Lenco turntable (Part II)

Great thing about bank holiday weekends is that you have more time to work on your projects! At least I had this time round.

After doing the initial calibration to the Lenco idler mechanism, I got the turntable finally assembled on the new plinth. It runs very quiet and smoothly despite having not dampened the idler wheeler  as recommended in the Lenco Heaven forum. I need to look into it but so far the turntable runs really quiet.

The arm board is a bit lower than it needs to probably. I can adjust the VTA with the micrometer but I should look at raising it to allow the arm to be rotated to the right for best placement. The micrometer is preventing this to happen. Will refine later.

GL75 now on its final place
Audiomods tonearm
First test
Clearaudio virtuoso cartridge

Ok, impressions so far after all this work then. I did many upgrades this time to my system so it is hard to quantify which part contributed the most to the sound improvement. Overall tone and clarity has improved. I suppose that the heavy plinth makes a big difference to the bass clarity. The overall tone improvement and detail is quite likely to be the Audiomods tone arm as well as using the Clearaudio Virtuoso Wood MM cartridge that Tony kindly give me to try.

Lenco turntable (part I)

A rare occasion in my hi-fi life is too see me switching from electronics to basic mechanics. I’m not either good with woodworking or mechanics :). Either way, the GL75 rebuild is coming along quite fast. I sealed the original arm holes in the top plate and painted it yesterday. Today, I spent all morning waxing the plinth. The afternoon was devoted to rebuilding the Lenco turntable mechanism and testing the re-lubed motor and spindle. I still need to do further calibration, but so far so good.

Here are some pictures of the building steps:

plinth and arm
top plate ready for paint
painted top plate
waxed plinth

Barry French’s 26 DHT preamp

26 DHT Preamp (by Barry French)

After owning several high quality pre-amps & modifying some of them to some degree or another, I decided to have a go at building a top quality unit, so here goes first I needed to build the power supplies for filaments & B+ in a separate case, I already had some idea of the look & design that I wanted to follow, so I sourced the following components from these suppliers:

Continue reading “Barry French’s 26 DHT preamp”

4-65a SE Amp: output valve

One of the output 4-65a was actually a military JAN-8165. Wanted to test a pair of them but in the rush of building the amp I ended up with a mixed of the two. Not an orthodox approach buy who cares! After playing the amp extensively for a couple of weeks I noticed that this valve droped anode current after 1hour or more of playing. Anode current could go down by 10-15mA. Perhaps it has to do with the pin oxidation, but I also suspected on the filament regulator due to the heavy current required on this. The regulator heatsink gets really hot as you can imagine so I was already blaming on the regulator FET before I even suspected on this output valve.

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A simple test was to replace this valve by a new NOS 4-65a. I did that and surprised to find that the filament regulator wasn’t to blame. The 4-65a SE filament stayed rock solid at 100mA after 1h30m of fantastic music. Yes, the bass of this SE is unique. It exceeded everything I previously listened to. Is the 46 driver in filament bias? Not sure yet, but hey ho. What an amp!

Adjusting the shunt regulator

One of the problems I found when using the 4-65a SE amp was that my version of the Salas SSHV2 drifted significantly with temperature. This was due to the 01N100D characteristics (see extract from datasheet below). We can see that @ -1.5V VGS the temperature dependency is as much as 50mA/100°C. With the smaller heatsink I had before and the significant voltage drop given raw supply voltage levels this caused a problem:

01N100D temperature dependency
IXYS IXTP01N100D datasheet extract

I then reduced the input raw voltage level to 300V to ease the power dissipation across this FET. This was not sufficient so I proceeded to look at various things on the regulator as described below:

Change log

  1. Replaced M1 for DN2540. The DN2540 has a better temperature response. The voltage levels now allowed me safely use the DN2540 and no need for a 01N100D.
  2. Added bigger heatsinks for M1 and M3. Even 2.5W across M1 would need a bigger heatsink if we are looking to run the CCS as high as 90mA.
  3. Removed TP resistor to allow higher CCS current
  4. Set CCS current to 85mA
  5. Load will be now 30mA per channel (30mA each 46 driver valve)
  6. Set output votage to 270V

Image

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Testing the SSHV2

All tested well in the workbench. I used an external CCS set at 60mA and the output voltage stabilisation was rock solid at 270V. Temperature of the heat-sink was around 40-45°C after 30min of continuous use.

