Filament transformer secondary center tap at cathode level - diyAudio
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Old 14th December 2012, 12:33 AM   #1
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Default Filament transformer secondary center tap at cathode level

I know that Weber, instead of DC filaments, either connects the filament secondary center-tap to cathode, or if a center tap is not available makes a resistor network between the filament lines and the cathode, so that the filament alternates around cathode level but never gets too far from the cathode level. From what I hear it's pretty effective at reducing hum to inaudible levels.

I think Weber does this on their Champ kit. I have a Champ kit from STI that someone else assembled. I don't have the book or even a schematic, but it's not like it's complicated. So tonight I'll take a look tonight and see whether the filament winding has a center tap.

Someone mentioned something about checking whether there is a bypass cap on the cathode resistor before I attempt this; but I'm not really sure how that is relevant.

I would think that hum would only be an issue in the first stage, but I could be wrong and haven't investigated. I assume I'd want to float the filament supply near the 12AX7 cathode, not the 6V6?
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Old 14th December 2012, 01:22 AM   #2
Enzo is offline Enzo  United States
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No, use the 6V6 cathode. The point of this is that the heaters be more positive than the cathodes of the sensitive early stage tubes. That prevents the heaters from boiling off electrons that get attracted to the cathode. That can happen when the cathode is more positive than the heater. By running the heaters at +12 or +25 or whatever, they are then a lot more positive than the cathodes of the 12AX7, which is typically only a volt or two above ground. It is not a matter of being within a couple volts, we want the heaters to be substantially more positive.

If you referenced the heaters to the 12AX7 cathode, you wouldn;t have that more-positive-than relationship.

I can't imagine why a bypass cap on the 6V6 cathode would matter.

Yes, it would only be the first stage that this matters.
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Old 14th December 2012, 01:49 AM   #3
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Oh, 'more positive' not just nearer in potential! I must have read something wrong somewhere, either in the material or my reading of it...

Thanks again for your time and direction!
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Old 14th December 2012, 03:07 AM   #4
Enzo is offline Enzo  United States
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Some old circuits just grounded one side of the heaters. Not as good as a center tap ground, but better than nothing, and didnlt require the more expensive tapped transformer or other parts. So if you make this modification, first make sure neither side of the heaters is grounded.
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Old 17th December 2012, 05:57 PM   #5
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So if I convert preamp filaments to DC it eliminates that source of 60 cycle hum. If I raise the filament + above preamp cathode it also confines any filament electron cloud so there is no flow to other elements except from the proper cathode.

I found this in Tube Amp FAQ 1/20/99 (Frames supported) under section "Amplifier Modifications" "A. OK/Recommended amp modifications"
"Gerald Weber advocates using a 270K/27K resistor divider from B+ to raise the filament windings in a DC sense above ground. This keeps electrons from the filament from hitting the plate, another source of hum."

Well, if it's DC there's probably not 60 hz hum involved, but isn't there still some random noise or hiss involved which might be further suppressed? In other words, I see people utilizing one mothod or the other, but not both. I imagine gains might be small but I don't see why both methods couldn't be applied; DC heaters raised by a resistor network off B+.

The description sounds like it doesn't raise the filament very high? And it sounds like it doesn't require any center-tap on the filament secondary?

My big fender Super Twin has two filament secondaries, one at 6.5 V for the output tubes and another at 6.1 volts for the preamp; (looking at the almost identical studio bass schematic). But doesn't a 12AX7, 7025, 12AT7 need 12 volts for the heater? Or are such dual triodes 6 volts on each half and able to be wired either way (parallel or series)?

BTW I'm pleased with my little champ clone as it is; it doesn't use either technique, but has so little gain and so little output that any noise is nearly inaudible. I'm not going to do anythning to it for now, except build a cabinet.
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Old 17th December 2012, 10:59 PM   #6
Enzo is offline Enzo  United States
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You say if it is DC heater supply there will be no 60Hz involved, but that is only the case if that DC supply is free of ripple. Gerald Weber writes some interesting books, but I take what he suggests with a grain of salt, let's just say.

If you run the heaters on DC, they yes, there is no AC radiating from the heaters. But you can run the heaters on AC and as long as they are offset from ground by a more positive voltage than the AC heater waveform, you will also prevent that hum. You want to add a couple resistors off the B+ instead? Go ahead. Seems to me the cathode of the power tube is already there with a handy voltage.


Hum and noise are not generic, hum can come from MANY places, and each place has its own cure, just as a rattle noise in your car could be inside the door, under the dash, or in the trunk. If you fill the door with insulation it mat stop the rattle there. But no matter how much you stuff in the door, it will do nothing about a rattle in the trunk.

Heater hum doesn;t alweays happen, just in some cases. Heater hum is only one source of hum, you could have the stage sharing ground copper with power suppyl filter or something, or just a shielding problem. Hiss/white noise comes from entirely different things than hum. SO elevating the preamp heaters solves ONE type of hum when the voltage is elevated to the appropriate level. If I decide that level is +25v, then that is what I need. B+ voltage divider resistors or cathode resistor, either one results in my 25v, then the job is done. I don;t know how you'd do both. If the resistors lift the heaters to +25, then what would ALSO connecting them to the cathode resistor accomplish? Hiss is a separate issue with its own course of repair.

A 270k/27k divider is a roughly 10 to 1 divider, so if you have 300v B+, your divider would make 30v.

It doesn't matter where this elevation voltage comes from, it still has to be connected to the heater wiring. For that, you need either a center tap or a "virtual center tap", in other words the pair of resistors. No matter what elevation supply source you chose, you will still need one of these ways to connect it.

OH, I take that back, if the AC heater wiring totally lacks ground connections, you could simply connect the + voltage to one side of the wiring. That would work too.


Look at the data sheet or just the base wiring diagram for a 12AX7 type tube. The two 6v triode heaters are in series with the center poin on pin 9, so you can wire the tube for either 6v or 12v operation. Usually we run them on 6v. SOme cocmpanies like Peavey, like to run the preamp heaters on DC, and they run the heaters in series. There 12v wiring makes more sense. For example in their 5150 amp, four 12AX7s wired as 12v are in series across the +24v and -24v supplies.
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Old 18th December 2012, 12:47 AM   #7
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My experience has been that if I employ good "lead dress" or "layout" of my heater circuit with the center tap merely grounded or by making a virtual center tap (connecting each leg to ground trough 100 ohm resistors) I have no issues with hum from the heater supply. If I'm worried that a hum might be caused by heater wiring I simply turn the amp on and while I am listening to the noise, disconnect both heater supply leads from the tubes. Obviously this requires some prep work in most cases to do this safely. If the noise goes away immediately it is caused by a heater problem (the tube swill stay warm enough for a while for the amp to continue with out it). If the noise stays it is something else. I would try that first before I reengineered the heater circuit. I really do think that in most cases where there is a genuine heater hum it is due to poor layout.
I would be more inclined to redo the layout than add more components or circuits. (-:

Last edited by jeeptechfred; 18th December 2012 at 12:50 AM. Reason: because I simply must!
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Old 18th December 2012, 04:51 AM   #8
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I'm trying to learn...

The little Champ kit was assembled with horrilbe layout, the heater lines are not even twisted.
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Old 18th December 2012, 04:52 AM   #9
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Thanks guys.
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