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Need Some Help With My Amp Project




 
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W2BX
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« on: February 27, 2020, 03:25:17 PM »

Hey folks,

Looking for some help with a project I'm working on.
A while back, I aquired a partial Ten-Tec Titan 425 RF deck. No tubes, no power supply, missing wiring, control board, and other various components.
What it does have is a complete enclosure, tank circuit, front panel, and metering. Not wanting to reverse engineer and replace the missing wiring, PCB's and other components in addition to not wanting to spend mucho bux on a pair of 4CX800A7's, I put the thing aside.

Fast forward to today, I just scored a great deal on some NOS Eimac 4CX350F's. I have several low level exciters that could benefit from the low drive requirements on these tubes so I've decided to build a tetrode grid driven amp using three 4CX350F's in this Titan RF deck. So far so good. I was able to fit three sockets on a metal plate at the back of the amp where the 800a7's used to be. I'm going to use an external remote blower arrangement vs the original on-board blower. I am aware of IMD issues. I will be running this at reduced anode voltage in addition to building a very well regulated screen supply and a stiff HV voltage supply. This will be mostly a linear for 160 80 and 40m AM lightly driven and run at the 300 watt carrier level. Should be enough headroom, yes?

I had this wild dream that I could use the Titan tank circuit as-is and everything will be wonderful!. I'd would very much like to make that dream a reality. First hurdle.. plate resistance..

(x2) 4CX800A7 in original titan 425
Anode voltage = 2200v
Anode current = 1.2a
RF plate resistance = 1100 ohms
-------------------------------------------------
(x3) 4CX350F in modified Titan
Anode voltage = 2000v
Anode current = .9a
RF plate resistance = 1333 ohms

-------------------------------------------------
Formula;
Effective RF plate resistance ~ .6 [V/I],
Where V is the plate voltage at carrier, and I is the plate current at carrier.

Did I get this right? I want to use the method of connecting an swr analyzer to the tank circuit output with the correct value resistor connected between the anode connectors and ground. I know this is going to sound like a stupid question but, I CAN do this without the tubes in the sockets, correct?. and what about the plate choke? that says in place during measurement?

Thanks. Any advise or suggestions greatly appreciated!

Glenn W2BX
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W1ITT
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« Reply #1 on: February 27, 2020, 04:01:10 PM »

Glen...
Do the backward tuning trick with the tubes and plate choke in place.  The tubes have 6 pf output capacitance apiece for 18 pf total.  The plate choke will look like some impedance in parallel and it should be accounted for as well.  If you have one of the way-cool NanoVNA fifty buck analyzers you can do it while watching the Smith Chart, as it's easier to see things happening as you change Ls and Cs in the plate tank.
Using an HP analyzer a few years ago, I tuned a 300 kw refurbished transmitter over in one of the countries named after "Stan" by this method.  We had a number of frequencies to "find" and only two of the special high voltage fuses so we couldn't afford to blow any.  It worked well and made short work of a potential problem. 
After you put the covers back on the rig things will change a bit, but on the 160, 80 and 40 meter bands the change should not push you out of the ballpark.  The 4CX350 is a cool series of tubes, designed with excellent IMD figures in mind.  It should be spiffy clean on the bands.
73 de Norm  W1ITT
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W2BX
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« Reply #2 on: February 28, 2020, 09:22:46 AM »

Hi Norm;

Thanks for the reply and info. Unfortunately I don't have a VNA, just an MFJ swr analyzer. I don't actually have the tube socket plate installed yet, I assume I can just connect an 18pf cap between the anode connection and ground in place of the tubes along with the resistor for measurements?

Frank GFZ mentioned it would be a good idea to take a look at the manual for the TMC amplifier that's using neg feedback with 4CX350's.



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KD6VXI
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« Reply #3 on: February 28, 2020, 09:44:09 AM »

An antenna analyzer will work fine business.

Stuff it into the antenna jack.

Key the relay.

Good to go, tune for 50 ohms.

I couldn't tell you how many amplifiers I've set up with nothing more than an MFJ259b.

--Shane
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W1ITT
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« Reply #4 on: February 28, 2020, 10:11:15 AM »

Glen....  We may be going out on a limb with just substituting the 18 pf capacitor for the set of tubes, but give it a try.  Put it out at the end of whatever you plan to use for a connection and plate parasitic suppressor, if any.  When you get the whole amp built, you can clip in a resistor and give it a double check. 
As with Shane, I've used simpler test equipment to do this, once even using an old Omega-T noise bridge which was pretty snazzy ham test gear back in the '70s.  It just takes a  few more minutes of twiddling.  What Shane is missing is that this is an opportunity to establish a "need" at least in your own mind to acquire a new and shiny radio toy.  I own two "real" network analyzers, but the coolness of having one that I can put in my shirt pocket was too much to resist.  And the things really work.  Keep bashing along on that amp project as I think you're going to end up with something pretty nice and learn some things in the process.  That's what amateur radio is about.
73 de Norm W1ITT
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W2BX
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« Reply #5 on: March 01, 2020, 10:15:29 AM »

Thanks again for the replies Shane and Norm.
I'm just now fitting the socket box at the back of the amp.
I need to find some ceramic standoffs to support the parasitic surpressor connection point and the HV coupling caps.