Back in the 4-65a SE Amp, the raw supply level at full load was 297V. The regulator played nicelly at 270V and the CCS current drifted only 5mA from 90mA down to 85mA after 2 hours of continuous play. The temperature of the CCS increased only to 47°C whereas the shunt FET got up to 54°C given the continuous 7W dissipated on it. I may try to reduce the CCS current a tad to ease here.

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The regulator is fitted back into the 4-65a SE amp

Played the amp for about 2 hours and slowly started to appreciate more and more the sweet sound of this beast. Bass has increased significantly in my systems and previously haven’t been able to extract the deep bass out of the SE amp through my Fostex FE167E full range drivers.

I’m really going to enjoy this amp for a while until I start looking at changes. Oh yes, many come to mind, but easy for now. Other projects are awaiting for a long time so far.

Ale

4-65a SE Amp: testing it finally!

The much-awaited moment finally arrived. After yesterday’s driver tests, I did a lot of work this morning to assemble cables and test the output stage. What I clearly know now is that I won’t be needing any heating this winter! What on earth was on my mind when I decided to build this amp? God only knows…

Here are some pictures of the first tests in the workshop and then when I hooked it up to my system downstairs in the sitting room:

26 preamp and 4-65a SE Amp just before testing
Burning in the 4-65a!
Setting output bias
Measuring output

I did a quick measurement of the output THD without burning in the 4-65a or the amp. The operating point is not optimised but clearly shows a nice picture. First of all, the amp is absolutely quiet. The Rod Coleman regulators plus the extensive filtering on all supplies (LCLC and CLCLC) make this the quietest amp I’ve ever made! The distortion is higher than predicted. With the valves at 100mA/540V and with a non-inductive resistor load of 10Ω, the THD is about 2.7% for nearly 6W of pure class A power. Only 4% of the harmonic content is H3 and with a nice H2 component. The footprint of an SE amp is clearly on this amp.

4-65a SE Power test 5.6W test1

Hooked it downstairs and after a lot of wiring I finally got to play some good records on this amp. I used my 26 DHT preamp. First record to be played was “a love supreme” (John Coltrane). Here are my impressions so far:

  • I’m surprised with the bass. It is powerful and not something I was used to in a single ended amp
  • Definitely needs some burn-in time. The amp improved after 1 hour of use
  • It’s loud! You can get 10W easily in class A2. Very loud for my room!
  • The tone is warm and very sweet. you get the sound of the DHT clearly
  • Dynamics are its forte. This amp responds very well to them

Some more pictures:

4-65a Amp finished
Cherry red anode!
First time up
A power supply madness!
Input transformers and 46 drivers

Now is time for proper listening after so much work. A real accomplishment and I’m feeling very proud. The amp fits within my cabinet so wife is happy 🙂

 

4-65a SE Amp: first driver test

20130810-172222.jpgAfter completing the last power supply, I finally did some real tests on the 4-65A SE amplifier. Given the DC coupled design it is a bit tricky to do the initial calibration. I had to set the operating point of both 46 DHTs by adjusting the anode voltage through the individual gyrator load presets. Also had to balance at the same time the Salas Shunt current and output voltage to the desired levels. After playing a while with it I managed to stabilise the Salas shunt regulator.

Set the 46 to drive the output stage to 200Vpp with a 3.7Vpp  (1.33Vrms) input. That is a gain of approximately 54. Here is the distortion profile:

46 driver test1 200vppBreadboard is really quiet with the 50 and 100Hz noise below -95dB. It’s great to see the nice 46 Super Silvertone performing only 0.09% at 200V peak to peak!

Results are promising, just need final tweaks to 600V supply and then hook the 4-65a!

Ale

 

4-65A SE Amp: A2 grid current supply

Finished today the penultimate power supply of the 4-65a SE amplifier. This one is the A2 grid current one and doesn’t need a lot of filtering as the 46 driver gyrator will do most of that job. The DC stacked design of this amplifier will allow an independent loop of the A2 current between this supply and 4-65a grid-cathode.

Just only one more supply to go and I’m done!!!

40uF and 8uF oil caps
Mains transformer and bridge recitfier
Main switch and bridge rectifier
Choke and output connector