The AM Forum

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

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

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.

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
KD6VXI

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

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.

Norm,

Shane didn't miss that :)

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 :)

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 :)


--Shane
KD6VXI

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?

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?

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

If the 4CX350 is that sensitive be careful of it taking off!
Good candidate for swamping and untuned 50 Ohm input load.

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

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

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.

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.

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
KD6VXI

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

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 (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.

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

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

maybe mr Vu will weigh in .... he knows about low distortion with 4cx350  8)

You mean the grid load resistor,  correct?.
So tie the the 3 grids together and hang a 50 ohm resistor to ground....

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?

Hi Glenn,

A few years ago I built up a 4CX-350J in class A/AB1 linear service.  It was part of a class A/AB1 linear chain that started with a tap off the FT-1000D at 100 mW, into a 1 watt lab amp, into the 4CX-350J, and into a pair of 8877s.   I was able to get -55DB 3rd order IMD numbers, which is incredibly clean for an analog amplifier system.

The "J" version of the 4CX-350 is at least 10-15 DB cleaner than the standard models, but costs more too. It is the optimized "linear" version, probably demanded by medical or military service when IMD was important.  BTW, if you are using an SDR radio with high-level-derived predistortion then the "F" tubes will work just as well.



** Some of the following may or may not apply to your situation, but in general:


What I did:  

I built up a simple adjustable current overload sensor - controlling a separate relay for the screen and grid that shut down these two elements instantly when either is overdriven.

I used a string of diodes in the cathode lead to supply grid bias... super stable voltage.

I used a well regulated (electronically-regulated) screen supply. You will want to run recommended screen current for best linearity, thus the need for exquisite screen voltage regulation.

Being in class AB1, there is no grid current, ever.

Watch your plate voltage for any sag and take appropriate measures to keep it stiff thru heavy plate supply filtering and a 240VAC feed, etc.

You basically need to follow all the rules of linear tetrodes to keep them safe from harm and clean for minimum IMD.  Run two-tone IMD tests thru until you are satisfied with at least -35 DB 3rd for "F" tubes -   and -45DB+  3rd IMD for "J" tubes..

Load the amplifier heavily (less C2 mesh in the loading cap) to get the best IMD numbers.

If you break any of these rules, it is very easy to have a splatter-maker -  or destroy the fragile grid or screen.

I realize you will be using the "F" version of the tube and not the "J", but I would put out feelers for the "J" tubes and ya never know what you may find cheap.... ;D  Keep ads out in the ham wanted listings.   Worst case, the "Nebraska" parts place had them new when I built mine a few years ago.

Good luck with the project, Glenn!

Tom, K1JJ

Tie the three grids together.

Run your rf drive to one side of the resistor.

Ground the opposite side.

Connect the rf drive side of the 50 ohm resistor to the Junction of the three grids with a couple .1 uF discs.

From the Junction of the disc's and the grids, you apply bias voltage via an rf choke.

Hope this makes sense.  I have a pair of 250B tubes doing this now. 

--Shane
KD6VXI

Look at the TMC PAL-500 amplifier. Uses two 4CX350A's. They use separate bias supply feed for each tube to balance idle plate current. Also, TMC used a parasitic suppressor in each grid. The grids are tied together for RF but actually dc isolated by blocking capacitor. This amp uses a 6CL6 and 6146 driver. That was because it was designed for a drive level of 100 mw for the military. Where the tube driving circuits are, use the 50 ohm load instead. I suspect that to 4CX350's tending to "take off", so they added the Parasitic grid chokes. Usually a few turns of tinned wire across a 1 W 47 or 100 ohm carbon resistor. In a Collins design with 4CX250's (TRC-75) they just tied the grids together. Maybe the 4CX250"s aren't as touchy.   ST

I used to have a TMC PAL-500 linear amplifier here.  I especially liked the RF negative feedback from the plates of the 4CX-350As back to the earlier driver stage. Nice way to clean up the 4CX-350s into something even better.  I even added a little more NFB by increasing the feedback capacitor value.  It became a little harder to drive but remained stable, so I knew it was working.  This will decrease IMD.   Of course, negative feedback requires a driver stage which is not planned in this thread project...

And yes, I used parasitic suppressors in the grid circuit too. (as well as in the plates)   Nice precaution.

T

The need for parasitic suppressors on the grids can be pretty much removed by grounding the screen at DC

You hook the positive side of the supply to the ground and the negative return to the cathode.

This does nothing for imd, you'll still need a regulated supply for that.  However, it sure does give a high gain tube a hell of a lot more stability.

I can't claim credit for this method, but it sure cured a partially stable 4cx5000 I built years ago.

That used a 3 turn 9:1 input.  140 in would net 14 large into the dummy load.

Still ran PSupp on the anode.

--Shane
KD6VXI

Use a dc blocking cap between 50 ohm load and bias feed. Feed bias through RF choke. See the TMC PAL-500 amplifier schematic. They use a driver tube, but adapting the input circuit for swamped input dictates the DC block. 

Thanks to all who replied. Some great info here.... and a special hello to Master Vu on the channel, cawman!  ;D. Some of you may not know that Master Vu only likes "fresh sushi". Hello Tom, it's been a long while.. Almost 15 years since I moved to NC from NY.
Hope you're doing well. I'm hoping to get back on AM soon.

