Re: Bias problem
- From: Chief_Billy@xxxxxxxxxxx
- Date: Tue, 21 Aug 2007 18:40:23 -0000
On Aug 20, 4:37 pm, White Spirit <wspi...@xxxxxxxxxxxxxxxx> wrote:
Pete wrote:
He's running these in a 6100, which has a 470 screen grid and a 5.6k
on the grid (stock factory setup) with around 470-475volts at the plates.
It's my understanding that 470 ohms are normal for 6L6 types and 1K for
the EL34s. The schematics I have for a Fender Pro Reverb are 470 ohm
screen grid resistors (6L6GCs) and the Marshall 2203 has 1K 5W (EL34s).
1K screen resistors work for everything . Since their function is to
drop voltage if the screen grid
current starts running away, and therefore 'throttle back' the current
through the tube, it stands to reason that, within limits, 1K is
better than 470. Of course, the wattage rating is important or else
those resistors may fail while trying to do their job, which is why 5
watt is better than 1 watt.
Some builders have suggested that the 470's sound more 'open', and I
can't comment on that...it's subjective. 470 is the 'vintage' value
used in the old JTM's and Fender amps. The original JTM45's used a 1K
off the screen grid supply rail, and then split to discreet 470's to
each tubes pin 5. I just use 1K/5w on each tube as a no-brainer
anymore.
http://www.drtube.com/schematics/marshall/jtm45tr.gif
.
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