OT: modifying your amp for lower drive requirement
Locked
OT: modifying your amp for lower drive requirement
 
|
Almost 30 years ago now, before I was 99.9999% a QRPer, I had an Alpha
76CA (3 tubes). It had a giant swamping pad on the input (I still have it - the swamping pad, that is). Just take it out and like magic, you could drive the amp with QRP. My intention was never to exceed legal limit, just to be able to use the amp when driven with a QRP rig. I don't remember how much out I could get with QRP in, but it was nowhere near 1500 watts. I have no clue what they have on the inside of the KPA500, but if there is a swamping pad, you can probably take it out. What is Elecraft's motto? Hands-on radio? :-) When I used to use a Ten Tec Titan Amp, it required 36 watts of drive for 1500 watts out on 20m. That was probably 25 years ago. Finally, if you want to fly under the radar, there are certain high quality amplifier manufacturers overseas (closer to W6 than W2...hint hint) who will custom make you an amp for low power drive. Check the FT817 Yahoo group archives. de Doug KR2Q had my fill of QRO ages ago ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
Re: OT: modifying your amp for lower drive requirement
|
|
It's very unlikely any solid-state amplifier uses an input attenuator: Transistors and FET's just don't have that much gain.
Grid-driven tube amplifiers may indeed have enough gain (without the input pad), but there are a couple potential problems. Without the input pad, the amplifier may not be stable and/or its input impedance may not be anywhere near 50 Ohms. If that isn't a problem, spurious outputs may be. Spurious outputs from a QRP transmitter that cause no trouble barefoot might be way above the FCC limits when amplified by 25 dB. 73, Scott K9MA On Dec 20, 2010, at 4:09 PM, DOUGLAS ZWIEBEL wrote: > Almost 30 years ago now, before I was 99.9999% a QRPer, I had an Alpha > 76CA (3 tubes). It had a giant swamping pad on the input (I still > have it - the swamping pad, that is). Just take it out and like > magic, you could drive the amp with QRP. My intention was never to > exceed legal limit, just to be able to use the amp when driven with a > QRP rig. I don't remember how much out I could get with QRP in, but it > was nowhere near 1500 watts. > > I have no clue what they have on the inside of the KPA500, but if > there is a swamping pad, you can probably take it out. What is > Elecraft's motto? Hands-on radio? :-) > > When I used to use a Ten Tec Titan Amp, it required 36 watts of drive > for 1500 watts out on 20m. That was probably 25 years ago. > > Finally, if you want to fly under the radar, there are certain high > quality amplifier manufacturers overseas (closer to W6 than W2...hint > hint) who will custom make you an amp for low power drive. Check the > FT817 Yahoo group archives. > > de Doug KR2Q > had my fill of QRO ages ago > ______________________________________________________________ > Scott Ellington Madison, Wisconsin USA ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
|
|
In reply to this post by DOUGLAS ZWIEBEL
Well... keep in mind that an input attenuator also reduces the SWR seen by the exciter.
Even a relatively small 3 dB pad means that the SWR can't go above 3:1 even if the input impedance of the amplifier was zero or infinite! On 12/20/2010 2:09 PM, DOUGLAS ZWIEBEL wrote: > Almost 30 years ago now, before I was 99.9999% a QRPer, I had an Alpha > 76CA (3 tubes). It had a giant swamping pad on the input (I still > have it - the swamping pad, that is). Just take it out and like > magic, you could drive the amp with QRP. My intention was never to > exceed legal limit, just to be able to use the amp when driven with a > QRP rig. I don't remember how much out I could get with QRP in, but it > was nowhere near 1500 watts. > > I have no clue what they have on the inside of the KPA500, but if > there is a swamping pad, you can probably take it out. What is > Elecraft's motto? Hands-on radio? :-) > > When I used to use a Ten Tec Titan Amp, it required 36 watts of drive > for 1500 watts out on 20m. That was probably 25 years ago. > > Finally, if you want to fly under the radar, there are certain high > quality amplifier manufacturers overseas (closer to W6 than W2...hint > hint) who will custom make you an amp for low power drive. Check the > FT817 Yahoo group archives. > > de Doug KR2Q > had my fill of QRO ages ago > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html -- Vic, K2VCO Fresno CA http://www.qsl.net/k2vco/ ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
|
|
In reply to this post by DOUGLAS ZWIEBEL
On 12/20/2010 2:09 PM, DOUGLAS ZWIEBEL wrote:
