The AM Forum

Picked up a 2nd FT101E with an AM filter and did Tim's audio mods. Got it for the house so can use when not in the outside shack. Put together this G.G. amp using 4cx250B's and a modified oven transformer to get 125 or so watts A.M. There's a push pull sw. that disconnects about 3/4's of the main cap for the higher bands. It's working but needs some tweaking on the opti-coupler screen trip circuit.  I set it with a resistive load to trip at about 50 Ma but sharp peaks unseen on the meter will trip it. A similar amp I built some time ago has a simple latching relay set about the same and works fine. I figure a small electrolytic on the 24v trip relay might do it. we'll see. Haven't been on much but have been listening and hearing some fine sounding signals on A.M.
                                                                 best wishes to all, Jay-           

You have the same amount of plate dissipation as my new 4x813 amp has.
I run it between 150 to 200 watts carrier out.

What kind of maximum peak power do you get out of it?
At 2000 volts, I get about 900 watts pep, at 2500 I get 1200 or more, at 3000 I get about 1500.

There seems to be no advantage in running it above 2000 volts, as I get 800 watts pep with 150/175 watts carrier at under rated plate dissipation...

On ssb, I could crank it up and benifit....

The 4cx250b's are cool tubes, you could get 150 watts and 600 watts pep out of them with zero driving power (AB1) like collins did in the KWS1.
Maybe drive the grids with some sort of AM unbalanced modulator chip!

ppp (4 tubes) would be the bomb, 300 watts carrier, 1200 watts pep with no driving power...

Brett

Hi Jay,
Great job on the 4CX250B amp.  I like the layout and your construction work.  Hope you have enough inductance to put it on 160!
Regards,
Joe, W3GMS

Nice job, Jay.  Looks like a very compact and strapping little rig.

I wonder how loud the cooling is?  

External anode tubes are a good way to go for linear service.  I use glass for AM class C - and metal for linear... ;D

T

I don't have a p.e.p. wattmeter. Will put out 150 watt carrier with good peaks on scope and rf amp-meter. Readings are approximate. Used meter on T-368 dummy load and also measured voltage across heath oil filled load. All different. Within maybe 15 % on the lower bands. Plate voltage is about 2kv with 47 mfd of filter. Plate current -.2-.3 amp, screen voltage w/solidstate reg. is 300v current resting is low peaking up to 10-20 ma. Bias has shunt reg. and resting current is 160 Ma. Grid current of course is zero. Fan noise is acceptable for me anyway. Has 2 speed fan hooked to ptt. Might check air pressure/tube temps to see how slow I can go. I Like Tom's external air input fan setup on his "see thru" rig. 160 mtrs works fine on 1880 kc and up. Haven't tried below that. Cap is about 395 mmf on 160 mtr band. Cheated alittle with a bit more L. For higher power ssb would need heavier coils but not interested in that. They don't get hot. Parasitic chokes got hot on ten so shunted with a second load resistor. Need to find time to get on the air!! Thank you all for your interest and comments, Jay--

Nice layout.

I like the Hemi Orange xfmr.

Speaking of that color, turbocharger silicone hose can be used to plumb hot air out of the tubes. It is all over ebay and other places. You can get some to fit right around the tube anodes and poke the connection through. It could be ducted to the inlet side another blower. You can quiet things down and have better cooling at the same time.

Make sure the hose has no metal reinforcement for obvious reasons. I found out about this when an Alpha Alpha amp I was working on was missing the original duct. 

So its grounded grid, but with voltage on the screens?
Is that the way its supposed to be run?
I wonder why something like the 813 has all the grids grounded, but the 4cx250b has screen voltage.

I wonder what the distortion is like they way you are running it.
Two 4cx250b tubes in tetrode connection were used in the kws1 in AB1, Collins said the distortion was very low that way, and they take no driving power. From memory, I think I got about 600 watts pep out from that rig, which would be 150 watts of carrier.

I use them in audio service in AB1 for 600 watts out at 2000 volts/500ma.
Resting current 90ma per tube, 2000 volts, 350 volts on the screens, about -55 volts bias.

