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View Full Version : That 500k Grid Bias Mod [5e3]

petemoore
06-05-2006, 05:32 PM
Please put me in my place if this is'nt the place for 5e3 mods..
What I wanna know is, can a 'standard' pot be used for replacing the Grid to ground 220k's for bias modding my 5e3, or do I need a heavy duty number.
I guess If I were to hit a dead spot on the pot wafer, being a grid bias it'd just hum or buzz, no real danger of burning up stuff as long as I can keep an eye on that
Bruce / Mission Amps
06-05-2006, 05:47 PM
It will do nothing for the bias.
Bruce / Mission Amps
06-05-2006, 05:49 PM
You have a misunderstanding of what bias is and what it does.
These amps are cathode biased. The grid resistors are not for bias at all... they are not for bias in any amp for that matter.
If you sub the 220K resistors with a dual gang pot to alter the resistance here, you'll have a form of a master volume.
Guitarist
10-16-2006, 12:42 PM
That really interesting. So if one were to add caps to block the bias -DC you could use a daul gang 100K or 220K as a MV?

Those always were a mystery to me.
Bruce / Mission Amps
10-20-2006, 05:50 PM
There is virtually no real DC voltage on the grid load resistors in a cathode biased amp.
No caps other then the stock .1uF phase inverter coupling caps are needed so I think you can just use a dual gang 250K pot with the top of the pots connected to their respective .1uF coupling caps, respective wipers to the power tube grids and the bottom of the pots grounded.

:rolleyes:
To be safe, you might want to stick a grounded 680K to 1M 1/2w resistor on each power tube grid in case of a dual gang pot breakdown....
Guitarist
10-20-2006, 09:42 PM
Well as that's the case you've (we) invented the latest PPI MV. That's so incredable and what's nice is that it's a lower Ohm pot so your tone doesn't get too blanketed as with the 1Meg for eg.

I wish I still had a cathode bias amp BUT, please would this work with a normally -'vely biased amp as I sort of described in the last post with the added caps?

calm down, calm down..
drewl
10-23-2006, 05:18 PM
I did the same thing on my Marshall 18w clone.
Stock it has two 470k grid resistors that I replaced with a dual 500k pot.
Works very nicely.