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Vintage threads from the first ten years

Output tube Grid Resistors

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9/13/2000 8:55 PM
Peter S	Output tube Grid Resistors Anyone have any opinions on changing the output tube grid resistors(bias feed) on EL34 Plexi Marshall style amps from 220k to 100k?

9/13/2000 10:49 PM

Peter: Seems like they use different values for the type tube that is used. In the JCM 800..use the 150k for the 6550..and 220k for EL34.. And i see in fenders they use different values.. If you look at the schematics.. you'll see that usually the plate resistor on the PI/driver will be say 82k and 100k when useing the 220k bias resistors.. if the plate resistors are different..like in some of the Fenders CBS amps they use 47k and 82k in marshalls or 100k..then the bias resistors are different.. Now.. all this jibberish.. My opinion is i like the 220k and the 82k 100k plate resistors..Because this is tested, tried, used, and works in marshalls and bassmans,etc..
It must be the voltage of the plate resistors to the gain of the PI/driver of the tube used, that makes this setting.
I'm sure someone has the answer..but it must be to balance the voltage gain according to bias resistors used..to match what power tubes and how much gain is feed into them..And the other thing is the type PI tube used...if it is 12AX7 or 12At7 etc..
Richie{~}==:::

9/13/2000 11:37 PM

The ouput tube grid-to-ground (or bias supply in most cases) resistors are there to stabilize the tube's operation. You need a path to ground to "drain" off accumulated charges on the control grid.

Different tubes require different Rg1 values. Higher quality tubes with fewer residual ions and secondary emissions will tolerate higher values than recommended in the spec sheets on the tubes.

But a lot of recent production tubes aren't of the same caliber as back in the day. (early 90's tubes were often notoriously bad - the situation is generally better today).

9/14/2000 1:44 AM

I've found alot of different values of these resistors that work best in different amps I've designed and built. I have always used the 220k resistors on EL34 Marshall/Bassman style amps. Recently I was reading something someone wrote about getting better tone out of Plexi's and Bassmans using 100k resistors in this position. I also read something Ken Fisher wrote years ago about using 100k resistors extending tube life, but not changing the tone. I know what it does to the tone of the Kimerik K-50 amps, but I was just wondering if anyone had any input on whether they thought 100K resistors sound better, worse, the same? It does give the output signals going to the output tubes a little better balance, which may or may not be a good thing.

Peter S

9/14/2000 3:13 AM
Frank Clarke	Time constant of the AC coupling to the output tube grids and suchlike: http://www.aikenamps.com/BlockingDistortion.html

9/14/2000 3:47 AM

Frank,
thanx for your response and the link to Randall's excellent article. I know how to calculate the time constant of the circuit. What i'm looking for is something much more subjective, such as various peoples opinions on how they percieve the effect on the tone of the amp due to changes in the RC time constant. I've already done quite a bit of research on this subject over the years, and some AB comparisons on the effect of these changes using myself and my custormers as test subjects. I'm just trying to expand on already existing dats with an admittedly unscientific survey.

Peter S

9/14/2000 1:19 PM

Grid leak resistors of 100k or less are usually found on amps with parallel output tubes. If 200k is the maximum recommended grid leak for one 6L6 two in parallel would need 100k. The PI now has to drive 100k instead of 200k so the drive current required is doubled. If you go too low in value for the grid leak resistors the PI may not be able to provide the extra current and it will clip before the output stage reducing the power output of the amp and changing the overdrive tone (more PI clipping less power tube clipping). The PI is set up with more standing current and lower plate resistors (47k) so that it can provide the extra drive current without clipping.

Dave

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