The AM Forum

Hello all,

This is my first post on the forum so please take it easy on me. I decided to try building a plate modulated AM transmitter about two months ago. I had never built anything using tubes before so I started by getting a copy of the 1949 ARRL Radio Amateur's Handbook and started on page 1. I've attached the schematic of the rig as it stands now. I have it working and have made a number of 40M contacts with good reports.

So to the problem. For some reason the 6146 is dropping output power after about 30 seconds to a minute of transmitting. With the present screen and cathode resistor values it is initially putting out about 15W and then it drops down to below 10W. I have checked all of the RF stage components and none are overheating. I checked the output of the oscillator and it is generating a steady output during transmission. The B+ value of 380v is under load. The voltage values for the tubes are taken in relation to the cathode. I've tried three different 6146s and even a few 2E26s with no improvement. I noticed that the efficiency of the PA stage is not high enough. It's in the 50%-55% range before power loss.  

I'm thinking that the 6146 plate might be heating up and changing its Rp causing the issue. I've measured the plate voltage and current to try to get a clue as to the issue. While the output power is dropping, the plate voltage goes down and the current goes up. Thinking that I might not be coupling enough power out of the plate I've tried different variable caps and inductors in the PI tank. Currently I'm using a borrowed pair of variables from a Globe Scout. While the power output is dropping I'm able to bring it back up a bit using the load capacitor until it runs out of range in the open position, so 10pf. I'm also thinking that I might not be deep enough in Class C and that this might be causing the extra heat due to an increase in anode current conduction angle. Getting consistently accurate grid voltages via a volt meter has proven difficult though. I've been working on the grid biasing for a few days with no success in fixing the issue yet.    

I'm starting to run out of ideas as to what and how to correct this issue. I don't want to complicate the design by adding a bias supply. I know that this transmitter can be made to work in this current state. I want to keep things simple at all cost. It would be great if someone here can point me in the right direction as to what is causing this problem and possible things to try to correct.

Thanks!
Joel
W4GON

You need to by-pass the RFC in the grid circuit.  Junction of the RFC and the grid resistor add a 0.001 to 0.005 ceramic cap to ground.  Usually you can rely on just grid leak bias to provided enough grid voltage.  So, why the large cathode resistor, usually the cathode of the 6146 is just grounded.  Grid current should be 2-3ma.  Your grid resistor looks about the right value.  I think I have 24K in my HB 6146 xmtr.

2E26 as a crystal oscillator is a bit much.  Sure you're not causing too much crystal current.

Not sure why you have a 4.7uf cap across one of the screen resistors.

Looks like real nice low power rig.

What type of cap are you using for the plate coupling and RFC by-pass??

You can estimate the grid voltage by the grid current through the total DC resistance of the RFC and grid resistor.

Fred

Thanks Fred! I'll bypass the grid RFC. I don't recall seeing this bypass cap in the ARRL book. Is it to ground any RF that makes it past the RFC?

I added the cathode resistor to lower the current to the 6146 thinking I was overheating it. I actually had the cathode directly grounding before and I was going to go back to that tonight to see if it helps.

Regarding the 2E26 oscillator, good question! In the 6146 datasheet it calls for 100v peak RF Grid No.1 voltage. I was having a hard time getting that much drive with the smaller glass tubes so went with the 2E26. What should the crystal current be for safe operation? I'm assuming I just check the current on the ground side of the crystal?

I'll bypass the RFC and remove the cathode resistor to see if that helps. I have a variable resistor as the grid leak right now while I try to find the correct value for it. Not sure if this is a good idea... I have it set to 28k per the datasheet right now.

The 4.7uf cap across one of the screen resistors is to bypass some of the modulating signal into the screen. I've been reading W8JI's write ups and he recommended experimenting with the plate/screen modulating ratio. It helps with increasing the modulation to 100% on this rig anyway. I don't claim to have the ratio right on it!

The plate coupling cap is also "borrowed" from my Globe Scout so it's a 6kv 1nf cap. The plate RFC by-pass is a ~5nf ceramic cap.  

Very good on the grid bias voltage calculation. I was doing that as well, but it was not matching up with the volt meter so wasn't sure which one to believe. I'll just use the calculation going forward.

Joel

Not sure about the crystal current,  Make sure the crystal is not getting warm over time.  The problem is you're losing power,  what type cap are you using for the coupling to the pi-network??  Some type caps don't work well with high RF currents, they'll get hot over time.

