The 6CB5A Back in Black!

Hi!

After all the recent amplifier builds with expensive triodes and multiple chassis, it has been a welcome change to build a single chassis stereo power amp based on the 6CB5A tube again.

I have built many versions of amplifiers with this output tube, from a RC coupled low cost version to some ultimate versions with the best possible transformers and separate power supplies. This one is in the middle. It's single chassis stereo, interstage transformer coupled with Lundahl transformers.

I have written before that I consider such an amp as a smart choice when the budget is limited. With tube amplifiers people have always been way to focussed just on the output tube alone. It is very common that the sound of an amp is attributed to the tube. This is simply wrong. I have written before that the output tube contributes maybe 10-15% to the overall sound of an amp. Nothing can be said about the sound of an amp if only the tube is known.

Yet I am getting increasing questions about the sound of tubes. It's is probably partly my own fault because of my rants about certain directly heated triodes and especially the Elrog 300B which in my experience still holds the title for best sounding output tube. But this sound quality can only be obtained if all other aspects of the amp allow for it.

If the budget does not allow to match such directly heated triodes with very good transformers, an excellent power supply, including elaborate filaments supplies, much better overall sound can be achieved with other tubes, like the 6CB5A triode wired.

The cost of the tubes is very low and frees up budget to use better output transformers and other parts. The 6CB5A is indirectly heated and does not require a special filament supply. It is happy with a simple AC winding on the power transformer and can be operated hum free like that. Combined with a suitable driver tube, in this case the 6N7 which is my preferred choice for the 6CB5A, it is an excellent performer.

In this particular version a pair of input transformers has been added to allow balanced connection to the preamp.

I am always happy if someone does not insist on a 300B, but is open for such a smart choice like this one.

Best regards

Thomas

211 -The Next Generation- Part 14 : 6HS5/211 assembly

Hi!

In Part 13 of the series of posts about my next generation line of 211 amps I showed how the layout of the refreshed 211 mono blocks with 6HS5 driver is going to look like. This post will illustrate the assembly process.

Contrary to the 801A/211 amplifiers this one has the amplifier section and power supply on one chassis. This means a lot of iron needs to be fitted in.

Everything is mounted on a single metal plate:

Here the sockets, connectors and switches are already fitted. The underside:

The interstage transformer is the only piece of iron which is placed inside. Many rails and bolts are needed to hold the capacitors which will all be inside:

For the further assembly steps the plate is fixed in a rig which makes it easier to handle:

Here all the chokes and transformers are already mounted on the top side:

The inside with some initial wiring before the capacitors go in:

Everything in place and wired up, ready for testing:

The amp can be tested in the assembly rig, before the transformer covers and enclosure are added. 5 covers are needed to hide these heaps of iron:

Stay tuned for photos of the finished amps!

Best regards

Thomas

Gramophone Company Ltd. "His Masters Voice" 78 RPM record player

Hi!

Today I'd like to show something completely different. My good friend Frank with whom I did a 78 rpm Shellac demo at last years ETF visited and brought a marvellous piece of equipment to play with.

An 78 RPM record player for Shellacs, made by The Gramophone Company Ltd. in 1935. The player was named "His Masters Voice" and has the Nipper Dog logo prominently displayed on the side.

View from the top:

Tonearm with Nipper logo embossed:

The screw at the front of the arm fastens the needle which needs frequent replacement. There is a small trough underneath the arm to catch the needle when the screw is unfastened:

The inside with the motor:

The mechanics beneath the platter:

Frank also brought some shellacs from his collection to listen too. We started with a record from Rex Stewart on the label Amiga from 1947:

There was a concert planned 1947 in West Berlin. Due to a power outage they needed to find a new location and wound up in a Club which was located in the eastern part of the city. There several recordings were made among these the 'Linden Blues':

Next we played a record with Peggy Lee and Benny Goodman from 1942:

And this is how it sounds:

And finally a record from my collection. The Beggars Blues on Vocalion. A very old label, probably recorded in the 1920ies or 30ies!

And a video which shows it playing on the HMV:

I hope you enjoyed this post about historic 78 RPM records. Thanks a lot to Frank for the fun session!


Best regards

Thomas

211 -The Next Generation- Part 13 : 6HS5/211, Layout

Hi!

Not only the 211/211 and 801A/211 amps got a fresh design, also the 211 mono blocks with 6HS5 driver are being redone.

The 211 mono blocks with 6HS5 driver are currently my entry level version in terms of 211 amplification. Although the design of the previous version received a lot of praise, I need to redo it since I am running out of stock of the capacitors used. The capacitors were mounted on the top of the amp and were a major part of the visual look of the amps.