> Almost 30 years ago now, before I was 99.9999% a QRPer, I had an Alpha > 76CA (3 tubes). It had a giant swamping pad on the input (I still > have it - the swamping pad, that is). Just take it out and like > magic, you could drive the amp with QRP. Grid-driven vacuum tube amplifiers can exhibit much more gain than the grounded-grid triodes we are all very familiar with. The problem is that, while grounded-grid amplifiers are "sort of" inherently stable [except for parasitics], grounded-cathode amplifiers are not. Their input impedance is also variable and can be nowhere near 50 ohms. The input pad corrected that and help stabilize the amplifier. The [in]famous B&W "all band folded dipole" capitalized on this -- big 50 ohm load resistor = 50 ohms everywhere. I still see them around National Guard Armories. At an AFB club station, we had a tribander that was 50 ohms everywhere. Coax was full of water. Needless to say, it was a lousy antenna. > > I have no clue what they have on the inside of the KPA500, but if > there is a swamping pad, you can probably take it out. What is > Elecraft's motto? Hands-on radio? :-) I'd be stunned if that was the case. > > When I used to use a Ten Tec Titan Amp, it required 36 watts of drive > for 1500 watts out on 20m. That was probably 25 years ago. The current FCC gain limit in the US for commercially manufactured and marketed linear amplifiers is 15 dB. We can all do the math. > > Finally, if you want to fly under the radar, there are certain high > quality amplifier manufacturers overseas (closer to W6 than W2...hint > hint) who will custom make you an amp for low power drive. Check the > FT817 Yahoo group archives. See above if they are to be legally marketed. And, that sounds pretty scary. Many QRP radios have adequate spur suppression and phase noise ... at their normal low power outputs. However, their spurs and phase noise get amplified along with the desired signal. Raise the phase noise by 15 dB [or more if the amp is illegal], and you're probably putting a lot of trash into your neighbor ham's receiver. 73, Fred K6DGW - Northern California Contest Club - CU in the 2011 Cal QSO Party 1-2 Oct 2011 - www.cqp.org ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
Re: OT: modifying your amp for lower drive requirement
|
|
Fred and all,
That is something to be heeded. The Elecraft example is that the K2 key-clicks were not a problem until the KPA100 was introduced. The added gain of the KPA100 caused that problem to be amplified (along with the signal). Elecraft responded with the Keying Waveshape Mod kit which corrected the situation. 73, Don W3FPR On 12/20/2010 7:57 PM, Fred Jensen wrote: > > See above if they are to be legally marketed. And, that sounds pretty > scary. Many QRP radios have adequate spur suppression and phase noise > ... at their normal low power outputs. However, their spurs and phase > noise get amplified along with the desired signal. Raise the phase > noise by 15 dB [or more if the amp is illegal], and you're probably > putting a lot of trash into your neighbor ham's receiver. > > 73, > Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
Re: OT: modifying your amp for lower drive requirement
|
|
In reply to this post by Scott Ellington
"It's very unlikely any solid-state amplifier uses an input attenuator:
Transistors and FET's just don't have that much gain." The Ameritron ALS-600 has about a 15 ohm input impedance. Then they put a series 35 ohm resistor for matching and reducing gain. Phil - AD5X ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
Re: OT: modifying your amp for lower drive requirement
|
|
Phil,
Your statement amplifies the difference between "ham assumptions" and reality. The data-sheet indications of gain do not consider the input impedance (and loading of the driver), that parameter is quite important in actual use -- if the amplifier does not provide a good load to the driving transmitter, then "all bets are off". 73, Don W3FPR On 12/20/2010 8:28 PM, Phil & Debbie Salas wrote: > "It's very unlikely any solid-state amplifier uses an input attenuator: > Transistors and FET's just don't have that much gain." > > The Ameritron ALS-600 has about a 15 ohm input impedance. Then they put a > series 35 ohm resistor for matching and reducing gain. > Phil - AD5X > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
Re: OT: modifying your amp for lower drive requirement
|
|
In reply to this post by k6dgw
On 12/20/2010 4:57 PM, Fred Jensen wrote:
> The [in]famous B&W "all band folded dipole" capitalized on this -- big > 50 ohm load resistor = 50 ohms everywhere. I still see them around > National Guard Armories. You can see one in my back yard as well, 30 ft AGL. It's the best that I can do for 80m NVIS at this time. -- 73 de K2ASP - Phil Kane Elecraft K2/100 s/n 5402 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
|
|
In reply to this post by Don Wilhelm-4
CONTENTS DELETED
The author has deleted this message.
