Need help on 5E2 filament center tap and what to ground

King Fan

Poster Extraordinaire
Ad Free Member
Joined
Jan 1, 2013
Posts
8,755
Location
Salt Lake City
I totally agree the bleeder/voltage divider is elegant -- and needed to elevate in fixed bias. In cathode bias, because Merlin -- and millions of quiet amps -- use cathode voltage for elevation, I don't worry about that method either.
 
Last edited:

KeithAmpsIt

TDPRI Member
Joined
Aug 15, 2022
Posts
21
Age
60
Location
Olathe, KS
Yes you are right! Max signal amplitude on cathode biased power tube cathode depends of bypass capacitor value and typical 10uF I recall comes about 5Vpp at 80Hz. After I saw that on oscilloscope I quit using it as filament elevate although I did not do any measuring whether it have any adverse effect.

I usually install a bleed circuit which first has a 220k resistor and series to Common with it parallel 56k resistor and 10uF 100V electrolyt where I connect filament CT. Then depending which B+ the bleed is connected and its voltage the filaments elevate to 20% which usually comes somewhere 50V to 80V. When amp does not have cathode follower(s) bottom resistor can be 47k.
So is this a voltage divider you are describing? I'm having difficulty visualizing what you have described. Is it like what the Valve Wizard shows in this diagram? And this is better at mitigating possible hum than a CT to ground? I bought a pre layed-out turret board and don't have a place for extra components. Is this only done after an issue with hum arises?
IMG_0525.JPG
 

andrewRneumann

Friend of Leo's
Joined
Mar 22, 2020
Posts
2,153
Location
Cincinnati, OH, USA
The power for the heaters, then, is developed by the difference in potential between the legs of the heaters and that ground reference.

One nit to pick. The power is generated by the 6.3VAC across the heater winding alone. The center tap or virtual center tap can be connected to anything (AC or DC) and there will always be 6.3VAC across the winding providing power to the heaters.

Elevation of the heater circuit to reduce hum operates on the principle of saturation. As long as the voltage is high enough to keep the leakage currents in saturation, the small amount of AC on the cathode under signal conditions should be inaudible. Even if it were audible, it would be at the same frequency as the signal itself and would completely blend in with the audio signal would it not?
 

chas.wahl

Tele-Holic
Joined
Dec 8, 2012
Posts
595
Location
NYC
Well, OK. The whole "leakage currents in saturation" thing (I freely admit) I don't really understand; will have to follow that up somewhere. Any leads? I can't find an online version of the article in Electronic Engineering referenced by Merlin Blencowe (July 1944). Several sources refer to this, but nobody seems to have made it available on the internet.
 

King Fan

Poster Extraordinaire
Ad Free Member
Joined
Jan 1, 2013
Posts
8,755
Location
Salt Lake City
So is this a voltage divider you are describing? I'm having difficulty visualizing what you have described. Is it like what the Valve Wizard shows in this diagram? And this is better at mitigating possible hum than a CT to ground? I bought a pre layed-out turret board and don't have a place for extra components. Is this only done after an issue with hum arises?
View attachment 1020815

You're right, that Merlin bit, exactly. In that same short chapter, you'll note he explains why heater "elevation" is better than CT straight to ground. But in a small cathode bias amp (the 5E2) there's no need to install a bleeder/divider circuit -- just putting the heater CT on the 6V6 cathode pin will elevate nicely.
 

KeithAmpsIt

TDPRI Member
Joined
Aug 15, 2022
Posts
21
Age
60
Location
Olathe, KS
You guys are great! Thanks for all the advice and background info! Didn’t expect someone to find relevant info on this subject from 1944! Kudos to @andrewRneumann. I guess that was a time when research on tubes like this was at its peak probably.
 




Top