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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

input resistor on the JE LAB 300B

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ixe13

Member
Joined 2006
Hi,

I dont wanna use the volume pot on the input of this amp, do I have to replace it with a resistor of the same value?
I've looked around on different schema on many sites, and the value are quite different...

If not the same value, What would be the best value ?

Thanks for your help...

Yves

schematic.gif
 
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cjkpkg

Member
Joined 2008
I am using a 300ohm resistor direct from the input to the grid of the 6sn7, then a 100K resistor from input to ground. No pot on the inputs - signal comes from a preamp.

This combination helps to prevent oscillations and gives the grid a reference to ground.
 
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k9swc

Member
Joined 2009
You'll need to replace it with TWO resistors in series, the sum of which equals the original pot, connecting the point the two fixed resistors are connected to the point in the circuit where the wiper arm of the pot is connected (V1A grid). The problem will be determining just what values to use; I'd try a 68k + 33k or 82k + 18k, with the higher value connected to the input (lower volume) or reverse them for higher volume.
 
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k9swc

Member
Joined 2009
True, Pieter, but using just the 100k resistor would be the same as running the amp with the volume control all the way up. I haven't found many amps that are run that way - I run most of mine around 3-4. Wide open (10) would probably blow my eardrums if not the speakers...Not to mention making the neighbors very unhappy!
 
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pieter t

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Joined 2009
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k9swc,
Of course, but Yves might have some other volume control already.
I don't like to attenuate input signals with plain voltage dividers; then look at the schematic and see where you can lower the gain of the driver stage, for instance by removing the 47 uF cathode capacitor.
This amp has un unpractical input sensitivity of 250 mV RMS to begin with.
 
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ixe13

Member
Joined 2006
Hey guys,

Thanx for the infos,

I forgot to mention that I'll be using a passive preamp with this amp.

So the idea would be to have the amp input wide open and the control the volume with the preamp.

But then, use that way, am I gonna get end up with noise picked up on the way to the amplifier?

Also, I dont mind adding a cap as suggested by DF96. What would be the value (although I could look around on different schema on the internet)

I guess the one would go in parallel with the input resistor?

I'm not that good to make the calculations and make the right decision.
 
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cjkpkg

Member
Joined 2008
I have been researching this a little lately and from what I understand - however correctly - the grid resistor will have more or less of an attenuating effect at the cost of bandwidth versus the grid stopper. Increase value = increased attenuation & decreased bandwidth

Typical values for the grid stopper are from 200ohm - 1kohm...from what I have seen and has little to no effect on attenuation and are presumed optional.
 
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