Fairchild 670 PSU for tube preamp

[drtechno]

Not sure how you'd be using EL34 tubes in a preamp...

If you didn't need EL34s and 400+VDC, I might suggest using a transformer with a 240V secondary, then run a bridge rectifier after that. 230-240VAC, being very commonly available as either an isolation transformer for countries that use 230-240V, or was a step up from 100/120VAC to 230-240.

You'd get ~340VDC in a well filtered supply, and you could do a number of things to regulate the HV, if you choose to. Inductor chokes were cheap and capacitors were expensive back in the day.
A series diode will do more to cut ripple than a poorly tuned choke.
Use good capacitors & proper grounding (Negative of the bridge rectifier should connect directly to the negative of the first brute force filter cap, or with a large diameter wire, as this is where the main noise/ripple will be).

You could run a separate 6.3V filament transformer and a separate HV transformer, which in this case might save you some money, and allow separate AC turn on on and B+ voltage turn on.

The EL34 is used as the 340V regulator tube in the Fairchild power supply, which is adjustable as well.
There will always be cross-talk when the circuit is unbalanced and on the same power supply. Splitting them into regulators somewhat solves it but not really because the design and layout has more to do with cross-talk. The Fairchild circuit has no cross talk because the circuit is fully balanced and the signal is not referenced to the power supply so the power supply is not going to induce cross-talk.

So, what this guy is doing is a waste of time.
The grounding scheme of an unbalanced circuit is what induces cross-talk and not its power supply. Power is only 1/3 of the signal grounds(common) in an unbalanced circuit.

 

[Walter66]

Thanks for your input, good point.

Although the project is not about designing a no crosstalk PSU, it's simply to create a lifelike sounding PSU, doesn't matter if tubes are being used or not.
The topic we discussed here lately, using a cap multiplier prior to a LM317 regulator is another way to achieve good measurements.
What I'm unsure is, how will it compare soundwise with a fully tube regulated supply?

When comparing my actual choke- BJT cap supplier PSU vs. a fully EL156 tube regulated one, the sand one wins soundwise. Its able to produce sharpness and contours to the sound where the tube regulated one just produces blurr and clouds.

Excellent sound quality and good measurements is high priority in this project. Simplicity in design often wins when it comes to good sound quality.
I don't regard the Fairchild as a complex circuit schemo, but very effective and it has it's own beauty.

That's why I came about the Fairchild PSU, not because it had excellent channel separation due to the balanced and not PSU referenced circuit.
I just did a survey of the best tube regulators and tried destillation of a practical audio circuit.

But I'm still wondering why Fairchild used the EL34 as a regulator while there were special low impedance regulator tubes which don't eat that much of voltage (200V) to achieve good SNR and line regulation.

Btw, some PSU regulators use line- and some only load regulation. What's preferable in an audio circuit, unbalanced, single ended design?
The Fairchild has line and load regulation, the simple 6BM8 only load regulation.

 

[Cqwet Dbdfte]

Yes, use whatever tech best suits the purpose.
Power supplies should not have a "sound", contribute to it, nor improve it.
0V does not have a sound.
Regulation is all about removing any of trace of a sound.
A better power supply would besides sourcing a voltage also sink current, essentially be a power amplifier with DC output.
In the tube era that would be a complicated, big, and expensive gizmo reserved for lab use.
Now not too crazy to add a shunt stage on the output.

 

[scott2000]

Maybe something like Heikki's?

https://www.ghr.fi/proaudio/6L60/660ps.pdf

 

[Cqwet Dbdfte]

Maybe something like Heikki's?

https://www.ghr.fi/proaudio/6L60/660ps.pdf

That looks like a straight up tube PSU, with silicon replacements.
Nothing new or original.

 

[scott2000]

That looks like a straight up tube PSU, with silicon replacements.
Nothing new or original.

is there a point to this statement?

I have been using HV MosFet + Lm317 + zener etc for 30+ years for low noise regulation.

Power supply only need to put out low noise DC with minimal load fluctuations.
SiC diodes like vacuum rectifiers have no reverse recovery.
An all tube regulated power supply will be as big as large as the typical power amp. No need for that.

 

[Cqwet Dbdfte]

@scott2000 Yes, Nothing new nor original.
Same old. No precision reference reference, Zeners have a soft knee, HV ones even more, no bypassing of it, massive input RC filter, loses a lot of voltage, weak or incidental short circuit protection, Q2 gate threshold temp dependent, gate threshold vary part to part necessitating pot to set voltage, also likely to drift.
May be OK for some.

 

[scott2000]

@scott2000 Yes, Nothing new nor original.
Same old. No precision reference reference, Zeners have a soft knee, HV ones even more, no bypassing of it, massive input RC filter, loses a lot of voltage, weak or incidental short circuit protection, Q2 gate threshold temp dependent, gate threshold vary part to part necessitating pot to set voltage, also likely to drift.
May be OK for some.

interesting... I haven't tried it yet but something tells me it will be aok...
Hope so... I've got a stack of the pcbs...lol

 

[drtechno]

When comparing my actual choke- BJT cap supplier PSU vs. a fully EL156 tube regulated one, the sand one wins soundwise. Its able to produce sharpness and contours to the sound where the tube regulated one just produces blurr and clouds

that is expected when you build with the wrong type of parts .
But a fairchild is suppose to blur the sound stage little anyways when you push them.
The time I saw a commercial master engineer use it, they use lightly, but loop trough it several times instead of just slamming it and runing it once through.

But the power supply doesn't bear anything to a fairchild circuit because the signal is not referenced to the power supply. So what circuit are you trying to use? Let me guess: a grounded cathode.

