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K6JRF's Page formerly W6FZC AL-1500 EBS1 Ckt Analysis |
[Just added . . . Spice RF Analysis of the AL-1500 EBS Ckt!]
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This page details some improvements that can be made to the Ameritron AL-1500 Electronic Bias (EBS) Ckts. By use of 5Spice
analysis, both RF and AF, the ckt's operation is analyzed and detailed with recommendations for improvement. I also switched from MSim's "PSpice" to a newer, more compact analysis program called "5Spice" written by Andresen Software. It features almost intuitive operation, has many powerful features and and allows the import of various Spice models so that the ckt can be simulated using the actual real world parts. Click here to see more information on this fine circuit analysis product. The link below is an detailed analysis of one of the earliest (1975) ckt configuration. It didn't work as it was described and a detailed Spice analysis shows the required ckt component changes so that it does! Click here to download the WORD doc file. |
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Received this email . . . Hi OM, Your model is all messed up! The analysis is meaningless because you applied audio frequencies to the input instead of radio frequencies. The lowest possible frequency on the input is 1.8 MHz, not 10 Hz or even 3,000 Hz. Why would anyone model an RF detector with audio frequencies applied? 73, Tom W8JI An SSB signal consists of a "modulation" envelope, not a 'carrier' and this carrier-less signal is produced by both older and modern day transceivers. An AM signal has a "carrier" meaning that when the modulation stops, there's still a signal at the central RF frequency. With SSB, no-modulation means no-output and no carrier! Additionally, the modern day transceivers have wide band RF and AF ckts, allowing them to to make an "ESSB" [Extended SSB] RF signal output that contains modulation data typically from 50hz to 6Khz. Some specially modified transceivers can produce SSB envelopes with spectral info from 20hz to 9Khz! "Flex" radios can extend the modulation envelope to 20Khz! If a 50hz tone (from signal generator) is inputted into the rig's mic jack, the RF output is a SSB RF signal whose frequency is 50hz removed from the center frequency. It is this signal, when sent through an AL-1500 linear with an EBS ckt, that can emerge with severe envelope distortion. Since low frequencies (50hz) are easily outputted from the newer broadband radios (FT-2000, FT-9000, FT-5000, etc), the problem centers on the inability of most EBS ckts to pass this SSB envelope esp at low level b/c of three (3) main factors: low gain, incorrect time constants and passband skew. Perhaps W8JI has not seen the comments from Richard, AG6K on RF actuated EBC ckts; "This is a desirable departure from the RF-actuated electronic cathode-bias switch circuits that have appeared in ham magazines. At first, "RF-actuated" sounds like it might be wonderful. However, these "RF-actuated" circuits result in the cathode bias-voltage being rapidly switched between nonlinear-cutoff and linear operation while the RF relays are in transmit. This often causes transmit audio to sound rough on softly spoken syllables and increases the IMD products that the amplifier generates." As he correctly stated, many bias ckts produce MORE distortion than is acceptable, so they are quickly abandoned by the user. The distortion consists of the tube's bias being switched from cutoff to normal causing the signal to sound muffled with what many observers call a "paper-crunch" sound. The power saving advantages of the ckt are offset by the distortion produced so the ckt is abandoned. If a Spice envelope analysis is undertaken, it can show where the ckt problems are and allow you to 'test' fixes to make the ckt operate as intended. The envelope analysis looks at the gain, passband of the EBS ckt at audio frequencies and that has W8JI upset. Certainly the SSB envelope is an "RF" envelope that contains the audio frequency info when demodulated in a SSB rcvr. Despite what W8JI incorrectly thinks, there's nothing wrong or incorrect about analyzing a ckt's response in this manner. Doing this ensures that the ckt works properly at both RF and Audio frequencies. Further, optimizing the ckt in this manner makes it "bullet-proof" when used in actual operation. To prove this, the section below uses a conventional Spice "RF" analysis to show the same EBS ckt problems as the envelope analysis does! This section features a Spice ckt analysis run at 2Mhz. This