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 6AG9

Hi!

Last month I showed the 6GE5 compactron beam pentode and mentioned it's usability as an output tube in a single ended amp. I went through the compactron series tubes to find a suitable driver. And this is what I found: The 6AG9

Since I have a preference for all triode amps, I concentrated on triodes only. There are quite a few of them in the compactron series. Some combined with other triodes, diodes or pentodes in one bottle.

The 6AG9 contains a triode and a pentode. The triode section has an amplification factor of 39 and a plate resistance of 8500 Ohm. Quite similar to the values of the 6N7 which I like to use as driver for small output tubes. This would make it usable in a 2 stage amp with transformer-, LC or RC coupling. The heater current is a bit on the high side for such a triode, but that is because it also needs to supply the heater of the pentode section. A quick glance into the data sheet shows some nice plate curves of the triode section. A check of prices showed a nicely low cost of $3-4 for the tube. It seems to be an ideal companion for the 6GE5! These are the plate curves as shown in the data sheet:

And this is how it looks like in real life:

Quite alright if the region to the left is avoided!

Most 6AG9s in my stock came unmarked in bulk pack like those above.

Here a 6AG9 mage by General Electric:

The 12-Pin base:

Let's have a look how it is constructed inside!

Removing the glass reveals a better view of the details:

The pentode section takes up most of the space inside. The triode section (top) is separated by some space and a screen from the pentode (silver sheet of metal).

A view of the side of the pentode section:

The top:

The heater wire can be nicely seen coming out of the top and connecting across the tube between the sections.

The heater wire removed:

The part with more folds heats the pentode.

The triode section:

The triode electrodes separated:

The grid:

Cathode:

The triode grid is circular shaped:

The shield which separates triode and pentode:

The pentode section, cathode, control grid and screen grid on the left, plate and suppressor grid on the right:

The grids separated from the cathode:

The control grid has an extremely fine pitch between the wires:

A very well made tube, completely overlooked by audio designers!

In the meantime the tube serves as driver in a 6GE5 amp. Stay tuned for a report.


Best regards

Thomas