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RE: triode strap d3a:can wire wound plate resistor cause oscillation ?

Posted by shane. (A) on November 6, 2015 at 00:08:11

In Reply to: triode strap d3a:can wire wound plate resistor cause oscillation ? posted by genelex on October 24, 2015 at 14:01:03:

Tricky tube, and I've experimented with it a lot as low level signal stage - seems that it really likes to oscillate.

Having said that, people I know load it with a plate choke/transformer - so its a bit horses for courses, and an inductive plate load (WW resistor included) is IME not your first point of investigation.

My best advice is to start with layout - separate input and output to distinct sides of a 'sub board', use copper 2mm as the sub board and tie it to signal common. Enclose the stage suitably in what would best represent a Faraday Cage.

DC heat the tube, raise the H/K potential to ~35V via potential divider with low AC impedance from that node to 0V (cap), ferrite beads on the heater pins (over the pins ie. cut the solder 'eyelets' in half and slip the ferrite over the socket pin, heat shrink it. Bypass the heater pins (both end of the heater) to ground at HF with small value bypass caps.

100R CC resistors to plate (and screens tied to plate if triode, IME tying both screens to plate in lieu of the data sheet cathode connection sounds better YMMV), from the plate load, resistor bodies as close to the pins as possible (tubes pins = cut the socket pins back and heat shrink for isolation). Also ferrite bead on G1 in addition with (and this is variable) CC resistor of some value. I found ferrite + 100R was not enough, 1k was fine but limits noise performance for low level (mV) stage, 270R was fine and all is well.

Its worth it - but as others elude to - just because you don't hear squelch or gain reduction WRT channel balance, or AF pickup with a transmitter near-by doesn't mean that its not oscillating. CRO will tell you, for sure.. just be looking at the right frequencies !!

Again, well worth it - IMHO couldnt be happier with the D3a tube as DAC output - but it wasn't always that way.

Shane

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