ICL/Softone Model 1 Bias Instructions 2A3 300B

Click pics to enlarge. Top, innards of ICL Model 1. Note the 2 trim pots to the right of the triode sockets (V103). Note to the pots' right are printed Plate Current (Max and Min). Note towards the Upper Right corner the test point TP1.

ICL/Softone Model 1 Bias Instructions

This post came about because my friend wosirsir, whose audio forays (Quads and then my Tannoy Canterbury and EAR 509) I have chronicled many times in this blog, sent me a wechat pic of ICL/Softone Model 1 that he's listening to (see pic below).

It must have been at least 20 years ago that I bought from Japan the Model 1 2A3 and rather liked it. I just loved the quirky look. Later, when a Model 1 300B became available on the second-hand market I also grabbed it. They are basically the same amps (maybe a few components have different values, likely the anode resistors, and slightly higher Plate Voltage for the 300B; of course bias current differs).

I had so many amps, like Sun Audio, that I spent very little time with my ICL's. But when I returned to Hong Kong in 2020, towards the end of my stay I decided to fire up my SET amps and Ruark Crusader III and was pleasantly surprised, even shocked, that they worked so well together. The ICLs just sailed with the Ruark‘s, very musical.

The Model 1's have long been discontinued and can only be had second-hand. Here's the problem. This is a FIXED BIAS amp, and when swapping tubes one must re-bias. This article is to show how to do it.

As I use old stock tubes I had to re-bias the amps. For my 2A3, I used RCA double plates. No, I have never tried NOS WE300B in my Model 1. I emailed ICL/Softone and they replied. So I was good. Some years later, my hard disc crashed and I wanted the info and wrote them many times with no answer. As a last resort, I asked on AudioAsylum and the good folks helped me out (Tre' had the same email reply that I had before, and he rescued me). Here it is again:

***The procedures of the adjusting current of Model1 amplifier******

There is a place of a high voltage in the amplifier.
Open the bottom panel.
You can see the two pots near by 2A3 Sockets on PCB.
Two pots are used for adjustment of the plate current of 2A3.
Pot is set for the plate current of 2A3 to 45mA.
The operation of 2A3 of us is as follows.
Voltage of plate 310V from 300V
Current of plate 45mA from 40mA
Load resistance 5K ohm

At first, set two pots to MIN position for setting the plate current
to minimam.

And,You can see TP1 terminal right side of PCB.
TP1 is an output of the high voltage stabilization power supply.
TP1 has the voltage from 310 to 315V.
The direct current resistance value of the output transformer of
Model1 is 120 ohms.
The current of the plate is obtained from the difference of the
voltage of TP1 between the plate of 2A3.
Pot is adjusted so that there is a difference of the voltage of the
TP1 between plate of 2A3 in 5.2V from 5.4V when the current of the
plate is adjusted to 45mA.

120ohms * 45mA = 5.4V

So as to wait for the stability of operation, you must watch the plate
current for 10 or more minutes.

Because it is a high voltage, the inside of the amplifier is
dangerous.
You mast pay attention and be carefully.

When the short-circuit even a little is occurred, the high-voltage
stabilization power supply is broken.
Please pay attention.

For Model1-300B amplifier.

Two pots are used for adjustment of the plate current of 300B.
Pot is set for the plate current of 300B to 55mA.
The operation of 300B of us is as follows.
Voltage of plate 335V from 325V
Current of plate 55mA from 50mA
Load resistance 5K ohm

Pot is adjusted so that there is a difference of the voltage of the
TP1 between plate of 300B in 6.2V from 6.6V when the current of the
plate is adjusted to 55mA.

120ohms * 55mA = 6.6V

So as to wait for the stability of operation, you must watch the plate
current for 10 or more minutes.

Application Notes: 1) You must have a load, so connect the upside down amp (make sure not to topple it, very easily done) to some loudspeaker; 2) Make sure the volume is ZERO; 3) Turn the Multimeter to DC Voltage; 4) Apply the Red Probe to TP1; 5) Apply the Black to Chassis; 6) Turn the Trim Pots so that they show the desired Voltage; 6) Keep in mind you can use the Formula to run the tubes even more conservatively, which I prefer. e.g, for the 300B amp, if running at 50 mA current, Voltage needs only be 6V.

This will not be the last of my articles on ICL. This article has a brief.summary.

Circa Dec 2020, HK. Front left, 2A3 - Note the Glass bottle 6SJ7, which is a MUST, way Superior to Metal Tube Stock. Front Right, 100V San Ei 6BM8 amp. Rear left, Studer A730, right 300B.

At Wosirsir