Author: Ale Moglia

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

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

Improving the valve curve tracer (part 3)

Didn’t take as long as I thought to build the new anode current sensor for the valve curve tracer. A simple PCB was used to solder directly the components for a quick solution. Here is how it looks:

DPP_0008

 

valve-tracer-sense02

I tested this new circuit with some valves from high transconductance to low current ones. The current amplifier works like a charm. I used a CCS first to calibrate the x10 output and found a great linearity from 1mA up to 100mA. Currently I used my bench power supply to test this but I’m planning to do some major surgery to the tracer and include this sensor inside the main chassis.

With a dual 15V supply the AD8479 can work with up to 600V in common mode. This is perfect for my tracer and can be used to sense anode and screen currents.

Tracing a 49 NOS valve
The proof is in the pudding!
Wired anode current sensor
Curve tracer and new anode current sensor

 

Improving the valve curve tracer (part 2)

After some feedback from the DIYaudio forum clearly the INA122 was a killer in this application as no differential signal was used at the output stage. Despite having an INA122 around, I looked at an alternative option with a good op amp I had at hand. The LM833 is a dual low noise op-amp that can do the trick here:

valve-tracer-sense02

Continue reading “Improving the valve curve tracer (part 2)”

4-65a SE Amp: Fitting the HV regulators

A bit of further progress today as managed to build two Salas Shunt HV regulators (SSSHV2) with the slight tweaks I tried recently. Both regulators will provide a very stable voltage reference (+280V) for stacked supplies. Now time for completing the wiring of the 46 drivers and do some further testing…

Salas SSHV2 Regulators