Tag: 814 triode

814 SE Amplifier: Custom Output Transformers

 Improving the 814 SE Amplifier

photo 3After more than a year listening to this fantastic amplifier, it was time to do the first significant improvement to it despite I resisted to modify it after so much work and effort put into the design and build. The evident upgrade was the output transformer. When frequency response was measured, it was evident to see that the HF response was lower than expected. This is the result of the transformer and its configuration in this circuit. The LL9202 is a better OT for higher impedances and in this circuit, it is used in the 6KΩ /8Ω mode.

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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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A super-linear output valve and fireworks :)

Testing the 814 in triode mode

Worried about the G2 maximum voltage limit as per datasheet in class B, I decided to test the 814 carefully. So carefully that had a bit of smoke and fireworks (but not inside the valve)

Paul Leclercq suggested adding the grid resistors in G2 and G3 and do some baby steps whilst testing this lady.

Here is the test rig used:

The valve is astonishingly linear. At 600V anode voltage and 50mA quiescent current the valve only produced THD=0.01% at 15Vrms output!

Internal pannel meter wasn’t disconnected and apparently didn’t survive the 600V exposure so blew up and a domino effect provided some interesting fireworks inside my bench supply (series MOSFET regulator) 🙂

Evidently something went wrong with an unexpected short to ground, couldn’t be bothered with doing some proper root cause analysis so shut everything down and brewed myself a cup of tea….

814/VT-154 triode mode curves

First set of characteristic curves in triode mode:

A first attempt to produce a SPICE model was quite successful:

The valve glowing in the dark, nice….