#39684

Hi, I've been trying to gather parts for a 3CX1200D7 build. I had trouble locating a sufficient filament transformer. Tube requires 6.3V @ 25A. Ended up finding one for 6.3V @ 28A - Would like to have more overhead but thought it would be the best I'd find, and only $50 brand new! Perfect!

BUT In my haste to complete an order of needed parts, I wasn't paying close enough attention. It turns out that it doesn't have a CT. (it's dual secondary - parallel is 6.3V@28A and series is 12.6V@14A) - D'oh!

Now what... when I used to play with guitar/audio amps, a semi-common thing to do would be to take a pair of low value resistors, put one end of each to each end of the filament winding, and ground the joint in the middle for a fake CT. Unsure how this will affect a cathode bias.

I've also heard of using the secondary winding of a totally separate transformer that does have a CT, and using it in parallel across my filament winding, then using that transformers centre tap. Further, I've heard of using a pair of diodes, each anode to each end of my filament winding and tie the cathodes together in the middle... or maybe that was t'other way around... either way I dunno about this either.

Anyways, obviously the best way forward would be to actually use a proper transformer, but since I can't find one and I already made this oopsie... I'm wondering what the next-best-way forward is? :)

Thanks :)

#39685

开云体育

Still another easy way out is to unwind those heavy windings and put in your own center tap. There can't be more than 20, 25 turns of wire per 6V winding?? Transformer secondaries are always wound on the outside, meaning modifying them is very easy.

Steve, K0XP

On 12/1/2023 10:56 AM, va3dxv wrote:

Hi, I've been trying to gather parts for a 3CX1200D7 build. I had trouble locating a sufficient filament transformer. Tube requires 6.3V @ 25A. Ended up finding one for 6.3V @ 28A - Would like to have more overhead but thought it would be the best I'd find, and only $50 brand new! Perfect!

BUT In my haste to complete an order of needed parts, I wasn't paying close enough attention. It turns out that it doesn't have a CT. (it's dual secondary - parallel is 6.3V@28A and series is 12.6V@14A) - D'oh!

Now what... when I used to play with guitar/audio amps, a semi-common thing to do would be to take a pair of low value resistors, put one end of each to each end of the filament winding, and ground the joint in the middle for a fake CT. Unsure how this will affect a cathode bias.

I've also heard of using the secondary winding of a totally separate transformer that does have a CT, and using it in parallel across my filament winding, then using that transformers centre tap. Further, I've heard of using a pair of diodes, each anode to each end of my filament winding and tie the cathodes together in the middle... or maybe that was t'other way around... either way I dunno about this either.

Anyways, obviously the best way forward would be to actually use a proper transformer, but since I can't find one and I already made this oopsie... I'm wondering what the next-best-way forward is? :)

Thanks :)

--
See my QRZ.com page at

#39686

开云体育

Quote: “I've also heard of using the secondary winding of a totally separate transformer that does have a CT, and using it in parallel across my filament winding, then using that transformers centre tap.”

?

Yes I have done this and it works very well.? Using the resistors for a fake CT will add a variable bias as the cathode current varies which will hurt the IMD performance.? Also as suggested, you might be able to make your own CT.? Try probing the secondary winding with a DVM fitted with a pin to see if you can find the ? voltage point.? I have also done this and found a CT at a very easy point to get to between the two layers of the secondary at the physical end between the two layers.

?

-Chuck K1KW

?

Show quoted text

From: [email protected] [mailto:[email protected]] On Behalf Of va3dxv
Sent: Friday, December 01, 2023 1:57 PM
To: [email protected]
Subject: [ham-amplifiers] Artificial centre tap for GG 3CX1200

?

Hi, I've been trying to gather parts for a 3CX1200D7 build. I had trouble locating a sufficient filament transformer. Tube requires 6.3V @ 25A. Ended up finding one for 6.3V @ 28A - Would like to have more overhead but thought it would be the best I'd find, and only $50 brand new! Perfect!

BUT In my haste to complete an order of needed parts, I wasn't paying close enough attention. It turns out that it doesn't have a CT. (it's dual secondary - parallel is 6.3V@28A and series is 12.6V@14A) - D'oh!