frequency is "inside" of the Ham Radio Band allocation as it sits between 160M (1.8Mhz) and 80M (3.5Mhz) ham bands. The "period" of 2Mhz is 0.5us, so it's easy to use that time to construct an RF signal to be used as the stimulus input to the EBS ckt. Most Spice programs allow the construction of "piecewise" linear signal, as does 5Spice. This gives the user a powerful tool to use with Spice's "transient" analysis to completely characterize the ckt under test. A sample of the "text" file used to produce a 10 watt RF signal appear below. It is simply the time representation of the RF amplitude. The excerpt shows two (2) cycles of the 2Mhz waveform. A "text" editor can construct the file needed. *SSB_2Mhz@10Watts * *enter waveform data points *comment: first data point is at time = 0 *data format: *time amplitude 0.000u 0.0 0.125u 31.6 0.250u 0.0 0.375u -31.6 0.500u 0.0 0.625u 31.6 0.750u 0.0 0.875u -31.6 1.000u 0.0 At time "0", the amplitude is "0" volts; at 0.125us, the amplitude is 31.6 volts which represents 10 watts into 50 ohm load. The total duration of this test signal is 50us. This allows one to determine the sensitivity (gain) of the ckt along with its "turn-on" and "turn-off" time. All this with just one stimulus signal. Since the EBS ckt is supposed to respond to " . . millewatts of signal" per W8JI, then 10 watts of RF drive should be overkill. Stock EBS Ckt RF Analysis The chart shows the close-in detail of the RF signal applied to the stock EBS ckt. The "blue" line represents the RF signal input; the "magenta" line represents the internal switch state; the "red" line represents the output of ckt. The stock EBS ckt schematic is here. This revision has added the cathode ckt's zener diode for completeness. The zener is a 1N5343B, 7.5V @ 5watt. The actual zener is a 10W model but this chosen model does not effect analysis since the total current through the zener is kept to apx 300ma, which equals apx 4 watts of dissipation. 
Disturbing results . . already! Carefully observe the "charging" action of the input signal (blue) in relation to the output (red) chart
line. The output follows the input due to the short time constant of the ckt. The good news is that the output (red line) starts to
switch "on" at apx 8us, so the ckt responds quickly. Compare that with the modified EBS ckt below, apx 23us. The bad news is that at 50us, the stock EBS ckt only attains apx 18V level and it immediately starts to rise indicating it's starting to turn off! Within 50us (from 50us to 100us elapsed time point), the ckt has risen to apx 19V level. That says it will be completely turned-off in about 500us. So the tube is some where between operating and cutoff bias . . Can you say "distortion"! This chart shows what AG6K described as marginal bias levels. This response is at a drive power of 40Watts! Lower input drive gives even poorer results. 
This chart shows the elapsed time up to 1msecs. Note that between 50us (end of signal) and 550us, the ckt's output slews to 25.8V and
then continues to rise, slowly, from that point. The tube is in cutoff bias. Short hang time of apx 550us. Not what I'd call a
properly operating EBS ckt.
This is confirmed in the last chart showing the elapsed time to 100msecs. Note that the output (red line) has continued to rise to 35V
bias level in apx 45ms. That is the ckt's total "hang-time" even though the tube has been in cutoff since the apx 350us point. At this point, it's obvious that the stock EBS ckt does not have enough "gain" b/c transistor Q5 doesn't switch on fully and turns off immediately when the signal stops. That low gain plus very short time constants equal a poorly functioning EBS ckt. Modified EBS Ckt RF Analysis 
If you wish to see the Spice schematic of the modified EBS ckt, it's here. Again note the zener diode, 1N5343B has been added for completeness in the Spice simulation. The chart shows the close-in detail of the RF signal applied to the modified EBS ckt. The "blue" line represents the RF signal input; the "magenta" line represents the internal switch state; the "red" line represents the output of ckt. The ckt's output starts to turn-on at apx 23us, slower than the stock EBS ckt's 8us. Note that at 80us, the modified EBS ckt has attain operating bias of 10V [ The turn-on time is dependent on C10 value; 0.33uf = 50us; 0.68uf = 65us; 1uf = 80us. Graph shows C10 = 1uf]. The output continues to decrease even though the RF input stimulus signal has now stopped. The output attains the 9.5V level at the 100us time point and does NOT start to turn off! What you would expect from a EBS ckt. 