Fred.

The coupling cap to the PI network is a ceramic from what I can tell. It's whatever WRL used for that component. I removed it from my Globe Scout 680 and put it in the HB rig on Monday. Actually, I took the parasitic choke, plate RFC and plate coupling capacitor all from the Scout and used them on this rig. I did this to remove any possibility that I had a bad component on that stage. The Scout has been putting out rated output power for years so I know those three components are good now at least...

Joel

You should add a 2.5uh RFC to ground on the output line to the coax connector.  This is a safety feature.  If the plate coupling cap shorts, B+ will be on the antenna.  The choke will short the B+ and blow the fuse.  Hopefully you have some fuses in the rig somewhere.

Fred

Hi Joel and welcome to AMfone.  Very friendly folks over hear and a wealth of knowledge. 

My suggestion is to use a scope to find out what is sagging from an RF point.  I would start at the plate of the XTAL oscillator and work your way through the transmitter.   I think with that method you will quickly find out what in the chain is dropping off.  If you don't have a scope, you could use an RF voltmeter.  If you don't have one of them you could make an RF probe and use it with one of your analog VTVM's. 

Since you have no negative fixed bias supply on the final, if the drive drops off the plate current will increase and then your B+ will drop since your pulling a lot more current from your supply. 

73,
Joe-W3GMS         

Joel
Fred

Since the is no fixed bias on the 6146 grid, and no "clamp" circuit... the cathode resistor may be required to protect the tube from run-away plate current if the RF grid drive is not present (or too low in amplitude).

The 392 ohm value seems okay... but, with 87mA of total 6146 cathode current, the dissipation will be 3W. Therefore, a resistor rated at 3W creates (what some call) a "Chernobyl" situation.

Stu

Yes, he can use some cathode bias, especially with no fixed bias or clamp tube.  The problem he posted is he has about 20 watts at first but it drops to about 10 watts after 5 or 10 minutes.

So it seems something is not handling the RF, possibly a bad cap or the type of caps used or something with the crystal oscillator.
Could be a bad resistor or one of the RFC is breaking down.

Hopefully he'll find the problem

Fred

Correct, the issue is that after 30 seconds to a minute I start loosing output power from about 15W to less than 10W. I do have an Oscope and will do what Joe recommended. I've ran that test on the rig in the past, but will do it again very carefully to see if I can pin it down. Fred, your comments on the bypass caps has me wondering about the plate bypass. I'm going to replace it with a higher voltage orange drop just in case. I replaced the plate RFC already so I don't think that component is the issue.

I'm starting to run out of spare parts for this rig... That's why the cathode resistor is only 3W as that's all I have right now in that range. The power output problem was present when I didn't have a cathode resistor so I don't think this is the issue. I'll be placing an order for more parts once this issue is resolved.

Joel

All the RF by-pass caps should be ceramic disc capacitors.  Don't think orange drop caps are good for use in RF circuits unless the newer modern ones are but I doubt it.  Same with the all important RF coupling cap.  Use a high voltage mica or ceramic disc.   I use old TV high voltage door knob caps, but they're probably hard to find today.  You can buy new RF door knobs.

Fred

If the PA plate current is rising and the output dropping what is the PA grid current doing? Sounds like you are slowly losing grid drive. I notice on your schematic the grid current is 2 MA? That's way too low. Try 3.5 MA (the tube manual will say 2.5 but that's not enough). Your efficiency will increase and modulation will improve. I don't think 2MA is enough for the tube to start to run away at that plate voltage though. If in fact the grid current is dropping then start to look at the oscillator for the problem. I also would expect 20 to 25 watts out of that circuit, more with higher plate voltage. I would also suggest getting rid of that cap across the screen resistor. If you can't get 100% modulation without it you may not have enough audio power, or the mod transfomer is mismatched.
 

I bypassed the grid RFC and ensured all RF stage bypass caps are ceramic per Fred's suggestion. The coupling caps between oscillator and PA are mica.

