01a Preamp Gen2 finished!

First build of 2015, just after the New Year. I’m pleased to listen to the 01a preamp gen2 finished. It’s a beauty, only the warm sound of the 01a thoriated tungsten filaments can provide. A perfect match to the Starlight CD player that was lacking of a bit of gain.

It’s dead quiet, the frequency response is superb and the dynamics are there. Very good bass that improves what I was getting out of the discrete DAC. I love listening to this preamp. A great pleasure. Thumbs up to the return of the 01a. My best Sylvania globe valves are now in use, yay!

Will soon write a proper blog entry for this ultimate preamp.

01a Preamp: Build (Part 5)

Finished the preamp build yesterday, still have one power supply to build though. That is the benefit of the Christmas holidays as you have some proper time to work on your projects.

Magically this time all worked fine from start. The preamp measures really well using the noisy bench power supply:

As expected the HF response is really good thanks to the design of this stage. The -3dB pole is above 100kHz. LF response is actually better than the shown above as the soundcard LF pole is playing as well.

From a distortion point of view the preamp is performing well at maximum level which the Starlight discrete DAC will be playing at (i.e. 400mV):

The nice decay of harmonics is characteristic of this fantastic DHT.

When protection fails

“Don’t Stop Til you Burn Enough!”

Merry Christmas to you all! Christmas Eve ended up with some smoke and the party was over a bit early. We played music really loud last night and got carried away with some dancing around whilst playing “Don’t stop til you get enough” from Michael Jackson. And literally, the 814 SE didn’t get enough!

Let me explain briefly what happened. The 814 output stage has a crowbar protection circuit. It is configured to trigger around 200mA. The crowbar works brilliantly well, however, what is tricky on my design is the fact that you do get proper grid current in A2 operation. This grid current adds to the cathode current and flows through the crowbar sensing resistor. Well, volume was so loud so am sure that when the drum or bass kicked in, the crowbar was triggered. Interestingly enough, the shunt resistor is a 330Ω / 50W piece which should (in theory) blow the 500mA fast fuse. Well, it didn’t. The current peak wasn’t big enough before the output voltage of the 600V supply dropped significantly. Bear in mind that 330Ω should take serious current out of a 600V supply in theory!

The result was evident in a couple of seconds. The shunt resistor went madly hot, burned the plastic stand-off isolator quickly and fell over one of the current meters and burned the plastic cover badly as you can see on the image above. Luckily only one channel crowbar got activated.

When I rushed into the 600V supply mains switch I saw the secondary pair of dampening resistors (100Ω 7W wire-wound) melting and red hot. They were acting as a fuse and obviously preventing the supply to deliver further current.

Of course, the 814 was intact as it was the OT. At least the crowbar did it job, but far too early 🙂

Fixed it this morning after doing the Christmas present stuff. I couldn’t end up without music in Christmas!

Lesson learned here. Crowbar needs proper testing! I will buy smaller fuses – probably a pair of 200mA slow burn will work fine next time.

Happy Christmas!

Merry Christmas!

It is so great to have another Christmas. It gives us an opportunity to wind down and spend proper time with our loves ones. The ones who are physically with us, and remember the ones who are no longer here, but still very present in our memories.

In fact, we all look forward to the holiday period to crack on with our projects. We all have a pile of endless projects and ideas and no better time than Christmas to start working on them.

Continue reading “Merry Christmas!”

01a Preamp Gen2: Build (part 4)

Further progress today with the final top plates for the preamp. Now is time to do what I really enjoy: soldering!

Valve Leakage Test

Years ago when I built my analogue curve tracer I added a small, yet very effective valve leakage test circuit. Due to my laziness, I failed to test a transmitting tetrode which I bought on-line and despite being NOS it damaged my uTracer. I followed the repair and re-calibration process and got the tester back again running, however, I regretted not having used this simple one step test I normally did before. Lesson learned, now I do use it back again!

Here is the circuit in case you don’t have a proper tester and you want to build something similar yourself:

You can test for leakage current using a simple amplifier made out of a NPN transistor and an indicator. In this case I used a Russian Neon (80V/0.5mA) and the existing supply on my tester (+/-80V). You can replace all this with a simple LED and the supply you have at hand. The circuit is designed to turn on the bulb when 5 μA leakage current is provided on the base of Q1 thanks to grounding the valve element next to the one under test. So for example if we want to measure cathode to grid leakage, we simply ground the cathode and we connect the tester to the grid. Same process is repeated with the other valve elements.

When I asked for some help in the DIYAudio forum, someoone gently recommended this text. Unfortunately I don’t read German, but what I got out of this adivce was:

valves with poor vacuum (i.e. failed the test described on the procedure in 18A)
1. preamp valves with less than 4 μA are usable
2. output valves whit less than 10 μA are usable
Valves that are good and show little Gas on the gas test:
1. should have less than 0,6-1μA for preamp valves
2. And should have less than 1.5-2μA for output valves

So the 5μA threshold was good enough in my view. It does work well and the beauty is that when neon light is very dim is an indication that it may be a workable valve despite the tiny leakage in particular with output valves.

Hope this helps

Ale

01a Preamp Gen2: Build (Part 3)

The preamp build is progressing, albeit slowly. I tried a new breadboard construction approach. A nice Ikea chopping board was used to build the front and back wooden panels. A pair of aluminium squared section rails are used to provide support and hold a pair of top plates for the power and audio connectors. As a bonus given the low power consumption of the filaments, I will use these rails as the heat sink for the Rod Coleman filament regulators as well 🙂

All in a very compact design as I literally ran out of space for further equipment in my living room!

Think

814 SE Amp: upgrading the gyrator loads

The upgrade made to the 814 SE was quite simple, I replaced the gyrator carbon 4M7 for a Holco one and the mu resistor for a Mills 820R MRA05 5W. The capacitor was finally reduced from the outrageous 1uF Mundorf Mcap EVO Oil Gold to an 220nF 450Vdc Mundorf Mcap EVO Silver Oil.

The main change was the capacitor and proved to have no loss impact on the bass. I have recently made the OT change so cannot comment on the overall impact in the sound of these changes. I can say at this stage that I cannot notice any big change. It sounds as good as before!

I’d have expected a noticeable change by reducing the size of the cap, but in this case I didn’t.

Custom Transformer Supplier in Europe

Custom transformer supplier

Given I had several emails asking about the output transformers I used in the recent upgrade of the 814 SE Amp, I asked Mr. Vincenzo to kindly provide an email address so you can reach directly to him. You can send your request via the form below which protects his email address from internet SPAM :).

Alternatively, if you don’t want to use the form below you can browse his website (in italian only sorry).