Power output for Zetagi B300P with attenuator

So, after getting my Zetagi B300P repaired I am now trying to work out exactly what power it puts out from the 6 position switch.

I was running the amp directly from my Grant 2 which puts out just over 20w on SSB, however I was told this would overdrive the unit and blow it up again, something I don't want to do.
I placed a Zetagi RP2 attenuator in between rig and amp. It has a 2 position rocker switch which attenuates at 1/4 or 1/2. I assume that means the unit attenuates the rig output power to the amp by either 25% or 50%. Currently I use the 50% position and assume that this only allows 50% of 20W which in an input to the amp of 10W.
Please correct me if so far I have got this wrong!

Now onto the 6 position switch on the B300p.
I assume, and assumptions are always dangerous which is why I am taking some advice from you good guys on here with more experience.
My assumption is that the amp output power is divided, either equally or not, by 6 and each switch position puts out 1/6th more power for each position.
Doing my basic maths the following might apply:-
Pos1 = 50w (300w divided by 6 = 50w each position)
Pos 2 = 100w
Pos 3 = 150w
Pos 4 = 200w
Pos 5 = 250w   (I was advised not to run pos 5+6 on FM/SSB)
Pos 6 = 300w.
Does this sound about right?
I am sure there must be some diabolically hard and complex calculation that will give the answer, alas I have not found it yet.

Today I ran the amp on position 5 using the attenuator set at 1/2 (50% power reduction), for a quick qso on UKFM lasting no more than 5 minutes. I could smell the same electrical burning smell I had when I burned the pre amp out.
As the amp has not been used very much at all, should it smell like burning until it's been used for a while???. A bit like a new car smells for a while untill the kids get in it and fart and puke in it then the nice new smell goes away.

It is home based on a 30A mains power supply and the antenna is a Sirio Gainmaster 5/8 with a SWR of 1.1:1.
I intend to transfer it to my mobile station wired through a fag lighter socket and currently use a Tank Whip. I will also be using a T2LT.
Should the amp be "hardwired" to the car battery rather than fag socket?

I appreciate that there is an awful lot of waffle above. I am just trying to give a clear picture of set up so answers will be easy!

Thanks in advance.
Simon.
CT2379.
12.27.2019
22:48:01

Hello Simon,

I would have thought the best way to check any power levels for this or any other piece of kit is to use a decent power/watt meter and a 50 Ohm dummy load capable of handling the amount of power your testing. I think it's unlikely that the six power levels are in equal 50W increments.

The burning smell is indicative of something getting too hot and not a good sign. Some people put fans on these things to help with cooling, they are well known for overheating.

If you put this amp in your car you will need to wire it directly to your battery, the fag lighter socket will not provide enough current for it and you will probably smell burning again this time the wiring to the socket!!

73's Gary.

I can't remember the size of the "steps" on the amp - the 6-position switch basically switches out resistance to attenuate the input signal - but I wouldn't assume them to be equal steps. In any case, all amps eventually go into "compression" - where the gain (and linearity on SSB) of an amp starts to decrease above a certain power input. 10W PEP on SSB will give you around 250W out and it should be fairly clean. Don't put in any more than about 5W on FM/AM on position 6 for about 120W out, and don't try to drive the amp above that sort of level. You can screw a bit more out of the amp but especially on SSB things start to get messy.

A common feature of these amps used to be that the low-power-rating resistors on the attenuator burn out - before any abuse (ie putting too much wattage in) actually took out the transistors.

As Gary says, wire directly to the battery and use the shortest and thickest cable you can - you can get 10AWG (American Wire Gauge -the lower the number, the thicker the cable) cable quite easily. Thicker cable = less voltage drop = a better performing and longer-lived amp.

Hi Simon,

Gary's right if you want to go full depth analysis of your gear and the way any decent Ham or RF tech would go.

You can go shelling out for dummy loads and RF wattmeters, use them once, never have a use for them again and ultimately be disappointed with what you find......

The first thing you'll find is that your linears are not pushing anywhere near 300 watts....

I won't get into the whoop-ass arguments about P.E.P (peak envelope power) or other such nonsense.

You are getting much more power out than you put in so be happy with that

The front switch labelled 1 to 6 is a simple resistor attenuator control to limit how much power you put into the linear, 1 being the most attenuation, 6 being straight through. It's "sort of" a power control.
(Those resistors can get really hot too, I've seen them blackened to hell!)
It's pretty much the same job as your RP2 is doing. You ain't ever going to figure it out, just accept 1 is the lowest and 6 is the highest.

Heat - you're gonna get heat.
Those front end resistors are getting hot and so are the transistors and that heat has to go somewhere. You just gotta hope the heatsink has a good enough airflow to take that heat away. As Gary has again mentioned, people put fans on them to help draw that heat away.
(Again , I won't get into the poor efficiencies of those transistors, the push-pull circuitry or losses through the class B biasing!)
Put it this way, my rig has a similar circuit of those linears built right into the radio....most of the radio casing is a big-ass heatsink, with ducting and a fan....designed for Hams. Hams are notorious for rag chew long overs

You're also right, they're gonna smell a bit funny to start with just a like a new oven does.

I'd run the RP2 at the quarter setting and the linears nearer 6 for best results. You should be able to chew the rag all day long.
(Don't do the trucker trick of running them at 24 volt - you'll get more power, just briefly before they blow up!)

As for wiring to the car, Gary again is right, (he is a helpful bloke)
DON'T use the ciggie lighter socket! You have been warned.
My rig at full power runs some 19 amps, your homebase is running on a 30 amp pack, figure for direct to battery wiring along those lines. Most guys run this sort of wiring for their big-ass music systems so there's plenty of info out there.

Just a word of warning - I've known guys park on top of a hill, run their linears on a night time DX or net, pack up for the night and the car won't start........flat battery!! Take a spare or jump pack with you just in case

Sheesh, waffled as long as you...hee, hee

Anyway, hope that's of some help.


All the best,
Victor

Ah, someone else popped in with helpful advice too

This forum's great, haven't seen a wildly wrong piece of advice yet!

You're lucky Simon.....go to somewhere like eHam and you'll be more lost than anything. That's if they haven't berated you for asking a "CB" question or wrapped you in the middle of a flaming war.

Go Charlie Tangos!
Proud to be a member

Thanks fellas all good advice, Victor that's what I've always liked about this forum, there's absolutely no snobbery or acrimony between C.B.ers and Amateurs we all jog along together just fine. I think many of us here do both anyway after all radio is radio.

73's Gary.

Spot on Gary. Loving this forum for just such things, as you say radio is after all radio.

I was many moons ago one of those whizz-kid electronics enthusiasts with a passion for radio and took it upon myself to join a local Ham club......all went to pot when I took up CB, pretty much shunned by the club for my "illegal" activities
Landed up joining a CB club, Foxtrot Papas - their motto was "Friendly People" and never looked back.

If only it had all been a bit more harmonious back then, might have taken my ticket years ago.
Just glad the times have mostly changed.

All the best to you mate

Gary and Victor.
You have both enlightened me, thank you very much for taking the time th help me out. I appreciate your honest and forthright advice.
This forum is indeed a good place to be. I have been elsewhere to try to find answers and failed miserably, been baffled with science and confusing/contradictory links....this place is the first port of call from now on.

I will take all those pointers above on board and can now start to make some headway in setting up my portable set up with a clearer picture of what I need to do and why I am doing it.
That's enough waffling for now, I have already done enough!

Very much hope to have a QSO on the radio one day soon.
Kind regards.

Simon.