Need help with Homebrew linear toob selection

[Glenn NY4NC]

I'm not worried about wasting electricity, but 20% effeciency is a bit silly! 

I suppose if we were really worried about wasting electricity, we wouldn't be on the air at all, or even have our receivers powered.

[W2VW]

I'm not worried about wasting electricity, but 20% effeciency is a bit silly! 

I suppose if we were really worried about wasting electricity, we wouldn't be on the air at all, or even have our receivers powered.

You lost me. What is 20% efficient??

[WA1GFZ]

Who cares about efficiency when the basement shack is under 65 and heat rises.

[Glenn NY4NC]

Excuse me, not 20%, but  17%  ...2500 watts input 375 watts output.

Steve's comment

"You can run 2.5 kW on AM. You can run as much input as you want. It's the output the FCC measures."

How you say.... tounge in cheek? 

I'm not worried about wasting electricity, but 20% effeciency is a bit silly! 

I suppose if we were really worried about wasting electricity, we wouldn't be on the air at all, or even have our receivers powered.

You lost me. What is 20% efficient??

[Steve - WB3HUZ]

I think you are planning to run a linear there Glenn. Guess what, 20% efficient will be about it on AM. You can go for 25 or 30 but this leave little headroom for the audio peaks. Of course, this is 20% efficient only when there is no modulation. The efficiency goes up with modulation.

[K1JJ]

I think you are planning to run a linear there Glenn. Guess what, 20% efficient will be about it on AM. You can go for 25 or 30 but this leave little headroom for the audio peaks. Of course, this is 20% efficient only when there is no modulation. The efficiency goes up with modulation.

You wanna hear even more ridiculous efficiency?

My new linear amp is gonna have three driver stages, all running pure class A, grid driven.  The efficiency is down around 10% for them.  Usually, take the power dissipated in the plate and multiply it by 10% to come up wid the power out in class A.

A 1W op amp at 500mV out, driving a  6550 tube  running 20W resting idle, putting out 10 volts swing, driving a  4CX-250 running 125W resting idle, putting out 300V monkey swing - all sweating it out to cleanly drive a 4CX-XXXX with no grid current.  The final runs AB1, thank God -  with neg feedback.  Sick, huh?

Caw mawn.

[Glenn NY4NC]

Exactly what I was thinking Steve when I hit the enter key... PEP vs CW carrier...

1500w pep output w/2500w input... dats looks better... so it's 17% efficient with no modulation and 40% efficient on modulation peaks??

I think you are planning to run a linear there Glenn. Guess what, 20% efficient will be about it on AM. You can go for 25 or 30 but this leave little headroom for the audio peaks. Of course, this is 20% efficient only when there is no modulation. The efficiency goes up with modulation.

[Glenn NY4NC]

Whats the anode voltage and current on that 4CX-xxxx  T?  also, what's the max carrier output?

I think you are planning to run a linear there Glenn. Guess what, 20% efficient will be about it on AM. You can go for 25 or 30 but this leave little headroom for the audio peaks. Of course, this is 20% efficient only when there is no modulation. The efficiency goes up with modulation.

You wanna hear even more ridiculous efficiency?

My new linear amp is gonna have three driver stages, all running pure class A, grid driven.  The efficiency is down around 10% for them.  Usually, take the power dissipated in the plate and multiply it by 10% to come up wid the power out in class A.

A 1W op amp at 1V out, driving a  6550 tube  running 20W resting idle, putting out 25 volts swing, driving a  4CX-250 running 125W resting idle, putting out 300V monkey swing - all sweating it out to cleanly drive a 4CX-XXXX with no grid current.  The final runs AB1, thank God -  with neg feedback.  Sick, huh?

Caw mawn.

[K1JJ]

Whats the anode voltage and current on that 4CX-xxxx  T?  also, what's the max carrier output?

Glenn,

Well, the 6550 driver tube will have  250V and the 4CX-250 will have 1500V on them. The 1500V is the same regulated supply that is also used for the 4CX-XXXX screen... kinda slick.

Notice how conservative the demand is on the drivers... the 6550 is a 50W? tube with 250V - and only 10V swing needed, no power -  the 4CX-250 is a 250W tube with 1500V and only 300V swing is needed out, no power.  We can get clean IMD figures in the -60db++ range like this, so I'm told.

