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Old 19th January 2014, 02:40 PM   #1
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Default TUBE HEATER WIRING OPTIONS

Should the 6.3VAC heater winding be grounded, or wired off-ground directly through the tube filaments? Fender seems to want to ground one side of the winding immediately, and ground one side of each tube (pin 9 for 12AX7s, pin 2 or 7 for power tubes). Other designs connect one winding to ground through a small value resistor (to give it a small DC component, thus raising it above ground?). I assume this is to minimize hum on the ground potential. What are you guys's thoughts on this? I am wiring up a chassis right now, and am being very conscious of my grounding methods (some prudent, some risky just to see what happens). Thanks
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Old 20th January 2014, 02:00 AM   #2
Enzo is offline Enzo  United States
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Fender grounded one side of the heaters in very old models and usually smaller ones at that. They have not wired one that way in 50 years though.

The common method is to wire all your sockets in parallel with a twisted pair of wires. You do need a ground reference though, for hum abatement. If the 6v winding is center tapped, we ground that center tap. If the winding has no center tap, we create an artificial center tap. That is nothing more than a 100 ohm resistor to ground from each side of the 6v winding. Usually that was done at the pilot lamp socket for convenience.
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Old 20th January 2014, 02:47 PM   #3
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It is also common practice to apply a positive DC offset to the heater winding to minimize hum coupling into the cathode. This is also used to protect cascode or cathode followers whose cathodes are at higher DC potential (there is a maximum heater to cathode voltage spec).

DC offset is simply applied to the centre tap or an artificial CT using 100R resistors between the legs of the 6.3V winding. A simple, low current voltage divider from B+ can be used to create the offset since no current flows.
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Old 20th January 2014, 04:01 PM   #4
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Thanks for the suggestions! I want to keep my designs "historically accurate" for their sound, but incorporate improvements when needed. Will be rewiring my heaters off-ground, and ground the winding CT (my PT has one). Just got it working yesterday, and DID shortcut some grounding - on purpose to see what I could get away with - and yes it hums! Oh, and the amp didn't make a sound when I thought I had it all wired....(expected it). Part of learning, but after finding a missed ground, an un-wired preamp pin, and lastly realizing I had forgotten the 1meg resistor on the input jack, it worked! So...will also insulate the input jack from the chassis, separate cathode resistor grounds from heater grounds (hey, they were right next to each other, just run them together!....NOT). Have a tremelo eyelet board ready to add later when I get the amp working. And put extra pot hole in the front for tone later too.
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Old 27th January 2014, 03:58 PM   #5
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Hi!!
A little off topic but some of you may find interesting to add this in your next build.
I usually place a high current diode in series with the heater chain and use one of the circuits of a DPDT standby switch to bypass it. When the standby is on, you get the heater voltage half wave rectified and your tubes will last much longer!
It's a good way to make a standby switch useful, by the way.

That said, this was not my own idea, I saw this somewhere else and decided to include it in my builds, for I think it's great!

Regards,
Fran
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Old 27th January 2014, 04:45 PM   #6
DF96 is online now DF96  England
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Quote:
Originally Posted by FranciscoPerez
It's a good way to make a standby switch useful, by the way.
It may be a good way to stop a standby switch from being harmful!
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Old 28th January 2014, 03:19 PM   #7
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Hi DF69
Could you elaborate on that?

I only see a standby switch harmful for a rectifier valve, if the amp is not designed properly considering the inrush current to the first power supply cap.
Otherwise, I don't see any disadvantages ( other than the extra money that the switch costs).

Thanks!!
Fran
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Old 28th January 2014, 03:36 PM   #8
DF96 is online now DF96  England
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Two things can damage valves:
- running with full supply rail voltage, but reduced heater power - cathode bombardment or stripping
- running with full heater power, but zero or significantly reduced supply rail voltage - cathode interface also known as sleeping sickness
Standby switches often do one of these.

For long life the best options are:
- switch off - no heater, no supply rail
- switch on - normal heater, normal supply rail

If desperate for a 'quick on' feature for people who lack patience then try
- reduced heater, no or very low supply rail
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Old 28th January 2014, 03:52 PM   #9
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Thanks for that!

I agree, sleeping sickness can definitely be an issue in case you leave the amp in standby for a long time.
However, I don't think cathode stripping would exist in a guitar amp, considering the tubes we usually work with here.
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Old 28th January 2014, 04:00 PM   #10
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Thanks for all the "food for thought" on standby sws and tube life. Speaking of diodes and heaters, why couldn't you full-wave rectify the 6.3VAC to make it DC (less hum), but then switch out one diode for "standby" mode? And add a cap to smooth it out some.
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