Now what... when I used to play with guitar/audio amps, a semi-common thing to do would be to take a pair of low value resistors, put one end of each to each end of the filament winding, and ground the joint in the middle for a fake CT. Unsure how this will affect a cathode bias.

I've also heard of using the secondary winding of a totally separate transformer that does have a CT, and using it in parallel across my filament winding, then using that transformers centre tap. Further, I've heard of using a pair of diodes, each anode to each end of my filament winding and tie the cathodes together in the middle... or maybe that was t'other way around... either way I dunno about this either.

Anyways, obviously the best way forward would be to actually use a proper transformer, but since I can't find one and I already made this oopsie... I'm wondering what the next-best-way forward is? :)

Thanks :)

Virus-free.

#39690

On Fri, Dec 1, 2023 at 11:58 AM, Chuck Neal wrote:

Yes I have done this and it works very well.

Ahh that's good to hear, Thanks! Any idea on whats needed of the required transformer? I'm kind of thinking that as long as the winding can handle the DC current, and doesn't have a huge DC resistance, then it's probably ok? Like a 12V 1A maybe?

I'll poke around and see how easy it would be to add a CT, didn't really consider that at first, but yeah. Thanks Guys.

#39692

开云体育

A 6.3 VAC @3A ?CT transformer will work just fine.? That’s what I used

?

-Chuck K1KW

?

Show quoted text

From: [email protected] [mailto:[email protected]] On Behalf Of va3dxv
Sent: Friday, December 01, 2023 5:07 PM
To: [email protected]
Subject: Re: [ham-amplifiers] Artificial centre tap for GG 3CX1200

?

On Fri, Dec 1, 2023 at 11:58 AM, Chuck Neal wrote:

Yes I have done this and it works very well.

Ahh that's good to hear, Thanks! Any idea on whats needed of the required transformer? I'm kind of thinking that as long as the winding can handle the DC current, and doesn't have a huge DC resistance, then it's probably ok? Like a 12V 1A maybe?

I'll poke around and see how easy it would be to add a CT, didn't really consider that at first, but yeah. Thanks Guys.

Virus-free.

#39693

开云体育

Dear ‘1200 Builder,

?

When dealing with Heavy Iron it is not wise to

Jury-Rig.

?

Save for the ability to purchase the right iron.

?

In fact, HARBACH may just have the perfect item in

stock, or would be able to wind up something in a heartbeat.

?

Hal Mandel

W4HBM

#39694

开云体育

And if we simply ground one of the terminals of the 6.3V transformer, what happens?

--

F1AMM

Fran?ois

#39696

On Fri, Dec 1, 2023 at 04:27 PM, HaL Mandel wrote:

In fact, HARBACH may just have the perfect item in

stock, or would be able to wind up something in a heartbeat.

Well friend, I'm not sure if a dollar is worth the same to you as it is to me :) Keep in mind also; Something that you can get for two hundred bucks is going to be triple that for me, once I factor in shipping, import tax, exchange rate, etc...? I didn't pay more than a hundred bucks for any other part of this build including the vacuum variables! The xfrmr I got is new and it was fifty bucks, which is about what I'd expect to pay for the "correct" one at a hamfest, so until that happens I'll have to make it work :)

Though I didn't know Harbach was still in the game. Hammond has a suitable 30A listed in their Peter Dahl list but it's custom order. If I really like how the amp turns out I might contact them.

#39697

开云体育

For a few years now, I've been using Antek toroids for my "custom" transformers: ? Where necessary, I've bought a suitable low-voltage unit and rewound it for my requirements. Since the secondaries are always wound on the outside even on toroids, turning one of their standard transformers into one that fulfills your exact needs is simply a matter of unwinding all the external tape and kapton insulation, then carefully unwinding the low voltage winding in question, then rewinding as appropriate.

