26 Preamp Gen2 completed

Finally, here it is.

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Improved sound compared to LL1660 version. This OT (LL2745/8mA)is better suited for the 26 in this configuration. Sacrificed output impedance a tad to get more gain. Filaments starved at 760mA. Distortion is as low as 0.02% @ 3Vrms output.

First listening session with Mingus, exceeded my expectations. Rounder bass and more space. Like this sound!
Will do some proper measurements but for now will enjoy and listen to it…

A brief 600V experience

Andy Evans predicted it when heard the news ” I sense a cartoon is coming”. Well, it wasn’t long till this came up. I guess we all have to experience the fireworks and domino effect of silicon failing above 600V. I laugh now, but it was annoyingly enough that best option was to go for a cup of tea.

Lars needs the 6P21S pentode curves and some other requests are piling up as well, so will have to repair the tracer sooner than later…

 

New modular preamp 26/01a and 4P1L

Today made some progress in building the modular pre-amp using my latest design on the 26. I’m planning to make this preamp a modular one to enable quick changes and replace pair of valves and OT. This will enable to use 01a and 4P1L as well…

I used a floor plank I found around as the base. The front is a simple aluminium piece that holds the stepped attenuator. Both 4-pin sockets are standing with the rubber suspensors that DHTRob kindly sent me. The two resistor arrays for filament bias are placed between the valves and the OPTs.

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There is plenty still to be done: connectors, filament regulators and HT bits.  Once all drilling is done, I will then move onto wiring…

 

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….