On closer examination of grid current, I am indeed slowly loosing grid drive. It's slow and only about .5ma, but apparently that's enough at these power levels. I also checked oscillator output using the scope and peak voltage is slowly dropping over time. I measured about a 7v peak to peak drop. I tried three different 2E26s and then rewired for a 6V6 and tried two of those with no change in grid drive loss. I checked for overheating components and the only one that's getting hot is the plate resistor. That would be the case since I'm right at its rating. I replaced it with a 10K 20w resistor as I've done before to drop the output of the oscillator. This resistor drops plate voltage to around 150v and it's also getting hot. No other components in that stage are hot to the touch. I connected the scope to the plate of the oscillator and I'm seeing the voltage drop at this point. I then removed the 6146 and modulator tubes and ran the oscillator without load. After an initial drop in p-p voltage the signal stabilizes and does not drop further.

I'm not sure what to try next to get to the bottom of the issue aside from throwing it out the window.

Joel

What happens if you put a resistor in series with the crystal.  Start with 10K and maybe go higher if the oscillator keeps running.  Problem could be a bad cap that is changing value as it warms up or maybe a bad RFC.  The one in the grid circuit, if it is bad will load down the oscillator.  BTW what type crystals are you using??  The two cap voltage divider in the osc.grid circuit would be the first caps I would check.  What type caps are they??

Another thing, as the voltage drops does the oscillating freq stay stable or is that drifting??

Fred

Attached is the schematic of a low power (CW) transmitter that I used to own. I modified it to add plate modulation... and I used to operate it for long periods of time on key down/AM. It worked fine on 80m. For 40m operation, I had to add a tuned circuit to the output of the oscillator because the input capacitance of the 6L6 was too great a load on the oscillator on 40m.

Comparing to yours...

1. I think that the 50k ohm resistor that is in parallel with the crystal in your transmitter is a significant contributor to crystal heating in your transmitter... and the problem you are having. I also had a DX-20 that I modified for plate modulation (schematic attached)... and I had to reduce the value of the DX-20's 6CL6 oscillator's grid to ground resistor (across the crystal) from 100k to 18k to solve a crystal heating problem that was present on long periods of key down.

2. Separately, the attached "six lima six" transmitter schematic does not employ a tuned circuit at the output of the oscillator. As mentioned, for 40m operation, I had to add a tuned circuit to the "six-lima-six" oscillator's output to resonate out the input capacitance of the 6L6 power amplifier stage

The DX-20 does include a tuned circuit at the output of its oscillator.

I suggest that you reduce the value of the 50k ohm resistor to 10k ohms.

Stu

Excellent! I have homework to do now. Let me digest all the info provided and apply the mods and I'll report back.

Fred. The two cap voltage divider on the grid uses small ceramics. While I was on the air and watching my signal on a number of websdr receivers I could see my carrier drifting about half a Khz. So yes, there is some drifting of the carrier signal. 

Thanks guys!
Joel

OK great,  Stu thinks the problem is with the crystal current, that's why I suggested adding a resistor in series with it.  I thought it was a crystal heating problem when you first posted the problem, that's why I questioned the 2e26 oscillator.  Try what Stu suggested, reduce the 50K resistor.

He did this with a DX-20, the last time I had a DX-20 was 56 years ago.

Fred

Will do. Forgot to add that the crystals I'm using are FT-243 types from AF4K.com.

Joel

Joel

I believe the "real" FT-243 crystals are physically larger... and able to handle more crystal current than the AF4K crystals. The AF4K crystals are a newer, smaller type of crystal packaged inside of a reused, surplus FT-243 crystal package.

Stu

OK FB,  You have to hope that they are real FT-243s or a lesser crystal in a FT-243 case, which could be part of the problem.

Brian Carlings crystals are a known problem in some situations.  His FT243 are not the same as a 'old skewl' 243.

Search the archives.  Some people flatly refuse to use them.

I have no experience.  Just relaying what I see about the boards.

--Shane
KD6VXI

Well, I was really hoping that we had this thing figured out. I replaced the 50k resistor in parallel with the crystal with a 10K. There was no change to the power loss problem. I have an old school Texas Crystals FT-243 that uses the large crystal wafer so I tested with that one and the problem was still present. I then added an 8k series resistor to the ground side of the crystal. The grid current dropped to less than 1ma, but I still had the expected output power. It then dropped down to less than 10W within a minute. So I then wired my Heathkit VF-1, plugged it in and shorted the oscillator cathode RFC. I got 3ma grid current and output, but it then dropped to less than 10W as it has been doing. I aso replaced the two cap voltage divider caps with micas with no change. I also replaced the grid leak variable resistor with a 33k 1/2w since that's the closest value I had to 28k just in case, but same problem. I'm staring to think that it's not the oscillator now.