This chart shows the elapsed time up to 1msecs. Note that the ckt's output is active sitting at 9.4V operating bias level. Look
carefully and you'll see that the it's starting to rise, albeit, very slowly. This is the start of the hang-time. All is as it should
be.
This chart shows the elapsed time to 300msecs. Note that the output (red line) has now risen to 35V bias level in 300ms. This time
represents the total hang-time of 280ms. A smoothly operating EBS ckt indeed!If you wish to shorten the hang-time, select a cap value shown in the "C10 = Hang Time" table shown here. For example, C10 = 0.33uF gives apx 90ms hang-time. Note that this is the ckt's stock value, so no change is necessary. The value for the Spice simulation is 1uf which gives apx 280ms. I will stop here but I could show other RF stimulus inputs (2Mhz @ 5Watts for example) where the stock EBS ckt barely responds; the hang-time is even shorter (apx 30ms) than you've seen in the 10Watt analysis (apx 45ms). Is this a realistic input?? On 75mtrs, I drive the AL-1500 with 20 watts from the FT-2000D, so using 10watts and 40watts is meaningful. BTW, the modified EBS ckt operates identically with the 2Mhz @ 5watt test signal. The reason for the poor stock EBS ckt performance is explained in the "envelope" section below. Low ckt gain and non-optimum time constants chosen are the culprits. Low ckt gain is neutralized somewhat if the input power is increased; for example, at 2Mhz @ 40Watts drive level, the turn on voltage is 4V with a hang-time of 45ms. But as you've seen, the hang-time is meaningless since the tube is cutoff 225us after signal is removed. In defense of the ckt, it probably was designed MANY years ago for "AM" operation. So with the proper drive level, it would probably operate marginally. I'm being kind here b/c if the ckt's time constants and ckt gain were analyzed for an "SSB" signal, the ckt would work with any form of signal transmission! The stock EBS ckt performs very poorly b/c it never switches fully on to provide operating bias. Then it drops out almost immediately when it's removed. Same as a "bias-power" saver ckt. In all cases, its performance depends on the input power of the signal. The higher the input drive power, the better it performs. The modified EBS ckt works with as little as 5 watts input drive due to the use of a high-gain transistor used for Q5, MPSA29. W8JI incorrecty treats a SSB Tx signal as a "carrier" . . but it's not! If this were an "AM" signal, the stock EBS ckt could operate better if not perfectly depending on carrier power level. Why? The low level modulation 'rides' on the carrier so Q5's low gain could be mitigated by the carrier's power level. But for SSB, the low amplitude 50hz audio produces low amplitude SSB RF envelope and now the EBS ckt's gain is critical as to whether it can fully switch on and retain operating bias for the length of the input. Due to low gain, it can't, thus the distortion! The ckt's RC time constants are equally important b/c they can also produce the same effects. The ckt improvements make the EBS ckt a truly superb EBS ckt for any linear with any operating mode including QSK CW. Note that it fully meets the requirements for a "fast" turn-on (80us) and "long" hang-time (350ms) as stated in the design goals. And to boot, the cost of the modification parts is . . . . $0.88! So which EBS ckt do you want in your linear? Please continue to read the "envelope" analysis to see "how" to make it into a great operating EBS ckt w/ just a few mods. |
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Electronic BiaS Ckt [EBS]: 
As you read in the AL-572 DBC analysis section, the AL-1500 linear also supports an a "electronic bias ckt" which is called the
EBS (Electronic BiaS) ckt. To me, all linears should have one! It saves the useless waste of "idling" power of
480 watts (3850V @ 125ma)! That said, recently, I found that this ckt restricts SSB Tx audio b/c of various ckt problems. This section is devoted to analyzing these problems and has suggested ckt improvements. It's mounted near the opening of the fan plenum area and has a small "detector" board mounted on a standoff, just above it. As you see, I've marked the "missing" components which makes a difference in the simulation. EBS Schematic: The schematic was copied from the user's manual and translated into "5Spice" so it's not the original but it's much easier version to read. It similar to the AL-572 DBC ckt and generally operates the same manner.  The EBS-1 features a remote detector board that uses a pair of Schottky diodes, driving a bipolar transistor switch (Q5). Q5 is a fast-attack, slow-decay switch with C10 setting the decay time and R4 limiting the charging current. Delay or “hang-time” is an additional period when bias remains at normal operating levels after drive power goes to zero. The EBS-1 remote detector board serves as an "RF" high impedance sensor. It's mounted near the source of RF, preventing the EBS-1 ckt from having a large effect on input SWR. The switching and control board is mounted directly below the RF detector board. This latest version shows the Zener diode, 1N5343B, 7.5 volts nominal breakdown voltage, in the cathode circuit. This adds to the completeness of the analysis. A 'good' electronic bias ckt, should switch on quickly so that the SSB Tx wavefront is fully allowed to pass. This implies that all frequencies within the wavefront should pass equally and it should delay in turning off so that so any audio "effects", if any, are fully "passed". The stock EBS ckt does not do this! 