Before starting the process, I check the volts-per-turn ratio by winding ten or twenty turns of ordinary hookup wire on the toroid then applying power and measuring the voltage. You can easily do this while applying an appropriate load to the existing or your "new" winding. Thus, I used one of these: as the basis for a 7.5VAC/21.5A 3-1000Z filament transformer, installing my own center tap while at it. The price of their toroids is extremely reasonable, especially considering all the hassle involved in finding, buying and perhaps modifying a standard iron lamination E-I transformer. Antek even has a 1500 VA 950-volt high voltage transformer that can become an excellent basis for a voltage doubler, high voltage power supply: ? (Unfortunately, that is their largest and highest-voltage transformer.)

An interesting bit of trivia is that the high voltage transformers used in the Alpha 8410 (which uses a pair of 4CX1000As or 4CX1500Bs) appear to be a pair of 1500VA toroids "wound" together to form a single unit; I've been wondering whether they might actually be a pair of those 950-volt units mentioned above... Several years ago, I bought a pair of the 950 volt units and used one of them to built a compact high voltage power supply for my Heath HA14 amplifiers; the core hardly even gets warmer than lukewarm even at 700W output in CW contests.

After rewinding several of the low-voltage units, you develop a technique of easily unwinding the outside kapton and other insulation layers then unwinding the copper wire while keeping it straight and unmarked, then rewinding the wire and insulation as necessary. It's gotten to the point where I was able to modify that 12V toroid mentioned above into my desired 7.5VCT/21.5A? transformer within a little more than four hours, including rewinding all the kapton and insulation tape back onto the core.

And I just can't beat the prices. Shipping is also reasonable. Everything I've bought so far was in stock but you can easily check for stock status on their web site.

I am not an Antek owner or stockholder, nor do I have any other affiliation with the company or its owners/stockholders, other than that of a very satisfied customer.

Steve, K0XP

On 12/2/2023 11:40 AM, va3dxv wrote:

On Fri, Dec 1, 2023 at 04:27 PM, HaL Mandel wrote:

In fact, HARBACH may just have the perfect item in

stock, or would be able to wind up something in a heartbeat.

Well friend, I'm not sure if a dollar is worth the same to you as it is to me :) Keep in mind also; Something that you can get for two hundred bucks is going to be triple that for me, once I factor in shipping, import tax, exchange rate, etc...? I didn't pay more than a hundred bucks for any other part of this build including the vacuum variables! The xfrmr I got is new and it was fifty bucks, which is about what I'd expect to pay for the "correct" one at a hamfest, so until that happens I'll have to make it work :)

Though I didn't know Harbach was still in the game. Hammond has a suitable 30A listed in their Peter Dahl list but it's custom order. If I really like how the amp turns out I might contact them.

--
See my QRZ.com page at

#39701

开云体育

Ceramic Disk Capacitors are used as RF bypass capacitors in RF circuitry--that is to short RF to ground.? And Capacitors can be also used to block DC and low frequency current form passing through them. How to use the in your situation is described below.

?

Use a set of High Voltage Ceramic disk capacitors, wired from each of the two filament leads to ground—this will allow the high frequency RF to pass through and block the low frequency AC filament voltage..?

-Disk ceramic Capacitors are also used in High Power Tube Amplifiers as a DC blocking capacitor to Pass through only the RF from the plate to the PA plate circuit and then the antennas system.?

-Since this works in the plate circuit and it will work in the ground/common circuit too.?

-I am not certain about the voltage requirement for this filament grounding circuit. ?Look at the Filament transformer’s insulation value.? You aren’t going to need much higher voltage for the capacitors than that.

?

Turns out, high voltage disk capacitors are becoming hard to find:? I found this at RF Parts:

? 0.01 micro farad @ 2KV

-Assuming you will use a 0.05 microfarad capacitance value you will need 10 of these to make your filament grounding system.? Double than quantity if you need a high voltage of 4KV.

-Alternate choice but more expensive are doorknob capacitors.? While these might be awkward to mount a capacitance above 2000pf may work for you, these will certainly work and look impressive.? You are going to need one of these door-knob capacitors for your amplifier plate circuit too.---just buy 3 and you’ll be set :>).

?

