Velleman K4040 mods.

[awkwardbydesign]

I'm trying to educate myself concerning valves, but still need help with some questions. Such as, can I alter R13 from 33k to 47k (or even to 100k) without upsetting the gain or rolloff of this ECC82 input? My original K4000 circuit used 100k, but I would like to use this later version which does away with the input cap, while still keeping a high impedance for my passive preamps.

[Firebottle]

If you look at the valve data sheet http://www.mif.pg.gda.pl/homepages/f...10/e/ECC82.pdf
you will find under 'LIMITING VALUES' a max figure for the grid resistor.

For the ECC82 it is 1 meg, so a 47 (or 100) K value is fine.

[awkwardbydesign]

Thanks Alan. I thought that would be the case, but wanted to check.

[awkwardbydesign]

And I discovered why one channel was twice as loud as the other. NFB resistors are supposed to be the same value in each channel!
When I was modifying it a couple of months ago, I must have changed one and not the other. So hopefully it will work properly now.