Elevate filament center tap: yea or nay?

Browneyesound

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In going through my superlead and JTM45 offset circuits (that I made ~15 years ago, surprising how much I have forgotten about the specific circuit tweaks and construction in all that time), I am contemplating referencing the filament CT to positive voltage, likely tapping off a voltage divider from the screen supply @~40-50V.

This is mainly to keep the heater/cathode voltage in a better range for the CF than for noise, though it might possibly help with that as well.

Is this just a nothingburger/wasted effort on my part, or am I helping prolong the life of my CF tube by doing so?
 

Jon Snell

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If it was worth doing, I would have though the manufacturers would do it automatically.
The heaters are insulated from the cathode. Depending on the valve in use the heater - cathode safe area is usually north of 100volts, so never an issue unless you have a faulty valve with a heater cathode leak and then you may get some hum.
 

Pete Farrington

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I think it’s very beneficial on several fronts, especially so in typical Marshall type designs, with only partially bypassed early stage cathodes and a DCCF cathode idling up near 200V (hence with large signal, instantaneous voltage will be significantly higher, far above the typical 180 or 200V instantaneous limit).
Also my hypothesis is the reducing the potential difference between pin2 and 3 of the output valves can only help to reduce the risk of flashover / arcing.
I suggest a higher elevation than 40V though, there seems no downside to going for around 80V.
 

XTRXTR

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And the protection diode on the DCCF grid to cathode help after a blackout or loss of power returns while your standby is still closed. The public power substations more often than not are out dated, old etc. A simple heavy draw and rebound on that station can cause a brown out or a brief blink out followed by a voltage surge in recovery. The caps for the B+ stay high while the heater drops. For a brief moment the VHK becomes larger and zap goes the arc. With a raised heater and cap on the CT you reduced this potential damage. I do the diode as well, may be over kill protection. Frankly you just never know what may happen to the power grid, protect for extremes. Crazy weather events, constant high wall voltages, idiotic gender reveal parties playing around with aluminum Mylar balloons into the high voltage lines. My wall AC is 125 sometimes 126VAC

Sarcasm:
Remember the Texas two week plus blackout during a bomb cyclone that froze the lines 100s died - oh well, that is just what happens now.

Who knows maybe Russia will just start sending missiles to random countries for no reason...Right when you got your beautiful Marshall back from a tech and you're jammin', can't hear the warning alarms.
 

StingRay85

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Anyone ever tried to put this elevation of the heaters on a switch, and A/B it for noise?
 

Pete Farrington

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Yes, a friend that plays rockabilly with amp set totally clean was having trouble with his volume pot becoming a hair trigger to achieve that (Fender normal channel!).
I thought easy peasy, lifting the cathode bypass on the input stage should make a noticeable difference.
Unfortunately doing that allowed buzzy heater hum on the input stage valve to break into the signal path (previously the cathode decoupling shunted it to 0V common).
I thought to experiment with my variable bench power supply to see whether heater elevation might help, so connected it between the heater winding CT and the amp chassis. Starting from 0, as the elevation voltage reached about 3V, the buzzy hum began to fade out and had disappeared by 4V, any higher made no difference (my bench power supply goes up to 30V). I was surprised an elevation of only 4V was sufficient, I expected it to be at least 4.5V (Vpk of 3.15Vrms).
Hence my comment that heater elevation allows the use of valves that would otherwise cause hum with partially or unbypassed preamp cathodes.
Interestingly, it shouldn’t make any difference where a elevation circuit has its 0V common reference. The mechanism is that the the elevation current saturates the h-k leakage resistance. Provided that the DC component is large enough to saturate the leakage path’s current carrying capacity, any AC component can’t modulate it.

PS I ended up just replacing the nearly 60 year old volume pot, using a regular Bourns 10% audio taper to achieve better resolution at low settings (compared to the rather worn 30% stock pot taper).
 
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