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Filament Wiring




 
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W9ZSL
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« on: September 20, 2018, 05:09:13 PM »

Dumb or not so dumb question time. Part of my heart rehab has been doing some much-delayed building. I decided to build the parallel 807 amp from the 1957 ARRL Handbook.  It's going to be primarily for AM on 80 and 40. Just finished with a power supply for the RF. While it has a couple 6.3 V filament windings, rather than run the leads from the bottom of the rack, I decided to put a separate transformer under the RF section chassis. It will be about 8" away from the 807s. The diagram assumes 1/2 of the filament winding is grounded and calls for a ground of pins 4 (cath) & 5 (fil) at the socket. The iron has a center tap. If I grounded that instead, could I wire a twisted pair to the tubes (or not twisted) or just ground one of the 6.3 leads at the xfmr and run a single wire to the tubes? Shielded? Is one method better than the other?  I know a twisted pair is common for audio amps. Speaking of which, I have a second (older) 600 V supply for the modulator. That will run a pair of 6146s because I'll get about 95 watts...more than enough to do the job. The 807s will run about 120 watts inpoot.
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DMOD
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« Reply #1 on: September 20, 2018, 10:46:22 PM »

Here is the way it is some times done with a single 6.3V secondary.

It has a Unipotential (sleeve) cathode so the cathode can be grounded anywhere the filament is grounded.

If the filaments are wired in series then you would need a 12.6 volt filament transformer.

I don't know what schematic you're looking at but the second schematic may be what you're talking about.

Phil - AC0OB

* 807 Filament Supply.pdf (54 KB - downloaded 45 times.)
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W9ZSL
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« Reply #2 on: September 21, 2018, 10:14:05 AM »

The original shows both cathodes grounded at the socket and 1/2 of the 6.3 secondary grounded at the transformer. A single wire feeds each tube and filaments are in parallel.  One terminal of the filament of each tube is grounded at the socket also. My xfmr is 6.3 VCT and I'd like to ground the center tap and then run each side of the secondary to the tubes. They would not be in series. I've seen it done both ways and am wondering if either one has any particular advantage over the other. I'd rather ground the Center Tap and run the other two wires to the filaments. There are only the two 807s in parallel with no other tubes on the chassis. There's only one schematic.


* 8075.jpg (1070.28 KB, 1940x1192 - viewed 76 times.)
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DMOD
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« Reply #3 on: September 21, 2018, 11:30:02 AM »

Quote
My xfmr is 6.3 VCT and I'd like to ground the center tap and then run each side of the secondary to the tubes.

It won't work. A 6.3VAC center tapped secondary has 3.15VAC on either side of center tap. Your filaments would be running at 3.15VAC each.

Get a 12.6VAC CT transformer and wire according to my schematic #1.


Phil - AC0OB

* 807 Filament Supply.pdf (65.58 KB - downloaded 28 times.)
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KA2DZT
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« Reply #4 on: September 21, 2018, 12:57:13 PM »

You can use your filament xfmr with the CT, just don't use the CT.  Leave the CT taped off and follow the schematic you posted.  Filament wiring is usually the easiest part of any tube project and normally doesn't require 20 questions.  Wake us back up when you get to the harder parts of this amplifier project Grin

Fred
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DMOD
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« Reply #5 on: September 21, 2018, 07:38:49 PM »

The Filament supply schematic below should suffice for a grid-driven 807 Linear Amplifier.

No need to complicate things.


