Marshall Amp clone. - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

Steve_P · 14th Jul 2009, 1:51 pm

Seeing as we haven't got a circuit it's hard to say. Can you leave that one - it might well be OK.

Cheers,

Steve P.

M0XNA Neil · 14th Jul 2009, 4:23 pm

The negative grid bias supply in these amps is typically -65V. The EL34 datasheet states a grid bias of -38V for a quiescent cathode current of around 30mA (i.e. with no signal applied). You can insert a 1 Ohm resistor in series with the cathode to ground and measure the voltage across the resistor in mV will give the cathode current in mA.

Neil.

Ed_Dinning · 14th Jul 2009, 8:22 pm

Hi Peter, conect 2 of the 500pf ones in series, this will give you 250pf at twice the working voltage. A lot close in value.

Ed

lenaghans1 · 14th Jul 2009, 8:44 pm

ed dinning thanks for your reply.do you know what the voltage would be.one end of this capacitor has a wire going to the treble pot right lug.

peter.

lenaghans1 · 14th Jul 2009, 8:54 pm

moxna neil thanks for the reply.i already have a bias probe to measure the bias.if i use a 47k or a 56k bias resistor i get 23ma, but i get red plating. if i use a 82k bias resistor i get 14.6ma on three valves and 13.8ma on the other.i can notice a difference in the tone.

thanks peter.

14th Jul 2009, 1:51 pm	#41
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Marshall Amp clone. Seeing as we haven't got a circuit it's hard to say. Can you leave that one - it might well be OK. Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

14th Jul 2009, 4:23 pm	#42
M0XNA Neil Tetrode Join Date: Apr 2009 Location: Towcester, Northamptonshire, UK. Posts: 92	Re: Marshall Amp clone. The negative grid bias supply in these amps is typically -65V. The EL34 datasheet states a grid bias of -38V for a quiescent cathode current of around 30mA (i.e. with no signal applied). You can insert a 1 Ohm resistor in series with the cathode to ground and measure the voltage across the resistor in mV will give the cathode current in mA. Neil. __________________ An engineer makes for a penny what a scientist makes for a pound!

14th Jul 2009, 8:22 pm	#43
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: Marshall Amp clone. Hi Peter, conect 2 of the 500pf ones in series, this will give you 250pf at twice the working voltage. A lot close in value. Ed

14th Jul 2009, 8:44 pm	#44
lenaghans1 Retired Dormant Member Join Date: Apr 2009 Location: Isle of Wight. Posts: 34	Re: Marshall Amp clone. ed dinning thanks for your reply.do you know what the voltage would be.one end of this capacitor has a wire going to the treble pot right lug. peter.

14th Jul 2009, 8:54 pm	#45
lenaghans1 Retired Dormant Member Join Date: Apr 2009 Location: Isle of Wight. Posts: 34	Re: Marshall Amp clone. moxna neil thanks for the reply.i already have a bias probe to measure the bias.if i use a 47k or a 56k bias resistor i get 23ma, but i get red plating. if i use a 82k bias resistor i get 14.6ma on three valves and 13.8ma on the other.i can notice a difference in the tone. thanks peter.