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W1VD
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« Reply #25 on: June 24, 2010, 03:16:54 PM »

I'll make the measurements if someone provides an amplifier.

Some folks fixate on the 3rd order products (because they're the most prominent and the ones normally reported) and don't pay attention to what's going on further out. It's the 5th, 7th, 9th etc that make the signal really wide. An amplifier with a bit higher 3rd order products (within reason) but with a rapid attenuation slope of the higher order components is preferable to the opposite condition.

Device types / amplifiers exhibit widely varying slopes so each design really should be analyzed. Idling current, device matching and proximity to saturated output influence IMD level and attenuation slope.   

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« Reply #26 on: June 24, 2010, 03:35:50 PM »

Yep, Agreed.  When I say 3rd order, I really mean them all, but it wud be too much to type... Wink  Maybe "IMD products" is a better phrase.

When I test my rigs for IMD, I use my reference FT-1000D driver barefoot to mark the crud levels up the band. It is very clean running at 20 watts pep. Then I add the amplifier and see how much it changes, to the limit of the noise floor up the band. I've found my highly modified Henry running reduced power and voltage - and Dr. Love, generally do not add to the trash level more than a few db. That's only when they are running conservatively.  I did run some two-tone tests, but found by playing taped voice programming through the system it is more revealing  for the infinite variations in dynamic range added distortion vs: a steady tone. A voice, at certain "sour spot" audio frequencies to the rig, will make the rig dirtier from what I see.


I'd be curious as to what Bob/KBW's amp shows for efficiency, cuz he said last night the heatsink was getting rather hot. At 33%, you wud think it would. 300w of heat is no small job to sink.

Maybe he'll swing over to your place with his amp when it's ready.  I know lots of guys are interested in hearing about the specs.  I'd be one of the first to knock one out with 12 pills++ if it tests well..

T
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« Reply #27 on: June 24, 2010, 04:23:14 PM »

I know that I have to pad the input because of the expected PEP.  Not a problem.  Right now I'm just working on just the carrier and trying to clean things up and make the Retro happy. 

The schematic that came with the kit is different than what is on the website.  According to the assembly instructions some changes have occurred with the amp.   

Not sure how you determined that a .001uf is on the input transformer.  The only .001 in this kit is on the biasing portion of the circuit.
 
The input transformer has an 18pf on the primary and an 1100pf on the secondary.  The output primary has a 910pf  and the secondary has a 24pf. 

Should I still knock down the 18 and 1100 down by a factor of 10?
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« Reply #28 on: June 24, 2010, 04:52:07 PM »

Bob

Ok... It appears that the noisy schematic on Communication Concepts site makes "C2" look like "C7". I found a clean schematic and associated documentation on another web page of their site:

http://www.communication-concepts.com/appnotes/eb63300sharp.pdf

The values you listed, above, for the capacitors are reasonable.

Try putting a 50 ohm, non-inductive resistor in series with the output of the Retro 75, looking into the input of the amp. I.e. center-pin-to-center-pin, not center-pin-to-ground
.
Stu



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« Reply #29 on: June 25, 2010, 04:30:01 PM »

Bob,
I have a Motorola app note from 1995 that says 1100 pf on the secondary of the input transformer and 910 pf on the primary of the output transformer. My guess is your transistors and not a matched pair and one is hogging the bias. I have seen this effect playing with bigger 48 volt devices.
So one device may be in class B and the other AB. Usually there is a color dot on the device case. Both should have the same color dot for hfe match.
The efficiency will be around 50% so expect heat. I suspect EB63 isn't great for IMD or input VSWR without feedback. I can scan the app for you after the weekend if you remind me.
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K1JJ
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« Reply #30 on: June 25, 2010, 05:22:01 PM »

The efficiency will be around 50% so expect heat. I suspect EB63 isn't great for IMD or input VSWR without feedback. 