A capacitance of? 0.05 microfarad will present only 3.8 Ohms (2 x 1.8) resistance to the RF signal at 1.8 MHz even lower resistance as the frequency goes up.? But will present 106K Ohms (2 x 53K) ohms to the 6.3 volts 60 Hz AC voltage.? Use this calculator to determine alternate capacitance values if you desire:

?

73,

Dave, w6de

?

Show quoted text

From: [email protected] <[email protected]> On Behalf Of va3dxv via groups.io
Sent: 1 December, 2023 18:57
To: [email protected]
Subject: [ham-amplifiers] Artificial centre tap for GG 3CX1200

?

Hi, I've been trying to gather parts for a 3CX1200D7 build. I had trouble locating a sufficient filament transformer. Tube requires 6.3V @ 25A. Ended up finding one for 6.3V @ 28A - Would like to have more overhead but thought it would be the best I'd find, and only $50 brand new! Perfect!

BUT In my haste to complete an order of needed parts, I wasn't paying close enough attention. It turns out that it doesn't have a CT. (it's dual secondary - parallel is 6.3V@28A and series is 12.6V@14A) - D'oh!

Now what... when I used to play with guitar/audio amps, a semi-common thing to do would be to take a pair of low value resistors, put one end of each to each end of the filament winding, and ground the joint in the middle for a fake CT. Unsure how this will affect a cathode bias.

I've also heard of using the secondary winding of a totally separate transformer that does have a CT, and using it in parallel across my filament winding, then using that transformers centre tap. Further, I've heard of using a pair of diodes, each anode to each end of my filament winding and tie the cathodes together in the middle... or maybe that was t'other way around... either way I dunno about this either.

Anyways, obviously the best way forward would be to actually use a proper transformer, but since I can't find one and I already made this oopsie... I'm wondering what the next-best-way forward is? :)

Thanks :)

#39702

开云体育

I forgot to put in these RF Parts door-knob links

?

Dave, w6de

?

Show quoted text

From: [email protected] <[email protected]> On Behalf Of Dave w6de via groups.io
Sent: 3 December, 2023 00:58
To: [email protected]
Subject: Re: [ham-amplifiers] Artificial centre tap for GG 3CX1200

?

Ceramic Disk Capacitors are used as RF bypass capacitors in RF circuitry--that is to short RF to ground.? And Capacitors can be also used to block DC and low frequency current form passing through them. How to use the in your situation is described below.

?

Use a set of High Voltage Ceramic disk capacitors, wired from each of the two filament leads to ground—this will allow the high frequency RF to pass through and block the low frequency AC filament voltage..?

-Disk ceramic Capacitors are also used in High Power Tube Amplifiers as a DC blocking capacitor to Pass through only the RF from the plate to the PA plate circuit and then the antennas system.?

-Since this works in the plate circuit and it will work in the ground/common circuit too.?

-I am not certain about the voltage requirement for this filament grounding circuit.? Look at the Filament transformer’s insulation value.? You aren’t going to need much higher voltage for the capacitors than that.

?

Turns out, high voltage disk capacitors are becoming hard to find:? I found this at RF Parts:

? 0.01 micro farad @ 2KV

-Assuming you will use a 0.05 microfarad capacitance value you will need 10 of these to make your filament grounding system.? Double than quantity if you need a high voltage of 4KV.

-Alternate choice but more expensive are doorknob capacitors.? While these might be awkward to mount a capacitance above 2000pf may work for you, these will certainly work and look impressive.? You are going to need one of these door-knob capacitors for your amplifier plate circuit too.---just buy 3 and you’ll be set :>).

?

A capacitance of? 0.05 microfarad will present only 3.8 Ohms (2 x 1.8) resistance to the RF signal at 1.8 MHz even lower resistance as the frequency goes up.? But will present 106K Ohms (2 x 53K) ohms to the 6.3 volts 60 Hz AC voltage.? Use this calculator to determine alternate capacitance values if you desire:

?

73,

Dave, w6de

?

From: [email protected] <[email protected]> On Behalf Of va3dxv via groups.io
Sent: 1 December, 2023 18:57
To: [email protected]
Subject: [ham-amplifiers] Artificial centre tap for GG 3CX1200

?