Stu, I have a tuned circuit in the coupling stage. Do you think that this is resonating out the input capacitance of the 6146?

Just thinking out loud, but I'd like to load the oscillator without actually having the 6146 in place to see if the grid current drops. I want to take the Rp of the 6146 out of the equation. Is it a bad component or something up with the tube I wonder?

Joel

Remove the plate and screen voltage from the 6146, leave the 6146 in place.  Turn on the oscillator and tune the grid tank for max grid current up to 3-3.5ma.  See if the grid current stays stable.

Also make sure your oscillator plate and screen voltages are not dropping for some reason.

This is a simple straight forward xmtr, you shouldn't be having this much difficulty. Somewhere there is a component that is failing under circuit conditions,  don't give up we'll find it.

Fred

Thanks for the encouragement Fred. Iv'e put a lot of time into this rig and I really don't want to give up on it. I'll run the test above and see what I get.

I have a 47ohm 1/2w resistor across each amp meter. This should be ok I think... I might have made a mistake in the build process so trying to think of anything that might be the issue.

Joel 

Joel

According to its specification sheet, a 6146 has an input capacitance (looking into the grid, with the cathode bypassed to ground) of 13.5pF. The 2E26 has an output capacitance of 7pF. If one adds the driver-output 60pF tuning capacitor, one obtains a minimum capacitive reactance at 7MHz of: 282 ohms; and a maximum capacitive reactance (assuming the variable capacitor has a minimum capacitance of around 6.5pf) of: 842 ohms.

If one considers the 5.7uH inductor in parallel with the 7.5uH inductor, one obtains a maximum inductive reactance of 142 ohms.

Therefore... one cannot make the capacitive reactance equal to the inductive reactance... and one cannot achieve the desired resonance at 7MHz.

I suggest that you replace the 5.7uH inductor with a 2.5mH RF choke (or whatever you have on hand that is 100uH or larger).

Put the 5.7uH inductor (now available for other uses) in series with the 7.5uH inductor

This will increase the maximum inductive reactance to that of a 13.2uH inductor ... i.e. a maximum of 580 ohms at 7MHz.


This should allow you the resonate the 6146 input by adjusting the 60pF capacitor.

You should adjust the 60pF capacitor (i.e. not set exactly at resonance) to obtain a target of 2.5mA of grid current on the 6146.

Having (not quite) resonated the 6146 input, you should be able to obtain much more drive voltage. The 6146 grid leak resistor (which should be a value that is reasonably close to 27k ohms) will "self bias" the grid to a DC value of -2.5mA x 27k ohms = -67.5VDC with respect to ground. The cathode resistor will bias the 6146 cathode to 392 ohms x 90mA (total cathode current) =35VDC with respect to ground. The total DC grid-to-cathode bias will be -67.5V - 35V = 102.5V... which will make the 6146 operate very far into class C.

You can always reduce the value of the 6146 grid current if -102.5V turns out to be too large a grid-to-cathode bias. The normal target grid-to-cathode bias is -85V. Taking into account the 392 ohm cathode resistor (and the associated 90mA of DC cathode current) and the 27kohm grid leak resistor... you would have to reduce the 6146 grid current from 2.5mA to 1.9mA if you want the grid-to-cathode bias to be -85V.

As was previously suggested by Fred, you should add a .001uF (or larger value) bypass capacitor between the bottom of the 2.5mH RF choke on the grid of the 6146 and ground. This will allow you to make an accurate measurement of the negative DC bias (relative to ground) on the grid of the 6146 (measured at the point where the bypass capacitor is connected) by removing the RF that would be present at that point in the absence of this bypass capacitor.

Stu

If your current meters are direct reading, meaning the grid meter is about 5ma full scale and the plate meter is say 150-200ma full scale you should not  have any resistors across the meters.  Resistors across the meters will alter the readings.  You can put a cap across the meters but you don't need them.

To add to what Stu suggested,  you have to make sure your grid tank circuit tunes to 7mhz.  You should see max grid current somewhere within the tuning range of the tuning cap, not at min or max capacitance.  Next you have to be sure it's tuned to 7mhz and not a harmonic (14mhz).

