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In Reply to: RE: what's the best stopper approach to avoid trouble with 5842q? posted by Triode_Kingdom on July 18, 2017 at 19:59:45
4X 10 Kohm carbon comp. stoppers arranged in a "cone", whose apex is the I/P point. A 100 ohm carbon film stopper on the plate. Last but not least, ferrite beads on the heater wires.
Tubes like the 5842 and 6C45П (6s45p) come distressingly close to oscillating in their cardboard boxes. Types of this sort are intended for grounded grid RF service. When operating these ultra-high gm types common cathode, EXTREME precautions against parasitic oscillation are very much in order. "My o'scope shows nothing." Your o'scope is not nearly wide band enough. I would not be shocked if microwave frequencies are produced.
Eli D.
Follow Ups:
I have joked somewhere on the web about the grid stopper "cone of silence" for 5842's. Worked for me.
"Your o'scope is not nearly wide band enough. "
And even if it is (some now work into the GHz range), it will be difficult to see a transient signal at -60 dB. Only a microwave spectrum analyzer can guarantee a clean amplifier.
--------------------------
Buy Chinese. Bury freedom.
With the 417A/5842, I've had good luck with 180 ohm carbon comp grid stoppers (in combination with a ferrite bead).
YMMV.
I implemented the same for my builds. Stopper on all 4 grids and a ferrite bead on the heater. No audible issues.
Stuben
~!
The Mind has No Firewall~ U.S. Army War College.
what's your audio -- power supply and ground schemes? my K-horns sound "muddy" but part of that comes from very resonant walls - I can tap on a wall and "boooooommmmm" like the thud of a poor car subwoofer
Karlson Evangelist
PS 8 henry {25 ohms DCR} /Vishay 45 uF/10 Henry {35 ohms DCR}/Vishay 45 uFLED bias
45 output tube. Star ground to a 1969 Kennedy half dollar {40% silver/60% copper}
was not necessary to lift ground anywhere @ all in this build
sometimes is necessary to lift circuit from PS...{to make it dead quiet}
I never know why.
AC on the filaments
The Mind has No Firewall~ U.S. Army War College.
Edits: 07/22/17
Kennedy 1/2 dollar for star ground.Firefox was acting strange and edit/delete was missing when I tried to amend before. This has happened before.
The Mind has No Firewall~ U.S. Army War College.
Edits: 07/22/17
hi Eli D. and Triode_Kingdom
when I first built it sans any 5842q grid stopper, it oscillated strong enough to pretty much block any audio. I think I removed several tube socket grid pins and just used one stopper around 120 ohm - it seemed to play music pretty well on my old Edgarhorn system but would not be surprised about lurking oscillation.
about where would you expect the 4-10K stoppers to roll things off? What spec/size bead would be appropriate for the heater wiring?
thanks
Karlson Evangelist
Eli,
Like the diagram? What advantages and application (transmitting RF?) would use this?
Thanks!
The control grid, is now screening the input( the cathode ) from the output. Dealing with C_Miller was a major driver of the tetrode( and then its improvements, the beam tube and pentode ).
It is not w/o its drawbacks; input impedance is cathode||bias resistor in that particular circuit...which is OTO a positive grid in a grounded cathode circuit.
There won't be a step change in loading when the grid goes positive though...and for power tubes, this is a non-trivial consideration. My next amps are grounded grid finals.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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