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 Post subject: SB-220 Has output on only one band
PostPosted: Feb Thu 01, 2018 11:40 pm 
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Location: Allen, TX
I come here when I'm stumped and boy am I stumped!

The SB-220 I'm working on for a friend works perfectly on 20 meters ONLY. There is little to no amplification on all the other bands, and the plates get quite cherry red on the non-working bands.

Naturally, I looked at band specific components attached to the band switch and thoroughly cleaned all the contacts three times. The band switch wafer inside the RF compartment was corroded worse probably due to baking in the high heat. I took off the front panel and check all of the band specific coil/cap components and all looked fine.

The owner says it used to work on all bands, but you never know!

My question is what, other than components connected to the band switch, could cause this amp to work only on one band. My other thought which rules out components connected to the band switch or the switch itself is that it's highly unlikely for ALL the other bands to stop working, if for example it's a bad switch contact. Could it possibly be a resonance issue with the plate choke, which looks unmodified?

Thanks,

Rick
W5RAA


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 Post subject: Re: SB-220 Has output on only one band
PostPosted: Feb Thu 01, 2018 11:52 pm 
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I assume you are in the CW/Tune position. What is the plate and grid current showing on a non-working band, i.e. 40 meters? On 40 meters with the load capacitor full meshed and a moderate amount of drive (around 20-40 watts from the exciter) is there a dip in the plate current when you adjust the tune capacitor?

The output tank side of the band switch is extremely separate in the SB-220. On 80 it adds a fixed padder in parallel with the variable load cap and on the other bands it shorts part of the tank coil so there isn't much to go wrong.

I would be very suspicious of a build issue as in reversing the band switch wiring for the other bands so it only works on 20 which coincidentally is the center position. Visually check for this problem. A few years ago I repaired a Johnson Viking 500 (also was available in kit form) that wouldn't work on 40 meters because the original builder had the tank tap one turn off; a quick check with a dip meter confirmed it resonated below the 40 meter band.

A small possibility is the RF safety choke was damaged. It provides a DC short to ground which will trip the breakers if the output coupling/DC blocking cap fails shorted. My homebrew amp had its choke damaged when an antenna came down in an ice storm preventing proper tuning; I assume some turns shorted and it no longer acted as a choke in the HF bands. I wouldn't suspect the RF choke in the anode feed circuit because a failure there would create different symptoms than what you are seeing.

Rodger WQ9E


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 Post subject: Re: SB-220 Has output on only one band
PostPosted: Feb Fri 02, 2018 2:23 am 
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Location: Allen, TX
Thanks Roger!

Yes I'm in the TUNE position.

The plate current for the non working bands is the same as the 20 meter band which works, but there is no dip for the non working bands.

It couldn't be a build issue because the owner insists it used to work on all bands. The reason I got it is because he incorrectly installed a new diode/meter board and incorrectly wired it. Since 20 meters works I am assuming the diode/meter board is working correctly. The miss wiring actually go to the same place on the meter board in the HV supply to the plates.

I'll try temporarily bypassing the safety choke to see it that's it.

How about the RF chokes on the grids?

Rick
W5RAA


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 Post subject: Re: SB-220 Has output on only one band
PostPosted: Feb Fri 02, 2018 2:41 am 
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Rick,

I guess the grid chokes could be the problem also. Check to see if you are getting normal grid current on the working bands.

80 would be a good test band to check for output network switch problems because all it does is add the padder for the load capacitor and you could temporarily jumper that to the output network and either rule the switch in or out as the problem.

I wasted a lot of time looking for problems with the Viking 500 because I assumed the owner had been using it on 40 in the past but he hadn't.

If you have a MFJ antenna analyzer or its equivalent you can connect it to the RF output, put a resistor from one of the 3-500Z anodes to ground to simulate the normal operating load, and then test the network without HV/RF applied which allows you to safely move things with your fingers or substitute low power components for test purposes.

I guess the problem could be in the cathode drive circuit but off the top of my head I can't think of anything that would allow you to drive the tube without also allowing for the output tank to tune if the problem were just on the input side to the amplifier.

You can just temporarily disconnect one end of the safety choke but do this only for a test and reconnect it to avoid a situation where the full HV ends up on the output connector.