Having said all this, the issue you're having is the power drops after a few minutes of operation.  Your problem is not a design issue,  you're looking for a component that is failing under load.  If it was a design issue you wouldn't see higher power at first then dropping off.  You wouldn't see the right power at any time.

Fred

Maybe this has been covered, but have you looked at the grid DC and cathode DC levels to see if there are any increases (charging effects)  over time?

The reason I axe, is that over the years I've had several plate modulated rigs using fixed/ grid leak bias that did the same thing - the power output slowly went down after a minute or so.  The grid leak bias circuit (actually the fixed bias capacitor)  kept charging higher and higher with no end in sight. In my case, it took a diode in series with a 10H inductor coming off the fixed bias supply to keep the charging effect from occurring. (blocking effect) The fixed bias supply DC filter cap was simply charging up from rectified RF getting into it. The Tron suggested this fix to me and it worked like a W1GAC charm.

I see you have a different biasing arrangement and if no help to you, maybe this suggestion can help someone else who comes across this problem.

Tom, K1JJ

Looking at your schematic, I agree with Stu, that 5.7uh coil in the osc. plate circuit is a mistake.  It's actually in parallel with the grid tank coil because of the large coupling cap between them.  Replace it, like Stu suggests, with another RFC.  Try not to use the exact same type you have in the cathode circuit to try to avoid additional oscillations at some other frequency.  If you don't have another RFC you can just use a resistor, say 10K.  Check the power dissipated in the plate resistor.  You may need something greater than 2 watts.

Fred

Thanks Stu! Your detailed explanation was much needed and greatly appreciated! I've printed it out.

I'll switch to an RFC in the oscillator plate and dial in the coupling stage tank values. I'm using an inductor on the oscillator plate now because I'm out of RFCs. I'll have to "borrow" one from... I wound the tank inductor myself so I can redo it for whatever value if needed. The bias explanation was awesome and with it I'll be able to confidently dial in the 6146 bias now.

Thanks again
Joel 

Is that 2.5 mH ferrite plate choke?

Ferrite core chokes tend to heat up too much and saturate.

Try a 425 uH air core choke.


Phil - AC0OB

Joel
et al.

Recall that it takes two things to make an oscillator

1. Positive feedback

2. Saturation

The positive feedback is controlled by a combination of the 126pF and the 10pf capacitors... working in conjunction with the crystal ... to select the frequency where the phase shift around the feedback loop is 0 degrees (i.e. equivalent to 0 degrees)

The saturation occurs because the plate current of the 2E26 flows through the impedance of the tube's output load... to produce a plate voltage drop.

If the output load impedance that the 2E26 is looking into is too low (as it is with the 5.5uH inductor in parallel with the 7.5uH inductor... and not enough parallel capacitance to resonate the output circuit)... the saturation will occur at a relatively high 2E26 RF output current. For illustration: at 7MHz and 200mA of peak RF plate current, the peak of the RF voltage drop across the output load is only j142 ohms x 0.2A = 28 volts).

The high value of RF plate current at saturation goes along with a high value of grid-to-cathode voltage. For illustration, with the transconductance of the 2E26 being only 3500 umhos (which is much lower than that of a 6CL6)... and with at least 200mA of peak RF plate current... the peak RF  grid to cathode RF voltage will be greater than 200mA / .0035 mhos = 57V.

This large 2E26 grid to cathode voltage will correspond to a very high crystal current. Also, the plate dissipation associated with the high plate current will cause problems for the tube.

By making the changes needed to present a resonant output load to the 2E26, the peak RF plate current of the 2E26 oscillator will be reduced (i.e. the oscillator will saturate at a much lower amplitude RF plate current), and the amplitude grid-to-cathode voltage will be correspondingly lower... leading to a correspondingly much lower crystal current... and therefore much less crystal heating.

Stu

Stu,

The circuit would work better if the grid tank was in the oscillator plate circuit but probably the tuning cap is grounded to the chassis.

In both my HB 6146 and HB 813 rigs the grid tuning caps are isolated above ground.

Joel, in his last post, said he didn't have another RFC, so he replaced the 5.7uh coil with another inductor of some unknown value.  I suspect this is not going to work any better.

Possibly if he used the slug tuned coil and put the right size cap across it to have it resonant.  Problem is the tuning of the grid tank will greatly affect the resonance.  To reduce these affects the coupling cap between the two tank circuits would have to be greatly reduced.  Right now I think there is a 500pfd coupling cap, reduce it to 100pfd.  The best way to do it is to use link coupling between the two coils.