Hi, I've been trying to gather parts for a 3CX1200D7 build. I had trouble locating a sufficient filament transformer. Tube requires 6.3V @ 25A. Ended up finding one for 6.3V @ 28A - Would like to have more overhead but thought it would be the best I'd find, and only $50 brand new! Perfect!

BUT In my haste to complete an order of needed parts, I wasn't paying close enough attention. It turns out that it doesn't have a CT. (it's dual secondary - parallel is 6.3V@28A and series is 12.6V@14A) - D'oh!

Now what... when I used to play with guitar/audio amps, a semi-common thing to do would be to take a pair of low value resistors, put one end of each to each end of the filament winding, and ground the joint in the middle for a fake CT. Unsure how this will affect a cathode bias.

I've also heard of using the secondary winding of a totally separate transformer that does have a CT, and using it in parallel across my filament winding, then using that transformers centre tap. Further, I've heard of using a pair of diodes, each anode to each end of my filament winding and tie the cathodes together in the middle... or maybe that was t'other way around... either way I dunno about this either.

Anyways, obviously the best way forward would be to actually use a proper transformer, but since I can't find one and I already made this oopsie... I'm wondering what the next-best-way forward is? :)

Thanks :)

#39703

Va3dxv, do you need a filament transformer for a 3cx1200a7? ?I’ve got about 3 of them here right out of AL1200 amplifiers. ?If this will hflp you contact me direct email. ?Lou

Sent from the all new AOL app for iOS

Show quoted text

On Saturday, December 2, 2023, 2:40 PM, va3dxv <va3dxv@...> wrote:

On Fri, Dec 1, 2023 at 04:27 PM, HaL Mandel wrote:

In fact, HARBACH may just have the perfect item in

stock, or would be able to wind up something in a heartbeat.

Well friend, I'm not sure if a dollar is worth the same to you as it is to me :) Keep in mind also; Something that you can get for two hundred bucks is going to be triple that for me, once I factor in shipping, import tax, exchange rate, etc...? I didn't pay more than a hundred bucks for any other part of this build including the vacuum variables! The xfrmr I got is new and it was fifty bucks, which is about what I'd expect to pay for the "correct" one at a hamfest, so until that happens I'll have to make it work :)

Though I didn't know Harbach was still in the game. Hammond has a suitable 30A listed in their Peter Dahl list but it's custom order. If I really like how the amp turns out I might contact them.

#39704

YOU NEVER WANT A FILAMENT TRANSFORMER RATED MORE THAN?NECESSARY!! ?here’s why: Filament transformers are designed to deliver the rated voltage ONLY when the rated current of the tube is drawn of transformer, if you have a filament transformer rated at say 10v?@ 10 amps. ?The 10volts?output will only be when 10 amps is drawn! ?If less current is drawn the result voltage will be higher than 10 volts. ?If more than 10 amps is drawn?it will be lower than 10 volts!!!! ?Never over rate?the transformer beyond what the tube requires. ?Take any filament transformer and measure the voltage with no load on it. ?It will be way more than when it’s loaded down properly . ?Similarly the same applies, for instance a single 3cx800 needs 13.5v at 1.5 A + or - .6v

It’s hard to find a 13.5 1.5a filament transformer. ?But 12.6 transformers are a dime. Dozen in all current ratings. ?Using the reverse principle it’s possible to use a 12.6 v 4 amp transformer for a single 3cx800 tube. I do it all the time. ?The result is almost exactly 13.5v when a 4 amp transformer is pulling 1.5 amps. ?
Henry amplifiers used a 3cx1200D7 in some of their amps. ?They used ECA transformers. ?If you look up the part number on that ECA transformer you can call out for that specific transformer. ?I’ll bet ther are a few floating around.?

Sent from the all new AOL app for iOS

Show quoted text

On Saturday, December 2, 2023, 7:58 PM, Dave w6de <w6de@...> wrote:

Ceramic Disk Capacitors are used as RF bypass capacitors in RF circuitry--that is to short RF to ground.? And Capacitors can be also used to block DC and low frequency current form passing through them. How to use the in your situation is described below.

