The Octal Preamplifier Mk3

Hi!

Today's post will show the assembly process of a new incarnation of the Octal Preamplifier line, the Mk3.

Previous versions of the Octal Preamplifier have already been presented on this blog. The Mk1 was the first in the line which got an upgraded version, the Mk2 (photos here). The Mk3 version merges the circuits of the Octal Phono Preamplifier and the Octal Line Preamplifier into one chassis. In order to fit it into a single chassis the simpler version of the power supply has been used without any chokes.

The Mk3 version does not render the Mk1 or Mk2 obsolete. They are different circuits and represent preamplifier implementations at different cost and sound levels. The Mk3 represents the lowest cost version, the Mk1 is in the middle and the Mk2 remains the best sounding (and highest cost) version of the three.

The assembly starts with the top metal plate, here with tube sockets and power transformer mounted:

The tube sockets are mounted with vibration damping rubber elements:

First the heater wiring is done. I decided to leave the line tubes on AC and the phono tubes are heated with DC.  The high voltage AC leads to the power transformer are also in place already:

Next some solder terminal strips get mounted. These will hold all the passive components:

Solder terminals stuffed with all the passives:

The face plate holds on/off switch, volume control potentiometer and the input selector switch. As much as possible is prewired before the face plate is inserted into the chassis:

The same with the back plate, all wires from the input jacks and to the output jacks are already soldered to the connectors. The MC step up transformer is mounted directly besides the phono input jacks:

The MC step up is a Lundahl LL1681. It is connected to two switches, one per channel, which allow selection of the step up ratio between 1:13 (for higher input impedance) and 1:26 for low input impedance and maximum gain. Switching is done as already shown on a spearate MC step up unit.

Front and back plates inserted and ready to be wired up:

wiring all the inputs to the selector switch is the most tedious job during the build process. All input wires are twisted with a ground wire for shielding. This ground wire is only connected at one end. There are separate ground connections for the inputs and the output. One wire for all inputs and one for  the output. These are connected to the ground point of the line stage part.

Mains AC wires are covered with an extra insulation sleeve. Also the input and output wires for protection:

 Wires are purposely not bundled to minimize crosstalk. Looks messy but works better. Now the bottom plate and transformer cover remain to be attached and the preamp is ready to play:

This preamp is the first to use my new transformer covers.

Top View:

The back side with all connections:

Phono input on the left (MC only). Two switches to select between 1:13 and 1:26.  Next three line level inputs and the output jacks. On the bottom left is the ground lift switch.


Some views of the tubes:

The next project will be an integration of this preamp circuit with the low cost variant of the 6CB5A amplifier. This will be a fully integrated amplifier with phono on board. Stay tuned!


Best regards

Thomas

Back in Black

Hi!

Here some photos of a D3a LCR phono preamp and 10Y line stage in a black/silver color combination.

This preamplifer combination is the very similar to the previously shown D3a phono stage and 10Y line preamplifier. Separate phono (left) and line stage (right):

Phono and line both have external PSUs:

Although the PSUs look a like, they are quite different inside.

The complete preamplifier combination requires significant floorspace:

Each chassis is 27cm wide and 40cm deep. The bodies are 12cm high with parts and tubes extending out of the top. The PSUs reach the largest height with almost 30cm each.

The back sides:

The switches on the back of the phono allow selection of two different step up ratios of the silver MC step up which is integrated (LL1933Ag).

The 10Y tubes in the line stage:

The tube sockets are mounted on vibration damped sub plates.

The D3a in the phono:

Side view of the preamp:

A top view of one of the PSUs, showing the 6AX4 rectifier bridge:

Best regards

Thomas

Transformer Covers - Trafohauben

Hi!

The new transformer covers are finally available in all sizes. I just received the first batch back from varnishing. They are finished in a high quality matte black. Other colors are in preparation.

Hallo!

Die neuen Trafohauben sind nun endlich in allen Größen verfügbar. Gerade kam die erste Charge vom Lackierer. Vorerst gibt es nur matt schwarz. Andere Lacke sind in Vorbereitung.