That's an interesting point. I wonder if the 11N90 linear amp could be made to work well (acceptable IMD) with a large amount of RF negative feedback?  Maybe a few 11N90's driving a group of twelve (6X6 p-p) with NFB around the whole bunch?

T
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #31 on: June 25, 2010, 06:51:54 PM »

Yes Tom, I have simulated a 22 FET linear with source resistors and RF feedback. It will do 5500 watts out on 75 with 130 volts on the drains. About 2400 with 80 volts. I have a 7000 square inch heat sink area all machined waiting for me to assemble it. IMD looks pretty good but need to run simulation in FFT mode that Jay just pointed out.
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« Reply #32 on: June 25, 2010, 07:02:05 PM »

This is using 11N90's ?  Should be quite the amp if it works well. Better than fils and blowers during hot wx.

T
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« Reply #33 on: June 26, 2010, 07:09:39 AM »

The transistors are matched according to the assembly instructions and the former packaging they came in.  I don't seem to have power hogging on one transistor.  They seem equal with an infrared thermometer and to feel.  I have the same app note too.  Heat not a  problem to deal with.  The MRFs have a nice hefty heat sink.  On the subject of heat sinks, if anyone is looking for a cheap hefty heatsink go to Allelectronics.com.  They have some 10" x 7" x 2" heavy duty heatsinks for $18.00.  They're used but beefy.  PN: HS-620

I guess what I'm having a hard time understanding is why the amp works fine with 2 other rigs but not the Retro75.  Unless I live in bizarro world I would think the Retro75 is the suspect rather than the amp. 

I'm going to try Stu's suggestion with the resistor and/or pad.  May even take a stab at messing with the network on the Retro75 by putting in trimmer caps.
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« Reply #34 on: June 26, 2010, 09:04:45 AM »

Bob

The transistors in the amplifier represent a very non-linear load for the Retro 75. I.e. the impedance from base to emitter of each transistor varies a great deal as its collector current varies. The emitters are connected directly to ground (no emitter resistors to provide a more stable resistance looking into the base of each transistor).

The Retro 75 output stage appears to be interacting with this non-linear load (which also has reactance) to produce the instability you are seeing.

The other transmitters you tried are apparently less sensitive to looking into this kind of a load. For example, the high Q tank circuit of the FT-102 is apparently acting to reduce the tendancy toward instability associated with the transistor amplifier. The Retro 75 output circuit is a low pass filter (to remove harmonics) and not a high Q bandpass filter.

The series resistor and/or an attenuator will provide some isolation between the Retro 75 and the amplifier... and will hopefully tame the instability.

Stu
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« Reply #35 on: June 26, 2010, 09:27:47 AM »


Great analysis Stu...

So is it also possible we're seeing regeneration through the 12v line since the Retro-75 and the EB-63 both run off 12V? If powered from the same supply, and if 12V decoupling is not optimal (daisy chained power versus star power), I see opportunities for instability via the power rail(s). Powering the Retro-75 from a 12V gel cell as a test might verify/eliminate this possibility.

Also, a P-P AM linear does not always have to be biased class AB. Remember the Vacuum Tube class BC P-P linears where each tube is biased at cutoff? This only works with AM (not SSB). The non-linear transfer around cutoff results in the RF output of the AMP to have a higher modulation percentage than the driver, maybe 70% driver, 90% AMP output.

I have run those CB type P-P solid sate amps on 10m AM, and turning off the AB bias seems to make little difference on AM whereas on SSB the results are horrible. No bias requires a bit more drive though for the same RF output. It seems that the thresholding cross-over issue is overcome with the presence of the carrier where it don't really matter.

Jim
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« Reply #36 on: June 26, 2010, 09:58:11 AM »

Jim

I think that an interaction associated with a shared power supply is also a possible source of this instability. However, I would expect the instability, in that case, to take the form of a spurious audio frequency modulation of the carrier (either a periodic modulation or a chaotic/noiselike modulation at audio frequencies). In any event, if I were directly trouble-shooting this problem, I would try powering the Retro 75 from a separate power supply... as you suggest. I have a small storage battery in my shack (the size used with a garden tractor)... and that's what I would use for testing.

