4-channel Tube Pre Layout

Working on a 4-channel tube preamp layout, and I'm trying really hard to get the PSU inside the case without noise problems. External PSU increases the cost way too much.

My 2-channel version required a bit of MuMetal to quiet it down, but I'm trying some different PSU transformer types.

Let me know if you see any pitfalls in this design.

And the front panel...

Dave's  EF85 project has a nice power supply set up using toroids though.....

https://groupdiy.com/index.php?topic=62689.40

In old tube tape recorders it was common to rotate the mains transformer by 45 degrees. You could try this and also rotate the Jensen OP transformers by the same amount in the opposite direction so they are at right angles to the mains transformer.It is surprising how much unshielded OP transformers will pick up magnetically.

Cheers

IAn

I'm going to try a Don Audio toroid versus a custom Edcor EI and see which one is better.

Another EI option is to mount it on either the outside or inside of the rear panel.

Like this, Ian?

BluegrassDan said:

I'm going to try a Don Audio toroid versus a custom Edcor EI and see which one is better.

Another EI option is to mount it on either the outside or inside of the rear panel.

Click to expand...

I wouldn’t stick a toroid inside that box unless it’s tucked away nicely and shielded well. Preamps can be sensitive. That’s just me though..

BluegrassDan said:

Like this, Ian?

Click to expand...

That's it.

Cheers

Ian

BluegrassDan said:

Like this?

Click to expand...

It's a start but if you have the time don't immediately drill holes for any of the transformers and experiment with the orientation at least for the channel closes to the PSU transformer. The case internal reflections (if it's iron based) can create nasty surprises. Or the reflections from where you end up installing the preamp case ie. everything is nice on the lab bench but noise goes up 10dB when it's in a rack.

It's going to get annoying when you most likely find out the 50/60hz hum will not go away. Consider the external PSU even if it costs more and save a lifetime of annoyance. You already have high end transformers there, it's not going to break the budget.

[edit]

With external PSU you need DC heaters, which can complicate things. Don't want the dirty 6-12VAC running along any length of clean DC high voltage lines.

BluegrassDan said:

I'm going to try a Don Audio toroid versus a custom Edcor EI and see which one is better.

Another EI option is to mount it on either the outside or inside of the rear panel.

Click to expand...

Don-Audios toroids are very fine but I wonder if they have enough juice for heating eight tubes though?

I have used them very much and I also place them inside the box but I haven't built a four channel preamp based on those transformers.


Best regards

/John

Don Audio sells a version with 4.5A 12v.

And how much current is drawn by the 8 (?) preamp tubes?

Heater current that is



Best regards

/John

John Hardy once told me at an AES that he has a test rig in which he places his toroid transformers energized in circuit and listens for the quietest relationship while rotating them, when he finds that spot he marks each one so they can be installed accordingly. His transformers are located on the channel 4 side of his 4 channel mic pre.

johnheath said:

And how much current is drawn by the 8 (?) preamp tubes?

Heater current that is



Best regards

/John

Click to expand...

Four 6072s and four 12BH7s draw 1.9A of heater current at 12.6v.

I would not build a tube preamp with an EI power transformer in the same box.

A toroid will be easier to get to acceptably low hum levels.
I've used Antek for several builds, like this:
http://www.antekinc.com/as-1t250/

Has 250v with two 3A heater windings.
It looks like your front panel is 3U, which means you can side mount the transformer.
Antek sells metal shields to put over them or you can also build something yourself with mumetal.

I used a toroid like this in my V76 build which you can see here:
https://groupdiy.com/index.php?topic=68498.msg871702#msg871702

The Jensen output doesn't pick up any noticeable hum.

I had to mount the transformer on the bottom since I was doing a 2U.

If you do want to use EI output and power transformers, they should be mounted so they are off axis at right angles.
Look at how Fender guitar amps had the power transformer on it's side so the EI core was pointing up, at a right angle from the output transformer.
You'll find minimum hum with them in this orientation.
Increased distance decreases hum with the inverse power law so the further apart the better.

I tried using a EI transformer initially in that V76 and the hum was awful. I'm not sure you'll be able to do it in a box that size without really fighting a battle.

As things go, I'm now working on an external PSU. Stay tuned.

You might want caps at the top of the plate resistors to avoid crosstalk between channels, via the power rail.

Use a heavy copper wire (~12 gauge) for the ground buss wire as well. 

if you are going external, have you considered using smpsu's?  would be less $$ and can easily give you the current you need for all the heaters...

pucho812 said:

if you are going external, have you considered using smpsu's?  would be less $$ and can easily give you the current you need for all the heaters...

Click to expand...

Please tell me more...