The 80mm covers can be used for small chokes or transformers. Inside dimensions: 70*70mm. Can be used for example for Lundahl LL1667 and LL1668 plate chokes or LL1660 interstage transformers. These must be mounted sideways as shown on the photo.


Die 80mm Haube ist geeignet für kleine Drosseln oder Trafos, Innenmass 70*70mm. Z.B. passend für Lundahl LL1667 oder LL1668. Auch geeignet für kleine Zwischenübertrager wie LL1660. Diese müssen jedoch hochkant montiert werden wie auf dem Foto gezeigt.

The 100mm covers can be used for small power transformers like my filament transformers or small B+transformers. I use this size on the  Octal Line Stage

Die 100mm Hauben sind geeignet für meine Heiztrafos oder auch kleine Hochspannungsnetztrafos für Vorstufen. Diese Größe verwende ich z.B. in der Oktal-Linestufe.

This cover can also be used with the smaller Lundahl output transformers like LL1663 or the Lundahl chokes LL1673 if mounted sideways.

Diese Haube kann auch für die kleinen Lundahl Ausgangsübertrager verwebdet werden, wie z.B. LL1663 oder auch für die Siebdrosseln LL1673, sofern diese hochkant montiert werden.

In case these output transformers or chokes shall be mounted flat on the surface, the 120mm cover is to be used. Also two chokes or transformers would fit under this cover, but they would need to be mounted closely together.

Sollen diese Übertrager oder Drosseln flach auf der Oberfläche montiert werden, passt die nächst größere Haube mit 120mm. Unter diese Größe würden auch zwei solche Übertrager oder Drosseln passen, sofern nahe beieinander liegend:

This cover also fits my medium sized power transformers which I use for 45 amps or which are used in the low cost 6CB5A mono amplifiers.

Diese Haube ist auch für mittelgroße Netztrafos geeignet, wie ich sie für 45-Endstufen verwende oder in der low cost 6CB5A Monoendstufe.

And finally the large 150mm cover, suitable for big power transformers as for example used in a stereo 6CB5A amp and others. Also large output transformers can be fitted under these.

Und schliesslich die große 150er Haube für große Netztrafos, wie sie z.B. in stereo 6CB5A-Endstufen zum Einsatz kommen.

The corners at the top are available with two different radii for the 150 and 120mm covers.

Die 150er und 120er Hauben gibt es mit zwei verschiedenen Verrundungsradien an den oberen Kanten.

Drop me an email if you need any of these. Initial quantities are limited.

Best regards

Thomas

The Octal Line Preamplifier, part 2 : Power Supply

Hi!

As promised in the first article about the Octal Line Preamplifier I will present the power supply schematic today.  Similar to the Octal Phono stage, there are two possibilities. One using a choke and a very simple one using only resistors and capacitors for the smoothing.

Both power supply options are very similar to the power supplies of the Octal phono stage. They use the 6BY5 as rectifier tube. The 6GL7 can be AC heated. Since we are dealing with much higher signal levels as in a phono stage, no DC heating is necessary and still the preamp is hum free. This is the schematic of the version with choke input filter as is used in the first version which I built:

Very straight forward, a hybrid rectifier bridge with four UF4007 diodes augmenting the 6BY5. I used two of them in series in each leg (4 total) to increase peak inverse voltage capability. With choke input filters some very high peak inverse voltages can occur at turn on.

The rectifier feeds the input choke, a 40Hy unit, followed by a 47uF electrolytic cap. Two RC sections with 500 Ohms and 47uF each provide sufficient filtering for hum free operation. The 100k resistor serves as bleeder resistor. Together with the two other 100k bleeders, one in each channel of the signal section, critical current is always guaranteed even if the signal tubes are unplugged. This avoids a steep voltage rise which could otherwise occur in case the tubes are unplugged and the PSU is turned on.

The rectifier tube and the signal tubes are heated from separate windings. The 6GL7 heaters are referenced to ground via the two 100 Ohm resistors.

