Mono Tube Amplifier.

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Pim Driessen

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Dear Forum Members,
I own two Unitran output transformers 9U13 and I would like to make a few mono amplifiers with EL84 tubes.
I am not completely new with tube amplifiers, I have rebuilt a Lafayette amplifier in a distant past, it still plays, but it is not convenient that it is a stereo amplifier because of the cables for the speakers and preamp.
Here an image of the newly built amplifier.

And as it stands now.


Now comes my question, what is a nice design for the Unitran transformer with 2 × EL84?

If I could make the choice, a rectifier tube would be preferable.
Does anyone know a good design, have already looked around but can't see the tube forest through the tubes.
Greeting,
Pim
 
Have a look at the "GEC 912+" circuit.

That's listed in the John Linsley-Hood "Valve and transistor audio amplifiers" book as one of the later and more advanced designs, with a pair of EL84s and a tapped output transformer.

This is an example of that design:

In the JLH book its shown with the input stage and PSU omitted, with a 12AX7 for the dual triode and EL84s, with 270 Ohm cathode resistors.

I also found the Unitran catalog with the info for that transformer:
 
His transformers have feedback taps (which I believe are supposed to improve linearity), which is why I selected a design that uses them.
 
rjenkinsgb said:
His transformers have feedback taps (which I believe are supposed to improve linearity), which is why I selected a design that uses them.
Click to expand...

It was known as 'ultra linear' back in the day. It's a trivial mod, and 5-10's were often built using UL transformers.

Mullard 5-10 stereo

Hi All I am building a Mullard 5-10 stereo kit for a client. I am following the original circuit, but the usual issues will present themselves: how to reduce the gain on the amplifier and improve the sound etc. I'll be posting some photos next.
www.avforums.co.za

I don't know as they were really 'feedback'?, but more like a little extra drive to the transformer from the screen grids.
 
Thank you for your comment, go deeper into it,
Greeting,
Pim
 
Pim Driessen said:
Thank you all for your response, go deeper into it.
Greeting,
Pim
Click to expand...

What is your amplifier above built on?, it doesn't look to be a metal chassis?. Must admit, the only valve amp I ever built (back in the 60's) was built on a piece of hardboard from the tip, and mounted in an old suitcase (also from the tip), with a speaker and grill also mounted in the case. Pretty well everything came from the tip, including most of the components - with the exception of the 6BW6 output valve, and mains transformer, which I was given (as those weren't commonly available at the tip).

I still have the old suitcase, complete with hole where the speaker went - I've no idea what happened to the amplifier out of it?.
 
Pim Driessen said:
The material is Corian, widely used as countertops, is from Dupont.
Click to expand...

I suspect it's screening properties aren't too great?

Which is the reason for using a metal chassis, particularly for hum reduction.

Tubes are becoming more and more popular.
Click to expand...

I don't know as 'popular' is a word I'd use, you haven't been able to buy decent quality valves for a VERY long time.

Although I will admit they are useful for some guitar amplifiers, where their low quality is an advantage. But even there, where they might use a valve amp (or big stacks of them) on stage, they often use transistor amps in the studio for recordings.
 
In terms of foreclosure, Corian probably won't do much.
I just asked Corian Nederland today, it is being checked whether studies have been done about it.
But otherwise I have no problems with switching noises or radiation from the outside.
Plan to clad the inside with aluminum for the mono blocks to be built.
Here is a picture of the bottom of this stereo tube amplifier.
 

Interesting busbar type construction.
 
crutschow said:
I don't understand the fascination with antique tubes and vinyl, but to each his own
Click to expand...
Some people like old cars
Some people like old houses
Some people like <insert item here>
As you say, each to their own.

Nigel Goodwin said:
Interesting busbar type construction.
Click to expand...
I was thinking that myself.

All in all it is a strange form of construction, I would not choose it myself,
but it is very nicely done, looks good.

JimB
 
JimB said:
All in all it is a strange form of construction, I would not choose it myself,
but it is very nicely done, looks good.
Click to expand...

I was thinking you must need a substantial soldering iron?, or at least a large bit.
 
A neighbor from my previous house had a Camaro from the late 1960s. You know, 6.5L pushrod V8, with a four-barrel Holley carburetor, tricked out camshaft and headers.
Four speed manual transmission with a Hurst shifter.
He claimed it was the best car.
Best at what? It had worst mileage than an aircraft carrier, guzzling premium gas and polluting like crazy.
Drum brakes, solid axles and severe oversteer and no factory installed seat belts. No creature comforts other than a puny A/C system.
When you parked it, it would leave an oil stain.

Yet, I must admit that hitting 120 MPH, with all the big engine madly growling and vibrating, while the suspension struggled to keep all wheels on the road, it was exhilarating as hell. White knuckled adrenaline all the way.

At the same speed, It definitely beats the leisurely, cocooned sensation of a Mercedes S-class in its plush, conditioned environment while listening to Vivaldi via a 5.1 Dolby system.

I totally get why people love tubes.
 
I went to look for what is present in my house in terms of diagrams of tube amplifiers.
I came across a diagram in a book by R. zur Linde that I really like, what do you think?
As an extra I would stabilize the filament voltage with an Lm338K TO3 housing.
Greeting,
Pim
 
Pim Driessen said:
I came across a diagram in a book by R. zur Linde that I really like, what do you think?
Click to expand...
Looks like a standard circuit, that should work for you.
Pim Driessen said:
As an extra I would stabilize the filament voltage with an Lm338K TO3 housing.
Click to expand...
Of course you will need a rectifier bridge and capacitor to generate the DC voltage required.
The total required filament current will determine the size heatsink you need for the regulator.
6.3Vac will generate about 7.8Vdc with a Schottky diode bridge rectifier, so you will likely need a low-dropout regulator (which the LM338 is not) to get the regulated 6.3Vdc you need.
 
Go find a transformer with 10 volts extra, then the LM338 TO220 housing can be used, if I don't find such a transformer, then an option is to Amplimo to have a transformer wound to specification.
Greetings,
Pim
 
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