williamson Valve Amplifier

MENTION THIS here again because this is different to the copy of it that is in the old GEC projects book. The old "Trident-66" was based on this, and also appeared in the Maplin magazine around 1994. Again that version of "Trident-66" is "obsolete" for various reasons, not least because, as aforementioned, it turns out not to be an exact copy of a proper Williamson, but GEC's version of it. That used 6SN7's, as well as some different component values, whereas the original Williamson used all single triode 6J5's.

It was suggested to me only recently that the 6J5 is actually a 6J7 pentode wired internally as a triode.

This is untrue, see here: RCA Metal Valve (6J5) Striptease

Note that as it stands, and assuming true triodes, then this design 'contravenes' the advice given in Wireless World article¹ 'Negative Feedback In Amplifiers'. But not necessarily if low-capacitance valves are used.

Except of course using 6J5's means six octal valves per amplifier, which is probably why GEC opted for 6SN7's, not realising why the 6J5 was originally specified. Or simply ignored it. Plus the 6J5 is metal shielded, 6SN7's aren't. However 6SN7's are cited as an alternative in the original Williamson Wireless World article.

Therefore V1 – 4 are 6J5, V5 & 6 are of course KT66 (which as triodes have nearly identical characteristics to PX25). Alternatively V1 – 4 could be EF86's wired as triodes. Mains transformer is 410 – 0 – 410 (e.g., VT1038) and rectifier valve is 5V4. But can be GZ34 et al.

Williamson specified that the incremental inductance of the output transformer must not be less than 100 Henries. Tests have shown (using Wireless World's own suggested method) that say even the old Maplin type VT425 has 130 Henries, while VT1041 has 250 Henries.

The balance and bias adjuster network in the KT66 cathode circuits can be largely ignored, when I tried this the KT66's would keep drifting off anyway so frequently needed readjusting. Instead use one common cathode resistor of 340Ω (2 x 680Ω in parallel). This is conservatively high thus ensures that whichever is conducting more, if either, its PA is not exceeded.

WARNING: ELECTRIC SHOCK HAZARD Working with high voltage is dangerous! Never poke your fingers into a live circuit! Adopt a proper safety procedure and follow it every time – before the circuit may be safely touched, switch off the mains and pull the plug. Then make sure that the HT voltage has discharged to at least 50V; if necessary discharge with a resistor (not a screwdriver!). The proper industrial procedure includes shorting the HT with an earthing strap, you could do this but don't forget to take it off before turning the power back on. (Alternatively a high value discharge resistor, say 1MΩ, may be permanently incorporated into the power supply.) This procedure follows the acronym S.I.D.E. – "Switch off, Isolate, Discharge, Earth". Don't cut corners!