Fred

Hello Stu, Fred and rest,

I've made all the changes suggested. Here is the latest schematic. After removing the oscillator plate inductor the oscillator output is way down. I'm measuring 16vp-p out of the oscillator now. I was at over 100v before. With the 6146 online I'm not getting any grid current measurement on the meter. I'm still loosing power also. I put a 10k resistor in place of the plate RFC to make sure that it wasn't due to some interaction between the two RFCs, but same low output from the oscillator. I lowered the blocking cap to 100pf mica and the coupling cap to 1nf mica. I have the tank set to 60pf and 7.9uh which puts me right around 7.290MHz and 364 ohms. What am I missing here?

I disconnected B+ to the plate and screen of the 6146. Grid current went up to about 2ma and stayed there. Reconnecting B+ drops grid current to less than .5ma with accompanying eventual loss of power. I removed the shunt resistors on both meters. The transmitter is a few inches closer to my open window now. 

Joel

Joel

You said you removed the 5.7uH oscillator plate inductor... but what did you put in its place? You need to replace it with a larger inductor (100uH or more) to feed the DC plate current, but to block the 7MHz RF component of the 2E26's plate current. This inductor needs to be rated to handle the 2E26 plate current.

Alternatively, as Fred suggested, you can just move the 7.5uH inductor and the 60pF variable capacitor to the place where the 5.7uH inductor was. That way, the 7.5uH inductor will provide the path through which the DC plate current is fed to the 2E26.

Are you able the resonate the oscillator plate load by adjusting the 60pF tuning capacitor to produce an oscillator output voltage swing that is large enough to produce 2.5 mA of 6146 grid drive?

Hi Stu,

I put a 2.5mh RFC in its place. I attached the latest schematic as a pdf on my last post since I've made a number of changes.

No, I am no longer able to adjust the tuning capacitor to produce anything close to 2.5ma on the 6146 grid. I'm a bit confused as to why this is the case. I think I'll try moving the tank over to the oscillator plate. I'll have to use a fix value capacitor and rely on the inductor for tuning.

Joel

Joel

Okay

Moving the tuned circuit to the output of the oscillator will allow you to avoid the extra 2.5 mH RFC.

Resonating the tuned circuit is critical. As you approach resonance, the 6146 DC grid current will rise dramatically (following the rise in amplitude of the RF component of the oscillator's output voltage)

Stu

OK FB,  I studied your schematic,  everything looks right.  You say you have the grid tuning set at the values stated.  Are you tuning and peaking up the grid tuning.  Do this by peaking up the grid current to max current.

With the B+ removed from the 6146 you have 2ma grid current and it stays steady.  Reconnecting the B+, you must retune the grid back to the max current, are you doing this??

You must tune the 6146 tuning to dip the plate current which may cause you to have to retune the grid again.  Dip the final with light loading to your 50 ohm output watt meter dummy load.  If you then increase loading, you must go back and retune the grid, re-dip the final again.

You should reach some level of power output (whatever 15-20 watts) that stays steady.

I think the grid current drops when you reconnect the B+ because of the cathode bias resistor.  Try your testing with the 6146 cathode grounded.  Just watch the plate current doesn't exceed 130 ma.  You must due all the tuning quickly.

You said earlier that you didn't have another RFC for the osc plate load and put an inductor for the plate load.  What is the final component you're using, coil or RFC??  This part is critical to getting enough voltage (power) to the grid of the 6146.

Fred

Ok, thanks Fred. Shorting out the cathode resistor is indeed allowing me to generate about 1.2ma of grid current with the B+ connected now. Why is this the case? I'm right around 1.4ma of grid current with B+ removed on the 6146. I followed your tuning process and that's the highest that the grid current will go right now. With B+ connected, grid current stayed at a solid 1.2ma while I watched my power output drop from 20W all the way down to 7W! The cathode resistor is off the "bad component" list since I have it shorted out right now...

One thing I'm noticing about the tank in the coupling stage is that the coil (7.15-13.8uh range) is not giving me the peak in grid current on either side of its sweet spot of 7.9uh (per the resonance calculation). Grid current rises as a go lower in inductance down to its limit and I'm sure I could get higher grid current if I could go down to say 5uh... This doesn't make me feel warm and fuzzy inside about this stage.

