4-400 Amplifier

[wx3k]

I have a commercial paging transmitter that was used on 35 MHz.....Aside from the other gear in the rack, the PA is a nicely metered assembly complete with HV power supply, blower, etc. It is Class C as it stands now. Of course the tank circuit needs to be modified for 75/160 meters. What approach would be best here ? Building an outboard modulator for this OR re-biasing the amplifier for linear service ?  I guess the modulation transformer would be the big expense with the outboard modulator. I am not looking to run a kilowatt, just some additional power beyond the Ranger output.

[WD8BIL]

Well Steph, you could run it as an amp while you look for a mod tranny and a pair of 813s.

[W3SLK]

What kind of power does it offer now? What are the voltages? You can modulate that with 811's as well as 813's.

[The Slab Bacon]

FWIW, I have dismembered several Quintron paging transmitters made for somewhere around 49Mhz. Lots of really good parts to be scrounged! they used a single class C 4-400. They had nice power supplies and metering circuits. However a single 400 is a little lame in leanyour service.

My suggestion is, since you have to rework most of it anyway to get it down to HF,
Rebuild the PA deck for a pair of 400s, and use robust and wide spaced tank components. Run it as a simple GG leanyour for now and as you accumulate parts later change it over to a plate mod rig.

Most of those things were in 6' racks, so you have plenty of room to add more later as you build it.

                                                        The Slab Bacon

[wx3k]

The amplifier will currently do about 350 watts out with the following voltages provided...

2600v on the Plate, 400v on the screen and -90 grid bias

[The Slab Bacon]

The amplifier will currently do about 350 watts out with the following voltages provided...

2600v on the Plate, 400v on the screen and -90 grid bias

Yes, but that is not in linear service. In linear service the efficiency will drop to around 33%. Also the 350w is a max outpoot with no consideration for am headroom. 350w peak output (PEP) will only equal out to 87 1/2w carrier output. Hardly worth the bother of building a linear.

The low plate voltage (2600v) in linear service will require more plate current to make things happen. the sinplest way to do this is by paralleling a pair of them.

                                                              the Slab Bacon

[wx3k]

Yes, this is a Quintron !  

[WD8BIL]

Frank, she's using a barefoot Ranger now. Even if this thing gets her a 125w carrier in AB2 it'll be better than what she's got now.

[flintstone mop]

Nice looking Quintron box.
My vote would be to keep it class C and plate modulate.
You have a T-368 type of RF signal. I think legal limit A.M. or close to it.

Fred

[The Slab Bacon]

Frank, she's using a barefoot Ranger now. Even if this thing gets her a 125w carrier in AB2 it'll be better than what she's got now.

Buddly,
          this is true. but a 6' rack amp to put out 100w is one of those "whats wrong with this picture" scenarios. that is the same transmitter that I cut up for parts. there are gazillions of them out there just about free for the taking since low band paging is just about extinct.

the power transformer out of one of those was what I was limping my 4x1 rig along with until I got the mega iron. It is definately good for 1kw PEP or better. It would hold 2600vdc with 400Ma load. I was beating it harder than that, but it was sagging on the voice peaks. So it would easily be good for 250W of carrier in AM linear service. Since she will have to rework most of the PA deck anyway to get it down to 80M, there is plenty of room in that chassis to shoehorn in a second tube. Used 400s are cheap and plentiful. I still see them at festers for $10-20. If you're gonna take the time to convert it over, why not take a few more minutes to get much more "bang for the buck".

Just my take on things.

                                                       the Slab Bacon

[W9GT]

Not sure what type of modulation was used in that xmtr could have been FM, but you might already have the means to put it on AM phone.  Many of the old "low band paging xmtrs, such as the Budelman used plate modulation for the tones and voice.  Check out the power supply and see if there is also a modulator on board.  The old Budelman paging xmtrs used a 4-250 or a 4-400 modulated by a pr of 4-125s.  N9HXW is running a Budelman paging xmtr on AM phone and it works great. 

I would also vote for building up a plate modulator, if you don't already have one in the rig.  Another option would be cathode modulation, which could be accomplished cheap and easy.  Ask W8NWF about that.  He is running an 833 cathode modulated by 3 6146s and it sounds great.  No need for a big modulation xfmr.  It would be a shame to not use that 4-400 in class C as it was intended.

73,  Jack, W9GT

[flintstone mop]

If you have a Junque box or even if you want to go out scrounging parts, it could be a nice project to build an external modulator, with a pair of 4-125's or a pair of 810's.
I know that putting an external modulator from scratch can get expensive.
You can get to legal limit and have a nice plate modulated transmitter much cheaper than spending an inflated price of a T-368 which is a 4-400 modulated by a pair of 4-125's.
Or you can save the dollars and the hassle of building and make it a linear. 125 watts is better than Ranger power.
With present conditions on 160M 125 watts and a reasonable antenna, you will have a respectable signal.
80M you get close to being a channel master.

