Author: Ale Moglia

"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)

300B SE Amp: build part II

Slowly making progress during the lockdown period. Over the past few weeks I managed to achieve quite more than I was expecting. The fixed bias and 300B filament supply board (board 1) needs to be wired. On the other hand, the driver filament supply plus the HT supply board is completed (board 2):

Board No. 2: HT Supply and DHT driver filament supply

I tested the HT supply and can deliver over 430V. It has more voltage capacity, however the filtering capacitors are 450VDC.  The DHT filament supplies can do 8V to 15V to accommodate different filament requirements (including some level of filament bias). It works beautifully!

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300B SE Amp – here we go

A Brave New World

Surely you’re as tired as I’m with COVID-19. One of the best things I can do to distract my mind is to keep myself away from social media. Every stone you turn, there is COVID or a statement about it. I won’t moan as I have a job for now and a healthy family. Some members of my family were infected but nothing major. I can only say is that the world has change. And so my day to day life looking after the young family whilst working is a real challenge. Starting my fifth week of lockdown, I have to distract somehow my mind at times, otherwise will go mad.

A New Concept

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3 versions of the gyrator board

Three hybrid mu-follower (aka gyrator) board generations

From left to right: 1) Standard Rev08 PCB with full flexibility of FET and TH components. 2) Rev 1.0s board with SMD except Rmu, protection drain resistor and LED as well as space for any nice big PIO capacitor. 3) the smallest version of all, all SMD except Rmu, film cap and standard TO-220 top FET and multiturn trimmer.

Very happy with the results in the board development. It does take more time and precision (you will need a microscope) to work with the MELF resistors and the SMD components in general. However, it’s worth the trouble if you’re looking to reduce the footprint.

46 driver

I love the 46 DHT. To the extent that I went on the crazy idea to implement it in filament bias. It was my winter heating indeed. Stupid idea, but sounded brilliant.

I had several questions about the 46 lately. As I’m rebuilding my 300B amp, I’d love to play with the 46 again. Here is how I’d implement it today:

The gain of the stage is about 40. The 46 has a mu of 5 in triode mode. Instead of burning loads of heat in the filament bias arrangement, I use a degenerative cathode resistor (unbypassed). The input set up transformer is the brilliant LL7903 wired in 1:8. I have a PCB made for this which it’s very handy. A zobel arrangement (C1+R2) helps taming down the frequency response at HF. Running the valve at 30-33mA is ideal. 200V at the anode is ok and the HT may be adjusted depending on the swing you need here. The stage has all the bearings to drive whatever output stage. I will add a source follower prior to the 300B as I run the 300B in fixed bias mode.

Hope this helps

Ale

The return of the Mule (RE084)

It’s been a while since I experimented with preamps again. I settled as a permanent setup with my 01a/ER801a which I love. However, I wanted to continue with my experiments so I built a new Mule, let’s call it “Mule 2” for now. It’s based out of multiple PCBs which simplifies the construction process and reduces build time:

 

The Mule is back!

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Goodbye McCoy

I hate saying goodbye. However, that’s life and it’s sad to see the great people jump on the next bus. I was very young when I first heard McCoy Tyner. I was learning saxophone when obviously Coltrane took me to him and his piano. Unique style, complex tempo and chords made Coltrane explore the deep end. That Coltrane Quartet was magic.

He did a lot for music indeed. More importantly he made a big difference to a few individuals on the planet, including me. Probably my love for music and thrive for innovation was due to him and Trane.

Many moons back went to see Mingus Big Band in New York. On the table next to us, McCoy sat down with Mingus’ wife. We were crazy about his presence but didn’t want to make him feel uncomfortable. We had a nice brief conversation and enjoyed the music as equal audience members. Still, his presence next to me disturbed my mind.

I will spin a Love Supreme and My Favourite things on vinyl tonight. Have a good trip McCoy, I will continue to enjoy your magic…

300B filament supply

In the process of rebuilding my old 300B amplifier, I decided to make a new filament power supply. It all came up around the components I had at hand, so it could’ve been improved but that meant extra cost:

I have a pair of custom-made JMS transformers with multi-taps secondaries. This helps me tweaking the right output voltage. Anyhow, any 15V transformer would do. Perhaps you want 14V to ensure you don’t dissipate too much on the filament regulators (e.g. Rod Coleman regulators)

I used my flexible LT supply PCB which allowed me to build this in less than 1 hour. I also used some existing chokes made in the UK by “Spirit” which are ok for this purpose. The Lundahls are in use, so can’t reuse them:

300B filament board ready

I used some SOT-128-2 schottky rectifiers but any other should work as well. Resistors are wirewound and the CMR choke is what I had in stock as well. A simple 15mH/3A should do fine.

The output measured well at 9V with a 6R load which dragged 1.5A. A bit more than the 300B but should be a good indication of performance. Also ripple level is good at 2.5mVrms. The rest will be cleaned up by the regulator itself 🙂