The 6AX5, which I presented last month, could be used as an alternative to the 6BY5.

If you want to save the cost of the choke, use this simpler PSU:

The same rectification scheme, but only RC sections are used for smoothing. This version needs a lower secondary voltage to get the desired B+. Also only two UF4007 are needed. In this schematic signal tubes and rectifier share the same heater winding. Therefor the heater winding is elevated to a positive voltage by connecting one side to a tap of a voltage divider between B+ and ground. This is not very elegant and separate heater windings would be preferable. But I used such a scheme already with the Octal phono stage and it works fine. This second option is not tested, so some minor adaptions might be necessary if you build it.

Here a view of the backside of the preamp showing the rectifier and the connectors:

Best regards

Thomas

Tube of the Month : The 6GL7

Hi!

This months tube is a dissimilar double triode, a kind of tube which was not presented in the Tube of the Month series yet. So it is about time since there are many interesting tubes of this species out there. Here is the 6GL7.

These kinds of tubes have been developed as combined TV vertical oscillator and amplifier applications. For this they contain a high mu triode along with a low mu power triode section. What makes these interesting for audio is the possibility to build an amplifier with a single tube (nick named spud or potato amplifier). The high my section can be used as driver and the low mu section as output triode.

The 6GL7 has an octal base which is shown on the left. It is especially interesting for audio since the low mu section has a plate resistance of only 780 Ohms at an amplification factor of 5. With it's maximum allowable plate dissipation it can provide almost 2W output in single ended configuration with an output transformer of 2.5-3.5kOhm primary impedance. The high mu section has an amplification factor of 66 with a plate resistance of 30kOhm. Just right to drive the output section with a well usable input sensitivity. All this combined with a moderate 1.05A heater current. See the datasheet for all technical parameters. The low plate resistance of the low mu section also makes it very suitable as a line stage output tube. RC coupled it will yield a low output impedance with a useful and not to high gain. In case only one section is used, it is recommended to ground the electrodes of the unused half. That's how I used it in the recently introduced Octal Line Preamplifier.

I have not found a datasheet with plate curves. So I measured them with a tube tracer:

High mu section:

This is a very linear triode system.

Low mu section:

Not as linear as the other section but still well usable if the operating region os carefully chosen.

This makes it very usable for audio and I already presented a line stage with this tube, the new version of my Octal Line Preamplifier. It performs very well in that preamp which encourages me to also develop a simple power amplifier with these tubes. The 6GL7 is surprisingly small considering the 10W plate dissipation rating of the power section. Including the base pins it is just about 70mm high. The diameter is the same as that of other Octal tubes. This would allow for a very compact power amp with just one of these in each channel. Earlier versions of the 6GL7 have the typical Octal base, which is about 20mm high (excluding pins). Later versions use the so called coin base also called wafer base. This only has a small plastic wafer attached to the glass which holds the pins. This wafer looks like a coin, hence the name. Below a photo of two Sylvania 6GL7 one with the old style base and the other with the coin base:

Another comparison between the two:

A close up of the coin base 6GL7:

RCA 6GL7s:

6GL7 manufactured by Lindal (Japan):

General Electric:

Westinghouse:

Now let's have a look at the internal construction. Removing the glass tube reveals more details.
Clearly visible the different plate size of the high mu section (left) and power section (right):

The high mu section from the side:

The getter ring attached to the plate of the power section:

The top mica disc:

A small strip of metal welded to the rods of the grid of the power section provide some extra cooling.

The connections from the electrodes to the base pins:

Another view, showing the heater wire going into the cathode of the high mu section:

The heaters of both sections are wired in parallel:

The heater wires removed from their cathodes. Since the power section has a stronger cathode it needs more folds of the heater to get it to the right temperature:

A close up:

The plates removed from their triode sections:

The grids and cathodes in comparison, power section on top and high mu section below:

Note the gold grid wire of the power section. The cathodes removed:

The grids in comparison:

A close up:

The high mu grid:

The cathodes:

close ups:

The plates:

Stay tuned for more projects with this tube!

Best regards

Thomas