Phil - AC0OB

* 807 Filament Supply.pdf (39.32 KB - downloaded 41 times.)
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WZ1M
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« Reply #6 on: September 22, 2018, 02:06:53 AM »

Is the filament transformer a "REAL" 120 vac primary? If its a 110, 115 or a 117 vac primary, your going to be running closer to 7 vac on the filaments. Not to worry on the filament side but if the transformer is not heavy enough, it will saturate and cause high current draw on the primary. If your house line voltage is running near 124 vac, which many now do, your looking at close to 7.5 vac filament.
Just my confusing two cents.
Regards,
Gary...WZ1M
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W9ZSL
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« Reply #7 on: September 22, 2018, 12:05:51 PM »

Hey Fred. Stay asleep. I guess you missed the point or didn't comprehend the question. I've seen diagrams where the filament transformer for an amp was wired so that the secondary center tap was grounded and both sides were run to the final. I've also seen (and used) ONE wire to the filament with the other grounded to the chassis. My question was IS THERE AN ADVANTAGE TO USING ONE METHOD RATHER THAN THE OTHER? Case-in-point: NOTE THE DIAGRAM BELOW.


* FIL2.jpg (2071.33 KB, 1944x1388 - viewed 66 times.)
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W9ZSL
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« Reply #8 on: September 22, 2018, 01:04:58 PM »

Gary. It's 117.
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KA2DZT
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« Reply #9 on: September 22, 2018, 08:17:18 PM »

Hey Fred. Stay asleep. I guess you missed the point or didn't comprehend the question. I've seen diagrams where the filament transformer for an amp was wired so that the secondary center tap was grounded and both sides were run to the final. I've also seen (and used) ONE wire to the filament with the other grounded to the chassis. My question was IS THERE AN ADVANTAGE TO USING ONE METHOD RATHER THAN THE OTHER? Case-in-point: NOTE THE DIAGRAM BELOW.

Mike,

The schematic you posted with the 4-250s is correct for those tubes because they are directly heated cathode tubes.  Those tubes do not have separate cathodes so you must use a filament xfmr with a CT.  Your amp is using 807s which do have a separate cathode so you would normally ground one side of the filament xfmr and run only one wire to the filament just like what is shown on your 807 amp schematic.

I'm going back to sleep,
Fred
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W9ZSL
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« Reply #10 on: September 23, 2018, 11:00:09 AM »

That's the answer I was looking for. To cathode or not to cathode...that is the answer. You learn something new once and awhile. Thanks. Nighty-night.
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KC2ZFA
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« Reply #11 on: September 23, 2018, 11:56:57 AM »

but you can always parallel-feed filaments of indirectly-heated cathode tubes
with a 6.3 VAC CT xfmr (with twisted-pair and proper bypassing of both sides of the filament)...no ?
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Nico and Chappie (Chappie is the dog...)


« Reply #12 on: September 23, 2018, 12:55:37 PM »

Absolutely, but that makes sense with audio amps to get less hum. In your case it doesn't matter
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W9ZSL
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« Reply #13 on: September 23, 2018, 01:08:59 PM »

Then again I've seen many modulator diagrams with only one wire going to the filaments and the other side to chassis ground. In the case of this 807 amp I can do either because I have an extra 6.3 volt transformer and the power supply has one side of the filament winding grounded. My point in mounting the extra transformer on the RF chassis is to avoid running a long filament wire from the bottom of the rack where the power supplies will live to the RF Deck on the top.
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PA0NVD
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Nico and Chappie (Chappie is the dog...)


« Reply #14 on: September 23, 2018, 02:03:32 PM »

When an extra filament transformer can prevent long wiring, seems a good reason to me. Long wires to the RF chassis may give more RFI problems. So long wire need good attention to grounding, bypassing and eventually shielding
In modulators the hum is less of a problem than it is in HI-FI amps. There you really don't want any background noise. Than a twisted pair to the filaments makes life more easy in this respect. If the filament transformer doesn't have a CT, than an artificial centertap using two resistors to the filament wiring to mimic a CT transformer. This center tap can be grounded but also can be bypassed to ground and connected to a positive voltage of 10 - 50 Volts. That prevents hum sometimes introduced by electron emission from the filament to the cathode if the isolation of the filament is less than perfect.
But all of this can be done in modulators as well if your like your design for max performance. Gives some extra fun....
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