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some-updates-on-the-1626-build-aiqmwxwlAlCeelwmHJHg.zip (13.37 MB)

I could not find suitable 5k 10W resistors for cathode duties at the audio stores. Both RS and element14 (Farnell) did not have stocks for the Dale aluminium power resistors. I looked around for a while, and eventually ordered Caddock MP820 series from RS. They were extended availability items and would have to be shipped from elsewhere. Total cost for 2 of these babies is around SGD25. I sure hope they would do the job well.

The MP820 resistors dissipates a bit of heat. Mounting the cathode capacitor on the aluminium plate is not ideal. I then looked around for suitable smaller sized capacitors that can be directly soldered onto the resistor leads rather than mounting the large Nippon Chemi-con. Anyways, there is not that much headroom for the Chemi-cons. It's a tall order to fulfill since the voltage would need to be around 200v, and yet be of a manageable size.

John Broskie happens to have an old post about calculating cathode capacitor value and the corresponding capacitor value to inject the right amount of power supply noise to counteract output noise in the circuit. The formula to calculate cathode capacitor is Ck = 1 / (2pFRk) or Ck = 159155 / F / Rk. I think this formula is pretty much standard. For my circuit, with a cathode resistor of 5k, this works out to 2uF for a 16Hz cutoff. Standard capacitor values guided me to this cutoff frequency... not that I play pipe organs that much in the system. There surely must be some mistake I thought. Is a mere 2uF enough?

Well... I tested with a 1uF capacitor that I had on hand and it worked pretty well to replace the 220uF Nippon Chemi-con that I used in my tests so far. It turned out to be quite an advantage for a direct coupled circuit as this is a small enough value to consider a quality polypropene capacitor in this location. The same post also mentioned a formula to calculate a corresponding capacitor (ultrapath capacitor) to inject ac signal to the cathode, thereby canceling the power supply output noise. The 1626 has a gain of about 5. This results in 1/6 of the 2uF Ck, giving 0.33uF.

I got some GAD-viva capacitors from Koba since they had these values (and others too... their GAD 0.1uF Silver Gold is around SGD105! ). Managed to wire them up tonight, together with the MP820. Dialed in the operating points and finalized the design. 6N6P (gain of 20) is running at 110v, -2v bias (actually the yellow LED drops 1.95v). 1626 is running slightly hot, at 185v, -18v, 26mA.

The final schematic is also in the pictures above. The schematic for the CCS used is the HV PNP CCS from diyaudio.com (pdf can be found here). I used an additional red LED for the reference voltage string (LED2). This increases the reference voltage, causing the emitter resistor to be larger, and raising the CCS impedance. The pdf for the Janus Shunt Regulator would provide detailed information on how to build it. My implementation used a Ohmite rheostat for R5, allowing me to adjust the voltage, and a JJ ECC832. C1 used is 22uF, C3 & C4 are 220uF. Bypass capacitors are GAD-viva 2.2uF ones.R4 and R7 are Kiwames.

I just need to finish up the IEC inlet and switch, plus the top aluminium plate. I guess these can be done at a more leisurely pace since music is flowing now. ;)

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it-sings-HCtbFjzallGhqfyzynqc.zip (4.71 MB)

Was wiring up some portion of the amplifier this afternoon, and proceeded to hook up the CCS, cathode resistors, and used my spare speakers as a test load. After some false starts, it managed to sing with my iPhone as a source. As expected, the gain was low. Strange thing is that voltages on the 6n6p is more or less in the calculated ballpark rather than the 85v previously observed when testing the CCS with the 6n6p alone. Together with the output transformer and 1626 tube in the circuit, it had different operating points. B+ was also lower. This was expected due to the extra current the 1626 is pulling. I just have to adjust the Broskie shunt regulator to provide for more B+. I ran it for a short while before switching everything off and going for my reunion dinner.

