[yestertech]

Hi Neil, the additional cap would go across the 47 mF, after the 1K smoothing resistor. The effect you hear is caused by stray coupling, either in the HT -ve ( chassis ) wiring, or in the HT +ve connections.
It is usually exacerbated by high gain circuitry and can be cured by careful layout and perhaps additional HT decoupling.
If the anode load resistor for the ECL80 triode section is returned directly to the same HT point as the pentode section ( a & g2 ) , try inserting an additional 10k resistor between them, and adding a 4.7mF to ground at the junction. This usually works.
Also, check that the earthing points you have used are good low resitance paths. If using tags, don't forget to insert a star washer between tag and chassis to improve contact, and bolt down tight.
If the chassis is coated, scrape away some coating before doing this.
Lastly, you may find the effect is made worse by excessive frequency response. Try inserting say, a 4k7 resistor between the coupling cap to the ECL80 pentode section grid and its grid leak. This will roll off the frequency response to prevent unwanted hi frequencies interfering with correct operation.
One , or a combination of the above is usually guaranteed to cure the problem.
ANDY