?

Use a set of High Voltage Ceramic disk capacitors, wired from each of the two filament leads to ground—this will allow the high frequency RF to pass through and block the low frequency AC filament voltage..?

-Disk ceramic Capacitors are also used in High Power Tube Amplifiers as a DC blocking capacitor to Pass through only the RF from the plate to the PA plate circuit and then the antennas system.?

-Since this works in the plate circuit and it will work in the ground/common circuit too.?

-I am not certain about the voltage requirement for this filament grounding circuit. ?Look at the Filament transformer’s insulation value.? You aren’t going to need much higher voltage for the capacitors than that.

?

Turns out, high voltage disk capacitors are becoming hard to find:? I found this at RF Parts:

? 0.01 micro farad @ 2KV

-Assuming you will use a 0.05 microfarad capacitance value you will need 10 of these to make your filament grounding system.? Double than quantity if you need a high voltage of 4KV.

-Alternate choice but more expensive are doorknob capacitors.? While these might be awkward to mount a capacitance above 2000pf may work for you, these will certainly work and look impressive.? You are going to need one of these door-knob capacitors for your amplifier plate circuit too.---just buy 3 and you’ll be set :>).

?

A capacitance of? 0.05 microfarad will present only 3.8 Ohms (2 x 1.8) resistance to the RF signal at 1.8 MHz even lower resistance as the frequency goes up.? But will present 106K Ohms (2 x 53K) ohms to the 6.3 volts 60 Hz AC voltage.? Use this calculator to determine alternate capacitance values if you desire:

?

73,

Dave, w6de

?

?

From: [email protected] <[email protected]> On Behalf Of va3dxv via groups.io
Sent: 1 December, 2023 18:57
To: [email protected]
Subject: [ham-amplifiers] Artificial centre tap for GG 3CX1200

?

Hi, I've been trying to gather parts for a 3CX1200D7 build. I had trouble locating a sufficient filament transformer. Tube requires 6.3V @ 25A. Ended up finding one for 6.3V @ 28A - Would like to have more overhead but thought it would be the best I'd find, and only $50 brand new! Perfect!

BUT In my haste to complete an order of needed parts, I wasn't paying close enough attention. It turns out that it doesn't have a CT. (it's dual secondary - parallel is 6.3V@28A and series is 12.6V@14A) - D'oh!

Now what... when I used to play with guitar/audio amps, a semi-common thing to do would be to take a pair of low value resistors, put one end of each to each end of the filament winding, and ground the joint in the middle for a fake CT. Unsure how this will affect a cathode bias.

I've also heard of using the secondary winding of a totally separate transformer that does have a CT, and using it in parallel across my filament winding, then using that transformers centre tap. Further, I've heard of using a pair of diodes, each anode to each end of my filament winding and tie the cathodes together in the middle... or maybe that was t'other way around... either way I dunno about this either.

Anyways, obviously the best way forward would be to actually use a proper transformer, but since I can't find one and I already made this oopsie... I'm wondering what the next-best-way forward is? :)

Thanks :)

#39708

Hi Dave, the need is for a dc return, nothing in the way of capacitors will work.
Using AC on the filament a center tap for the dc return will null hum voltage.
The concept was raised of using a small filament transformer with a center tap having around say a 3A secondary at a voltage the same or close to the tube's fil voltage.
What hasn't been noted yet is that the primary shouldn't be powered up on the added transformer. that would lead to high circulating current in it and overheating.
Reid

#39712

开云体育

Thank you, Reid.? I was only thinking about the RF part.? I ignored the DC plate current part of the equation.

Forum readers, please excuse my oversight.

?

Meanwhile, I have found in my spares a NOS in the box Stancor Filament transformer #P-6457, 117/107 Volt primary, 7.5 V CT @ 21 Amps, Insulation test 2500 Volts.? It weighs almost 8 pounds.? Make me an offer.

?

73,

Dave, w6de

?