Another problem was found when doing the 5Spice AC simulation; transistor Q5 (MPSA05) does NOT have enough current gain (BETA)
to be the key "switching" element in the ckt. That transistor determines whether the signal is "valid" by switching from "off"
to "on". A low-gain transistor further skews that ckt's response to the high-frequency side. This is evident for a low level
SSB drive of 10 watts! The new transistor, MPSA29, is basically a "darlington" transistor meaning that there are two transistors inside the part feeding each other. The current gain (BETA) is extremely large (typ 5,000 to 10,000) and that translates into "high sensitivity" for low level signals. This is mandatory for proper operation of this ckt. This part is similar to the TIP122 used in the AL-572 DBC ckt and was substituted for the MPSA05 (hfe = 100) that is very short on required gain! The chart above shows the frequency response of the EBS1 ckt. The BLUE [C1= 0.001uF] line shows that it's -28dB down at 50hz, so its response is skewed to the hi-frequency side. This causes a severe loss of low-frequency audio. The RED line shows the response centered within the passband allowing all SSB audio (from 20hz to 5Khz) to pass w/o attenuation. Both plots reflect the replacement of Q5 with a "MPSA29" transistor. Other parts such as 2N7052 or 2N7053 are compatible. 
In addition to a balanced freq response, it must turn on and off faster than the rise time of voice waveforms. This is not a
problem since the choice of the R-C time constant is easily chosen. The problem in this ckt (and in ALL half-wave rectifier
ckts), is the "hole" left by the "dead" time of frequency in question. For 50hz, it's 10msecs. In other words, the time
'delay-time' of the ckt must account for this so that it doesn't start to "turn-off". That's what happens using the "stock"
Rs and Cs. The chart show the response to a SSB signal envelope consisting of 50hz audio component at 10watt drive level into the stock EBS1 ckt. This, for my AL1500, produces apx 650 watts output. As the chart clearly shows, the ckt NEVER FULLY SWITCHES ON! After 210msec the ckt oscillates around 4V to 9V level making the 8877's bias fluctuate from 16V to 21V. It's clear that stock EBS1 ckt is never quite fully "ON". At 300msec, it continues to stay partially "on". For a SSB signal consisting of "1000hz", the ckt is fully "ON" and allows that to pass w/o attenuation. But for a 50hz SSB signal, there is severe distortion b/c the 8877 tube is varying between cutoff and partially-on bias. In the AL-1500 amp, the stock value for C10 is 0.33 µF and this results in apx 90mS of bias hang time as per 5Spice analysis depending on the input SSB drive level. To increase the bias hang time, C10 can have a range from 1uF to 10 µF. Since 250ms to 350ms would be more desirable, C10 should be increased. Check the table, C10 = Hang Time, for a range of values for C10. 
The values for C1 and C10 are critical of ESSB b/c they determine the audio passband response. A serious attenuation of
frequencies below 500hz occurs w/ the "stock" values for C1 (0.001uF). And the "half-wave-hole" requires a much larger value to serve as
an "integrator" over the time that ensures that the ckt stays "turned-on". This is important for ESSB radios and the changes should be
made if you wish to properly pass low frequency ESSB Tx audio. The chart shows the EBS-1 ckts response to SSB envelope consisting of 50hz with the C1, C10 and Q5 value changes. Now the ckt is fully turned on in less than 2msec and easily passes the 50hz ESSB Tx audio. There is some passband ripple but not enough to effect the class AB bias of the 8877 tube. 
