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Re: BassmanRI Grid Stoppers


 
6/8/1999 6:35 AM
Bruce
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Re: BassmanRI Grid Stoppers
The grid stopper resistors are also used to tone down the input stage a little.  
The resistor forms an RC filter with the grid to anode capacitance and actually limits the upper hi frequency response of the preamp.  
It also can help to control any high frequency oscillations created by weird grid capacitances.  
As far as the phase inverter/driver, there is all kinds of odd wires and junk surrounding the circuit so the resistors are installed right on the tube socket to limit anything ahead of them effecting the circuit.  
Why it was helping your "blocking" problem is a mystery to me as the frequency response effected by a 1k5 resistor has to be way above what I think you were hearing.  
I think Speedracers answer is more like it.  
Try a different grid load resitor value too and see if it is any better.  
Oh as for why... nobody cranked an dgrinded away on 5F6A Bassman amps when they were built and chances are you are using a 12AX7 front end instead of a 12AY7 anyhow.  
Do you have a small 47pF cap across the output of the PI/driver tube?  
 
Bruce
 
6/8/1999 12:37 PM
Ron Orlando
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hey guys..Yup, I have a 47pf cap on the inverter and yes I'm using a 12AX7 in the first slot. The RI has two 47 ohm grid stoppers in the circuit. When I wired up the new 'vintage'board I left them out as per the original. Well, the ol'girl didn't like that.so in went the 1.5K's. To be honest with you the amp sounds just fine with the 1.5k's in place..a Fender tradition. I don't remember the B+ voltages off the top of my head.but they are less then the RI schematic (like blackface voltages) but higher then '59 spec for sure. I was thinking, this may have some effect on the power stage behaving this way. Also, for the record,the Hoffman board uses 1.5k grid stoppers.
 
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