I agree that, in AM operation, running a linear amplifier close to Class B might work as well as Class AB (maybe even better)... depending upon the specific characteristics of the tube(s), and the peak modulation index being used. As has been discussed earlier in this thread... 3rd order IMD is a key factor in determining how to operate a linear amplifier. I.e. how faithfully does the amplitude of the output rf waveform (at the fundamental frequency) track the amplitude of the input r.f. waveform... over the range of amplitudes that are being employed. As you have pointed out.. the nice thing about AM, in this regard, is that (if you control the negative peaks) you can keep away from the region where the output rf amplitude is a very non-linear function of the input rf amplitude.  

Best regards
Stu

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« Reply #37 on: June 26, 2010, 01:34:33 PM »

Took some hints and tips from everyone.  

There is indeed an interaction between the Retro 75 and the amp using a single supply (battery).  Before going to dual power supplies I put 100ohms in series with the Retro and amp.  The Retro stablized and had a pretty descent sinewave at the input to the amp however, the sinewave appeared 'rough' as if something was very lightly superimposed on the trace.  Output looked better but not great.  And I think this is where an oscillation comes into play.

Went to dual power supplies and Retro works without the 100ohm resistor however, output sinewave is worse than above.  With the 100ohm resistor only a slight improvement.  Again, probably oscillation issues.

On a long shot I started probing the stages in the amp with a scope.  The amp is oscillating. It's very apparent on the secondary of T2 and more so on the collectors of each transistor.  It occurs both in standby and transmit.

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« Reply #38 on: June 26, 2010, 02:21:11 PM »

Bob

Does the amp oscillate if you disconnect the output of the Retro75 from the input to the amp... and leave the input to the amp open?

Stu

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« Reply #39 on: June 26, 2010, 02:31:13 PM »

Hi Stu,
Yes it does oscillate with the Retro disconnected. If I key the relay by shorting the CE junction on the relay driver the amp oscillates as well and does so without any RF at the input. Tried it with the input shorted as well. No change.
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« Reply #40 on: June 26, 2010, 02:56:49 PM »

Bob

What type of load is the amp looking into?

Stu
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« Reply #41 on: June 26, 2010, 04:10:25 PM »

Stu,  a 50ohm non-inductive load.  I 've 2 DL's.  One is a cantenna the other is a 2kw dry DL.  Both do the same thing.  Just so you know when the amp is in standby, the relay opens the input and output to the amp bypassing it so the input and output are floating.  However, even in Xmit with the input shorted or connected to the Retro75 (power off) the amp still oscillates.  The frequency of the oscillation does change somewhat.  Oscillation is in the 1.5Mhz range.  Also, I increase the bias temporarily to see what would happen.  Going from .645V to .675Vdc 'cleaned' up the oscillation. 
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« Reply #42 on: June 26, 2010, 04:37:21 PM »

Bob

Try adding some additional capacitance directly from base to ground for each of the transistors in the amplifier. Use something like .005 uf (5000 pF) from base to ground on each transistor. Keep the leads short.

Stu
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« Reply #43 on: June 26, 2010, 05:35:04 PM »

I'll have to try that next week when I get a new pair of transistors. Opened a base emitter junction on one.
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« Reply #44 on: June 26, 2010, 06:25:57 PM »


Looking at the EB63 schematic, the bias is there all the time. This means the AMP has an open input and output and the zitters are idling away trying to amplify something. It can be debated whether an AM P-P linear needs to be biased at all, but the need for bias during idle periods is unnecessary, and likely causing the issue.

Once you rebuild the amp, maybe add a fuse on the +12v power buss.  Huh

Also consider jumpering that bias diode to go to pure class B with NO idle current.