* titan_build_1_email.jpg (270.5 KB, 800x450 - viewed 98 times.)
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KD6VXI
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« Reply #6 on: March 01, 2020, 03:25:08 PM »

Norm,

Shane didn't miss that Smiley

I have a few VNA's here, along with a few other pieces of "needed" test equipment.

I figured we'd get him with a working amp, then inform him he needs more equipment Smiley

The Nano is an OK vna.  I MUCHO prefer my VNWA, although it's an order of magnitude more.  Does have better dynamic range, though.....

I do find myself using the VNWA to take measurements, then using VNASaver to do the actual horsepower and displays.

Glen,

You will find that you want more test equipment in the future.  It's great being able to sweep the parasitic choke and find out if it's working or not, among other things.

But, those will be "necessary" in time Smiley


--Shane
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W2BX
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« Reply #7 on: March 06, 2020, 11:14:09 AM »

I have a pretty decent collection of test Eq. Wish I had a Spec An. I'll look into one of those $50 VNA's in the future...

Question.... I'm making an anode connection bus bar which will feed the three resistors/suppressors to anode clips. Is it ok to use bare copper flashing? a piece of sheet copper cut into strips... without some sort of plating?..(I also have a sheet of brass i can use).. same with the 4 turns of wire on the suppressors, does this wire need to be plated as well? I'm thinking this was a common practice in amp building due to skin affect.

Thoughts?
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KD6VXI
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« Reply #8 on: March 06, 2020, 05:59:08 PM »

I use a single piece of brass or copper strip and I make the parasitic suppressor out of the same strip.

A horseshoe or hairpin with the resistor shunting it.

Works FB in all the amps I've built.

--Shane
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W2BX
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« Reply #9 on: March 07, 2020, 07:42:00 AM »

I use a single piece of brass or copper strip and I make the parasitic suppressor out of the same strip.

A horseshoe or hairpin with the resistor shunting it.

Works FB in all the amps I've built.

--Shane
KD6VXI

Do you plate or tin the copper?
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kc2we
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« Reply #10 on: March 07, 2020, 02:38:38 PM »

Note; 4CX350's are designed for AB1 SSB service only. No grid current. Grid driven ONLY. We used these tubes in Army Collins AN/TRC-115. 
So even in AM mode, linear amp operation only - grid driven only. Same for CW /  RTTY In AM linear mode de-rate substantially to avoid excessive plate dissipation. For AM, you're better off with 4CX250's or 4CX300A's that can run class C.
The 4CX350A/F is designed for grid driven service only. Need only 25 volts peak or 17.7 volts rms at grid for full output. Drive across a 50 ohm non-inductive terminating resistor. That's about 6.2 watts drive. Two tubes in parallel? Same, 6.2 watts across 50 ohms. return the grid to rf ground at cold end with bypass capacitor. Bias tube(s)  at -27 VDC at all plate voltages. Not too compatible with the "standard" 100 watt class transceiver / transmitters.


Check Eimac data sheet.  ST KC2WE
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Seth Taylor
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« Reply #11 on: March 07, 2020, 07:30:58 PM »

If the 4CX350 is that sensitive be careful of it taking off!
Good candidate for swamping and untuned 50 Ohm input load.
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kc2we
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« Reply #12 on: March 07, 2020, 07:35:58 PM »

Exactly. That's how Collins designed the amplifier in the TRC-115
Drive two tubes with 50 ohm globar load.

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Seth Taylor
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« Reply #13 on: March 08, 2020, 12:18:33 AM »

I do not plate or anything else.

Plating makes it look cool, but deposits such a thin layer it does nothing for rf.  At least the home based plating systems.

As others have pointed out, the 350 is a very fragile tube.  No grid current!

--Shane
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W2BX
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« Reply #14 on: March 08, 2020, 09:51:17 AM »

Yes, see my original post. This will be grid driven, linear service. I have several low level exciters, that's what it will be used for so not being compatible with 100w class transceivers is not an issue. I understand the issues regarding bias, grid current and swamping the input with 50 ohm resistor. It will loaf along at 300w carrier as an AM linear.


