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previous: gazza hi all,I'm stuck and need s... -- 4/9/2003 7:56 PM

Re: newbie homebrew confusion !

4/10/2003 12:20 PM
gazza	Re: newbie homebrew confusion ! does anyone have a link to a schematic of something like a marshall 18 watt of a matchless sptifire or a vox ac15 etc. that uses a B+ similar to mine and also has typical test voltages recorded on it ? Am I correct in the following calculation ? current through both el84s = voltage drop on cathode resistor / cathode resistor resistance ? hence 9V / 130ohms = 70 mA. B+ voltage - cathode voltage = anode voltage = 273 -9 = 264 V. so I'm dissipating roughly 264 0.070 watts through both tubes (at idle ignoring the grids) 264 0.070 = 18 W = 9 W each ? As i understand it I should have a AB pushpull amp biased to dissipate between 0.5 and 0.7 the max dissipation at idle ? so 9W/12W = 0.76. thus I'm biased quite hot at idle but not wayout ? My problem is that I get NO sound at all ! - there seems to be something going wrong with the phase inverter I think - the pre stages are OK. any ideas for how i should try to trouble shoot this ? I don't have easy access to a scope. cheers, Gareth.

4/10/2003 12:20 PM

gazza

Re: newbie homebrew confusion !

does anyone have a link to a schematic of something like a marshall 18 watt of a matchless sptifire or a vox ac15 etc. that uses a B+ similar to mine and also has typical test voltages recorded on it ?

Am I correct in the following calculation ?

current through both el84s = voltage drop on cathode resistor / cathode resistor resistance ?
hence 9V / 130ohms = 70 mA.

B+ voltage - cathode voltage = anode voltage = 273 -9 = 264 V.

so I'm dissipating roughly 264 *0.070 watts through both tubes (at idle ignoring the grids)
264 * 0.070 = 18 W = 9 W each ?

As i understand it I should have a AB pushpull amp biased to dissipate between 0.5 and 0.7 the max dissipation at idle ?

so 9W/12W = 0.76.

thus I'm biased quite hot at idle but not wayout ?

My problem is that I get NO sound at all ! - there seems to be something going wrong with the phase inverter I think - the pre stages are OK.

any ideas for how i should try to trouble shoot this ?

I don't have easy access to a scope.

cheers,

Gareth.

Replies:

wayne Marshall 18 watt:http://65.104.... -- 4/10/2003 8:00 PM