This chart shows the 5Spice simulation analysis with the component value changes for C1=0.047uF , C10=1uF
and Q5=MPSA029 using a single trigger "pulse".
These new values eliminate the low frequency response cut off and these values guarantee that the ckt will pass all low frequencies without
attenuation or distortion.The charts show the response of the EBS-1 ckt for a SSB single "pulse" signal. The "turn-on" time (green line) is a bit slower (apx 400usecs) than before but is fast enough to ensure that there is no attenuation of the low frequency signal. 
The second chart shows the same response except over a large time frame so the ckt's response to a single input trigger can be analyzed.
The "hang-time" (green line) is apx 90msecs but by 200msecs, the tube is 'cutoff'. It's slower than the stock (a good thing) b/c of the
larger value for C10 (1uF) and allows any audio-effects such as reverb, to pass thru the linear w/o distortion or being cutoff. To change the hang-time, pick a value from the table to get the desired hang-times. For C10 = 0.33uf, apx 90ms; C10 = 0.68uf, apx 180ms; C10 = 1uf, apx 280ms. This allows you to customize for the hang-time desired. Q1 is an MJE3055 transistor which has 60V collector to emitter breakdown (Vceo) voltage. I MAY replace it with a TIP122 with a Vceo of 100V. This was the main replacement in the AL-572 amplifier's bias ckt as you read in the that menu item. Time will tell if the present transistor can continue to perform without failure. So far after 1.5 years of linear operation, transistor (Q1, MJE3055) has not failed. Final Schematic: The schematic below has all of the mods incorporated. 
Parts Info: Some of these parts are very common and can be found in most electronic stores such as Radio Shack. However, much better prices can be obtained from Mouser Electronics and that's what I've used. Click the link for each part to see Mouser's price and availability. Ref Des ------ Value ------- Price/Ea ------- Link -------- Remarks C1 --------- 0.047uF ------- $0.57 ------- -here- ------- 600V rating is ok: 50W drv = 141Vp-p! C10 --------- 1uF ---------- $0.08 ------- -here- ------- 50V rating! Q5 --------- MPSA29 ------- $0.18 ------- -here- ------- drop-in replacement; beta=10K! R8 --------- 22K ohm ------- $0.05 ------- -here- ------- 22K ohm, 1% film resistor C1 and C10 should be removed and replaced with the new values. R8 is soldered to the back side of the board so it should be replaced with the new value (22K ohm) resistor. Q5 must be unsoldered and replaced with the new part, MPSA29, which is a pin-for-pin replacement transistor. You can use an alternate such as a "TIP110" but the pins require some bending to fit the existing PCB pattern; I don't recommend it. Comments: The original analysis and mods were completed on 7/17/09 using the "envelope" (audio) analysis. The EBS ckt has worked as Spice predicted! The SSB envelope processed with the AL-1500's EBS ckt has no discernable distortion. The RF Spice analysis has shown the "same" problems as the envelope analysis. What many fail to understand is that low frequency audio modulation (50hz for example) has much less amplitude than, say 1000hz signal. So with the EBS ckt's low gain and skewed bandpass, these low level signals are "distorted" b/c the EBS ckt can't fully attain operating bias as was clearly shown in the RF Spice analysis. W8JI incorrecty treats a SSB Tx signal as a "carrier" . . but it's not! If this were an "AM" signal, the stock EBS ckt could operate better if not perfectly depending on carrier power level. Why? The low level modulation 'rides' on the carrier so Q5's low gain could be mitigated by the carrier's power level. But for SSB, the low amplitude 50hz audio produces low amplitude SSB RF envelope and now the EBS ckt's gain is critical as to whether it can fully switch on and retain operating bias for the length of the input. Due to low gain, it can't, thus the distortion! The ckt's RC time constants are equally important b/c they can also produce the same effects. Reports from Hams say their SSB Tx audio is "clean and clear"! One such "proof" was demonstrated by Tom, K6NHK with his AL-800H. He exchanged the recommended parts and the low frequency distortion was removed. His SSB Tx audio is perfect (I've heard it!). Emails from other Hams say they had the same results. I have no doubt that they would! The RF and Audio analysis have shown that problems exist with the stock EBS ckt and that the recommended changes do eliminate them! |
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