On Wednesday, December 6, 2023, 8:10 AM, Steve <k0xp@...> wrote:

Old news to some, perhaps... I think he died some time ago; but W4ZT's
amplifier web sites, mostly showing big Russian tubes such as the GS-35B
but also with links to many European Russian tube sites, seem to have
been dropped by the ISP in the last month or so. He also advertised
sockets from K4POZ, who also died a decade or more ago, who used to
build professional-looking sockets for such as the GI-7B. I cannot
remember who I got them from, but I was able to buy new sockets as
recently as several years ago from someone that I found on the W4ZT site.

73,

Steve K0XP

?

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

On Sunday, December 3, 2023, 10:58 AM, Steve <k0xp@...> wrote:

On 12/3/2023 7:44 AM, Jim VE7RF wrote:
>
> BUT he should have NO grid current at all, when keyed, and no drive
> applied.? Something is still amiss.
>
Considering the condition of that modified amp, I wonder if perhaps the
grid current circuitry is miswired, and has always indicated something,
but the operator just didn't realize it wasn't reading correctly until
the troubles began and we began asking questions about what he was
seeing?? With wires running every-which-way, it looks like a mess, in
those pix.

And BTW, someone please remind me what problem suddenly reared its ugly
head to bring the owner-operator here for help, in the first place??

Steve, K0XP

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 :)

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.

?

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

?

?

?

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 :)

?

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 :)

Adrian and all
I sent a similar reply a few hours ago, but it doesn't show up, I assume it's lost.? And if not I hope the 2 versions are consistent.

I measured the voltage across the 2 terminals of the big zener.
? key down 9.3 volts
? key up? 9.1 volts

Individual Tube Idling Currents Keyed, no input signal
Rear Tube:? Plate: 40 ma? Grid 10 ma
Front Tube Plate; 60 ma??? Grid: 18 ma

All readings drop to 0 when unkeyed.

Cheers ? /John