 3-5 watts output.
Thank you for ideas in advance.:sly:


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I found a pair of these HST output transformers in my junk bin. I normally build tube audio equipment, I am not as familiar with solid state designs from scratch.

Nothing wrong with experimenting... it's one of the ways we learn.

But you should know that sold state amplifiers haven't used output transformers in a very long time. I suggest a google search for "Amplifier Schematics" and go into the images list... you'll see what I mean.
Both Nelson Pass and Jean Hiraga (L'Audiophile) had at one time transistor/FET amplifiers that used a nominally 60 ohms primary. It's an interesting exercise to design a transistor amp with a bit higher supply voltage and lower current, transformed down to 8 or 4 ohm level.
But the 5W capability is a bit limited. Maybe a headphone amp?

I'd give you a reference IF I could remember. Maybe after my 2nd coffee ;-)

Everyone who answered the call, I thank you!

My only question was "What suitable output transistor might be a good choice to start?"

*1st Google is IMO is a crap-shoot

*I have over a half terabyte of every magazine from the 1950's-1970... of the USA and UK..close but no cigar as of yet.

*5 watts is more than adequate with four way horn drivers super efficient.
I have spent the better part of six months searching and researching, and only ask here when I have done exhaustive research.
After spending forty years with tube equipment, I thought it would be fun to play in the "sand" with an older design with these OPT's. I have built many versions of the 1969 JLH class A circuits back in the 1970's and 80's and restored HK 330B's, but nothing with output transformers. Also, interstage transformers are not an issue, I have several pair, also phase inverter circuits have been used in the past without the need for the interstage transformer whilst still employing the output xfmr. I am starting to think these were upgrades for better than average transistor radios. I have not found any link to the actual use of these OPT's in past literature, it is like searching for a ghost. FUN

Thank you all again, I am going to make these work, and will post results when fully tested and running.
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This is the Nemesis . You can push pull it with an input 10k:10k transformer. If you ask my opinion in MOSFETS , the highest trans Conductance/ input capacitor is FQA28N15. If run a push pull in class A , very reasonable for 5W , than the even order harmonics which are the main if not only ones , get canceled out.


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Thank You

kokoriantz, Thank you kindly for the Nemisis information. ;)
I was intending on going for class A with this project from the start, but was unsure which output device to use. I have a lot of time and huge parts bin so this will become a great learning experience.

Fets might be the way to go, but will assemble some other parts I have on hand first. I'll have to check if any 2sk133,34, or 35's are in the bin.
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2sk135 , nowadays 2sk1058 are lateral Mosfets with good linearity but very low trans Conductance. It is suitable for single ended but in push-pull, must have driver to have sufficient gain for feedback/damping factor. Look in your stock maybe have IRFP150.


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....My only question was "What suitable output transistor might be a good choice to start?"...

Transistor is transistor. Apart from size/ratings, there is not as much difference as in tubes.

Can you write the stresses? 48rCT 5W? The battery will be about 12V. The peak collector voltage is 24V. Peak collector current is 0.91A. In class B the dissipation is <3W so 1.5W per device. In class A the dissipation is >10W so 5W per device.

But the pri bal DC in A is also 0.91 Amps but the can says 0.75A. I guess it could do 3W in A at 9V 0.75A supply. (In those days we "never" used two devices in A because they would run so much cooler in B.)

Speed is no great issue because the bandwidth of the transformer will be less than any but the oldest Germanium devices.

"GE Transistor Manual" has mass pages on transformer amps and I assume you have digested this. - Transistor Manual 1964.pdf

> upgrades for better than average transistor radios.

I lived through that era and never saw these used for anything, although they were widely listed in catalogs. The case type is more Broadcast or Military than consumer. The spec strongly imply vehicle battery.
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...sold state amplifiers haven't used output transformers in a very long time...

What about current Macintoshs?

McIntosh MC2KW Amplifier

And i have heard a couple diy amps that use big triodes to drive MOSFETS (running at very high voltages) thru some custom Onetics OPTs. One of the best amps i have ever heard.

Why is that? None of the schematics for such amps I've seen had driver xformers. Although you can use them - just another design decision.

Not sure which schematics you searched, but the transformer shown SCREAMS mid 60´s :) so Germanium transistors and driver transformers.

Basically this, with proper values for 48 ohm CC impedance and 5W RMS output.

Let´s do a little Math to see if it applies to my suggestion (my memory, really, I grew in that era :) ) ... or not.

48 ohm CC so 12 ohm Collector load in a PP and 5W RMS mean 10W peak so:
Peak V= Sqrt(W*Z)=Sqrt(10W*12V)=Sqrt120=11,95Vpk which beautifully matches 12V supply.

So that transformer was designed in the 60´s for a 5W RMS amplifier fed from a 12V supply .... a very interesting and modern project ... way back then.

Which in the 60´s would have used PNP Germanium transistors and a driver transformer, in a basic configuration very similar to my example.
By the way it´s a Mullard example and shows the way things were done in early 60's.

Tons of examples but sadly only in Philips/Mullard/Siemens/RCA/etc. yellowing paper books, none considered worth scanning and uploading so please don´t ask for Internet links.

I rest my case. :)

EDIT:just saw PRR's post.
Yes, that´s the way it was done.
Guess he´s also a "silver haired Citizen" like me, he he. :p

People weaned on Silicon NPN devices, MosFets or chipamps , let alone Class D stuff won´t know what are we talking about , but believe me, none of these modern wonders was even dreamt of way back then :eek:
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Found ERA Correct Circuit...

After nearly giving up searching month after month, I finally found a circuit that exactly matched the criteria and specifications of the correct ERA for these transformers.
I was hoping for a Class A circuit and it does exist!
It is a Mullard design.:cool: Persistance pays:)

BOOKSHELF HOBBYIST: Radio and electronics hobbyist books
Clive Sinclaire

Transistor Audio Amplifier Manual
Bernard's 175 page 35
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Not sure anybody actually made such amps, it was beyond weird , Mullard itself labels it "experimental".
Standard was way closer to what´s shown in page 39 of the same book. (a transformer driven Push Pull OT amplifier).

That said, thanks for searching and finding it and even more for bringing to the public view that treasure of old books.


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I made a quick simulation with IRFP150 and 600:10k standard input transformer. For 0.11% distortion it needs 13w dissipation per transistor. The Damping factor is over 15 limited by the transformer's resistors.


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You would not need a driver transformer with a push-pull Class A OPS. I'd start by choosing something like D44Hxx devices - the transformer looks like needing 375mA per side, 750mA total. The OPS can be configured as a diff output pair, suitably driven with differential stages.
It is also possible to design Tx OPS in Class (A)B without driver transformer, but the bias arrangement is more difficult. One of my designs for 50W /12V PA used capacitor coupling to the output stage triple.
You could also use 2N3055 (new epi type, though you probably won't find any slow RCA devices now anyway) at a push. At this current range they will be near peak gain and fT.
Each half of the primary would be 12 ohms, so 12v would deliver something around 10W (depending on primary reistance, saturation voltage etc) peak into the winding.
So 12V seems OK.
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