TUBECLINIC

LINZ / AUSTRIA

STC-technology in general

STC should be read as the abbreviation of

“Standard Tube Circuitry”,
which in its meaning almost tells everything about…

In the entire very first, I have to say, that this article is no stuff for the beginner. You needed
some experience in applied electronics to follow these explanations. You should also have
some experience with the design of tube circuits, to understand this material. Please have
a look for a skilled electronics tech, if you are not able to follow this material!

How STC started

I came to this technology, because I am playing electric jazz guitar and tried to find a way, to achieve this
nasty triode tube-sound by the use of semiconductors. First all my attempts failed and I was close to the
point to stop my research.
Well, sometimes only good luck helps. A musician from my neighborhood asked me to help with his solid
state combo-amp, which he wanted to use with his acoustic guitar. The built-in piezo pickup had too high
impedance for the amps input. He had tried by himself with an OP-amp as a preamp, but failed. It did not
sound like he wanted. I gave him a JFET high impedance frontend – and he was happy. Some days later he
came in again and meant, if we could not mount this JFET stage into his guitar. Now we had the additional
problem that it should use only as little battery power as possible. So I defined during tests that about 1mA
were the least possible. He again came back some days later, a smile in his face, asking what I had really done.
He meant that now his guitar produces on stage the sweetest tone he ever heard. So I again went into
research after I had proven the sound by myself. This was the point, were STC really started. All in all I can tell
today, that I was misled before by data books, application notes, tech articles and so on. The only real way is
to use a tube schematic, search for a JFET with similar characteristics substituting the tube and use exactly
the same operating point and idling current. Easy – if we know…

© Barbara E. Gerhold „TUBECLINIC“, 2011 rev. 1.4

 TUBECLINIC
LINZ / AUSTRIA
Have a look at the above schematic: Do you dig what I mean? This little schematic shows the typical input
stage of a Fender® tube amp. Translated to STC it sounds the same and works the same. You could translate
for example also a typical Marshall® input stage (below). It again will sound the same as the tube thing!

The formulas for the translation:

The JFET
It needs to be chosen in a way, that its gate-source voltage is nearly the same as the grid-cathode voltage of
the tube at the same anode current. In our above schematic, the 12AX7 has an Ugc of about -2V, so we
choose a BF245B, which will show nearly the same value.
JFETs should be bought in larger quantities and selected afterwards. I always buy them in bulk of 100# and
select each type to several characteristics. The selection process is described in behind. Please be patient.
The supply voltage
It has to carried out as about 10% of the original tube schematic. For example: If the tube circuit had a supply
voltage of 250VDC, you should use 25VDC.
“Resistor ladders” between several JFET stages should be kept at 10% of the tube circuit values for to get
same operation (e.g. if the voltage drop ahead a frontend tube is 10V => 10k at 1mA, we use 1k at the same
current value to get 1V drop).
The caps in the supply circuit
As we use only 10% of the supply voltage, caps have to be 10 times bigger in value, than in the tube circuit. If
the original tube circuit says 10µF we have to use 100µF.
The source resistor
If you have chosen the right JFET, it will have the same value as the cathode resistor in the tube circuit.
The drain resistor
Look at the tube circuit and use 10% of the value of the anode resistor from the tube circuit.
The operating point, the idling current
It will settle at compared the same value of the tube circuit. For example: If the operating point was 50% of
+Ub in the tube circuit, it will again be 50% of +Ub in the JFET circuit.
Well, and the sound will also stay the same! Easy – if we know…

© Barbara E. Gerhold „TUBECLINIC“, 2011 rev. 1.4

 TUBECLINIC
LINZ / AUSTRIA

Some typical, frequently asked questions and their answers

- How would you define the sound of such a STC-circuit?
Very, very tube-like. Maybe the sound will be a little crispier than the original tube sound. Here a small cap
with only some …pF across a drain resistor or against ground can help. If you use a hard coupled MOSFET
(see schematic of the STC Line-Preamp) as a drain follower, this is not necessary, because it has its own
gate capacitance. Some people told me, they associate the JFET-sound a little with a pentode in triode
mode, anyhow…
- If the supply voltage of the tube circuit is bigger than 300V, we have to manage. The maximum value of a
BF245 is 30V.
We could use only 30VDC and make the drain resistor a little smaller.
We could use a source resistor with a little bigger value (which would make drain current a little smaller
concurrently) and leave the drain resistor to is formulated value.
We could use another type of JFET (not so easy, because only a few types are really suited for STC).
- What – if the anode current is bigger than the chosen JFET can deliver?
We have to couple a small signal MOSFET (e.g. BS170). See schematic STC-Line-Preamp.
- Will the transition frequency be high enough?
Yes – BF245 reach 700MHz, truly enough for audio.
- Will the gate impedance be high enough?
Yes – it is higher than the grid impedance of a tube.
- Is a JFET as rugged as a tube?
No – but this problem can be handled easily. Max. power and voltage limits must not to be exceeded. Also
see next point.
- Are JFETs also due to oscillations like tubes?
Yes – and the same preparing will help. At inputs or outputs to connected items always use series resistors
(gate/grid-stoppers). Zener or Schottky barriers are used to protect against static discharge and couplings
to other equipment (e.g. tube circuits). Proper grounding should be regarded.

Now have a look at HiFi-Design

I pointed out a well respected and also well known circuit of a Tube Line-Preamp, to let you see, what is
today’s “State of the Art” in HiEnd tube tech. The following circuit’s schematic has a total amplification of
about 50x, a very high impedance input – only determined by the volume pot – and a balanced output
(+6dBm) together with an isolated but unbalanced ou

© Barbara E. Gerhold „TUBECLINIC“, 2011 rev. 1.4

 TUBECLINIC
LINZ / AUSTRIA

Some words about the parts needed

© Barbara E. Gerhold „TUBECLINIC“, 2011 rev. 1.4