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Post subject: Re: Vibrolux Reverb 1968 drip edge bias set-up question
Posted: Sun Oct 13, 2013 1:02 am
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Jimitrader, glad you got the tube situation sorted out.

" Changed input jack resistor to 1M 1/2 watt 5% "

This is the grid-to-ground single-tied resistor. Correct? Not the paired input 68k-ohm ones, sometimes soldered on the board, sometimes soldered between the two input jacks for each channel. Just to be sure. As the 68k-ohm resistors are really the input grid load resistors. The 1meg-ohm is the input bias-setting parallel grid-to-ground resistor. If the 1 meg-ohm is placed in series with the grid of the 12AX7 input tube, you'll get an impedance mismatch (between guitar and amp) and concurrent drop in gain.

See the layout of the following link:

http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: Vibrolux Reverb 1968 drip edge bias set-up question
Posted: Sun Oct 13, 2013 8:51 am
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BMW2002Ti wrote:
If the 1 meg-ohm is placed in series with the grid of the 12AX7 input tube, you'll get an impedance mismatch (between guitar and amp) and concurrent drop in gain.


+1

A significant drop in gain for sure.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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