SE el84 triode wired=hum as pentode=no hum ?

[walkman]

Hi , all

Is there any reason an el84 wired as a triode pins 7 & 9 together should have distinct hum , even with the preamp volume on zero? When its wired as a pentode there is no hum.

[pdf64]

This is most likely due to ripple on the B+, because the screen grid is an input just like the regular grid. As a pentode, I'm guessing that the screen grid goes through an extra stage of B+ filtering compared to the anode supply? As a triode, the screen grid will be missing out on that filtering, so the ripple signal will be getting input to the tube, modulating the anode current, via the screen grid.
It might be causing weird ghost notes by intermodulation effect also?
To reduce it you could up the filtering to the anode supply, either with more capacitence, or with a 'pi' (cap / choke / cap) arrangement. But a 100mA rated choke might be a bit pricey - Peter

[Enzo]

Have you actually wired the socket as a triode, or did you just short the plate and screen together in the existing circuit? If you did the short cut, you are defeating part of your filter.

[walkman]

i've shorted the pins 7 & 9 , originally via a switch but now hard wired.....
this means its lost the filtering at the screen. I already have 47uf on the B+ but also had 47uf at the screen .... i also have a 100ohm from the rectifier to the junction of the filter cap and the B+

[Enzo]

Instead of shorting the two together, try running the triode mode off the screen node of the B+. That will let you keep the two stages of filtration your screen used to have. How large a resistor was between plate and scren nodes? You might adjust the value in my scenario.

[walkman]

there was a 4k7 resistor ..... before you posted again Enzo, i re did the power supply so that goes recto then 47uf 100 ohms 47 ohms B+ ....... there is lot less hum ... im happy with that but it would be great to get it even quieter