Exciter power output

[KE6DF]

What power output should you have from an exciter to properly drive a linear with two 3-500z's like a SB220?

Driving it with an AM signal, that is.

What about a pair of 4-400a's in GG, if that is any different?

[Ralph W3GL]

With a pr of 3-500Z's properly matched to your exciter, with 3200 to 3400V on
them you can produce a 350 watt carrier with headroom to sustain 100% +
modulation peaks with 30 watts of drive as long as the HV supply has enough
reserve capability.  

In AM linear service you should be able to load them up to 350 or so mills
and they will blush so you need to put plenty of air past them...

You can push the SB220 up to 250+ watts of carrier as long as it has enough
air going into the case however, the stock fan needs to be replaced with a
bigger one like the Harbach unit and the cap bank on the doubler supply should by now, need to be replaced as well.  

The 220 properly modified/updated, with a good 25 watt AM driver (modified DX60 or Drake T4X/B), makes a fine 250/300 watt transmitter set-up...

As for 4-400's, you have 200 watts total plate dissipation less (800W vs 1000W).  While they will work if properly designed for GG service, they are no match for the 3-500's in linear service...


 

[K5UJ]

You'll probably see some variation by band.  manufactured ham amps may be hot on one band; require more drive and less efficiency on others.

pair 3-500zg here; 2.8 or 2.9 KV on plates; 18 to 20 w. drive = 300 w. on 75 m. with ~ 360 ma Ip.  On 160, a few more watts drive and 400 ma Ip for roughly the same carrier power.  As Ralph wrote, if you want to transmit for more than a minute or two you have to have a lot of air moving through the amp--you have to forget about the hammy obsession with having a "quiet amp." 

With some amps, a few other gotchas:  if output sags after a few minutes, especially on 160, look at the fixed value load caps that get switched in on the output pi network.  If they are cheap disc ceramics replace with RF doorknobs rated for 2 or 3 A on 1 MHz.  Watch the current rating for the h.v. plate transformer; some may only be rated for 500 ma continuous so for long transmissions with modulation it may at least, need air over it.   

Rob

[KM1H]

Gain on the SB-220 and most glass tube GG amps ranges 10-12dB. Since the SB-220 PS is a bit wimpy Id start at 20W and see how the heat looks. For windbags 200W is max, for most others you can run 250-300W carrier with a little cool down time. A muffin fan on the xfmr will help if your in for an allnighter.

Carl

[Ralph W3GL]

As Carl says, 20 watts drive to an SB220 is good,
(10db = 200 watts carrier) however I'm in the "windbag"
category with a 220 and push it to 300/325 watts output
but this box has been modified and will survive my buzzarding
transmissions...

My Amp Supply LK-500ZC however, will do almost full AM legal
limit as the power supply has a good transformer and full wave
bridge circuit but it does wimp out a bit on 160 due to the crappy
disk padding caps.  Some day I might get that problem fixed and
modify the final compartment to pass more cooling air but in the
meantime I just back off a bit from pushing it to the max.

As I said before the SB-220 with upgraded air and fresh PS caps and
a good exciter able to provide 25 watts of clean, well modulated, AM
makes a good 200 to 250 watt station...(Less 160 of course,,,)

[w1vtp]

Gain on the SB-220 and most glass tube GG amps ranges 10-12dB. Since the SB-220 PS is a bit wimpy Id start at 20W and see how the heat looks. For windbags 200W is max, for most others you can run 250-300W carrier with a little cool down time. A muffin fan on the xfmr will help if your in for an allnighter.

Carl

200 watts is about it what with the inadequate cooling.  Agree about the trannie too.  I had a SB 220 and it would run pretty hot -- I used mine mostly on ssb.  Built a HB 813 for AM & RTTY.  It would run key down @ 500 watts input all day / nite.

[KZ5A]

A good rule of thumb is that most linear amps can support 25% of their CW output as unmodulated AM carrier, modulated AM PEP same as CW output.

Adjust the drive to get the desired unmodulated carrier out from the amp.

73 Jack KZ5A

[kb3ouk]

And I can speak from experience on this, make sure everything on the output side of your amp (like antenna tuners, wattmeters, etc.) can handle the amount of power you are pumping through them.

