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Wednesday, February 24, 2021

Tube of the Month : The 6EA7


This month I'd like to present an octal due triode with dissimilar systems. Meet the 6EA7:

Like most of these dissimilar double triodes the 6EA7 was developed for vertical deflection oscillator and amplifier service in TV sets.

Section 1 of the tube is a high mu system with an amplification factor of 66 and a plate resistance of 30kOhm. Section 2 is a low mu triode with an amplification factor of 5 which comes with a low plate resistance of 920 Ohms. The latter has a maximum plate dissipation of 10W, which makes it useable as an output tube for a flea power amplifier or a headphone amplifier. The high mu section could be used as a driver for the output section so the entire amplification per channel would be done with a single tube. These parameters are very similar to those of the 6GL7 which was covered as tube of the month before. Many 6EA7 tubes are double stamped 6EA7 / 6EM7. Both types are quite similar but not exactly the same as can be seen when comparing the RCA data sheets: 6EA7 data sheet, 6EM7 data sheet . The 6EM7 according to this RCA data sheet lists 68 / 40k Ohm for amplification factor and plate resistance of section 1 and 5.4 / 750 Ohm for section 2. Even the heater currents differ slightly. Although within usual tube variations it is quite surprising that both versions have been combined into a single tube type. Interestingly the General Electric data sheet lists 65 / 34kOhm and 5 / 770 Ohms which puts it somewhat in the middle between the RCA values. Apparently such kind of deviation was not critical for the intended purpose. But when used in an audio circuit it is important to check the values especially between two tubes used in the 2 channels of a stereo amplifier to avoid channel imbalance. 

As always we have a peek at the plate curves and compare data sheet with actual curves. Section 1 (high mu) :

Section 2 (low mu) :

And now some photos of various 6EA7 / 6EM7 tubes.

6EM7 / 6EA7 stamped tubes made by RCA:

These are the more modern variant with 'coin' base.

The top, note the cooling fin for the grid on the low mu section:

The base, the small plastic disc at the bottom gave it the name coin base:

An older RCA 6EA7 with the full height octal base:

Sylvania 6EM7 / 6EA7 with coin base:

Full base 6EM7 made by Sylvania:

Here two Sylvania tubes both stamped with a single tube number:

6EA7, 6EM7 and double stamped tubes made by General Electric:

In terms of construction they all look identical:

Coin base GE tube:

Here an odd brand, Daltronix:

And lastly Zenith:

Since the tube is very similar to the 6GL7, I am not sacrificing one to show the internal construction. See the 6GL7 tube of the month post for photos of that.

The tube in operation.

Best regards


Wednesday, February 17, 2021

New Amplifier in development with TM300B


I have been testing a new concept for a power amp optimised for systems with extremely sensitive speakers since a while.

It will use the TM300B as output tubes. The circuit will be completely free of capacitors in the signal path.

Here a photo of the prototype build used for testing some variations and evaluation.

Stay tuned as the development progresses.

Best regards


Friday, January 29, 2021

Tube of the Month : The 6V3


In last months Tube of the Month post about the 5687 I mentioned that not many 9 pin miniature (Noval) tubes have been covered in this series. So I decided to pick another Noval tube for this month. Meet the 6V3.

Since TV Dampers are among my favourite vacuum tubes I looked through my tube storage and found some 9 pin specimens.

As far as I am aware the 6V3 (later released with an 'A' suffix) is the only miniature TV Damper tube. The pinout of the Noval base is shown on the left. The heater is connected to pins 4 and 5 which is common for 9 pin tubes. The plate is connected to pins 2, 7 and 9 while the cathode is brought out to a small top cap. The latter is obviously done since the high voltages which occur in TV Damper tubes would arc across the narrowly spaced base pins. The heater consumes a hefty 1.75A at 6.3V. Quite a lot for a tube in such a small bottle. The maximum DC plate current is 135mA (800mA peak) and the maximum inverse voltage is 6kV. Somewhat similar to other TV Dampers like the 6AX4. For the complete technical data refer to tech RCA data sheet. And for some technical background on TV dampers see the 6AU4 post. Like other TV Dampers the 6V3 and 6V3A could be used as rectifier in power supplies. 

Let's go through my collection of 6V3 tubes.

Here we have 6V3As made by Sylvania.

A close up to the top where we see the isolation spiral between heater and cathode which ensures the high voltage ratings:

Another Sylvania which came in plan military packaging:

DuMont branded 6V3A:

DuMont obviously sourced their 6V3s from Sylvania as they share the same construction style:



This one has a quite elaborate construction with 3 mica discs.

Some close ups:


Identical construction to the Raytheon above, but not sure who cross-branded whose tubes:

IEC Servicemaster 6V3 made by Mullard?

And lastly General Electric 6V3A:

I salvaged this one to have a look at the internal construction:

This tube has a peculiar detail which I have not seen before, a small metal disc mounted below the cathode:

Probably as a measure to prevent arcing?

Here we see that the upper mica disc centered the heating element in the cathode while the second disc holds the cathode in place to ensure distance to the plate.

Here we have a better look to the small disc at the bottom which is connected to one of the unused pins (nr 3).

Not all the 6V3 have this feature. Only the Raytheon and General Electric. If anyone knows exactly the purpose of this disc, please share.

Another surprise detail I have not come across before is the construction of the heater element:

A spiral of wire wound around an insulated center piece.

The cathode:

Quite massive for such a small tube!

The plate with massive cooling fins:

An interesting tube with surprising manufacturing details. Makes me want to use 6V3s in a project!

Closing the post with the tube lit up.

Best regards