Show quoted text

From: [email protected] <[email protected]> On Behalf Of Brandon DX via groups.io
Sent: 3 December, 2023 18:55
To: [email protected]
Subject: Re: [ham-amplifiers] Artificial centre tap for GG 3CX1200

?

Hi Dave, the need is for a dc return, nothing in the way of capacitors will work.
Using AC on the filament a center tap for the dc return will null hum voltage.
The concept was raised of using a small filament transformer with a center tap having around say a 3A secondary at a voltage the same or close to the tube's fil voltage.
What hasn't been noted yet is that the primary shouldn't be powered up on the added transformer. that would lead to high circulating current in it and overheating.
Reid

#39713

On Sun, Dec 3, 2023 at 01:58 AM, Dave w6de wrote:

Ceramic Disk Capacitors are used as RF bypass capacitors in RF circuitry--that is to short RF to ground.? And Capacitors can be also used to block DC and low frequency current form passing through them. How to use the in your situation is described below.

?

Use a set of High Voltage Ceramic disk capacitors, wired from each of the two filament leads to ground—this will allow the high frequency RF to pass through and block the low frequency AC filament voltage..?

-Disk ceramic Capacitors are also used in High Power Tube Amplifiers as a DC blocking capacitor to Pass through only the RF from the plate to the PA plate circuit and then the antennas system.?

-Since this works in the plate circuit and it will work in the ground/common circuit too.?

-I am not certain about the voltage requirement for this filament grounding circuit. ?Look at the Filament transformer’s insulation value.? You aren’t going to need much higher voltage for the capacitors than that.

?

Turns out, high voltage disk capacitors are becoming hard to find:? I found this at RF Parts:

? 0.01 micro farad @ 2KV

-Assuming you will use a 0.05 microfarad capacitance value you will need 10 of these to make your filament grounding system.? Double than quantity if you need a high voltage of 4KV.

-Alternate choice but more expensive are doorknob capacitors.? While these might be awkward to mount a capacitance above 2000pf may work for you, these will certainly work and look impressive.? You are going to need one of these door-knob capacitors for your amplifier plate circuit too.---just buy 3 and you’ll be set :>).

?

A capacitance of? 0.05 microfarad will present only 3.8 Ohms (2 x 1.8) resistance to the RF signal at 1.8 MHz even lower resistance as the frequency goes up.? But will present 106K Ohms (2 x 53K) ohms to the 6.3 volts 60 Hz AC voltage.? Use this calculator to determine alternate capacitance values if you desire:

?

73,

Dave, w6de

I bought? .01uf @ 1.4 kv disc caps from RF parts.? Huge things at 1" diam x 3/16" thick.? ?
Also bought a bunch of Ceramite brand? disc ceramic 4700pf caps (.0047uf) @ 15 kv? from Henry radio.? They too will handle a lot of current....and are big things.

Then bought dozens of the chinese .01uf @ 30 kv disc caps off their? ebay store.? Massive things? at 1 3/8"? diam x 7/16" thick.? These hi pot tests to? 15 kv? ( limit of my hi-pot). Max current of all of em.? ?11m ops use em for coupling caps into the cathodes of GG amps...and stuff several kw through em. The chinese disc caps also come in 10 kv..and also 20 kv.? ?They are so cheap, and shipping is dirt cheap,? I decided to just stock pile 3 x doz of the 30 kv types.? ?I hi-pot tested all of em....and also the C... which was typ 9200-9600 pf.?

?

I put 2 of em in series.... and wire that mess across the cathode of the 3x3, 3x6, YC-243, and bigger tubes.? Drive applied to the junction of the caps.? Done that way, the drive current splits in half...and makes for better use of caps.? ?Z at 60 hz is sky high.? Z at 1.8 mhz is low.?

?

For the OP's situation, a small fil xfmr with the CT on the sec? will easily work.? Then the? CT becomes your fake CT....that can be used for the cathode return current.? ? Sec of small xfmr wire in parallel with his existing fil xfmr( that has no CT).

And like Reid sez.... the 120 vac primary of the? small fil xfmr has it's 120 vac primary leads taped off...... not used.?

?

Jim? VE7RF