The Octal Line Preamplifier, Part 1 : Signal Section

Hi!

As mentioned in my previous post, I finished development of a new entry level line preamplifier to match the Octal Phono Preamplifier. In this post I will present the circuit of the signal section.

For many people the cost of a preamplifier with transformer volume controls is too high. So I came up with this rather classic and simple circuit, which keeps the parts cost down. This means it uses a regular resistive potentiometer for volume control and no transformer coupling. The signal is routed from the inputs through a source selector switch and from there into the volume control. In order to keep the input impedance high, a 100k pot is used. From there a single triode section provides some amplification and low output resistance. The goal was to avoid feedback circuits, cathode followers and the like, yet the output impedance should be well below 1k.

This is the circuit:

I chose the 6GL7 tube for amplification. This is a dissimilar duo triode with a high mu and a low mu system in one bottle. In this preamp only the low mu system is operational and the other section is left unused. Grid, cathode and plate of the unused system are tied to ground. The reason why this tube was chosen is because of the properties of the low mu section. It has a mu of 5 which will result in an amplification of about 4 (12dB) when RC coupled. This is not too high, so the volume control can be used in a sensible region, yet provides some gain which is needed to bring the output of the Octal Phono Preamp to suitable levels to drive a power amp. The plate resistance of the section which is used is a low 780 Ohms. It runs at about 25mA. So it provides plenty of drive for longer cables or power amps with low input impedance. Down to 10kOhm load should not be a problem for this preamp.

Let's go through the circuit in detail. Input selection and volume control should be pretty obvious. The 1M resistor from grid to ground provides a ground path also in case the wiper of the pot looses contact. The cathode resistor of 1k generates the bias. It is capacitively bypassed to keep the low output impedance and to avoid any loss of gain. With the 4.7k plate resistor the tube operates quite linear.

In the schematic a 1uF output coupling capacitor is shown. This is fine for power amps with high input impedances of 47-100k, as is typical for tube amps. If a solid state power amp is used which typically has an input impedance of 10-20k, the coupling cap should be increased to 4.7uF. The coupling cap should have a voltage rating of at least 450V. I used a 630V MKP. The 1M resistor from output to ground avoids any static charge build up if the preamp is not connected to a load. This avoids loud plops if the preamp is plugged into a power amp which is turned on.

Also shown is the B+ decoupling cap of 47uF and a bleeder resistor. The 1k to B+ functions as decoupling from the other cannel, since the preamp uses a common power supply. B+ voltage requirement is 300-350V.

The tubes can be AC heated without any hum. The heater should be referenced to ground with two resistors across them and their joint connected to ground.

A simple and easy to build circuit which does it's job well. I have two variants of power supply circuits for this which will be shown in part 2.

The 6GL7 which is used in this circuit will get it's own coverage in the Tube of the Month post of September. Stay tuned!

Best regards

Thomas

Cool Gales Bath Audio Fest 2013

Hi!

As in the recent years, Cool Gales is hosting it's annual Audio Fest again this year. It will be held again at the Bath & County Club, Queens Parade, Bath on Saturday, October 19th. I am pleased to announce that I will participate at the event and will show some of my latest preamplifiers.

I will bring a new build of the recently introduced Octal Phono Preamplifier, along with a newly developed line stage, The Octal Line Preamplifier:

These are compact designs with integrated power supplies. The linestage has been developed to match the phono stage in performance and chassis style:

This new line preamplifier will be introduced in detail in an upcoming article.

These preamps will be demoed with Ivan's single ended 6CB5A amplifier.

See you in Bath!

Thomas

Tube of the Month : The 6AX5

Hi!

The tube of last month was a directly heated rectifier suitable for power amps. This month I am presenting another rectifier, but an indirectly heated and rather small one, usable for small amps or preamps. The 6AX5.

The 6AX5 is a glass tube on an octal base, hence it is also named 6AX5GT. It has a common cathode and two plates in one bottle which makes it usable as a full wave rectifier with a power transformer with center tapped secondary. Of course it could also be used in a bridge configuration together with two individual diodes, either silicon or tube, for example 6AX4.


The pinout is pictured on the left. Based on it's electrical parameters it would be usable in power supplies designed for the 6BY5 unless the cathodes need to be separate. An advantage of the 6AX5 over the 6BY5 is the lower heater current of 1.2A versus 1.6A. As such it would be perfect as an alternative in the power supply of my recently shown version of the Octal Preamplifier. With it's octal base it would fit right into the theme of that preamp. See the datasheet for all technical details about this tube. This is a great little rectifier which seems to be rarely used nowadays. Availability is good and prices are low.


The 6AX5GT can be found from all the major tube manufacturers. Here for example from Tung-Sol:

Two versions from Sylvania. One in military packaging and the other in the commercial Sylvania box:

RCA:

General Electric:

I have seen two construction styles of the 6AX5. One has two cathodes (internally connected) :

And the other a single (larger) cathode in the center between the two plates:

Let's have a closer look at both these versions. First the one with two cathodes. Removing the glass:

Cutting off the top mica disk allows a look into the plates:

A close up showing the heater wire in the cathode sleeve:

Removing one of the plates:

A close up of the cathode:

Glass removed from the other type:

Bending away one of the plates reveals the cathode:

A great rectifier which deserves to be used in audio applications.

Best regards

Thomas

Silver 10Y Line Stage

Hi!

Yet another silver vs copper comparison. I finished two line stages which are supposed to work with the previously shown all DHT phono stages. One implemented with silver output transformers and silver autoformer volume controls, the other implemented in copper.

Both of them sound great and proove again the superiority of the 10Y/801A family of tubes for linestage use IMHO. Those thoriated tungsten filamentry triodes are capable of an incredible resolution and reproduction of all nuances of tone colours.

But yet again the silver version has an edge over the copper line stage. Especially in the goose bump section. For example Elvis' voice on the song 'fever' seems to come even more alive. The tone colors in the bass are more defined and show more nuances especially in very low notes.

So far my experiences with silver transformers have been quite consistent. They can improve an already very good sounding design. Albeit subtle the differences are easily audible. The next step will be a power amp with all silver transformers.

This comparison again showed why the 10Y/801A type linestages are my favourite. No matter if built with silver or copper transformers. The silver gives a slight advantage and is worthwhile to pursue if the budget allows it. However silver should only be considered if the entire amplification chain is already on a very high level.

Best regards

Thomas

ELROG ER211 triodes back in stock

Hi!

I have been slow with updates on the blog recently. I moved my workshop to a new place which kept me quite busy. Today just a quick update on the availability of Elrog tubes.

I have some 211 back in stock. No 845 in the moment. I expect to have 845 in stock again by the end of the month. I have 2 pairs 211 available at a slightly discounted price since they have been used for measurement purposes for quality control. These tubes are fully functional and have not been stressed. They might have slight scratches in the base from insertion into the test socket.

I am working on the Tube of the month article for August which will be about another rarely used but interesting rectifier. It will be posted soon. There is also a listening report coming up about an all silver 10Y line stage. Stay tuned!

Best regards

Thomas