6GE5 Stereo Amplifier, Part 2 : Assembly

Hi!

The previous post about this project showed the schematic. Now lets see the assembly steps and photos of the finished amp.

While the schematic showed a power amp, I decided to build an integrated by simply adding a volume pot and input selector in front.

The ultra path oil caps, power transformer and the chokes are placed on top of the chassis. Both chokes will be hidden under a single cover.

Wiring of the heaters:

Adding some solder terminal strips for the passive components:

Completed wiring. The output transformers are placed inside the chassis:

The finished amp:

Front view:

View from the top:

The same view with the amp switched on:

Glowing tubes:

I am also offering kits for this amplifier. The prices are for a power amp version.

Iron set (power transformer, chokes, output transformers):

EUR 640,- (EU incl. VAT) / EUR 538- (outside EU)


Full parts kit (excluding chassis and wire):

EUR 890,- (EU incl. VAT) / EUR 748,- (outside EU)


Best regards

Thomas

6GE5 Stereo Amplifier, Part 1 : Circuit

Hi!

In previous posts schematic and assembly steps of transformer coupled mono blocks with the 6GE5 as output tube have been shown. As promised a cost optimised version will also be presented in form of a stereo amplifier.

Going stereo instead of two mono blocks is already a considerable cost saving measure with minimal impact on sound quality. To reduce cost further we need to let go of the interstage transformer and use RC coupling in the driver stage instead. Next is the reduction of the number of chokes which requires to go for a cap input filter instead of choke input. And lastly the exchange of oil caps with electrolytics. Here is the schematic:

I wanted to keep the ultra path connection in the output stage, hence two separate chokes for each channel to isolate them. All other capacitors are electrolytic. The first cap after the rectifier sees the highest voltage peaks, hence two 22uF/350V caps in series. This results in 11uF capacitance which is still reasonably low and does not stress the rectifiers too much. The two 150uF caps after the chokes can be implemented with 3 47uF in parallel as I did. These caps can also be increased but the 150uF proved sufficient for quiet operation. The bypass caps at the cathode resistors of the 6GE5 are needed to suppress any residual ripple which would otherwise be coupled from the high voltage to the cathode. The driver stage is a standard RC coupled stage.

A rather simple circuit which is easy to build. I will show the completed amp in the second part and will also offer this version as a kit. Stay tuned!

Best regards

Thomas

6GE5 Mono Amps, Part 2 : Assembly

Hi!

Part 1 about the 6GE5 Monos showed the circuit. In this post I am presenting the assembly steps of the amplifiers.

So far the amps which received the most positive feedback for the design are my 211/6HS5 mono blocks.  So I reused some of the design aspects of those 211 amps for the layout of the 6GE5 Monos:

All the capacitors in the circuit are place in a single row on the left side. The 6GE5 output tube at the front in the middle and the 6AG9 driver beside it. Two separate power transformers are hidden under the black covers. Since the heater windings of the power transformer used does not supply enough current for all the tubes, I used a separate heater transformer for the signal tubes. The 6BE3 rectifiers are placed at the back side.

This is the top mounting plate with just the 12-Pin tube sockets installed:

Most of the resistors in the design are directly mounted to the bottom side of the plate:

All remaining resistors are soldered to the terminal strip.

Mounting of the power transformers:

Most of the wiring done:

Adding the capacitors:

The chokes, output and interstage transformers go inside the chassis:

The finished amp:

The two monos side by side:

Top view:

I have been using these amps for some time and am very happy with the result. Power output is about 4W. Solid frequency response. As mentioned in part 1, I am making a parts kits available for these.

Iron set (power transformers, chokes, output and interstage transformers):

Mono block       EUR 740,- (EU incl. VAT) / EUR 622,- (outside EU)
Stereo version    EUR 1120,- (EU incl. VAT) / EUR 941,- (outside EU)

Iron Set plus oil caps:

Mono block       EUR 860,- (EU incl. VAT) / EUR 722,- (outside EU)
Stereo version    EUR 1360,- (EU incl. VAT) / EUR 1142,- (outside EU)

Full parts kit (excluding chassis and wire):

Mono block       EUR 975,- (EU incl. VAT) / EUR 819,- (outside EU)
Stereo version    EUR 1525,- (EU incl. VAT) / EUR 1281,- (outside EU)

Too expensive? Stay tuned for the entry level version of this amp with just a single choke and RC coupled driver instead of the interstage transformer.

Best regards

Thomas

6GE5 Mono Amps, Part 1 : Circuit

Hi!

I often hear people complaining about tube prices and how much better it has been in the old days when prices had been more affordable. I wholeheartedly disagree with this point of view. In fact I think we are in the best possible times when it comes to tube audio. While it is true that the prices of some overly hyped tube types skyrocketed, there are still a lot of dirt cheap tubes which are available in abundance as NOS. Thanks to the internet it is easy to find suppliers for such tubes and data sheets of even the most obscure types are just a few clicks away. I remember when I started to get into tube audio that it took me almost a year just to get the most essential tube data books.

Also the availability of other parts is gerat nowadays. Audio transformers are available in a larger variety than ever.  And there are new tubes still being made with the Elrog 300B being the latest introduction.

If you think some tube types are expensive, compare the prices of them from 40, 50 or even more years ago with inflation in mind. Suddenly even many of the highly sought after tubes appear not that expensive compared to the old days. But as mentioned above, no need to hunt for those mainstream tubes if you are on a budget. I already introduced the 6CB5A as a cheap alternative to the 300B and many amplifiers have been build with this tube. In the Tube of the month post about the 6GE5 I suggested that it could be used triode connected in 2A3 designs.