After dinner, I came back and hooked up my preamp, cd player and Coral Beta 8 speakers. Volume was much louder, but could do with more. I did some fine tuning of the operating points and B+. The CCS now outputs 112v. This results in 110v, -2 bias for the 6n6p. The yellowish LED is a constant 2.048v. 1626 now runs at 189v, -16v. Cathode resistor is a Dale 3k9 power resistor. With 128v across the cathode resistor, this should provide ~33mA of current. This did not change the volume output of the amplifier much, but is within the calculated values and should be a safe operating point for the circuit.

Adding a cathode capacitor to the 1626 resulted in more gain and I thought it sounded so much better with it in place. I used a 220uF 450v Nippon Chemi-con that I bought in Hong Kong years ago. I almost finished listening to a complete cd, but kids got to sleep, so further tweaking will be for another day when the stereo is built (add a balance pot for the 1626 filament and lift it nearer within the 128v cathode. I also want to try it with the 7k output tap, but this means lesser power output, but greater linearity - probably need help to A/B the difference).

Next up is to mount and wire the CCS, wire the ground, yellow LED and 3k9 cathode resistor, do up a panel to hold the IEC, switch and fuse. Then I will start with the other channel, followed by the top plate for both amplifiers. I would say this is a successful build. The next post would probably not be soon. Got lots of work on hand. ;)

Happy Chinese New Year to all!

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slowly-but-surely-my-darling-uIqFrfjkkagcDEaoatpv.zip (5.95 MB)

It sure is taking some time since I posted about the tube purchase. The concept for the chassis is largely determined by the FT-10 output transformers. Nick suggested that I mount them outside as they are such beauties. So, it's gonna be mounted that way. However, doing it this way does not mean less chassis work. In fact, I think it is more... and doubled since this will be mono blocks.

I only managed to drill and cut some holes for the tubes last night. Voltages were tested more or less ok few nights ago in a rat nest of crocodile clips wiring. The Broskie regulated supply worked like a charm. The slow start up offered by the 6AX5GT rectifier complemented the regulated supply nicely.

The constant current plate load for the 6N6P drivers are working fine, however, will need to fine tune the operating points of the tube as it does not seem to match the stated curves as much. This will impact the cathode resistor under the 1626 tube since they are directly coupled.

Stay tuned for more updates.

One of my KIVed project would be the Mighty Midget. I only got it working on one channel. The B+ was not enough when using the 6X5 rectifiers. Just a tad short for regulation. It was at around 290vdc. I need 300vdc for the Mighty Midget, and more if I want to use the Janus Shunt regulator. Out went the tube rectifier and in came some CREE CSD10120 babies. I'm using these  1200v 5A Schottky diodes from my parts bin.

I've also redid the Shunt Regulator onto a board and these are tested working.

The 6T10 really need a muting circuit, otherwise the amp will make some very nasty squealing noises upon power off. Tony recommended Rod Elliot's Soft-start circuit. Will have to order parts for this.

The time consuming portion is the need for a new chassis since the previous one had a hole cut for the tube rectifier already... Shucks... wonder if I should make this an all aluminium chassis... or stick with MDF and aluminium for quicker chassis construction.

Some pics here.

This preamp was working well since it was built. There were a couple of mods done since then, namely, the Feedforward Shunt Regulator, and also the DACT 50K Stepped Attenuators. I was re-looking the design especially from the perspective of my entire chain, and thought I should try out a different operating point.

What I had in mind was to do a -2v bias. This meant removing one LED from each section of the 5687 and also adjusting the B+ voltage. I also wanted to try John Broskie's PS-2 solo design. I intend to reuse the 10M45S on the Feedforward Shunt Regulator and also the heatsink for this modification.

