FDP Forum / Princeton Reverb schematic question/ 9 messages in thread.

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Roly

Contributing Member
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Whitehorse Canada

I don't get out much
Jan 17th, 2017 09:45 PM        

Why is the 1K5 resistor in line to pin five of the 6V6s absent?<br /> <br /> Thanks folks.



Steve Dallman

Contributing Member
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Merrill, Wisconsin

Age is just a number...mine is big
Jan 18th, 2017 06:25 AM        

Cost saving. This was intended to be a "practice" amp. Leo was extremely cost conscious. They were rF protection and felt in this application were not needed. Not a bad idea to add them



Steve Dallman

Contributing Member
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Merrill, Wisconsin

Age is just a number...mine is big
Jan 18th, 2017 06:27 AM        

You'll notice that along with the absence of grid resistors, there are no screen resistors (470 ohm) and again, not a bad idea to add them as well.



amphead4



Cincinnati, USA

Jan 18th, 2017 11:26 AM        

The screen resistors become even more important in the circuits that use a choke across the top of the pi. IMO, they are optional for the Princeton since there is a resistor in the power supply to limit screen current.



Roly

Contributing Member
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Whitehorse Canada

I don't get out much
Jan 18th, 2017 03:36 PM        

" there is a resistor in the power supply to limit screen current"<br /> <br /> I knew about that.<br /> <br /> Thanks for your help guys.



pdf64



UK

Jan 19th, 2017 02:25 AM        

I think that Fender's policy seemed to be that the power tube grid stoppers were only fitted by default in amps with a 12AT7 LTP.<br /> The 12AX7 is only good for audio bandwidth, whereas the 12AT7 is suitable up to 300MHz!<br /> So with such an increased frequency range, the likelihood of phase shift / unintended coupling is much higher, which may result in instability / oscillation.<br /> Grid stoppers are the standard method to combat this.



amphead4



Cincinnati, USA

Jan 19th, 2017 10:32 AM        

^interesting, never knew that...



Hammond101

Contributing Member
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So. Cal. USA

Jan 19th, 2017 12:04 PM        

pdf64 is Mr. Grid Stopper/Snubber! He has helped my several times when I have had oscillation issues, which sometimes sound like distortion and are hard to find. Always helpful.<br /> <br /> Kudos!



pdf64



UK

Jan 20th, 2017 02:31 AM        

Thanks, I'm pleased to have been helpful:-)<br /> Grid stoppers certainly can be very beneficial but if their value to too high then treble may get rolled off and, especially for early stages, hiss may be increased.<br /> So what value to use?<br /> In the linked thread over on MEF, Gingertube puts forward a guideline of <br /> grid stopper value >= 8/gm<br /> So at least 1k3 for 6L6, 6k8 for 12AX7, 1k5 for 12AT7 etc.



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