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Tuesday, April 21, 2020

The Single Ended 6CB5A Amplifier


I have built quite a few 6CB5A amps over the years. Just finished a new one. Here some photos of the finished amp and the assembly process showing the latest construction style.

The amp is a stereo unit with internal power supply, so all in one chassis.

Power supply section on the left and the amplifier section on the right.

The 'naked' top plate:

The back side of the top plate and the support plate for the chokes and signal transformers,
the small pate are for holding the capacitors in place.

Sockets and switches mounted:

Some of the resistors mounted to the underside:

Wiring up the power supply section:

Capacitors mounted and some more wiring done:

First level complete:

The subassembly with all chokes, interstage and output transformers:

The subassembly mounted to the main plate and wired up:

The amp gets tested before it is inserted into the wooden frame:


Best regards


Monday, April 13, 2020

Tube of the Month : The 12B4A


This month I'd like to show a low mu 9-pin miniature tube. Meet the 12B4A.

Like many interesting tubes the 12B4 was originally designed for TV sets for use in the vertical deflection unit. But of course it is usable for other applications as well.

The 12B4-A is a single indirectly heated triode. The pinout is shown on the left. It has a center tapped heater, which allows to heat it either from 12.6V or by utilising the center tap and paralleling the halves from 6.3V. The only difference between the 12B4 and 12B4A is the controlled heater warm up in the latter which allows it to be used in a series heater string with other tubes which need 600mA heater current (or 300mA if the series connection is used). Being a low mu triode it has an amplification factor of only 6.5 which is coupled with a plate resistance of a only 1kOhm. This results in a healthy transconductance of 6300 micromhos. The 12B4A is designed for rather high current operation with peak currents allowed to reach 100mA.
Although it is designed for low voltage operation, it can withstand high plate voltages up to 550V with peaks allowed to reach 1000V. The maximum plate dissipation is 5.5Watts. All this in a tiny miniature tube. The parameters would make it usable for example in line stages or as output tube in phono preamps or DACs, offering nice low output impedance. See the General Electric data sheet for all specifications. I never used the 12B4 myself and only stashed away a few of them many years ago. The 12B4 was quite popular among tube amplifier builders in the 1990ies but I haven't seen it mentioned much lately. As always let's have a look at the plate curves in the data sheet and compare them to the curves taken from a tube with the tracer:

It looks quite decent. I would not use it in applications where it needs to deliver large voltage swings like driver stages of power amps, but in low level applications it should perform nicely.

Here we have a JAN-12B4A made by General Electric for the military.

Let's have a look at the tube in all it's glory.

The plate is split in two halves and exposes the grid at the sides.

A close up to the grid.

Nice gold pated grid wire.

Some more views:

The base:

The getter:

The packaging:

Next we have RCA 12B4A:

It shares the split plate structure but the grid is not gold plated.


Probably made by RCA.


JAN (Joint Army Navy) 12B4A from PhilipsECG:



I guess we need to open one up to see the construction in more detail.

The top exposed:

The glass removed:

The plate halves are bent in U-shape to have a large surface area for good heat dissipation.

Removing one half of the plate gives a good view to the grid and cathode:

One half of the plate:

Grid and cathode fully exposed:

Just the grid:

Now let's see the tube in operation:

As expected the heater glow is fully visible between the plates:

What a little beast!

Best regards