I went to Koba and got some 1% 10R and 1R resistors, Wima and Rifa capacitors, a pair of LT1085, and a pair of BI Technologies 7276 - 10 turn 50k potentiometer to be used to set the output voltage. I also threw in my Mundorf 47+47uF capacitor and bought a pair of 240R Kiwame resistors from AHFartaudio for the PS-2. All these goodies would make any power supply green with envy. ;)

The PS-2 solo worked like a charm upon power up. With a 10 turn potentiometer, it was very easy to dial in the voltage on the plate of the 5687. With this potentiometer, the working output voltage range would be from 50vdc to 250vdc. Next steps are to mount the regulator into the chassis, along with some minor wiring.

You probably want to get John's PCB for the PS-2 instead of hardwiring.

More pics here.

Finally got the 2 regulators wired and mounted to the chassis. Previously I wanted to use a GE 22.5uF oil capacitor as the first capacitor. But once the power transformer and the regulator are in place, there is simply no room for the oil capacitor.

Just got to cross my fingers and hope they work... I've checked the wirings many times and fixed some wiring errors. Just got to wire up the signal portion in order to use it as a load for the regulator. Good thing is that I have a rheostat to slowly increase the B+. ;)

Once this works, I intend do this to my preamp psu as well. Yeah...

the mundorf m-lytic have a common ground, rendering it unsuitable for the janus design as i wanted to use the capacitor in C3 and C4 positions.

bummer bummer... got to get a few more capacitors for the janus section. i ended doing everything up on a perf board as the whole lot is quite heavy to be mounted by double-sided tape and zip cord. also, there is not enough space for the GE oil capacitor as the first capacitor.

i'm doing up the amp bit by bit... slowly, but surely...

horror of horrors... the 6D10 compactron socket that I got are not the correct ones! I got the 9 pins instead of the 12 pins... argh...

guess i have to wire up the rest of the connections and the power supply sections first...

above is the work in progress janus shunt regulator, with 12w ohmite rheostats and kiwames in key locations. the power supply will be rectified by rca 6ax5gt, then 22.5uF GE oil capacitors, then the janus shunt regulator with mundorf m-lytics.

i reused the moskido chassis for the 6t10 mighty midget. had to drill new holes for mounting of the r-core power transformer (R0080-043) from vt4c.com and also additional holes for mounting of the james output transformer. the moskido chassis had 2 octal tube holes already drilled. i will have to use the front one for the rca 6ax5gt and the rear one for the ge 6t10 compactron. the janus regulator will be on a separate copper plate. i sawed it from my m10 preamp chassis (an early project that was decommissioned).

no matter how i planned, i will miss some parts... this time round, i missed the 200R cathode resistors for the mighty midget. all of the resistors to be used there are takman rex carbon film resistors. got to replace with kiwame...

some pics here.

The moskido schematics have changed since the last time I looked at it. Now they are Direct Coupled, using a DC servo to get rid of the large output capacitor. Apparently this new version is much better than the original version.

But this requires another pair of mosfet per channel and the heatsinks that I got are not enough to dissipate the additional heat... Guess I'll have to KIV this project.

In the meantime, I am going to convert the chassis that I did previously for the 6D10 Mighty Midget. Gonna go grab some parts later.

There are not enough audio DIY related posts... hopefully I'll update with some progress tomorrow. I need to work out the tube rectifier and also Janus Shunt regulator stages for my Mighty Midget.

Finally got it done! Yeah! Tested working with all 6 channels. I did not really run it in as I do not have XLR inputs nor Speakon outputs.

It was a good learning experience for me. The main aim was not to drill too many holes onto the bottom of the chassis when mounting the UcD and SMPS modules. I guess this has been accomplished. I had 6 holes in total at the bottom. 2 for each SMPS module. A 5mm aluminium plate was used as the heatsink for the UcDs. This plate was mounted securely via the vertical slots on the bottom plate. Another aluminium plate was used as a shield between the 3 x SMPS modules and the 6 x UcD modules. This, according to the SMPS manual, would not be required as the SMPS is supposed to have low EMF/RFI radiation. Well, I did not have the luxury to read the SMPS manual prior to ordering the aluminium.

Next project would be to try out John Broskie's Janus Shunt Regulator on my preamp.