• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

First Tube Project - S5 Electronics Preamp

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Joined 2000
Paid Member
For several reasons, I have selected the S5 Electronics preamp as my first valve project.
It's just about the simplest tube preamp schematic you can imagine. There's a cathode and plate resistor, cap on input and output.
Uses a 5670 twin triode. Tiny thing. Doesn't include a power supply.

So I'm looking to find anyone who's built this kit, like to know their experience and if there's any simple improvements without crazy modifications.
There could be others out there that have built similar simple preamps.
Like to hear their thoughts as well.

I don't know a lot about tubes, but I see many designs with a cathode resistor bypass cap.
I suppose I could just try it, but I'd rather ask than break the thing out of the gate. Are there any recommended values for the bypass cap?
The Cathode Bypass Capacitor Explained - DIY AUDIO BLOG, AUDIO WORKSHOP

For $25 more, I purchased slightly "better" resistors and caps: Clarity caps and PRR resistors as opposed to orange drops and carbon resistors.

In order to suggest a modification, it is really helpful to start with information about the preamp.
A schematic would prove helpful.

A link to the S5 Electronics would be nice too.

It is also helpful to know how you will use this preamp.

Your signal source into the preamp is ___________
Your amplifier the preamp output will drive is __________
Joined 2000
Paid Member
Thanks for answering.

Didn't include the schematic since it's a commercial product. Although it's probably no more than an applications note's design.

Sources into tube preamp: phono stage and line level DAC.

Tube preamp high gain output to solid state power follower (no voltage gain).
Tube preamp low gain to standard solid state power amps.

Amps range from SE mosfet to two stage with jfet inputs.

This preamp can be set to 10, 20 or 30db of gain, which is something I need.
S5 Electronics
Joined 2011
Depends on your load, but if the input impedance of the amp is over 100k, it's probably not
worthwhile, since smaller caps tend to sound better overall (if they are still large enough).
Make sure the capacitors are not Mylar (polyester); those should be replaced with pp types.

If the grids connect to ground through a resistor, you may be able to short the input capacitors.
Some biasing circuits do elevate the grid above ground though, even in a common cathode circuit.

Is this like the circuit (without the Ck)? If so, there's no good reason to have the input coupling capacitor.
Designing Common-Cathode Triode Amplifiers
Last edited:
Joined 2000
Paid Member
Was able to get the S5 circuit built over a few weekends.
Plenty of gain to drive the First Watt F4 without changing resistor values.
I think it sounds pretty good out of the gate.
Has the tube sound I was looking for.
Would have been disappointed if it sounded like a transistor preamp. :D


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Joined 2000
Paid Member
Question about the cathode resistor capacitor voltage.
I measured 1 volt across the cathode resistor.
Would a 47uF 25v electrolytic be safe or does the voltage across it change with a signal?

I picked up a 60uF MKP 300vac, but it will not fit in the case.
They are 2.25"D x 2"L.


Joined 2000
Paid Member
The 6922 filament are not lighting up.
Using the Electro-harmonix gold pin 6922.
Is the 6922 really a direct drop-in replacement for the 5670?
It's ok if it's not. Will send it back.

The 5670 still works, and the bypass caps are not affecting the gain negatively.
There's a bit more midrange clarity.

I'll let this cook for a little bit.
Pretty happy with it as is.


Joined 2000
Paid Member
It's a fantastic combo.
Replace the 5560 with a 2C51.
The tube costs as much as the kit, but it's worth it.
You might want to upgrade the caps and resistors that come with the kit but that's up to you.

You'll need to build a 200v dc supply and 6.3 heater.

Let us know what you think if you build it.

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