|
|
|
In reply to this post by Phil Kane-2
CONTENTS DELETED
The author has deleted this message.
|
Re: OT: modifying your amp for lower drive requirement
|
|
In reply to this post by Don Wilhelm-4
On 12/20/2010 5:08 PM, Don Wilhelm wrote:
> The Elecraft example is that the K2 > key-clicks were not a problem until the KPA100 was introduced. The > added gain of the KPA100 caused that problem to be amplified (along with > the signal). Elecraft responded with the Keying Waveshape Mod kit which > corrected the situation. Yes, and the TX phase noise from the K2/100 was not a problem until you used it to feed a power amp, at which time your ham neighbors started descending upon you. That happened to me when I moved in down the road from K6XX -- it took him about two days to let me know about it. And Elecraft's response was more great engineering in the form of the K3. 73, Jim Brown K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
Re: OT: modifying your amp for lower drive requirement
|
|
In reply to this post by Phil Salas
On 12/20/2010 5:28 PM, Phil & Debbie Salas wrote:
> "It's very unlikely any solid-state amplifier uses an input attenuator: > Transistors and FET's just don't have that much gain." > > The Ameritron ALS-600 has about a 15 ohm input impedance. Then they put a > series 35 ohm resistor for matching and reducing gain. Actually, input attenuators are fairly common in power amps. It is one of two good ways to reduce power amp gain to meet the FCC requirement. Another good method is to use a bit more negative feedback, which has the added benefit of improving linearity (that is, reducing distortion/splatter/harmonics) and reducing noise. As others have noted, most output stages are cleaner if run a bit below their maximum possible output. While one could safely pick up several dB of gain by removing the input attenuator, I'd still be careful not to overdo that, I wouldn't try to reduce the feedback, and I'd run the amp pretty close to it's published rating. 73, Jim Brown K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
Re: OT: modifying your amp for lower drive requirement
|
|
In reply to this post by Phil Salas
Sorry, I should have said, "...uses an input attenuator of more than a few dB." They help with impedance matching and stabilization, so you probably couldn't take them out anyway. In any case, it would only reduce drive requirement to maybe 30 W.
73, Scott K9MA On Dec 20, 2010, at 7:28 PM, Phil & Debbie Salas wrote: > "It's very unlikely any solid-state amplifier uses an input attenuator: > Transistors and FET's just don't have that much gain." > > The Ameritron ALS-600 has about a 15 ohm input impedance. Then they put a > series 35 ohm resistor for matching and reducing gain. > Phil - AD5X > > ______________________________________________________________ Scott Ellington Madison, Wisconsin USA ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
|
|
In reply to this post by DOUGLAS ZWIEBEL
"The Ameritron ALS-600 has about a 15 ohm input impedance. Then they put a series 35 ohm resistor for matching and reducing gain." This is exactly right. The MRF150 MOSFETs used in ALS600 are rated at 20dB. I know people who homebrewed similar amplifier without series resistor and have full 20dB of gain - no problem whatsoever, stable like a rock. Igor, N1YX ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
Re: OT: modifying your amp for lower drive requirement
|
|
Of course, 20 dB of gain is not legal in the US, at least for commercial amplifiers, so it makes sense that Ameritron would use an attenuator, as it also saves them the cost of a matching transformer.
Scott K9MA On Dec 21, 2010, at 12:08 PM, [hidden email] wrote: > This is exactly right. The MRF150 MOSFETs used in ALS600 are rated at 20dB. I know people who homebrewed similar amplifier without series resistor and have full 20dB of gain - no problem whatsoever, stable like a rock. Scott Ellington Madison, Wisconsin USA ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
|
|
This post was updated on .
Luckily - most of the world is not like the US and gains of over 15dB are legal and common in non-us amplifiers. Modern FETs have much more gain than older devices and it would be quite normal to have 20dB gain from a stage - even after some feedback. There is an interesting design in the latest Dubus for a 1kW 2m amplifier that requires only 5W input. That is tetrode territory.
The popular THP 1.2KFX and 1.5KFX amplifiers use a 6dB input attenuator. Bypassing this allows you to drive them with less power under 25W, though it is not encouraged. The attenuator serves a couple of other purposes - it prevents serious over-drive with a 100W rig and it improves the input match. Incidentally - someone said with a 3dB pad the VSWR can't be more than 3:1 - that is only true if the PA does not have a negative impedance or to put it another way, return gain. That can happen when an amplifier takes off. Mike |
«
Return to [QRO]
|
1 view|%1 views
| Free forum by Nabble | Edit this page |