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6L6 mono SE and preamplifier

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Hello to everyone,
some months ago I have been able to complete my first "diy" project (if interested click here) with great success.

Now that I acquired some more knowledge I would like to start a new project;
I would like to build a complete unit with PSU, power amplifier and also preamplifier.

Attached are the schematics that I'm considering (Correction; this is the original version not the final one).

As regard the PSU (that I personally have designed in order to obtain the indicated requirements as B+ voltages and smoothness) could anyone provides me some suggestions to correct or improve it?

Can also someone indicates me a suitable output transformer for 6L6 in SE configuration (induction value etc.)?
 

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PRR

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Joined 2003
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What do the 2 Ohm resistors add to the power supply?

40uFd 5H 40uFd may be enough filtering-- it was for many old radios.

2 Ohms and 40uFd is a 2KHz low-pass.... will do nothing for 100/120Hz ripple.

Why do you use a cathode follower to drive a simple 6L6 grid? Should not need that.

Is your 6L6 screen grid divider passing enough current (low enough impedance) to handle the change of screen current from idle to MAX? Old data says 3mA to 5mA, but real tubes (especially recent-made) may be far off these numbers.

Is it necessary to run the SE 6L6 way up at 360V? (Perhaps not...)
 
PRR and Allen have touched on very important details of your power amp. I would add that the screen bypass cap needs to go to the 6L6 cathode.

As far as the preamp goes, what you have there is a phono stage with adjustable EQ. If you're planning to run a source that isn't a record player or a tape machine with direct head outputs, it's not going to work particularly well for you.

Part of the tug of war that you are going to experience is that the small currents drawn by a tube preamp allow for much larger power supply capacitors with a tube rectifier (and a quieter preamp). My solution when I have built similar units is to use a couple of 0D3s in series to make a quite +300V regulated rail for preamp duty.
 
What do the 2 Ohm resistors add to the power supply? <snip>

As regard the PSU, the first schematic attached is completely wrong and you are perfectly right about the CLC configuration.

As regard the power amplifier schematic I'm not sure specifically why the author of it runs the SE 6L6 with 360V. What do you suggest? Indeed in designing the PSU I faced some problems in rising the B+ up to 360V while filtering it enough.
 
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Hello audiowize, I have the intention to use a record player as source indeed many suggest me this preamp schematic for that purpose.

As regard the use of a couple of 0D3s I was wondering why you suggest that? Isn't it just an increase of complexity (number of tubes, bigger chassis etc.)?

And finally would you be so kind to explain me more about that the screen bypass cap? The author modified this circuit several times in order to get a flatter response (aimed especially if you want use a adjustable EQ preamp) as you can see here.
 
The power supply you have is likely too noisy for an RIAA phono preamp, but the extra RC node isn't necessarily helpful for the speaker amplifier. The series string of 0D3 with appropriate dropping resistor will shave off some of your B+ voltage and provide very low ripple for the preamp stage.

For pentode operation, the screen needs to be bypassed to the cathode.
 
Sorry again I also noticed that the power amp schematic that I uploaded first is the version without the modification to flatten the frequency response.

These are the correct schematics which I'm intended to build. As PRR stated there is no need of 360V B+.
 

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The power supply you have is likely too noisy for an RIAA phono preamp, but the extra RC node isn't necessarily helpful for the speaker amplifier. The series string of 0D3 with appropriate dropping resistor will shave off some of your B+ voltage and provide very low ripple for the preamp stage.

For pentode operation, the screen needs to be bypassed to the cathode.

Is there another way to avoid the use of two more tubes?

Moreover isn't 20uF 450V cap the screen bypass cap?
 
Is there another way to avoid the use of two more tubes?
Sure, use zener diodes.

You can also find chokes that have a lot of inductance at less current, and one of those will knock the ripple way, way down for your RIAA preamp.
Moreover isn't 20uF 450V cap the screen bypass cap?
Yes, the screen bypass cap needs to connect to the cathode, not ground.
 
Yes, the screen bypass cap needs to connect to the cathode, not ground.

Just the capacitor or the parallel resistor too?

You can also find chokes that have a lot of inductance at less current, and one of those will knock the ripple way, way down for your RIAA preamp.

In the preamp schematic there is another inductance just before the B+ input. Should be this sufficient if I choose correctly?
 
Just the capacitor will do.

The 20H to 470uF should shave off a lot of PS noise. I don't understand the voltage spec ahead of the choke, and it's pretty tough to read the values on the fuzzy schematic.

It is stated 20H min 15mA. The B+ should be 320V to 350V.

So the second inductance will be sufficient?
 
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Works well as it is now. The cathode is bypassed fully with 470 uF capacitor, so there is no difference if the screen is bypassed to cathode or to gnd.

Thanks for your contribution. Also what you think about the PSU that I designed? And as regard the clean B+ for the preamp? (PSU and additional inductance would be sufficient?).
 
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