Rodger WQ9E


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 Post subject: Re: SB-220 Has output on only one band
PostPosted: Feb Fri 02, 2018 3:48 am 
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Greetings to Rick and the Forum:

Rick writes:

Quote:
and the plates get quite cherry red on the non-working bands.


and also:

Quote:
The plate current for the non working bands is the same as the 20 meter band which works, but there is no dip for the non working bands.


I am assuming from this that the dip on 20 meters results in lower plate current than the apparent off-resonance condition on the other bands.

If so, this is a classic case of operating at a frequency outside the tuning range of the tank circuit. Since you are almost certainly operating in the band(s) for which the amplifier was intended, failure to find resonance indicates a tank circuit failure.

Rodger is extremely knowledgeable; his advice is always sound. I would not assume that what the owner of the unit tells you is correct... as Rodger points out. I have been led astray more times than I care to count by not being careful in my initial assumptions, and accepting owner input at face value is a real good way to fall into this trap.

Rodger's suggestion of using a network analyzer to look backwards into the tank circuit is a good one. Failing this, a good grid dip meter should enable you to discover what is happening.

According to the schematic I was able to download, the output circuit looks like a straight-forward Pi network. If you use a grid dipper, you should be able to short the loading capacitor and end up with a simple parallel resonant circuit... which ought to tune through the frequency of interest. Do this with all power removed, of course.

Other than Rodger's suggestion of a miswire, the only other thing I can think of is to check the tap points on L6 and L7. I don't know how these are done, but look for bad solder joints or loose or corroded connections if clamped.

Examination with a network analyzer or grid dipper should tell you whether the tuning range is off high or low, which should give you a big hint as to what to look for.

One question that you could answer is: Where does the pointer on the tuning knob point when the amplifier is tuned to resonance on 20 meters? (This assumes that the knob is correctly installed so that minimum capacitance is at the top of the 10 meter segment [ 3:00 o'clock] and maximum capacitance is at the bottom end of the 80 meter segment [9:00 o'clock] ).

Good Luck,

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Jim T.
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 Post subject: Re: SB-220 Has output on only one band
PostPosted: Feb Fri 02, 2018 10:19 pm 
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If the plates are showing color with drive, then their is grid current and the grid chokes are not open.

Output on one band, while showing grid and plate current on others may indicate major damage to the band switch.

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 Post subject: Re: SB-220 Has output on only one band
PostPosted: Feb Fri 02, 2018 10:44 pm 
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One other caveat with the SB-220! The mechanical alignment between the input network wafer and the ceramic tank wafers can go out of alignment. Make sure all switch index positions are in alignment.

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 Post subject: Re: SB-220 Has output on only one band
PostPosted: Feb Sat 03, 2018 4:04 am 
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Location: Allen, TX
This is getting stranger by the day, but that's what makes it fun!

I checked all of the wiring and components connected to the band switch and everything is correct.

Disconnecting the safety choke had no effect.

I tried the antenna analyzer on the output after closing the relay. Here are the strange results.

Band .....Resonant Freq MHZ..... Freq MHZ Range with less than 3:1 to 1 SWR
80............5.0................................2.1 - 7.3
40............4.0................................1.8 - 8.6
20............4.6................................4.0 - 8.6
15............4.3................................1.9 - 8.6
10............19.0..............................6.5 - 36.0

Notice that on the only band that works, 20 meters, the SWR at 14 MHZ is infinite! The <3.0:1 bandwidths are quite broad and at least the 80 and 40 meter SWR's are reasonable within those bands, but the amp still doesn't work.

I'm wondering why most of the resonant frequencies are all around 4 to 5 MHZ the upper bandwidth frequencies are all at 8.6.

Any thoughts?

Rick
W5RAA


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 Post subject: Re: SB-220 Has output on only one band
PostPosted: Feb Sat 03, 2018 4:48 am 
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The way you can use an analyzer to test the input matching is to add a temporary 68 ohm non inductive resistor between the cathodes and chassis.

For the PA tank you need a carbon comp resistor of about 2500 ohms between an anode and chassis... done this dozens of times using a CIA-HF and RigExpert analyzer to optimize amplifiers pi networks.

