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dc heater/ filament supply

1/22/2006 9:35 AM
dc heater/ filament supply
Does anyone have an effective method of converting the ac heater supply of a 5E3 to DC?
1/22/2006 8:20 PM
Ray Ivers
The " target="_blank">"> Marshall 4001 amp has about the simplest AC-DC conversion I've seen. If you decide to use this circuit, build & use it exactly as shown, to power one 12AX7 only, and don't "improve" it in any way. It works quite well in my 4001.  
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1/26/2006 12:20 PM

Well, here's a real simple one I came up with, especially nice for retrofitting when there were only AC heaters to begin with. Only powers one 12AX7 since it requires a 0.3A load:  
Interestingly, the resistor needs to be on the opposite side of the filament that the diode is on.  
1/26/2006 6:11 PM
Ray Ivers
Your circuit is definitely simpler than the Marshall one, and I imagine the 60Hz ripple it generates will be more effectively removed by most guitar preamps' R/C coupling networks than 120Hz would, should any of it stray onto the cathode(s).  
As you know, it's important to keep the ripple balanced WRT the heater CT, which is why I made such a big deal about keeping the Marshall circuit stock - I fooled around with it a bit, and always ended up with audible hum (whereas with the original circuit, there was none). I remember being especially amazed that either raising or lowering the cap value caused hum to appear out of nowhere.  
The resistor-placement thing you mentioned may be related to this. I would be tempted to cut the resistor's value in half in your circuit and use two of them, one on each side of the heater, to balance the loading and thus any ripple - but hey, if it's quiet as-is, then it's good to go. I was also wondering if a bipolar cap might make a difference in the ripple waveform, too.  
1/26/2006 6:51 PM
Greg Simon
If you use DC filaments on the first tube, can you still use the elevated CT filament method?  
Also would you say the DC on the first stage is quieter in overall hum to the elevated CT method?  
1/26/2006 7:37 PM
Ray Ivers
"If you use DC filaments on the first tube, can you still use the elevated CT filament method?"
Sure; you can connect the heater-winding CT, or an artificial CT using two resistors, to a DC voltage source (with a low AC impedance to ground) and end up with elevated DC heaters.  
"Also would you say the DC on the first stage is quieter in overall hum to the elevated CT method?"
Using a battery, with both terminals AC-grounded, as opposed to an AC heater supply with elevated CT? Absolutely - assuming the AC heater supply couples hum into the signal path (individual tubes can differ greatly in heater-hum rejection). Otherwise, uh, probably, but maybe not. ;) All DC-heater-power circuits are not created equal, nor are DC-elevation circuits - or AC heater supplies, for that matter.  
The 12AX7 heater system is basically balanced around the pin-9 heater CT. If the heater supply is also balanced exactly like the 12AX7's heater it's powering is, it should be dead-quiet as far as heater hum induced into the cathode goes, regardless of whether AC, DC, or RF is being used to power the heaters.  
Back to the real world; PT heater-winding CT's can be off-center, hum-balance pots burn out or get resistive-track damage, and the perfectly-balanced 12AX7 heater probably doesn't exist.  
If the above are accepted as givens, then IMO the best way to ensure minimum heater-cathode hum pickup is by using a balanced-DC heater supply, preferably both regulated and elevated. This may not be necessary for every amp design, but if done properly, any hum you hear will not be from the DC-powered heaters.  
Sorry for the long-winded reply, but perhaps it will help to explain why some try using DC-powered heaters or DC heater-CT elevation, and experience no improvement at all.  
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