Jim
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« Reply #45 on: June 27, 2010, 07:20:51 AM »

Jim,
I was thinking of just turning the amp off all together when no RF is applied.  And rather than using a COR, switch it directly. The amp does have a fuse on the B+. Definitely a must.
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« Reply #46 on: June 27, 2010, 01:42:03 PM »

Bob
et al.

Take a look at the schematic of the biasing circuit of the amplifier:

http://www.communication-concepts.com/appnotes/eb63300sharp.pdf

A few observations (comments?)

A. The bias supply is a forward biased diode (D2), bypassed by a 500uF 3V electrolyic capacitor (C22). This feeds bias current, at 0.7 Volts, through the center tap of the input transformer.

It seems to me that an additional, low impedance (at RF) capacitor... in parallel with the 500uF electrolytic... is appropriate. E.g. .01 uF... which has an impedance of 4 Ohms on 75 meters... or maybe a .01uF capacitor in parallel with a 0.1 uF capacitor).

B. According to the document (referenced above)... the transistors are biased at around 500mA each. Assuming the DC current gain is 100 (i.e. the MRF454 specification sheet says that the DC current gain is a minimum of 40 and a maximum of 150) ...  each transistor has about 5mA of base bias current. In addition, each of the 10 Ohm physical resistors from base-to-ground of their respective transistors will draw around 0.7 Volts / 10 Ohms =70 mA. The total of: 2 x (5mA + 70mA) = 150mA of bias current has to be delivered to the center tap of the input transformer by the 33ohm resistor (R4). This is not problem, because the 33 ohm resistor is delivering a total current of (13.6 Volts - 0.7 Volts)/33 Ohms = 390 mA. So far, so good. [150mA goes to the center tap of the input transformer, and the remaining 240mA flows through diode D2].

When the transistors are delivering (for example) a total of 140 watts of RF output power... corresponding to a total of (approximately)  140/0.66 Watts = 212 Watts of DC input power...  then the total collector current is around 212 Watts/13.6 Volts = 16 Amps. This corresponds to a total average base current (assuming a current gain of 100) of 160 mA. Adding this to the 140 mA of current flowing through the pair of 10 ohm resistors... we get a total of 300 mA of average current that must be delivered to the center tap of the input transformer. This is still less than the 390 mA of current that the 33 Ohm resistor (R4) is delivering.

But, what if the current gain of each of the matched pair of transistors is closer to its minimum data sheet value of 40? Let's say the current gain is 75. Then the total base current corresponding to a total of 16 Amps of collector current will be: 213 mA. Adding this to the 140mA of total current through the pair of 10 Ohm base-to-ground resistors, we get: 353mA... which is getting close to the maximum current that the 33 Ohm resistor (R4) can deliver... while still maintaining a 0.7 volts bias.

So... I'm wondering if the oscillation might me caused by an instability in the base bias voltage.

Stu  

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« Reply #47 on: June 27, 2010, 04:15:42 PM »

Stu,
I agree on the .01uF.  I thought about that yesterday when I was messing around with the bias and happened to stumble upon the oscillation and looking at the schematic.  The lack of one raise a flag.  I was about to solder one into place when I discovered the blown transistor.

Also, I wouldn't go by the schematic posted on their website or the engineering notes.  The bias circuit has morphed a little bit over the years.  See the current circuit diagram.  I should have posted this way in the beginning.  

D2 is a transistor being used as a diode.  It is heat sinked or sandwiched between the PC board and heat sink.  R4 is now 82 ohms.





* eb63Schem.jpg (61.61 KB, 1123x816 - viewed 480 times.)
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« Reply #48 on: June 27, 2010, 04:23:36 PM »

BoB

I tried magnifying the schematic... but there is one key value I cannot read.

What is the value of R4 in your amplifier?

Stu
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« Reply #49 on: June 27, 2010, 04:30:33 PM »

Stu,
R4 is 82 ohms.  Also see my previous post. I've included some of the changed components.
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