[KM1H]

The linears I presently have in line are the LK-500ZC with the optional external transformer and thats fine at 350-400W for hours at a stretch and usually driven by a TS-950SD with a wide filter. Ive had it since 86 and the first time I took the cover off was to toss those drifty 160 padders.

Another is an Alpha 76PA 3 holer and thats happy at 350W driven by various low power controlled carrier rigs or a TS-940.

The third is an NCL-2000 prototype Ive had since 1964 driven by a HT-32B on AM and SSB. Another NCL-2000 will be driven by my old CE-100V once its finished its overhaul.

The pair of 250TH's and 304TL's for audio is temporarily apart for 6C21 upgrades and cranking the 304TL's up a bit so the only old style amp running now is the 2X HK354C's and 8000 modulators.

Customer work has slowed me way down on my toys!

[ke7trp]

*Ke7trp throws interference flag on field*


The 4-400 is rated to produce more power then the 3-500.   A pair is rated at 1740 watts right in the data sheet.  The 3-500 is 1400 for a pair.  Remember, Plate watt rating is the difference between INPUT and OUTPUT that is disapated by the tube. Not the output power. Class of operation is a big factor in just how much is disapated.



 

With a pr of 3-500Z's properly matched to your exciter, with 3200 to 3400V on
them you can produce a 350 watt carrier with headroom to sustain 100% +
modulation peaks with 30 watts of drive as long as the HV supply has enough
reserve capability.  

In AM linear service you should be able to load them up to 350 or so mills
and they will blush so you need to put plenty of air past them...

You can push the SB220 up to 250+ watts of carrier as long as it has enough
air going into the case however, the stock fan needs to be replaced with a
bigger one like the Harbach unit and the cap bank on the doubler supply should by now, need to be replaced as well.  

The 220 properly modified/updated, with a good 25 watt AM driver (modified DX60 or Drake T4X/B), makes a fine 250/300 watt transmitter set-up...

As for 4-400's, you have 200 watts total plate dissipation less (800W vs 1000W).  While they will work if properly designed for GG service, they are no match for the 3-500's in linear service...


 

[WD8BIL]

Now hold on there ref! Buddly throws the red flag!

Let's apply the rules evenly. At 3000VDC on the plate (and 500VDC screen w/ -80V biass) a pair of 4-400s only specs at 1375W.

 At 3000VDC on the plate (no screen and zero biass) a pair of 3-500Zs specs at 1420W !!!!

Replay shows I win the challenge and am not charged with a timeout!

The 4-400 spec sheet did not have data on GG applications. You can grid drive 3-500Zs but .... why???

4-400s grid driven in AB2 only requires 7 watts +/-. If you have a QRPp transmitter that's a good deal. But with most riceboxes and other 100W class exciters I'd stick with GG. And the easiest to do is zero bias triodes.

So, individual circumstances dictate which way one would best use his/her talents!

[KE6DF]

So what drive is needed for a pair of 4-400a's in GG?

That was what I was originally wondering about.

Any different from a pair of 3-500z's in terms of drive required in GG?

[WD8BIL]

So what drive is needed for a pair of 4-400a's in GG?

That was what I was originally wondering about.

Any different from a pair of 3-500z's in terms of drive required in GG?

OK, that's fair enuf!

There's a few possible configs for the 4-400s in GG.

One way is to tie both grids to ground and treat it like a triode in which case the drive requirements will be similar.

Another methode is to ground the control grid, put 350 - 500VDC on the screen and cathode drive the 4-400As. This would still require the 80VDC biass as a positive voltage on the cathodes with associated RF choke and bypassing.
The drive requirements will be slightly less than half that of the 3-500Zs but the circuit complexities are now starting to make it a questionable use of time and resources.

[KM1H]

The 4-400A in GG requires about 10-12% more drive than a 3-500Z.

BTW, the 3-500 makes an excellent grid driven modulator tube also and zero bias to 2500V

[WD8BIL]

The 4-400A in GG requires about 10-12% more drive than a 3-500Z.

That's true with triode configured pubes, Carl. Tuned input helps noticably!

But the second configuration with screens alive only required about 1/2 the drive my 8874s did. Much easier to drive with low power rigs when the screens are jumpin'!