How is Mr. Scooter doing?

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 Post subject: Re: SB-220 Has output on only one band
PostPosted: Feb Sat 03, 2018 5:40 am 
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Greetings to Rick and the Forum:

At a guess, it would appear that you forgot to attach the necessary resistance to simulate the tube plate impedance.

The Pi network is an impedance transformation device. It matches the tube plate impedance (relatively high - several thousand ohms) to the antenna input impedance (relatively low - normally about 50 ohms).

In order to use the network analyzer trick, you must add a non-inductive resistance from the tube plate to ground to simulate the tube plate impedance which is infinite when the tube is cold with no power applied.

I found this article on line:

Quote:
RF POWER AMPLIFIERS - TANK CIRCUITS & OUTPUT
COUPLING
by Lloyd Butler VK5BR


http://users.tpg.com.au/users/ldbutler/ ... upling.pdf

In it, he gives the approximate formulae for calculating this impedance for the various classes of RF amplifiers:

Quote:
LOAD RESISTANCE
For valve RF power amplifiers operating at power levels suitable for amateur use, load resistances (RL) in the region of 1000 to 7000
ohms are typical. The ARRL Handbook provides the following approximation for valve RF amplifiers:
Class A: RL = Eb/1.3Ib
Class B: RL = Eb/1.51Eb
Class C: RL = Eb/2Ib
where Eb = Plate voltage and Ib = Plate current (Amps)


Note that this resistor can be of a small size (1/2 watt is fine) but must be NON-INDUCTIVE and must connect from the tube plate cap to ground by way of the shortest practical path. Of course, this is done with all power removed. The tube(s) must remain in place for this measurement because their inter-electrode capacity is part of the circuit. DO NOT FORGET to REMOVE this resistor before applying power to the amp or some serious fireworks will result.

I am guessing from your results that you did not have the correct value of resistor in place. In its absence, the tube plate impedance would be theoretically infinite. Since any Pi network has a limited range of impedances that it can match, I am going further out on a limb and predicting that the network will transform the very high impedance of a cold tube to a smaller but still very high impedance at the output point (antenna connector). Therefore, the Pi network is in fact working at 20 meters, just as you experience with power applied. The other readings you get are probably highly reactive and represent spurious responses in a circuit that is not working correctly.

I am not familiar with your antenna analyzer, but if all it reports is VSWR, then it is presenting you with an incomplete picture. A true network analyzer that will graphically display impedance with resistive and reactive components (+ and -j) as well as return loss would be more helpful here.

Anyway, please let us know if you did use a resistor of the correct value according to the above formula (use class A... it is close enough) and if not, please repeat the measurements with the resistor installed and let us know the results.

Regards,

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Jim T.
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 Post subject: Re: SB-220 Has output on only one band
PostPosted: Feb Sat 03, 2018 4:40 pm 
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I must be on ignore lists. :wink:

As I mentioned earlier, a 2500 ohm resistance is a good value for an approximate plate impedance for a pair of 3-500Z tubes.

Approximate values for single tubes are given here:

http://home.earthlink.net/~wd7s/TU-6B.htm

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 Post subject: Re: SB-220 Has output on only one band
PostPosted: Feb Sat 03, 2018 5:16 pm 
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For plate load calculation I just use the easy formula with appropriate constants for the class of operation. It isn't exact but it is close enough for this sort of testing and far simpler than calculating the exact load based upon operating curves which is far more precise than needed for the kind of testing the OP is doing:

Resonant load = plate voltage/plate current (in amps) x K

K is a estimated constant plugged in for operating class and reasonable amounts are to use 1.5 for class AB1, 1.6 for class B, and 2 for class C.

So for the SB-220 a calculated value of 2,500 to 2,800 is correct as stated by Peter and Jim.

Note that this operating load is based upon the tube potential and not the number of tubes so ten 3-500Z tubes running conservatively at 2000 watts input (say 3000 volts at .666 Amp) would produce the same operating load as a single 3-500Z running 3000 volts at .666 A as long as you measured the single tube quickly during its very brief operating life :)

Of course the pi network would have different values for the 10 tube vs single tube case since the capacitance contributed by the multitude of tubes would make the tune capacitor value interesting.

