SWR & TUNDERBOLT 305

[Swamp Yankee]

Hello All,
I finally was able to get up and running today and did a little testing before I headed out for work. The following are my results. Please comment if you can.
1) Royce 642 + Dosy + Max 2000 W/ Optimizer. SWR on channels 1-40 is 1.5. How I understand it, that's good.
2) Royce 642 + Thunderbolt 305 + Dosy + Max 2000 W/ Optimizer. SWR on channels 1-40 is now reading about 2.0.
3) My questions? Is the Thunderbolt 305 creating a false SWR? Is my True SWR really 1.5? Should I be concerned? I do not want to damage any equipment.
4) What is a safe Wattage to drive the Thunderbolt 305? Currently the DK on the Royce is 4 Watt AM / 12 Watt SSB.

Thank You So Much

 

[BC Coyote]

An SWR of 1.5 barefoot is fine. No problem there. It's normal for SWR to rise a little with more power output, but 1.5 to 2.0 is an abnormally high increase, so something is not right somewhere.

The input tuning on the Thunderbolt may be the problem, so you also need to check the SWR going INTO the amp. Set the gear up like this...
radio ~ SWR meter ~ amp ~ antenna. Turn amp on. Now check SWR of amp input. If it's showing higher than 1.5 or so you may need to modify the amp input tuning. Some amps will have a trimmer or two that can be tweaked to do that, but many amps don't and component changes may be required.

If the input tuning checks out, then the other possibility is that the amp is generating harmonics on other frequencies than the fundamental one, and those harmonics are creating a high SWR. Low pass filter after the amp will help IF that's the problem (unless of course it already has one). Make sure the filter can handle the max output of the amp.

There are others on this forum WAY more knowledgeable about tube amps than I am, so I will let them chime in about drive levels.

 

[Swamp Yankee]

An SWR of 1.5 barefoot is fine. No problem there. It's normal for SWR to rise a little with more power output, but 1.5 to 2.0 is an abnormally high increase, so something is not right somewhere.

The input tuning on the Thunderbolt may be the problem, so you also need to check the SWR going INTO the amp. Set the gear up like this...
radio ~ SWR meter ~ amp ~ antenna. Turn amp on. Now check SWR of amp input. If it's showing higher than 1.5 or so you may need to modify the amp input tuning. Some amps will have a trimmer or two that can be tweaked to do that, but many amps don't and component changes may be required.

If the input tuning checks out, then the other possibility is that the amp is generating harmonics on other frequencies than the fundamental one, and those harmonics are creating a high SWR. Low pass filter after the amp will help IF that's the problem (unless of course it already has one). Make sure the filter can handle the max output of the amp.

There are others on this forum WAY more knowledgeable about tube amps than I am, so I will let them chime in about drive levels.

Thank You!!! Will give it a try...

 

[BC Coyote]

One quick question.... Are you using a cross-needle type SWR meter or the more common single needle type ? If you have a single needle meter you will need to recalibrate the meter every time you switch from the radio to the amp. You will need to do that every time you move the meter from one radio or amp to another.

 

[Swamp Yankee]

One quick question.... Are you using a cross-needle type SWR meter or the more common single needle type ? If you have a single needle meter you will need to recalibrate the meter every time you switch from the radio to the amp. You will need to do that every time you move the meter from one radio or amp to another.

Hi,
Single needle...Will make sure I'm doing just that...I'm also curious if a Dummy Load will make a difference. I'll try that as well...Thanks.

 

[nomadradio]

The radio most likely puts out a fairly clean signal, without a lot of extra frequencies besides your channel frequency coming out of it.

The amplifier will be pushing harmonic frequencies, double, triple and more times the channel frequency. The antenna is not designed to efficiently absorb energy at 54, 81, 108 and higher frequencies. The SWR for those frequencies will be high, and the meter only shows you the sum total of them all.

It's mostly a waste of time to check SWR with an amplifier keyed up. The reading you get with the radio alone is the only accurate reading you'll see.

Inserting a low-pass filter in line between the amplifier and the SWR meter will reduce the SWR reading.

73

 

[Swamp Yankee]

Hi,
Single needle...Will make sure I'm doing just that...I'm also curious if a Dummy Load will make a difference. I'll try that as well...Thanks.

Hello,
Busy Busy morning. I was able to do more testing this morning per our conversation. The line up's / results are below. Please feel free to comment. Thank You.

1) Radio + SWR + AMP + Dummy Load = SWR 1.2
2) Radio + AMP + SWR + Dummy Load = SWR 1.5
3) Radio + SWR + Dummy Load = SWR 1.2

4) Radio + SWR + AMP + ANT = SWR 1.2
5) Radio + AMP + SWR + ANT = SWR 2.0..."HOWEVER" I was able to "TUNE THE AMP" by turning "Drive" "Load" "Tune" controls and was able to get a SWR of 1.5. HOWEVER, the OUTPOUT of the Amp was reduced by 50% or so. I also understand that De-Tunning Tube Amps is not advised.
6) Radio + SWR + ANT = SWR 1.5

As Always!!!! Thank You!!!

 

[Swamp Yankee]

The radio most likely puts out a fairly clean signal, without a lot of extra frequencies besides your channel frequency coming out of it.

The amplifier will be pushing harmonic frequencies, double, triple and more times the channel frequency. The antenna is not designed to efficiently absorb energy at 54, 81, 108 and higher frequencies. The SWR for those frequencies will be high, and the meter only shows you the sum total of them all.

It's mostly a waste of time to check SWR with an amplifier keyed up. The reading you get with the radio alone is the only accurate reading you'll see.

Inserting a low-pass filter in line between the amplifier and the SWR meter will reduce the SWR reading.

73

Thank You...I will order that tonight...

 

[Swamp Yankee]

Well, so much for the filter, that didn't work. Going to start looking at the Tunning of the Antenna & Coax a little closer...Just ordered a Rig expert AA-35 Zoom. I've been using a Dosy & MFJ...If that doesn't work, I'll buy another Antenna...