Note; 4CX350's are designed for AB1 SSB service only. No grid current. Grid driven ONLY. We used these tubes in Army Collins AN/TRC-115. 
So even in AM mode, linear amp operation only - grid driven only. Same for CW /  RTTY In AM linear mode de-rate substantially to avoid excessive plate dissipation. For AM, you're better off with 4CX250's or 4CX300A's that can run class C.
The 4CX350A/F is designed for grid driven service only. Need only 25 volts peak or 17.7 volts rms at grid for full output. Drive across a 50 ohm non-inductive terminating resistor. That's about 6.2 watts drive. Two tubes in parallel? Same, 6.2 watts across 50 ohms. return the grid to rf ground at cold end with bypass capacitor. Bias tube(s)  at -27 VDC at all plate voltages. Not too compatible with the "standard" 100 watt class transceiver / transmitters.


Check Eimac data sheet.  ST KC2WE
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W2BX
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« Reply #15 on: March 08, 2020, 09:53:28 AM »

I do not plate or anything else.

Plating makes it look cool, but deposits such a thin layer it does nothing for rf.  At least the home based plating systems.

As others have pointed out, the 350 is a very fragile tube.  No grid current!

--Shane
KD6VXI

Good! no plating...I'll just use bare copper, just curious why commercial amps all seem to have plated tank coils.

....... and where the heck do you find 47 ohm 3w carbon comp resistors for the suppressors?..... All I'm finding is metal film and wirewound which of course is no good.

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« Reply #16 on: March 08, 2020, 12:12:40 PM »

I have used mox metal film before.  But the gold standard is YO series from ohmite.

.ake SURE you have a good supply in the screen. I'm in the process of reworking a two tube 250b amp now.  It has regulators on the G1 and G2.  For G1 I use a LR8 regulator.  Good for up to 400 volts of range.  For the G2 I used a typical LM 3 terminal regulator.  It regulates a range of 65 to 30 volts.

Also, a 5k 50 Watt resistor on the screens to ground is a good idea. This series of tubes will give negative screen current.  You need a sink to handle that.

That dumps 25 watts at 350 or so volts, but makes the screen supply bullet proof.  It also makes the tu es way less prone to be an IMD generator!

--Shane
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kc2we
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« Reply #17 on: March 09, 2020, 09:21:24 AM »

Another suggestion, use an electronic regulated screen supply. Linearity is best with a stiff screen voltage. Also, these tubes like lots of air flow. ST
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Seth Taylor
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« Reply #18 on: March 10, 2020, 02:50:21 PM »

Wow... starting to realize this may have been an easier project if I started from scratch vs modding this amp. Now that I'm into it, there's no turning back. So very many details to plan. Just about finished with the metal work, waiting for some parts to arrive to complete the anode HV buss assembly.

Of course, studying docs and schematics of 4CX tetrode grid driven amps built over the past years is a big help but also causes confusion because of the design variations.

I'm studying several designs including one from the VHF handbook for 50mhz that, like mine, uses 3 tubes. Part of the TMC PAL500 schematic, and this amazing amp by W8ZR;

http://www.w8zr.net/160amplifier/160overview.htm

The real challenge for me will be getting the metering working in the Titan. Plate current is already there including the shunt... need grid and screen current. I understand screen current can swing negative. Since the Titan doesn't have a zero centered meter, I may try and use that W8ZR  red/green LED circuit to monitor screen current.




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kc2we
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« Reply #19 on: March 10, 2020, 03:26:31 PM »

Since grid current is supposed to be zero, maybe a similar technique to verify if there's any current at all. the overkill method is to use a low range ma. meter.

ST
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Seth Taylor
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« Reply #20 on: March 10, 2020, 07:13:01 PM »

On silver plating: real commercial plating uses toxic chemicals (silver cyanide) not normally available to the retail consumer.

On load resistor, all u shud need is a nice round number like 10 watts at 50 ohms.

ST
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Seth Taylor
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« Reply #21 on: March 10, 2020, 08:23:39 PM »

maybe mr Vu will weigh in .... he knows about low distortion with 4cx350  Cool
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W2BX
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« Reply #22 on: March 11, 2020, 08:13:18 AM »

On silver plating: real commercial plating uses toxic chemicals (silver cyanide) not normally available to the retail consumer.

On load resistor, all u shud need is a nice round number like 10 watts at 50 ohms.

ST

You mean the grid load resistor,  correct?.
So tie the the 3 grids together and hang a 50 ohm resistor to ground....
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kc2we
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« Reply #23 on: March 11, 2020, 11:19:15 AM »

Yes. three grids together. Check other designs, but recall that's it. There's no power consumption taken by the grids. You only need to develop about 18 volts RF rms. Only power loss is in the 50 ohm resistor, about 7 watts.
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Seth Taylor
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« Reply #24 on: March 11, 2020, 01:45:53 PM »

Yes. three grids together. Check other designs, but recall that's it. There's no power consumption taken by the grids. You only need to develop about 18 volts RF rms. Only power loss is in the 50 ohm resistor, about 7 watts.

What about the grid bias voltage also applied to that 50ohm resistor?
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