The 4CX-XXXX will run xxxx volts at x amps = x * xxxx  watts in AB1 at ~60% eff..     I'm hoping it will be up around -50 to -55 db 3rd IMD once 20db of feedback is looped back to the drivers. We'll see. 

An interesting point:

IMD does not add linearly between stages. The driver chain needs to be at least 6db cleaner than the final so that the final IMD degrades only about 1 db. ie, if the final is -50 db  and the driver is -56db, then the overall IMD is -49db. 

If the final is the same IMD as the driver, then you will see a whopping -6db degradation overall. [-44]  So, you can see why I'm putting so much effort into a clean driver.

T

[Glenn NY4NC]

Vely interesting!... That's a lot of effort... I have to wonder if there will be a noticable/audible difference compared to the standard pair of 3-500Z's... the artifacts were gone after 5kc from what I remember reading in your post..

[W2VW]

 Real world amplifier numbers. 1000 Watts DC input gives you 300 Watts carrier out and enough headroom for 100% positive. If not then something isn't right.
2500 Watts P.E.P. input will give you 1500 Watts P.E.P. output AM or ssb.

Check your calculator batteries.

[W2VW]

Real world amplifier numbers. 1000 Watts DC input gives you 300 Watts carrier out and enough headroom for 100% positive. If not then something isn't right.
2500 Watts P.E.P. input will give you 1500 Watts P.E.P. output AM or ssb.

This doesn't apply to Tom Vu's current expelliment as it will be unusually clean.

Check your calculator batteries.

[Steve - WB3HUZ]

1500w pep output w/2500w input... dats looks better... so it's 17% efficient with no modulation and 40% efficient on modulation peaks??

Something like this. At 100% modulation or so, you should achieve the maximum efficiency capability of the amp (most GG AB2 linears are in the 50-60 % range). Theory states a Class B amp can have, at best, 78% efficiency. Real world loss keep that down in the 60% range (with AB1 and AB2 lower due to higher idling currents).

[K1JJ]

Vely interesting!... That's a lot of effort... I have to wonder if there will be a noticable/audible difference compared to the standard pair of 3-500Z's... the artifacts were gone after 5kc from what I remember reading in your post..

Glenn,

Well, as you know, there's no way to hear the difference on frequency between a -30db or -55 db 3rd order signal.   Someone here once mentioned that -30db 3rd equates to 0.1% distortion level.  It's all in the sideband trash.  I was listening last night to my 5mW ssb signal and it went from S9+40 over to below S1 only 3kc away. That is probably -65db++ 3rd.  But when I turned on the FT-1000D 200w amp and ran it up to only 10W, the trash [normalized] was S9+5  3kc away. Huge difference.  I could still hear it up 4.5 kc.  The 3-500Z amp is even worse than the stock FT-1000D, by about 5db or so.

Just to prove it was not receiver overload, I was able to turn the 5mW level up to 15mW and found the IMD degraded to the same as the 10W amplifier. Running class A and then demanding only 1/10 the power capacity is the ticket to cleanliness. But, the problem is the final. To get 1500W out of class A final requires a 4CX-15,000. Big waste. So a smaller final has to be run class AB1 for a tetrode or AB2 for a hi mu triode. Neg feedback seems to be the only way to bring that tetrode final up to the clean driver IMD level...   At least the GG amp has some inherent NFB.

Anyway...  I think one thing not many guys think about is the varying input impedance of a linear. Just like driving a triode modulator tube, you need a driver [or exciter] that can handle the load variation when going into a cathode driven GG amp. That cathode may vary from 50 to 30 ohms or whatever. I saw it on my own 3-500Z amp when I stuck a Bird in the input.  This will degrade IMD.

So, I'm thinking that an AB1 tetrode has this advantage... no grid current, thus a stable input grid load for the driver, unlike a GG cathode driven amp.

T

[Glenn NY4NC]

"1500w pep output w/2500w input... "  67% efficient...

yup yup yup yup!!

Real world amplifier numbers. 1000 Watts DC input gives you 300 Watts carrier out and enough headroom for 100% positive. If not then something isn't right.
2500 Watts P.E.P. input will give you 1500 Watts P.E.P. output AM or ssb.

Check your calculator batteries.

