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FDP Forum / Amp Mods, Repairs, and Projects / Princeton Reverb schematic question

Roly
Contributing Member
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Whitehorse Canada

I don't get out much
Jan 17th, 2017 09:45 PM   Edit   Profile   Print Topic   Search Topic

Why is the 1K5 resistor in line to pin five of the 6V6s absent?

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   Edit   Profile   Print Topic   Search Topic

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   Edit   Profile   Print Topic   Search Topic

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   Edit   Profile   Print Topic   Search Topic

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   Edit   Profile   Print Topic   Search Topic

" there is a resistor in the power supply to limit screen current"

I knew about that.

Thanks for your help guys.

pdf64

UK

Jan 19th, 2017 02:25 AM   Edit   Profile   Print Topic   Search Topic

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.
The 12AX7 is only good for audio bandwidth, whereas the 12AT7 is suitable up to 300MHz!
So with such an increased frequency range, the likelihood of phase shift / unintended coupling is much higher, which may result in instability / oscillation.
Grid stoppers are the standard method to combat this.

amphead4

Cincinnati, USA

Jan 19th, 2017 10:32 AM   Edit   Profile   Print Topic   Search Topic

^interesting, never knew that...

Hammond101
Contributing Member
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So. Cal. USA

Jan 19th, 2017 12:04 PM   Edit   Profile   Print Topic   Search Topic

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.

Kudos!

pdf64

UK

Jan 20th, 2017 02:31 AM   Edit   Profile   Print Topic   Search Topic

Thanks, I'm pleased to have been helpful:-)
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.
So what value to use?
In the linked thread over on MEF, Gingertube puts forward a guideline of
grid stopper value >= 8/gm
So at least 1k3 for 6L6, 6k8 for 12AX7, 1k5 for 12AT7 etc.

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FDP Forum / Amp Mods, Repairs, and Projects / Princeton Reverb schematic question




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