Question about filaments grounding

[mresistor]

I was reading some info on Tweed champs (or similar class A) and a guy mentioned to connect the filament ground up to the cathode of the 6V6, above the bypass/filter. Huh? when you do this do, do the filaments then run about 30 or so volts above ground? Anyone ever heard of doing this? What are the advantages and disadvantages?

[John]

Elevated heaters tend to be quieter, or hum less. An easy way, as mentioned in another thread, is if you're making the artificial CT with a pair of 100R resistors is to connect each one from the heater pins on the power tube to the cathode pin. This is assuming of course it's cathode biased.

I made a voltage divider for my last one since I used 6SL7s instead of 12ax7. Elevated about 70 volts. It's dead quiet, I'm tickled. HTH.

[Willabe]

It's not really running at 30dcv. It's still running the heaters at 6.3acv floating on top of the "stand-off" dcv of anywhere from 30dcv to 80dcv.

All the heater current is drawn from the 6.3acv, not the dcv.

I've done 3 builds with it and so far I like it, easy to do, no problems and very quite.

Look in here for more info on dc heaters;

http://www.freewebs.com/valvewizard/heater.html


                           Brad     

[John]

Oh, that link reminded me Brad - I used the humdinger too since those tubes have "unbalanced" heaters. (which I don't know what that means, but anyway) It definitely was worth doing, a big reason why the amp is so quiet in my estimation. Very possibly with 12a_7 it wouldn't be needed. But for less than 10 bucks in parts and a few extra minutes I think I'll be putting it in all of them from here out.

[mresistor]

I meant - the filament 6.3vac is now riding up 30 or so volts instead of at ground reference. I know they are still running at 6.3vac. I see now that this is a method of elevating the heaters on a cathode biased amp. Thanks guys.

[Willabe]

Hi John,

Yeah, I'd like to try that too on my next build. Keep forgetting to add it when I draw out a new build.    

I have adjusted a few later model Fenders that had a heater balance pot, humdinger. It was very easy and was very noticeable on reducing the hum level to not being there at all.

12A_7 heaters have the option of wiring their heaters in humbucking which cancels hum. Pins 4/5 are 1 end of 2 separate filaments and pin 9 is the other ends of both filaments. 6SL7's have only 1 filament.  


                    Brad      

[Willabe]

I see now that this is a method of elevating the heaters on a cathode biased amp.

Yep.

                     Brad       

[mresistor]

Thanks for the link. - So Merlins site is back up. I tried this past weekend to view a section of the VW and no-go, something about bandwidth or views exceeded.... and of course now I can't remember what I was wanting to read.... 

[John]

I meant - the filament 6.3vac is now riding up 30 or so volts instead of at ground reference. I know they are still running at 6.3vac. I see now that this is a method of elevating the heaters on a cathode biased amp. Thanks guys.

Oh, and just a thought, and remember this is only my opinion! But making the voltage divider was very simple since I was making a turret board anyway. I stacked a 470k over a 100k (that went to ground) and bypassed the 100k with a 22uf/450v cap to filter any HT ripple there might have been. All this according to I believe Merlin's article. I used 3W resistors which were overkill, but they're cheap.

[HotBluePlates]

... Anyone ever heard of doing this? What are the advantages and disadvantages?

It's in the RCA tube manual, but they simply suggested referencing the heater string to a positive voltage rather than saying "attach to your output tube cathode resistor." That specific method is just a convenient source with no extra parts.

It is also described in Getting the Most out of Vacuum Tubes (1960), hosted at Pete Millett's site.

Benefits:
1. If your tubes have heater-to-cathode leakage (which happens via different mechanisms depending on the individual tube), the positive-biased heater effectively reverse-biases the diode represented by the heater and cathode, reducing hum from this leakage. The benefits mostly input tubes operating at low signal levels, where any source of hum can be significant.

2. If you have any cathode at an elevated voltage (such as a cathode follower, split-load or long-tail pair inverter), the positive reference voltage on the heater reduces the amount of voltage difference between the heater and the elevated cathode. That can help reduce the chance that these tubes will develop heater-to-cathode leakage hum.