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Thursday, March 4, 2021

The Mono Phono Preamplifier for Historic Records


Just finished a new variable EQ phono stage and took some photos before it got shipped to its new owner.

Like previous builds of  mono phono preamplifiers, this unit has separate controls for the bass turn over frequency and the roll off at 10kHz.

This allows for a variety of settings to cover a wide range of EQ curves from early 78rpm records to 33rpm vinyl mono LPs.

The actual phono stage and power supply have separate chassis similar to the stereo LCR RIAA phono stage.

The EQ networks are done in LCR topology as in the stereo phono stages. The inductors are mounted directly behind the face plate besides the EQ setting switches.

Some photos from the construction steps. Here the top plate with vibration damped mounting of the tube sockets.

Some parts mounted:

Output and interstage transformers as well as the B+ smoothing chokes are mounted on the top side under transformer covers:

There is a second support plate for B+ smoothing capacitors, a choke for the heater supply and the remaining parts of the EQ networks.

About one third of the EQ networks mounted :

The power supply top plate:

Chokes go on a separate plate...

.... which is mounted to the underside of the top plate:

Finished preamplifier:

Bass turn over selector:

Treble roll off selector:

Vibration damped mounting of the D3a tubes:

Side view of phono stage:

Back side:

Some sound clips with a similar phono can be found in the post about a 78rpm demo at the Munich High End show.

Best regards


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