Fred

[The Slab Bacon]

Jack,
        those Quintrons were FM. Cathode modding it would be a viable alternative. A single 6080 with both sections in parallel would do it just fine. Or (God give me strength) a single power MOSFET would also do the job.

One could also consider scream (screen) modulation. I've been running a single scream modulated 4-1000 for years now.

However, with anything other than plate modulation, the plate efficiency thing comes into play. With 33% efficiency and only 400w of dissapation to play with you come back to the need for multiple tubes.

Ed, KB2NSP used to run a cathode mod rig with 3 (IIRC) 4-400s. It used to pack a whopping signal and sound quite good.

Phred,
         With current band condx, 125w aint gonna make anyone a channel master regardess of what their antenna is. (within reason) But every little bit helps!!

                                                    The Slab Bacon

[w1vtp]

Nice XMTR!  I vote for hi level plate mod with an intermediate step of screen mod.  There are a couple of ways to screen mod the amp, one being getting a hammond AF transformer and turning it around backward to a good AF amp and using the former primary as a secondary to mod the screen.  If you are not sure about setting up the screen for modulation, ask around but I think you start off at 1/2 the normal voltage.

Later as you collect the proper IRON which might include a choke for modified  Heising you could change it over to plate mod.  Just be careful to get the plate tuning components that will handle the plate voltage plus the voltage the mod deck imposes on the plate circuit on positive peaks (figure for 130 - 140% positive peaks).

Congrats on the acquisition.

Al VTP

[W3GMS]

Class C is the way to go.  Redo the Plate circuit, Grid circuit along with making sure the RF choke has enough inductance on the lowest desire frequency of operation.  Rotary inductors in the plate and grid circuit would simplify the mechanics a bit.  The Ranger would make an excellent audio driver as well to drive a pair of modulator tubes.  A good screen or cathode modulator would do well in the interum to get it on the air.  This weekend you were putting a very strong signal into S.E. Pa. with your barefoot Ranger. 
Joe W3GMS 

[w8khk]

That is the same transmitter that I cut up for parts. there are gazillions of them out there just about free for the taking since low band paging is just about extinct.

the power transformer out of one of those was what I was limping my 4x1 rig along with until I got the mega iron. It is definately good for 1kw PEP or better. It would hold 2600vdc with 400Ma load. I was beating it harder than that, but it was sagging on the voice peaks. So it would easily be good for 250W of carrier in AM linear service. Since she will have to rework most of the PA deck anyway to get it down to 80M, there is plenty of room in that chassis to shoehorn in a second tube. Used 400s are cheap and plentiful. I still see them at festers for $10-20. If you're gonna take the time to convert it over, why not take a few more minutes to get much more "bang for the buck".

Just my take on things.

                                                       the Slab Bacon

Stephanie,

If these are so plentiful, it would be a good idea to look for another one.  That would give you the extra 4-400, socket, and filament transformer for the RF deck upgrade.  The power supply could use two transformers in parallel, feeding separate filter networks, one choke and cap for the RF deck, and the other for the modulator.   This assumes there is plenty of room in the bottom of the rack for the additional iron, etc. 

That plate choke looks like a Barker and Williamson 800.  Good down to 75M or 80M, but need more inductance for 160M.

A pair of 4-125s, or 4-400s would make great modulators, and in class AB1 they would need very little in the way of a speech amplifier to drive them.  Ranger audio output would certainly be sufficient with the appropriate driver transformer.

If there is not enough space in one cabinet, strap the two cabinets together, put the modulator in one, and the RF stuff in the other.   You could even rack the driver in the RF cabinet, and have room to add professional audio chain components in the modulator rack. 

73,
Rick

[Mike/W8BAC]

That plate choke looks like a Barker and Williamson 800.  Good down to 75M or 80M, but need more inductance for 160M.

Rick,

I think that copper pipe 90 on top of the choke will bring it close to 160? Good luck with the project Stephanie whatever you decide.

Mike

[KA1ZGC]

Stephanie,

If the plate voltage were a tad lower, I'd suggest a pair of 811s and some 2:1-ish step-down iron of your choice. That configuration works well up to 2000V. 2600V is cracking the whip on 811s, so maybe a pair of 4-125s or 4-250s, or even a single 4-400, any of which would be triode-connected.

I'm not sure 2600V is a good unmodulated voltage for a 4-400 in AM service, either. Methinks the positive peaks might be a bit excessive. The amp was designed for Class C FM service, and the carrier was already clipped by the time it got to the tube, so they could safely run the voltage up a lot higher than you would want to in AM service.

Maybe I'm wrong, but I would want to see if I could dial the voltage back closer to 2000V B+ and go from there. Perhaps going choke-input would get you close, but there's probably a better way.