Brett


 

Jay,

Schematic of that set-up?

Brett, Apparently these tubes don't work well with all grids tied directly to ground. I did see an article that someone did just that though. They draw too much grid current. The overall performance is said to be much bettter with regular grid voltages applied. My grids are only at RF ground with a low Q tuned input circuit to the cathode which is also the dc return path. No heater choke. In another xmtr I built with 2 813's I do the same thing but have to use filament choke for isolation as they are directly heated. 750v reg on screens and -150 on grids. needs much less drive this way too.  The 813 xmtr was originally grid driven with bias and screen variable to run class c or ab1. I never did get plate modulation hooked up to it and ended up modulating the 6146 driver and driving the heaters. I would not have built the 813's with grid supplies if I knew it would end up in grounded grid. They seem to work great just as you are doing.  Dave, good idea abt silicone hose. run across alot in my truck repair business. Blowers always seem ok until the project is done and then their too noisy.         Jay-

Ralph, I'll try and schetch one out and post it. Power supply, pinet and ptt circuits are straight forward. I'll try to detail the two solid state regulated grid supplies etc.
                                                                 Jay-

Yes,

I'm really interested in what you did to the grid and screen to
take them above ground for DC.

Long  time  ago (mid 60's) I thought of using three or four '250's
in a final..

Have a transformer that was built in the lab at Pearl Harbor (yeah,
Hawaii): bridge designed, 2kv @ 1a, small footprint,  hypersil technology.

Collected a lot of parts but never went forward with the project.

It was to be built into a cabinet the size of my MARS issued  KWM-2 .

 

then grids are very delicate.. lots of care must be taken in loading to avoid burning them up.

KWS-1 is only low distortion due to inverse feedback in the circuit. Normally a grid driven 4CX250B linear is pretty disgusting. The cathode drive Jay is using is the way to go. This can be done with any tetrode. Tom Vu could even use it for his mudulator.

Tom Vu having HiHi Fine Business OM TMC PAL-500K amplificafire having 2 Hon. 4XC250s but never using it  ???

I thought that one had 4CX300s.

Didn't they make another one with different valves?

How's Lil?

Yep, still have it.  It wuda made a good IPA for a bigger amp, but I went to the SS FET HB amp instead.

I built up a power supply for it. It required about eight different voltages - big job. I had to do a lot of mods to get it to work with the HB supply, mostly interface stuff.  I had 2KV on it and even enhanced the RF negative feedback a little. It worked very cleanly, but I cud only get about 300w PEP out of it with max drive. (2 watts drive? grid driven, tetrode connected) Maybe the tubes were soft, dunno. Shud do more. But there it sits in the cellar unused.



T

I see no reason all that this amp cant be run at an easy 250 to 300 watts AM.  Its required to reduce Filiment volts abit at high power. I have not done this but Eimac suggests it and others have told me that if you do, down to 5.5 to 5.75 volts they will last a very long time at high power.

Its a great amp and you did a great job. But seems like a lot of effort for 125 watts.  The 4cx350 tube will go right in there also.  I have swapped them out before.

I have a dual 250B amp. I sent it to a friend for modification and repair. Its been over 2 years now. Going on 3. I dont ever think I will get it back. 


If you need spare tubes, I have a Drawer full of them. 

A friend completed Tims Ft101 mods.  Fed in his audio rack. Full broadcast quality audio. Maybe actualy better them alot of BC rigs out there.  Trouble is.. Its 20 to 25 kc wide now.. LOL

Clark

I'd have to agree.  The 250 is an <<ok>> tube.  It's lineage dates back directly to glass, so I believe it could be one of the first ceramic/steel tubes, but that's heresey on my part.

I built a lot of 250B and 250R based amps back in the 80s.  Mostly to <<10>> meter operators.  We bought so many xformers for them that Dahl carried them as a 'stocked' item for a number of years.

