The AM Forum

Hi All,

I am finally at a point where I can consider building my big rig.

I have amassed a good stock of 4E27 (HK257B) type valves, that will hopefully be OK to use in the PA of my transmitter.

So, questions,

1, Looking at the datasheet that N3DRB posted, there is a circuit that shows a HF multiband transmitter - this looks mildly interesting, but also looks like the poor 6V6 is getting a serious thrashing.

I was wondering if this circuit is really all that is needed - looks far too simple to me!.

2, Should I build an exciter unit, say using 807s, or 6L6 in the multiplier and final, and then use the 4E27 as an amplitude modulated PA - thinking of basing the PA on something that would usually have an 813 in it?

3, I am looking at a Pi output stage, probably looking at 80-10m coverage - any pitfalls using this tube and idea?

4, HT wise, I only have about 1kv to play with - the final design is looking at about 100w of carrier, I have lots of TZ40 tubes for the modulator, along with a suitable PSU and modulation iron, will this lot work together?

Cheers
Sean

I hope you have good luck with those H&K tubes. I never seen one without gas and usable. Better choice would be the Eimac 4E27/5-125B.

Conversely, I have quite a few vintage H&K transmitting tubes, and most have been just fine.

73DG

The tubes are all from a number of makers, I do have one or two Eimac ones, but they are used, so I was hoping to steer away from those.

Perhaps building up a test rig first would be a good idea.

Thinking ahead to the build, and a suitable multiplier chain.

The tenth edition of The Radio Handbook shows on page 283 a design for a "multi band frequency multiplier" This looks to be ideal as a fairly simple way to get from the VFO to driving the grid of the 4E27.

From reading the datasheet of the 4E27, it seems that i will only really need a couple of watts maximum to drive the grid, this being the case, I wonder if I need to use the 6L6 as a multiplier tube, perhaps use the 6AG7 instead?

I will not have the 400V B+ supply, more like 300-350V, so I appreciate this will lower the available power slightly.

I plan to have the multiplier and VFO built on one chassis, and the final amplifier on a chassis, sitting above this unit.

Will this multiplier be happy with around 12-18" of coax feeding the PA?

Knocked up a static test rig this afternoon.

Initially I have 5 tubes for test.

The rig is only able to supply a maximum HT of 500v, whereas I plan to be running around 1Kv.

Test results look promising on 4 tubes, one performed a lovely little light show at around 200v of HT, so a 20% attrition rate is acceptable.

Of the 4 that are remaining, running with 500v plate and screen, 0v on the supressor, and around -35 on the grid I get around 150ma of anode current, so, while not exactly to published specs, I think I will be able to progress with these tubes.

Cheers
Sean

Well Sean, that is fantastic! My experience is only with 250TH and 100TH triodes. I have about 1/2 dozen of both and all are gassy? I always liked the earlier version of the 4E27 tube for some unknown reason so I am glad they worked for you! I tried every way to de-gas them except for the way Dennis W7TFO does it with no success.

BTW, all your ideas above look good. I would use an 807/6L6 combo if I were building the project.