Tag: DHT Class A2

814 SE A2 Amplifier

Goodbye 4-65a SE, at least for now

IMG_1401After enjoying the 4-65a SE amplifier for many months, I couldn’t resist myself from upgrading the output stage to the 814s.  I just needed changing sockets and filament raw supply transformers to fit the requirements of this lovely transmitting valve. Needless to say, my recent tests on 814s were very encouraging. The 814 seemed to perform much better than the 4-65a in delivering 10W of class A2 sound at half the distortion levels. This to me, was only worth trying.

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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 🙂