The new version uses the caps shown above. This is a mix of caps which are not easily arranged on the top of the amplifier chassis. So instead I decided to place almost all the transformers on chokes on the top and move the caps inside. This heap of iron needs to find a space on the amps:

All the iron will be placed under transformer covers. Two large covers are needed for the main power transformer and for the output transformer. The rest can be placed under 120mm covers. This is the arrangement I came up with:

The 120mm covers are now also available in a version which is higher, just as high as the 150mm covers, so a uniform look can be achieved if they are combined.

The metal pate for this has been designed already:

The two small plates are for the mounting of the sockets of the 6HS5 driver and 6CG3 rectifiers. The assembly of the amp will be shown in an upcoming post. Stay tuned!

Best regards

Thomas

Tube of the Month : The 84 / 6Z4

Hi!

Over the last 4 months I presented a series of indirectly heated small signal tubes, the 27, 37, 56 and 76. I mentioned their usability for example in preamplifiers. So naturally the question comes up, which would be a suitable rectifier tube to go with these? Here it is, the 84.

Of course there would be many possibilities. But we would want something that is not over powered for such a preamp, which comes in the same shape, from a similar era and preferably with the same base.

The 84 ticks all these boxes. It was mainly made in the same ST shape as these triodes and some early versions even came in globe bottles. The current capability of 50mA is plenty to supply two triodes and this comes with a moderate heater current requirement of only 0.5A at 6.3V. It has a rather low maximum peak inverse voltage rating of 1000V and a max plate to plate rating of 350V. But again this is plenty for using it with these triodes since they operate below 200V DC. The 84 has the same 5 pin base. The pinout is shown on the left. It is indirectly heated which is perfect for using it with the indirectly heated triodes to avoid that the plate voltage comes up before the heaters are hot.

The ST version of the 84 has exactly the same dimensions, so this would also be aesthetically pleasing on the same chassis. For all technical details, see the RCA data sheet. The 84 was originally introduced with the 2 digit designation, however it never had the early three digit number like the UY227. At least I have never seen one. It was later named 6Z4 based on the RETMA naming convention. Most of them bear both designations. The 84 is available in good quantities at moderate prices. Again a tube which seems to be totally ignored by the audio crowd. I must admit that I have not used it myself yet. But I do plan to build a line stage with the 27 tube and I am considering the 84 as the rectifier. It's characteristics are similar to the 6X5 which probably is a development based on the 84/6Z4. However the 84 would be my choice to go with 27/37/56/76. I already stocked up on the tube not only for the line stage but to have some variety to show in this post. Lets start with the 84 made by Sylvania for the military:

Besides the commercial designations 84 and 6Z4 these also carry the military VT number VT-84. This is one of the few examples which have the same number in both naming systems.

Beautifully packaged:

In boxes which allow testing of the tube without removing it.

The tube in close up:

The 5 pin base:

The top:

The plate structure:

The similarity of the internal structure to that of some versions of the 6X5 is quite obvious.

The printing on the glass with the dual designation:

Here a tube made by Ken-Rad:

Again a similar plate structure as seen in 6X5 tubes.

Also with dual naming:

RCA 84:

Tung-Sol:

Two different 84 made by Raytheon:

One of them with straight sided glass:

The other ST:

Philco:

Arcturus:

General Electric:

Canadian General Electric, made for the military:

Close ups:

Next another tube made in Canada, by Westinghouse:

This is a special variant called 84M

Rogers, another canadian manufacturer developed a special M-series of tubes. These consisted of equivalents of a range of types with external spray coating.

All the M-series tubes came with Octal base even if the equivalent it was based on had a different one.

The spray coating even covered the base, and only left the top clear:

Quite an interesting variant!

84 made by Sylvania this time in commercial packaging:

And another variant from this manufacturer for the military:

This was probably the last version of 84 which was produced late 70ies or beginning 80ies as indicated by the date on the box. Different plate structure from the others:

And finally a beauty, the only globe shape 84 in my stock:

Made by National Union.

Very different plate structure from the more modern variants:

As always also some photos showing the glow of the tubes, starting with the JAN Sylvania 84:

The heater is quite visible not only on the top and bottom but also between the two plates which are arranged vertically above each other.

The Ken-Rad which has the plates side by side:

Close up to the bottom part:

And finally the globe shape National Union:

A nice little rectifier, just waiting to be used!

Best regards

Thomas