UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - 120W PP EL34 amplifier re-born.

bikerhifinut · 22nd Oct 2018, 5:14 pm

I noticed on your circuit diagram that the input valve ECC83 has no grid leak resistor to ground and is capacitance coupled to the wiper of the pot via a rather high value 100k grid stopper.
Hmmm I admit I would have used something more like 10 to 22k as grid stopper at most and omitted the DC blocking cap on the input as unnnecessary as all my sources have output caps to block any DC offset on their outputs.
Of course i don't know what you feed that animal with and you may have a DC component on the signal coming in. Nonetheless I'd still have a grid leak resistor of around 1meg.
Another point to watch is that 100k grid stopper will shunt the volume pot and at maximum volume you have a potential divider that is shunting 2/3 of the signal to ground if I have understood it right, unless that series capacitor somehow negates the potential divider even at AC?

Andy.

22nd Oct 2018, 5:14 pm	#29
bikerhifinut Octode Join Date: Nov 2013 Location: Penrith, Cumbria, UK Posts: 1,993	Re: 120W PP EL34 amplifier re-born. I noticed on your circuit diagram that the input valve ECC83 has no grid leak resistor to ground and is capacitance coupled to the wiper of the pot via a rather high value 100k grid stopper. Hmmm I admit I would have used something more like 10 to 22k as grid stopper at most and omitted the DC blocking cap on the input as unnnecessary as all my sources have output caps to block any DC offset on their outputs. Of course i don't know what you feed that animal with and you may have a DC component on the signal coming in. Nonetheless I'd still have a grid leak resistor of around 1meg. Another point to watch is that 100k grid stopper will shunt the volume pot and at maximum volume you have a potential divider that is shunting 2/3 of the signal to ground if I have understood it right, unless that series capacitor somehow negates the potential divider even at AC? Andy.