[Ralph W3GL]

Buddly,

My statement was comparing the two tubes in GG linear service only,
where most hammy  ham-bone  composite amps are constructed.

Yes, as you state, the 4-400's will perform well in conventional service
biased for linear operation and 3-500 will work fine in conventional
grid driven circuits but, as you said, why?  

We all know, 3-500's were designed (now I could be wrong here...) for
GG service and very easy to achieve a clean 10 to 12 DB gain figure in
that configuration ...

Any how, I have used the 4-400's in GG service and they must have the
screens powered to achieve  a good clean  output.

Just my slant on the subject.  4-400's, 3-500's, whatever your budget fits
have at it, but remember, the 500's, except for their pricing, give more bang
for ones hard earned buck...

[KE6DF]

Here is a little background on what led to the question.

I mentioned in another thread I was thinking of building a small TX that could be used on 10M.

I also intend the transmitter to be used as an exciter on other bands to drive a linear.

I was targeting 50W output at that time.

I have a few modulation transformers to choose from in the 40 - 75 watt range that would be suitable for that project.

But, since writing that post, I aquired a 20W UTC LS series modulation transformer.

I'm considering the possiblity of building a TX around the LS transformer using all broadcast quality components.

I would also use a Heising reactor to keep DC off the UTC Mod tran.

But that would mean lowering the input power to 40W or so, and the output to something like 25 - 30 watts.

So I was wondering if I could drive common tubes used for linears amps with such a low power exciter.

I'd probably use a pair of 6L6's modulating a 6146 powered by a TV power transformer (say 350VDC) for the low power exciter project.

Or should I go back to the 50 Watt goal and ham radio type modulation transformers?

I don't want an exciter that too PW to drive anything.  

[ke7trp]

Go for the low power rig. Most ham amps will be very happy with 25 watts of carrier input. Even a good running ranger is often to much drive for ham amps and you end up overcoupling or detuning the ranger.

I am not sure which ham amps you own but alot of the ham class amps that used 800s will also be pleased with 25 watts or less.

C

[kb3ouk]

i drive my Hunter 2000B (quad 572Bs GG) with a little over 15 watts, and that will run it to about 200 watts output.

[KM1H]

25-30W will send a pair of GG 4-400, 3-500, 4x 572B (Clipperton L, etc) or 813's (at least to 20M) to the max. Be sure to use a tuned input for linearity and max power transfer.

If you want to use old slugs such as 250/304 TH/TL then you will need a lot more drive, probably 100W for a pair. Makes good use of all those 100-130W rigs without needing an attenuator.

[WD8BIL]

Yes indeedee! 20 - 30W drive on most ham amps is gonna work fine for AM use.

17 watts from my T4X gets me 250W carrier from a pair of 8874s. 24W is 370W carrier!
You're right in the ballpark!

the 500's, except for their pricing, give more bang
for ones hard earned buck.

No arguement there, Carl!

[KM1H]

30W from a TS-950SD wound up with about 1200W out of a souped up LK-800. That was an interesting test!

One guy said I was 60 over about 200 miles away Not bad for a table top amp with the external Dahl transformer option

[ke7trp]

That amp was an AZZ kicker.  Its sold. It was the ISM with 14 pos band switch, cpx tubes and 3600 volt power pack.  It was used in a gov installation.  The only two things I did not like was the touchy tuning (all that high voltage) and the loud blower.

C

[flintstone mop]

I'm not sure if the original question was answered.
I do not know the PEP rating for the Heath amp.
A pair of 3-500Z was 1500 W Drive power needed for 1500W??

Tune and peak for that 1500W whatever is needed from the exciter and when you're ready to switch to AM mode, reduce the power of the exciter to produce 375W. Maybe the other posts were mentioning that the SB220 was perfect for 20 watt exciters.......dunno
I adjust for 300W carrier and let her rip.
I have an Ameritron AL1500 and it is not shy about giving me 2000 W PEP.
7W drive. But it is an entirely different animal with a ceramic tube and 3500 VDC on the plate.

[KM1H]

The SB-220 is a 2000W PEP INPUT amp for light duty use, the transformer is only 500-600VA depending upon what Dahl catalog you read. Its NOT a tube issue.