Galaxy/Hy Gain produced an interesting amplifier using 10 6HF5 tubes in parallel drawing around 2.5 amps at 800 volts for its rated 2000 watt input. This results in an unusually low plate load of around 200 ohms requiring some interesting design work for its output tank circuit and rather than design the tank itself for a 200 ohm source they used a capacitive divider to transformer between a higher impedance tank and the low source impedance.

It is an unusual amp that works reasonably well even with its very odd design that looks something like one of the bizarre CB amplifiers. A plug on the power supply configures it to either provide 600 volts for 1KW CW/RTTY operation or adds an additional 200 volt supply in series to provide 800 volts for the then legal limit of 2 KW. Linearity problems with so many tubes in parallel was addressed by using a linearity detector in conjunction with the bias supply to vary the operating bias when non-linearity was detected. It is an interesting design but a more conventional amp would have been easier to design which probably explains why the design wasn't adopted by competitors. Interestingly using the technology of the time it would have been fairly easy to design a no-tune broadband output network for the Galaxy amp providing it with a competitive advantage.

Rodger WQ9E


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 Post subject: Re: SB-220 Has output on only one band
PostPosted: Feb Sat 03, 2018 5:39 pm 
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rsingl wrote:

Galaxy/Hy Gain produced an interesting amplifier using 10 6HF5 tubes in parallel drawing around 2.5 amps at 800 volts for its rated 2000 watt input. This results in an unusually low plate load of around 200 ohms requiring some interesting design work for its output tank circuit and rather than design the tank itself for a 200 ohm source they used a capacitive divider to transformer between a higher impedance tank and the low source impedance.

Rodger WQ9E


QRO designed its amps for a Pi network load of 200Z; that allowed using smaller value load caps... they used a broadband transformer to match 50Z. Chuckled when I first saw it. I would have thought going to a Pi-L for the lower HF bands might have offered more benefits for the same C values.

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 Post subject: Re: SB-220 Has output on only one band
PostPosted: Feb Sat 03, 2018 5:47 pm 
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Interesting Peter, I didn't know that about the QRO design. I guess it makes sense if you are trying to save as much space as possible but since the load cap doesn't see high voltage it can be one of the smallest and lowest voltage rated caps in the tank. The British phrase, too clever by half, often came to mind when viewing both product design and the convoluted behavior of some of my fellow academic types :)

Rodger WQ9E


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 Post subject: Re: SB-220 Has output on only one band
PostPosted: Feb Sat 03, 2018 6:11 pm 
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rsingl wrote:
Interesting Peter, I didn't know that about the QRO design. I guess it makes sense if you are trying to save as much space as possible but since the load cap doesn't see high voltage it can be one of the smallest and lowest voltage rated caps in the tank. The British phrase, too clever by half, often came to mind when viewing both product design and the convoluted behavior of some of my fellow academic types :)

Rodger WQ9E


It is a trade off. Capacitance requirement decreases, but the peak voltage increases. There must be some size cost relationships at play. MFJ played the same game, but switched between a Pi to a Pi L on 80 and 160 meters in some Ameritron amps designs for the same reasoning. The AL-572 comes to mind.

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 Post subject: Re: SB-220 Has output on only one band
PostPosted: Feb Sat 03, 2018 7:29 pm 
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Greetings to Peter and the Forum:

Peter, you wrote:

Quote:
I must be on ignore lists. :wink:


No.... but when I started typing my 5,000 word essay and looking up the web site, you hadn't replied yet. By the time I got finished, your reply was there, but I just couldn't let all that work go to waste. :D

Regards,

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 Post subject: Re: SB-220 Has output on only one band
PostPosted: Feb Sat 03, 2018 8:41 pm 
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LOL! Pulling your leg, I've done it hundreds of times!

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 Post subject: Re: SB-220 Has output on only one band
PostPosted: Feb Thu 22, 2018 5:58 pm 
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To close out this thread, the owner finally admitted to have 'worked' on the band switch!

So...I gave it back to him and suggested he correct his work. :?

However, I learned a lot from all your inputs for future adventures!

Ricj
W5RAA


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