 

[drtechno]

But I'm still wondering why Fairchild used the EL34 as a regulator while there were special low impedance regulator tubes which don't eat that much of voltage (200V) to achieve good SNR and line regulation.

Back in those days, RCA held many power supply patents with its 6080 tube. Maybe that was the reason.

 

[drtechno]

Maybe something like Heikki's?

https://www.ghr.fi/proaudio/6L60/660ps.pdf

Solid state power supplies are bad on tubes, and you want to soft start a fairchild tube pig with all those transformers. However, it would be interesting how much inrush current is developed as the solid state power supply creates more places for holes to go with a loud bang.

 

[scott2000]

and you want to soft start a fairchild tube pig with all those transformers

I think the op is building a pre amp and just referenced the fairchild power supply as something to power it...

 

[drtechno]

I think the op is building a pre amp and just referenced the fairchild power supply as something to power it...

I bet his circuit is some sort of grounded cathode or some other unbalanced circuit that adds the power supply noise to the signal. That in reality when you make an unbalanced circuit power supply (regardless of amplification device) you would want the power to have the same impedance as the ground to the AC signal's common (zero crossing)

 

[Walter66]

that is expected when you build with the wrong type of parts .
But a fairchild is suppose to blur the sound stage little anyways when you push them.
The time I saw a commercial master engineer use it, they use lightly, but loop trough it several times instead of just slamming it and runing it once through.

But the power supply doesn't bear anything to a fairchild circuit because the signal is not referenced to the power supply. So what circuit are you trying to use? Let me guess: a grounded cathode.

My preamp design is single ended throughout, stereo, using one PSU. It's a full function tube preamp, pulling 60mA for both channels.

Well, the Wandel & Goltermann tube PSU uses a huge EL 156 as a pentode regulator while Fairchild used EL34 triode wired.


There can be many reasons why a PSU sound different. First, it's technical design. Building style and choice of parts is next. Iron can make a huge difference in sound. I experienced with the first choke after an oil cap behind the rectifier. Choosing cheap asian iron sounded quite inadequate, Lundahl was best, Tango iron great improvement, too. Fairchild just did things in the right manner. They choosed the design wisely, build with the best parts and the best available iron. Modern replica incarnations have a hard time to beat that.

 

[drtechno]

My preamp design is single ended throughout, stereo using one PSU.

I don't really understand what you mean with "the signal is not referenced to the Power supply".?

Well, I will show you a fully balance preamp circuit that is in most high end studio gear. Which has 40db of CMRR with no power supply reference of the signal:

 

[Walter66]

Solid state power supplies are bad on tubes, and you want to soft start a fairchild tube pig with all those transformers. However, it would be interesting how much inrush current is developed as the solid state power supply creates more places for holes to go with a loud bang.

You think it's inapropriate to go with a LM 317 regulator in combination with a big iron tubed preamp design? The mains transformer would be a 50W type at least. Maybe even bigger. I would use tube rectifier GZ34 anyway, so soft-start would be implemented.

 

[drtechno]

You think it's inapropriate to go with a LM 317 regulator in combination with a big iron tubed preamp design? The mains transformer would be a 50W type at least.

I Just think of the inrush current. The DIsA preamp example does have a solid state, but with a tube regulator that sags the voltage and soft starts it.
I would rework it with a 6X4 tube instead of the bridge so the amp soft starts with the heaters powering up ahead of the B+.
Because when the tubes are cold, the B+ potential is at the cathode, So depending on where the heater is reference will temporarily exceed the cathode to heater potential. Over time, this stresses the heater until it opens up.

 

[Cqwet Dbdfte]

Use a meg ohm resistor feeding the gate cap on the MOSFET and get whatever soft start you want 30 seconds no problem.
Soft start the tube heaters too with an ICL, startup current is x10 typ. on cold NiCr heaters.
I put those inrush current limiters (CL60, CL90 etc)
on my tube power amps 120V AC mains (amp has 4x 6550 and 1500uF photo flash caps in the power supply. )
Keeps the power switch from frying.

 

[Cqwet Dbdfte]

Well, I will show you a fully balance preamp circuit that is in most high end studio gear. Which has 40db of CMRR with no power supply reference of the signal:

The grid resistors have the same ground symbol as the AC mains in the schematic.

View attachment 120023

Well, I will show you a fully balance preamp circuit that is in most high end studio gear. Which has 40db of CMRR with no power supply reference of the signal:
View attachment 120023

Are not the tube grid resistors referenced to ground? Same ground symbol as everywhere else in the schematic.
A good design but not true differential circuit.

 

[drtechno]

The grid resistors have the same ground symbol as the AC mains in the schematic.

Are not the tube grid resistors referenced to ground? Same ground symbol as everywhere else in the schematic.
A good design but not true differential circuit.

The rack these cards were in don't have an AC ground. It only has two wires to it.
The AC transformer on each card is double insulated.
Now a days I would do the Earth ground from the outlet, and not tie common or the power leg to it with a safety cap. Ground it to the chassis. But not tie the power supply ground to the chassis. And the only thing tied to chassis grounds are shields.
A balanced circuit is symmetrical. so is a differential circuit which is a type of symmetrical circuit.
B+ power is not reference to just one polarity of the signal, and all noise on B+ is cancelled at the transformer. So there is no power supply noise on the output. So its not considered reference because noise from the supply is not permanently added to the signal.
Look at the fairchild 670 schematics, it has a similar affair going on in its circuits. A lot of the main stream somewhat low end pro audio like API312, LA2A, 610, 1176, 1073 are quick and easy unbalanced circuits. They were cheap to make and later easy to market as a kit.