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4-65a SE Amp: cherry anodes
Category: 4-65A SE Amplifier
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.
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
After 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:
Breadboard 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: 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
307a with schade feedback (Part II)
Some time ago I traced the 307a that kindly Vegard Winge sent me from Norway. As suggested by Vegard, I re-traced them with lower screen voltage and increased feedback to improve the linearity of the 307a in this mode.
The modification to the curve tracer is simple. A simple resistor divider (R1, R2) provides the feedback to the grid. The grid driver provides a constant impedance so works fine. With a potentiometer you can adjust the feedback ratio and look at the impact on the curves. Very handy!
With 15.5% feedback and 125V screen voltage I got the best curves as shown below:
Nice to see the anode resistance coming down to 1kΩ.I tried matching a triode model to these curves with a good result
4-65a SE Amp: Building the OTs
More work done this afternoon. Built the LL9202/100mA OTs in their supporting structure and fitted the crowbar protection boards for the primary windings. Also added the speaker posts. A heavy part of the amp!
LL9202/100mA with crowbars
Crowbar protection for primary winding
Crowbar fitted on the back of the OTs
Module completed
Speaker binding posts
Crowbars and LL9202/100mA
Crowbar protection
Protecting your valuable output transformer
When using fixed-bias you don’t want to take any risks and damage your output transformer. What happens if your output valve (e.g. a transmitting power valve) makes false contact at the grid or the bias supply fails to start for whatever reason. Well, your valve will conduct fully and probably melting the anode and if not damaging the primary winding of your OT. Both things are catastrophic and you don’t want to try it…too expensive!
You may think there is a simple solution: a fuse. We all know fuses are inaccurate and can be dangerous if they don’t blow properly. Also HT fuses are expensive and they don’t come up with a wide range of values.
4-65a SE Amp: Building process (Part 2)
Earth and grounding and input Kiwame resistors
Close look at the 46 driver sockets
front view
filament heat sink and SSHV2 anmeter
filament resistor array and supply connectors
Top view of the breadboard
Further progress today: more cabling done for the SSHV2 boards and A2 current raw supplies for the 46 driver gyrators….
4P1L / 6C6C SE Amplifier Design
Pushing to the limits
We’re constantly obsessed to get the most out of our lives. Not a product of the capitalist world we live in, but a fact of our human nature. Its evolution.
When it comes to sonic power, unfortunately we are not too distance from this thought. We want more Watts. Yes, pure power. My generation back in the 80s got misled by the audio product marketing and their unrealistic metrics (e.g. PMPO) to fudge the real power of a solid state amplifier.
4-65a SE Amp: 46 filament supply tested
Finished the second channel and tested the filament supply. The filament array which is formed by two paralleled pairs of 20W 10Ω wirewound resistors gets hot as expected. The array temperature is about 110-126°C at an ambient temperature of 24°C. The anode of the 46 gets to 49°C after 20 min of use and the heatsink stays at 42°C whereas the regulator TO-220 transistors are about 45°C. There is about 30W dissipated on each array. Yes I know, a lot of power but the filament bias is hard to beat in terms of sound in my view.