I'm pretty sure I can make -35 IMD by running a stiff, reduced voltage plate supply and a stiff well regulated screen supply as mentioned.

Does this look right? (attachment) ... and one more thing... I'm using SK630 sockets which directly DC grounds the cathode on pins 2,4,6 & 8.

Yes, like the sketch. OK. You get fancy and feed bias separately to each tube to balance idle current. That would require a dc block for each grid. Since there's no grid current, series resistance value in the bias supply isn't important. The individual adjust is closer to what TMC did. ST

YES!! double plus good on grounded screen for DC (and RF IMHO), with regular element-to-element operating voltages, doing wonders for stability! Let that screen be a screen!

Ok, I'm missing something... how do you dc ground the screen if you need to supply +350vdc to the screen??. RF ground yes, through the built-in bypass caps in the socket. Just to clarify, this is a grounded cathode grid driven design.
 

Interelectrode voltages are all relative.  It just depends on where you define your reference point.  If you set the screen at DC ground (chassis), and then you set the cathode at -350v, you have a DC ground at the screen, but it's still 350 volts above the cathode and the tube runs like a normal tetrode.  And you similarly have to adjust the reference point of the control grid and the anode.
If you have one of the old W6SAI "West Coast Handbooks" he does a good job of showing and explaining the technique.  I ran a 4-1000 tube in this configuration for many years and it was stable as a church.  The screen bypass capacitor can be a source of problems as it doesn't always look like a pure capacitor at all frequencies.  Bolting the screen right to terra firma solves that.  You have to pay attention to metering but it all becomes quite simple to implement.. 
By the way, the Bill Orr W6SAI handbook is definitely worthwhile for the amplifier builder and they show up often at hamfests, assuming we ever get back to having hamfests again.
73 de Norm W1ITT

I found my Billy Orr book at Barnes and Noble.

Was somewhere in the neighborhood of 30 bucks.

If you are going to have the screens floating, it really takes a ton of bypass caps to do the jib right, and then it's still unstable at times.

Grounded screen is the hot ticket.

And make sure you ground with nice wide strap.  None of this #22 hookuo wire crap.

--Shane
KD6VXI

There's a bunch of the William Orr Radio Handbook on Ebay.  I quit counting after ten.  I have the brown one and the gray one and I'm keeping them both!
73 de Norm W1ITT

I'm going to stick with the tried and proven design of grounded cathode and +v on the screen. I have the Bill Orr Radio Handbook 23rd edition right in front of me. On page 17-2 is "A universal Grid-driven tetrode amplifier". This design, like every other 4CX250, 350, amp I've seen, is a +V on the screen, grounded cathode design. I'm not doubting a grounded screen may be a better approach but..... considering the arrangement of the Eimac SK630 sockets I'm using, a low inductance path from screen to ground is not possible. If there was a socket that connected the finger stock screen contacts directly to ground and floated the cathode connections... then yes. Considering I'm already well into metal work and socket wiring, I'll stick with the original plan.

Shane, the screen won't be "floating" since it will be connected to a very stiff well regulated supply.

Check http://www.nj7p.org/Manuals/PDFs/Books/orr_radio.pdf

1959 Editor's Engineers 1959 Radio Handbook, pages 629-631. There's a 4-400A amplifier with a grounded screen application. It explains how to do this. I scanned the pages on to a three page word doc to show here. Better quality by downloading the pdf from NJ7P. Although this design is a grounded grid cathode driven, the screen circuit can be replicated by itself while still driving the grid.

and
https://archive.org/details/books?and%5B%5D=radio+handbook&sin=

Your screen is still floating for rf depending on the resonances of your screen ground caps with a screen floating from the chassis.

The DC supply doesnt effect the rf stability, other than changing the gain characteristics of the tube.

By grounding the screen you end up with a very stable amplifier.

By utilizing the capacitors that run from screen to ground you may or may not have a stable amp.  Those caps will have resonances, and if your layout exacerbates the problem, the tube may take off.

With a screen at ground potential, the grid and cathode are nearly isolated from the anode.  Hard to take off like that.

Either way works, but as I said.  I had a 4x5 that was squirrelly.  I changed it to grounded screen.  This involved tying the screen directly to ground, having the positive side of the screen supply tied to the ground and the negative tied to the cathode of the tube.  The screen then isolated at all frequencies.

Both methods work.  However, the grounded screen works mo better with high gain tetrodes, especially multiband.

AG6K, Rich Measures (SK) is the one who turned me on to this method.  He published it in his website.

--Shane
KD6VXI

I appreciate the info, and I understand the concept for the sake of discussion but a grounded screen configuration is just not practical with 4CX350's (this project) and the sockets they are designed to be used with. A 4-400a is a tetrode but a different animal. (besides the anode, all the connections are on the bottom ...big fat heavy thick pins)

You mentioned you're working on a 4CX250 amp, you must be aware that the Eimac sockets don't lend themselves to a grounded screen config. You would need a custom socket with fingerstock that grounds the screen ring on the tube.

You can make clips, but depending on the socket that becomes a problem.

Or, ground pin 1.

The Russian sockets that will fit a 250 make it easy ttto modify the socket with spring clips.  They are an erector set.

--Shane
KD6VXI