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Thursday, October 8, 2015

211 -The Next Generation- Part 12 : 801A/211 towers, PSU assembly


The assembly of an amplifier chassis has been shown in part 11 of this series of posts. Now let's have a look at the power supply assembly steps.

Shown above are all the major parts which are mounted in one power supply. Like the amps, construction is done top down with the parts mounted on several plates.

The top plate only holds the transformer cover for the main power transformer and the on off switch.

The power transformer itself is mounted on a sub plate with the rectifier tube sockets:

wiring of the full wave rectifier bridge which consists of 4 6AX4 TV dampers:

The transformer plate mounted to the top plate:

The next level carries the capacitors and chokes for high voltage smoothing:

Here it is mounted to the main assembly:

The filament supplies for driver and output tube share the last plate:

This gets mounted at the bottom end:

The power supply ready for testing:

In the next part I will show the finished amplifier set.

Best regards


Sunday, October 4, 2015

The Ultimate Line Preamplifier


Last year I was asked to build an ultimate line preamplifier which has the potential to even better the single ended 10Y preamp with silver transformers. But how to improve upon such a preamp which is pretty much maxed out already?

The single ended versions with silver use top notch transformers. So what is left to play with are the capacitors. But I think those ex military stock NOS paper in oil caps are extremely good and I prefer them stuff over the usual 'audiophile grade boutique caps'. So the idea came up to abandon all caps from the signal path and to go differential. A prototype version with copper transformers was built and proved to sound excellent.

But then the Elrog 300B came and with it the idea to use it in a line stage. Again a prototype with copper transformers was built first. For this a custom OPTVC was wound by Tribute. Again the sound was very good, but the 300Bs are a bit more microphonic than the 10Y and would require a more careful selection of tubes.

So finally the decision was made to go back to the differential concept and build it with the best possible parts, which means a whole bunch of silver:

Lundahl silver line output transformers and silver wound AVCs from Intactaudio. 3 Pairs of Lundahl LL7903Ag silver line input transformers will be wired for different step up ratios to provide the possibilities of different gain. Each of the 4 tubes has it's own plate choke which are also silver wound! So all parts which are involved with the signal path are silver wound. 14 transformers and chokes in total!

Of course for such a project only the finest tubes will be used. In this case a matched quad of the fabulous 1602, a special variant of the 10Y

So will this be the best line preamplifier in the world? There are many claims out there about the best this or the best that. This is highly subjective, especially in audio. No matter how much effort is spent, there might always be something better elsewhere. But based on my experience in audio and with tubes, parts and silver transformers, this will be the best I can currently think of. And of course I need to finish it first and give it a listen. Stay tuned!

Best regards


Wednesday, September 30, 2015

211 -The Next Generation- Part 11 : 801A/211 tower, amp assembly


The next set of 801A drives 211 amplifiers shares exactly the same circuit with the already finished amps. But it will be in a different chassis in 'tower style'.

Everything will be mounted on metal plates, stacked above each other:

The top plate carries the two tubes and the signal transformers:

The bottom side of the top plate showing al the signal wiring:

The small transformer on the top left is the input transformer which is wired 1:4. This way the amp has a sensible input sensitivity with only two stages.

Handles and vertical mounting bolts added:

Capacitors and chokes are mounted on a sub assembly and pre wired:

The sub assembly added to the amp:

The amp ready for testing:

In the next part I will show the assembly of the power supply.

Best regards


Friday, September 25, 2015

Tube of the Month : The 76


After the presentation of the types 27, 37 and 56 of the early indirectly heated triodes, the logical next candidate for the tube of the month is the 76.

The 76 is basically an equivalent to the 56 only with different heater specs. It needs 6.3V at 0.3A.

All other parameters are identical to those of the 56 which of course includes the same UX5 base and pinout and also the same dimensions. See the RCA data sheet for a complete set of technical data. So unlike the 37 which received slight bumps in amplification factor and transconductance over the 27, the 76 is exactly a 56 with 6,3V heater. If these kind of triodes are appealing to you but the 2,6V of the 27 or 56 are inconvenient, this tube might be the right choice. I have not used it myself yet so cannot share any actual experience. But that might change some time in the future. As always lets start by looking at the plate curves of the tube:

And actual curves from the tracer:

Another shot with different spacing for the grid voltage steps:

As expected these look identical to the curves of the 56. It is also very similar visually:

The tube above is from the Wards brand

RCA 76 still sealed in the box:

Different style RCA box:

Insert with floating tube removed:

The tube:

Another view:

Like the 27, 37 and 56 the 76 has circular grids and plates which are arranged in a concentric manner around the cathode:

The top:

Another RCA 76 with 'meat ball' logo on the base:

Triad 76:

The top:

Plate structure:

76 made by Sylvania:

The top:

The 76 shows a similar glow as the 56 when in operation.

A close up to the heater as it enters the cathode:

This post concludes this mini series about the early indirectly heated triodes. I hope you enjoyed it.

Best regards