This also brings up the screen voltage: if it's derived from a B+ dropping resistor, no problem. If it's a seperate screen supply, particularly a well-regulated one, you'll need some way to introduce modulation to the screen as well as the plate. The easiest way out would probably be to introduce a choke between the screen and the supply. Otherwise, you may need to go the tertiary-winding (or second transformer) route.

FWIW: I run a 4-250 at 1650V, modulated by 811s which are driven directly from my Ranger's mod iron (with the audio gain pot only up to 4). 15W out of the Ranger gives a comfy quarter-kilowatt with gobs of headroom that I can (and do) modulate the piss out of. As Rick said, anything from 4-125s to a 4-400 would work in the same configuration, but I would triode-connect them.

Just a few thoughts off the top of my head.

--Thom
Kilowatt Amplifier One Zero Grid Current

[W4RFM]

This sounds like a job for a Collins ART-13 mod transformer, already has the screen taps available.
Bob

[w8khk]

That plate choke looks like a Barker and Williamson 800.  Good down to 75M or 80M, but need more inductance for 160M.

Rick,

I think that copper pipe 90 on top of the choke will bring it close to 160? Good luck with the project Stephanie whatever you decide.

Mike

Gee, Mike, I wondered what that plumbing copper EL was for.  Thanks for setting me straight, I never would have figggered that one out!  Should have been intuitive.  Too many lunch Margaritas, for lack of a better excuse!

[Steve - WB3HUZ]

Every 1 kW BC rig I've seen with 4-400s ran 2500 + volts unmodulated.  No need to reduce the HV assuming proper components are used.

[w8khk]

Every 1 kW BC rig I've seen with 4-400s ran 2500 + volts unmodulated.  No need to reduce the HV assuming proper components are used.

Yup, I agree.   The higher voltage, and lower current in the final should provide higher efficiency, and less dissipation in the the final toobs.  Just a matter of proper turns ratio in the mod iron.  The higher voltage allows higher power positive peaks before reaching saturation.  Strapescent!

[KA1ZGC]

Fair enough... then it's just down to the choice of mod tubes and proper handling of the screen voltage.

[The Slab Bacon]

  Heee, Heee. the copper elbow just simply directs air through the tubular plate choke at the finned plate cap

That plate choke wont have enough inductance to get down to 80m. the 30Pf vacuuuum variable cap for the plate tuning is way too small for the lower bands, as is the loading cap as well.

the HV transfoma is set up with a FWB rectifier and produces about 3+KV with no load. It loads down to around 26-2700v under 400Ma load. However it does have a center tapped secondary, so you could reconfigger it to a FW center tapped setup and get the lower desired voltage for plate modulation.

However, broadcash transmittahs usually run 400s at well ovah 3Kv on the plates and still plate mod them. (1Kw outpoot 24 / 7/ 365) with lotz ov air ovah them.

                                                          the Slab Bacon

[W2XR]

Hi Stephanie,

My first homebrew plate modulated rig ran a single 4-400A, plate modulated by a pair of 833As. I ran the 4-400A at 2750 VDC at around 275 Ma Ip, for about 750 watts plate input on 75M. I got close to 600 watts output at this plate voltage, which represents an efficiency of around 78%; quite good in my opinion, considering the plate voltage I was running. 4-400As like to see a minimum of 3000 VDC for really good efficiency.

In Class C plate modulated service, when you run the 4-400A at around 2700 or 2500 VDC, the Esg must be reduced from the published Eimac specification at the normal level of output loading, or else the Isg will run too high. With 2750 VDC on the plates of the pair 4-400As in my rig, and  at the level of Ip I normally run, I pull about 80 ma of Isg, and the Esg must be held down to 400 VDC at this set of operating conditions. If you run the Eimac spec of 500 VDC Esg, the Isg will come close to exceeding the dissipation rating of the screen.

This is true not only with my rig, but similar type transmitters including the Collins KW-1 and the Johnson Desktop Kilowatt when run at their normal plate input level and an Ep of 2500 VDC. Both rigs ran the 4-250As or 4-400As at an Esg of 400 VDC for the reason I have described above.

My experience with BC rigs employing 4-400As is that the Ep is usually around 3100 VDC; this permits maximum efficiency, and the Esg can be run at the normal Eimac spec of 500 VDC for plate modulated class C operation, without exceeding the screen dissipation rating of the tube.

I personally would modify that Quintron rig for plate modulated service. Most of the sheetmetal work is already done for you, and you will of course have to change the grid and plate tuned circuits for operation on the lower frequency amateur bands, consistent with parts that can adequately withstand the peak voltages developed in plate modulated service.

Hell, if anyone comes across one of those Quintron rigs, please send it my way. I'd modify it for my own personal use,  add a 2nd 4-400A, and put 'er on the 160M band.

Good luck with what will probably be a fun and rewarding project.

73,

Bruce