Tube Amps / Music Electronics
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|6/7/1999 6:25 AM|
||BassmanRI Grid Stoppers|
Hey Guys, I just installed a Weber 5F6-A Board in my Bassman RI. I also modified the the power supply. I left the first stage as is but connected it before the standby switch (like a Blackface Fender) and then change the next to stages to 20uf like the original. Anyway, the main board (ted's) is a joy to work with and I wired the circuit as the original except for adding a bias control.
Now onto my question...the Ri has 47ohm grid stoppers. The original had none. I re-built the amp to the original spec and left the grid stoppers out....I found that when I really whacked the low strings I got some very bad distortion/clipping. I installed some 1.5K grid stoppers and me and the amp are now happy !!! Can anyone explain why I needed the grid stoppers and why the original didn't?
|6/7/1999 7:23 AM|
What brand 6L6-type output tubes are you using?
I was always puzzled that some early fender amps get by without grid stoppers, and I've come across the same large signal transient overload problem/complaint on some of these early tweed and brown panel amps. I'm not certain, but it might matter how the tubes are constructed, as to whether or not they need grid stoppers. The early fender amps were fitted with 5881s, mfd by TungSol. Those tubes have gold plated grid wire. Most other versions of 6L6 beam tubes do not. Electron cloud buildup around the signal grid can cause the grid to lose control during large signal input. Gold on the grid surface hinders the formation of this unwanted electron cloud.
What I can't explain is why the RI bassman works without a substantial grid stopper and russian 5881 tubes. Maybe there is enough shunt capacitance between certain pcb traces to snuff the transient. What about difference in grid lead wire path between the RI pcb setup and the eyelet board setup?
I hope someone else can shed some light on this interesting problem.
|6/7/1999 7:45 AM|
What makes this is little fuzzy is the term "grid stoppers." As best I know, these low value resistors are in place to minimize oscillation under certain conditions and with certain tubes - not to act as part of a voltage divider network to limit output tube grid voltage swing (which seems to be their function as described in your post). Am I more confused than usual?
|6/7/1999 8:13 AM|
I've got Svetlana's in there. I have a nice new pair of Philips 6L6WGC that I can drop in.
|6/7/1999 8:16 AM|
Now that I'm thinking about it...do you think this may have something to do with higher operating voltages in the Ri then appear on the real deal?
|6/7/1999 1:03 PM|
The RI has a 12AX7 where the original had a 12AY7. So the RI has more gain and you may experence grid blocking.
Also maybe the original was not designed to be played cranked with a guitar, but cleanly with a bass.
|6/7/1999 5:43 PM|
They do 2 things as I understand it..
First they help quench any unwanted osc due to layout capacitacne etc.
Second, they help stop the stage from "blocking" (just like in high gain pre-amps); if the power tube starts to clip and draw grid current, it gets a little bias across that resistor to help out. With bigger gmhos and higher gain you often see higher values (like the 5.6K common to later Marshalls).
On the early fenders there are even some BF amps without them from the factory (very early) and some think it's due to layout etc.. I dunno. Try with and without and see which you like better. (or can get away with!)
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