[Glenn NY4NC]

I guess I was thinking that you are hearing those sidebands depending on the bandwidth of the receiver ??

Glenn,

Well, as you know, there's no way to hear the difference on frequency between a -30db or -55 db 3rd order signal.   Someone here once mentioned that -30db 3rd equates to 0.1% distortion level.  It's all in the sideband trash. 

[W2VW]

"1500w pep output w/2500w input... "  67% efficient...

yup yup yup yup!!

Real world amplifier numbers. 1000 Watts DC input gives you 300 Watts carrier out and enough headroom for 100% positive. If not then something isn't right.
2500 Watts P.E.P. input will give you 1500 Watts P.E.P. output AM or ssb.

Check your calculator batteries.

Try another set of new batteries.

[Glenn NY4NC]

I don't know Dave, maybe your calc is in reverse polish notation mode,  2500w input 1500w output = 67%

2500w input 1250w out = 50%... maybe I'm not wearing my tinfoil hat today??

"1500w pep output w/2500w input... "  67% efficient...

yup yup yup yup!!

Real world amplifier numbers. 1000 Watts DC input gives you 300 Watts carrier out and enough headroom for 100% positive. If not then something isn't right.
2500 Watts P.E.P. input will give you 1500 Watts P.E.P. output AM or ssb.

Check your calculator batteries.

Try another set of new batteries.

[K1JJ]

I guess I was thinking that you are hearing those sidebands depending on the bandwidth of the receiver ??
[

Glenn,

Well, once you do a particular RX bandwidth setting, you keep it there and do relative comparisons against the standard.  You can use whatever bandwidth you want as long as the good and bad products sound totally different.

In my case, I used 2.8kc bandwidth on the receiver. The transmitter had a recorded voice as a program. When clean, I could hear the voice as clean tones that disappeared into the noise after it's ~3kc audio normal bandwidth.  However the trash has a distinctly different sound. When it is there, it is a raspy, buzzy sound that extends at and past the normal 3kc channel.   As you turn up the drive and the signal goes from clean to poor, the raspy trash products begin to get louder and creep higher up the band.

There are some that say a good reciever with a good ear is a BETTER instrument than a spec analyzer for identifying  an IMD problem. I happen to agree with that. The front end and other parameters can be better in a good RX than the spec anal  too.  Heck, your receiver can be used just like a spec analyzer-  using a two tone in the TX and identifying the 3rd and 5th tone mix where it should be on the dial. Use the atten pad to find it's value from one of the peak two tones to the 3rd or 5th you identify. You can actually come up with real IMD numbers. There is an actual  procedure written if you're interested.

Try listening on your own signal and see.  For a simple setup, put a 6" piece of antenna wire on the back of your receiver and dump the ssb TX into a dummy load.   Use a step pad and terminated 50 ohm pickup for the RX  if you have one for better control.  Set the RX at S9+40 over and see how quickly the trash disappears as you tune on your own signal.

Then listen to signals on the air that you normalize to S9+40. After awhile, you can tell who is clean and who is not... in a RELATIVE way. I would guess signals on the air probably average around -31db 3rd IMD. Some much better, some far worse.  You can hear it when someone is running a -45+ db transmitter. Like tuning a CW note... :-)

T 

[WA1GFZ]

you need to use the cw filter when using a rx as a spectrum analyzer this is because the bandwidth is too wide. then correct for the bandwidth. dB=10 Log BW in hz.
subtract correction factor  from reading. a wider filter has a higher correction.

[K1JJ]

you need to use the cw filter when using a rx as a spectrum analyzer this is because the bandwidth is too wide. then correct for the bandwidth. dB=10 Log BW in hz.
subtract correction factor  from reading. a wider filter has a higher correction.

Yes, that makes good sense.  Taking actual measurments on weak tones requires sharp selectivity.

T

[WA1GFZ]

A Step attenuator and RMS volt meter on the audio will provide much more accurate numbers.  You know you have a bad method when nimbers change from day to day.

[Glenn NY4NC]

I understand... If you use a wide bandwidth filter, You are hearing (seeing) everything (the trash + voice) at once, with a narrow CW filter you can tune across the signal to listen specifically to the IMD products.

So IMD is more of a co-channel interference issue, not so much distortion to the actual tuned-in signal/voice

I know with audio, it's all within the "passband" of what your listening to...