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DangerMan · 5th Feb 2016, 8:40 pm

It might be worth measuring the voltage on the grids of the EL34's as the amp heats up to try and see what's happening.
Probably best to measure at the other end of the 10k grid stopper, away from the grid pin so as not to cause parasitic oscillation.

The voltage should obviously be zero, but you might see it increase if there is any gas current within the valve as it gets hot.
Interestingly, Mullard quoted 700k as the maximum (limiting) value of grid leak with cathode bias but they used 470kohm in their own 5-20 design. Leak used 680k (+ 10k grid stopper) presumably to extract every last bit of open loop gain.

1 kohm screen grid resistors are a good idea as they reduce the risk of parasitic oscillation due to transformer leakage inductance, and in the Mullard book is a comment that they slightly reduce output stage distortion with a very small reduction in output and gain. They won't help if the fault is grid current from the valve though, except by slightly reducing the G2 voltage and running them a little bit cooler.

5th Feb 2016, 8:40 pm	#137
DangerMan Heptode Join Date: Jul 2009 Location: Stoke-on-Trent, Staffordshire, UK. Posts: 719	Re: EL34 power amp project It might be worth measuring the voltage on the grids of the EL34's as the amp heats up to try and see what's happening. Probably best to measure at the other end of the 10k grid stopper, away from the grid pin so as not to cause parasitic oscillation. The voltage should obviously be zero, but you might see it increase if there is any gas current within the valve as it gets hot. Interestingly, Mullard quoted 700k as the maximum (limiting) value of grid leak with cathode bias but they used 470kohm in their own 5-20 design. Leak used 680k (+ 10k grid stopper) presumably to extract every last bit of open loop gain. 1 kohm screen grid resistors are a good idea as they reduce the risk of parasitic oscillation due to transformer leakage inductance, and in the Mullard book is a comment that they slightly reduce output stage distortion with a very small reduction in output and gain. They won't help if the fault is grid current from the valve though, except by slightly reducing the G2 voltage and running them a little bit cooler.