In the November Tube of the Month post I presented the triode section of the 6AG9 as a possible driver for a triode strapped 6GE5 in single ended mode. All that was missing is a suitable circuit. In order to evaluate the sound potential of these tubes I decided to not use the cheapest possible design but something a bit more advanced, with oil caps and interstage transformer coupling. Once the tubes have been proven to work as expected, simpler circuits can be explored as well as more elaborate ones.

In order to minimize variables, I decided to adapt a proven circuit which is known to sound good. So it will be just the tubes which changed. The circuit which I usually use with the 6CB5A could be adapted with minor changes.

The same circuit description as in the article about the single ended amplifier concept applies, so I am not going to repeat it. The voltages are adapted to the 6GE5 and 6AG9. The output tube runs at about 50-55mA with 300V from plate to cathode, so about 15W plate dissipation. The driver operates at about 200V and 6mA. The power supply is basically the same as described in an earlier Making of a 6CB5A amp post. Read it for a description of the supply.

Just for the fun of it I decided to build two cute little mono blocks. So currents, choke and transformer ratings have been adapted accordingly. To stay consistent with the 12-pin compactron theme 6BE3 TV dampers are used as rectifiers.

The construction of the amps will be shown in the next part. Stay tuned!

I am making parts kits available for this design, both for mono and stereo versions.

Best regards

Thomas

Tube of the Month : The 6GE5

Hi!

Most of you probably expected the 300B to be this months tube. But after all the attention it got on my blog recently, I decided to set a counterpoint by selecting a totally different tube for October. A Compactron Beam Power Pentode, the 6GE5.

As fond as I am of directly heated triodes, I also like to work with 'sleepers'. These are tubes which are largely ignored by tube amplifier builders but which are very usable in audio applications. One of them is the 6CB5A, also a beam power pentode. Many years ago I proposed the use of triode strapped 6CB5As as low cost 300B alternative. Quite a few 6CB5A amps have been built around the globe since then.

In the 1960ies General Electric introduced the compactron tubes with 12 pin bases. This class of tubes was intended to rival the transistor which started to replace vacuum tubes. With the 12 pins more systems could be integrated into a single bulb. There have been triple triodes, quad diodes, twin pentodes and many combinations. The 6GE5 is a single beam power pentode, thus it does not need all the 12 pins. Yet many such types with single systems have also been put on 12 pin compactron bases to enable TV sets which only need a single type of socket. I already covered some 12 pin compactron types, the 6U10, 6HS5, 6HV5A and 6CG3. There are many more hidden gems in this series of tubes and I intend to present some of them during the next year.
So why the 6GE5? With the 6CB5A there is already a good alternative for the 300B so maybe something smaller would be nice. Also many people don't like tubes with top caps, so I picked one without. The 6GE5 was intended as horizontal deflection amplifier tube in TV sets, a so called 'sweep tube'. It has a lower plate dissipation than the 6CB5A, only 17.5W which also comes with a lower heater current of 1.2A, less than half that of the 6CB5A. The 6GE5 data sheet lists the triode amplification factor as 4.4. Plate dissipation and amplification factor are remarkably close to the 2A3, so maybe the 6GE5 could turn out to be a nice low cost alternative to the 2A3 and get us some 3.5W in single ended configuration? To find this out we need to get the values for plate resistance, transconductance in triode mode and of course we need to see the plate curves. The data sheet does not show the triode curves, so I measured them with a curve tracer:

This is about as good as you can expect from an indirectly heated tube. Quite good linearity in the mid range with the spacings of the pate curves getting a bit larger between the 0V and 10V grid step. How about the transconductance? This can be derived from the plate curves, but I took the easy way and just measured it on the tube tester. With 300V on the plate and 55mA current I got 5300 micromohs (5.3mS) almost identical to the 2A3s 5250. The plate resistance is easily calculated as amplification factor divided by transconductance which gives 830 Ohms. Again close to the 2A3s 800.

Before we go more into detail about the use of the 6GE3 in a single ended amplifier, lets have a closer look at the tube. So far I only have 2 of them so I cannot show many photos of different brands. The ones I have are Made by General Electric:

The 6GE5 has a nice shape with it's rather short but and bottle.

A view from the top showing the grid cooling fin:

Let's open one of them to see how they are constructed inside:

The top, the heater is visible inside the cathode:

The bottom:

Removing the heater:

The heater wire:

A close up:

The beam plate with the cathode still inside, the grids are already removed:

This photo shows how the beam plate is internally connected to the cathode:

A close up of the cathode:

The beam forming plate:

All the electrodes side by side, heater, cathode, control grid, screen grid, beam plate and plate:

The grids:

The control grid is gold plated and the screen grid has some kind of carbon treatment to increase it's dissipation capability. A close up of the grids:

Another view of the control grid:

So how to use this tube in a single ended audio amp? From the values discussed above it is quite obvious that the tube could be used with typical 2A3 operating points. But maybe this can be optimised a bit. I used the set of plate curves from the curve tracer and extrapolated some additional grid lines since the tracer only works up to 400V :

Operation into a 5kOhm load is represented by the blue line. I would pick 300V on the plate and 55mA as operating point. The bias voltage on the grid is -55V which is convenient since it can be obtained with a 1kOhm cathode resistor. The B+ voltage would then have to be set to about 350V. It would be 355V to be exact but no need to be picky. A solid 4W should be obtainable like this.

Expect more about this tube on my blog soon. After selection of a suitable driver tube from the 12-pin compactron series, I will build a prototype amp. Stay tuned!

Best regards

Thomas