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Sunday, April 23, 2017

The 6 Channel Field Coil Supply


Some photos of a field coil power supply which has been finished recently.

It is meant to supply a pair of speakers with 3 field coil drivers per side.

All in one chassis.

Completely passive choke filtered supplies, independent for each driver.

Voltages can be adjusted in 5% steps with a common set of switches per section (bass, midrange and tweeter). Digital meters monitor voltage and current of each of the 6 independent supplies.

Adjustment is done by selection of different primary and secondary taps on the power transformers to avoid any regulation circuitry.

LCLC filtering of each of the 6 voltages, so 12 chokes in total.

The finished unit weighs over 50kg!

Best regards


Tuesday, April 18, 2017

The Ultimate 300B Amplifier - Part 2


In part 1 about my interpretation of the ultimate 300B amplifier I showed all the silver transformers and chokes which go into the amp. Here the finished mono blocks.

 Here one mono block, just the signal section:

 The amp looks the same as the first differential 300B amps with copper transformers.

One channel with power supply:

 The whole set of 4 chassis:

Was this worth the effort and many silver parts?

Definitely! These give a ver smooth and detailed sound and improve upon the copper versions.

Ample detail and life like presentation! Come to the High End fair in May and listen to these in room F231e together with the matching ultimate phono and linestage!

Best regards


Thursday, April 13, 2017

Tube of the Month : The 40


This month I'd like to present a tube most of you probably never heard of: The 40.

The 40 is a directly heated triode with thoriated tungsten filament developed for voltage amplification stages. It is derived from the UX201A.

It is a high mu triode with an amplification factor of 30, which is unusually high for a directly heated triode. As it is derived from the UX201A it shares the same base and pinout as well as the same filament which needs 5V and consumes 0,25A. The higher mu is achieved by a different grid geometry. Such a high amplification factor does not come for free. The plate resistance is an astonishingly high 150 kOhm vs the 10kOhm of the UX201A. This corresponds to a transconductance of only 200 micromhos. In order to achieve a meaningful amplification the tube requires a very high impedance plate load and can drive next to nothing. In actual use this means that not much amplification will be left. This is probably why this tube never got very popular on and did not see widespread use. However it might be worthwhile to look at it with some more modern approach. A constant current source load should enable to realise the full mu of the tube. Since it runs at very low current of much less than 1mA it could also be loaded with a grid choke. Some of those can carry some DC current. However the winding capacitance of such a choke could already limit the high frequency response. Many years ago I built some test circuits with this tube directly coupled to a UX201A. I remember that I struggled to get a decent high frequency response which started to roll off at 10kHz. The sound I got from the test build showed some nice tone however. I never went beyond some initial attempt to use the 40. Maybe some brave soul will look at it and give it a try? It shows the typical plate curves of a directly heated triode:

Unfortunately I have too many other things on the plate. But if anybody takes up the challenge to use this tube in audio applications, please report back your findings.

I only have ST shape versions of the 40 in my stock but it was also made in globe shape as UX240.

These are some type 40 tubes made by RCA.

Like the 01A almost all of the glass is covered with getter.

Very little of the internal structure is visible through the getter.

A 40 made by Philco:

Different style Philco packaging:

When the filament tis lit up a little bit of the internal structure can be seen:

A beauty, isn't it?

Best regards


Sunday, April 9, 2017

The 10Y Linestage


The 10Y line preamplifier is the most popular among my line stages. Here some photos of 2 recently finished ones.

One of them in the classic silver/black/walnut color scheme and the other in gold/maroon red/smoked eucalyptus which is very popular lately. Here both side by side:

They share the exact same circuit and use Tribute OPTVCs with finemet cores.

Both these versions have separate volume controls for left and right allowing for some balance adjustment.

The gold one together with it's power supply:

On this one the OPTVCs are mounted under transformer covers to match the color of the caps.

A close up showing the metallic particles in the red varnish of the caps and transformer covers glowing on the sun:

The back side showing the in and outputs:

The silver/black preamp:

The Tribute transformers are visibly mounted behind the 10Y tubes.

Best regards