Did single and pairs of 250s.  With 2.2kv on the anode, 375 on the screen and about -75 to -80 on the grid, we could get about 700 watts out of a single tube.  A pair would net about ..tada.. 1400.  It was pretty wide, though.  Drop the drive level down to where it would PEP < 500, and it was a LOT cleaner, and a lot less people would come screaming from adjacent frequencies.

I got a line on a LOT of the F or G series 250, coaxial socket, surplus once.  They don't play very well at HF.

I've put about 2800 volts on the plate of a 250, but you have to be careful with them at that level.  Some of the CBers have tried > 3kv, but I don't think they are successful for very long.

Bill Orr did an amp with a 250 driving a 3-1000 in the handbook...  It was supposedly fairly clean, but they only ran the 250 at about 80-120 watts PEP output, IIRC.

Palomar Electronics (of CB fame) also had a line of amplifiers, built by Pride (Built with Pride by the Handicapped!) that had a 250 in them.  Pride DX-300 was pretty much a CB linear, with a bandswitch.  4 watts carrier damn near overdrove it.  They also made a KW-1, which was also grid driven, but was just a swamped input, with a bunch of resistors across it.  The DX-300 was grid driven with an xformer.  About 10 watts would drive it to full output.

I'd be interested to see the schematic as well....  I have heard the 250 series of tubes where in a lot of DAF designs, along with 4-400s.  The 4-400 is a good tube, but the 250 is so dirty, I wonder if it didn't contribute to the history of the G2DAF being such a bad amp design?

--Shane

They dont seem dirty as modulators.
The eimac book does not list them as bad.
Intermod products are listed at least 35 db down, at least when you run them the way they say.

Brett

-25 in grounded cathode is the number I've always read.

There's a comparison chart here.

http://www.cpii.com/docs/related/22/C&F4Web.pdf

The bandwidth is twice the highest frequency transmitted.   That makes sense and alot of AMers dont get that..  3000hz of audio is 6kc wide. 10K audio is 20!

Thats another subject and off topic.. But it was in that link.

C

Here's a somewhat condensed diagram. Was testing last night into load and got 200 wt am out without transformer getting to warm. Yes it was alot of trouble to build it etc for a not so big increase in power but i got pleasure out of doing it. Thats all that matters to me. \
                             jay-

Clark,
thanks for the offer on the tubes. I have several but have seen the prices on e-bay for them going up the last year or so. Another rig I have uses 4cx250F which have 26v heaters. these used to be cheap also. I used tim's mods on two rigs (101's) and have been very pleased with the results. Too bad on the lost amp. I have to admit that in the distant past I often took on things and was slow to complete. Eventually got them back or done though.
                                                                Jay-

Yeah. Maybe one day I will get that amp back.   I was going to use it with the ranger. 

I think I have some "F" tubes also.  I got a stash of them at a ham fest.  Could be bad ones though! 


C

I heard someplace that the 4cx series went bad with  run time, the inside of the tube gets coated with stuff that boils off the cathode, then they short out.

I have a bunch of hamfest specials, about 20 of them, and used to run them, but after a short time they would arc over inside, the arcing getting more frequent the more I used them.

I tried about 5 sets, and all did the same thing, they would work for a while, then zorch.
I got four NEW ones in the cans, and have not had a problem since (15 years).

I think the ones at fests are pullouts, and they got pulled because they tripped someones breaker...

Brett
 

Maybe.. Good point though!  I have some in the bags still.  Old MIlitary metal/ paper bags. I am sure those are good. 


C

Thanks Jay, for the schematic, very interesting set-up.

Important point with the 4X150 and 4CX250B is to run the filament AT or slightly
below 6 volts.  The cathode "boiling" problem is caused by over voltage here...

Most hams just grab a standard "6 volt" filament transformer and never bother
to look at the voltage at the socket!

Yes..I was going to post that..  They run them at 6.3 or higher.. It toasts the tube.  6.0 is suggested and down to 5.5 if you have lots of disipation going on.. Like in an SSB amp.  I understand they last for years and years if you lower the Fil.


C