The RFC in the oscillator plate is the one I had in the 6146 grid stage. I "borrowed" the grid RFC from the Globe Scout and put it in the grid circuit of this rig. I then moved the spare RFC over to the oscillator plate. So I'm using legit RFCs on all the stages.

I'll try moving the coupling tank over to the oscillator plate to get rid of the RFC and see if I can get good resonance with the 60pf & 7.94uh values.

Disclaimer: No classic vintage rigs where destroyed in the making of this HB rig.

Joel

Joel

Grid current flows when the grid voltage is positive with respect to the cathode voltage.

With the 6146 cathode resistor in place (now bypassed), the cathode voltage is positive with respect to ground by an amount equal to the cathode resistor value (392 ohms) x the average cathode current that is flowing.

If the average cathode current is, for example, 90mA... then the cathode is positive with respect to ground by 0.090mA x 392 ohms = 35.3V.

Therefore, with the 392 ohm cathode resistor in place (not bypassed) and with 90mA of cathode current flowing (i.e. the plate voltage supply and screen voltage supplies are connected), the grid-to-ground amplitude of the RF grid voltage (from the oscillator output) has to be 35.3V higher to produce the same grid-to-cathode voltage (and therefore the same average grid current) as when the plate and screen supplies are disconnected (no 6146 cathode current).

Separately, the 100pF capacitor between the plate of the oscillator and the top of the tuned oscillator tank circuit... that is in your latest schematic... is likely preventing the tuned circuit from providing a resonant (high impedance) load on the oscillator's output... and, therefore, preventing you from getting sufficient peak grid-to-cathode voltage on the 6146. Its reactance at 7MHz is: 227 ohms.

Relocating the tuned circuit to replace the 2.5mH inductor that is feeding the plate of the oscillator (or even just increasing the 100pF capacitor value back to a few nF) should allow you to produce the necessary high impedance load for the oscillator

Stu

When you re-connected the B+   the current thru the 6146 created a positive bias on the cathode which put the grid more negative.  This condition requires that you have greater drive voltage, which you don't have.  That's why the grid current dropped to .5ma.  The grid of the 6146 only draws current on positive peaks of the drive voltage.  This grid current creates the neg. grid bias thru the grid resistor.  This pushes the 6146 into class C.  Grid voltage has to be high enough to get about 3ma of grid current.  The 6146 only conducts on about 70 degs of the peak voltage.  The 6146 only puts out pulses of energy into the pi network.  The flywheel effect of the output tank circuit creates the output sine wave to the antenna.

So, you can see that you need more drive than you are now getting.   One thing you need to look at is the coil you are using in the grid circuit.  This coil along with the tuning capacitor must resonant on frequency.  If you are using a slug tuned coil this could be part of your problem.  The grid coil needs to be an air coil of the correct shape.  Something about 3/4"- 1" in diameter is about right,  close space winding with maybe 20ga enamel wire.  This type of coil will have a greater Q that any slug tuned coil.  The Q of the coil is what determines the peak voltage created by the grid tank circuit.

Once we get the drive working correctly it seems you still have the original problem, the power dropping off.  Are you sure your power meter and dummy load are working right.  If you are sure the meter is OK and reading correctly,  I would next look at the plate coupling cap.  If this cap is not up to the job for a plate coupling capacitor it will quickly overheat and probably change value.

You mentioned moving the grid coil to the osc plate with the right cap.  Problem here is that a small cap in the osc. plate tank circuit may also get hot,  an air cap will work better, if you have one.

Fred

 

I think another design change that would again work better is where you have the grid tank circuit.  A better design is to have the grid tank coil and cap in series with grid RFC and grid resistor.  This requires that the grid tuning cap be isolated above ground.  Not sure what type cap you're using.  Why is this design better you ask, I think the grid resistor loads down the grid tank thru the large coupling cap that you have going to the grid.  This lowers the Q of the circuit which lowers the peak voltage developed across the coil.  Not sure how much the RFC lessens this.

So you would have the grid coil and parallel tuning cap connected directly to the grid, the 100pfd coupling cap (from the osc. plate circuit) would also connect to the grid.  At the low end of the coil you would have a by-pass cap to ground with the RFC connected there also.  Another by-pass cap to ground at the low end of the RFC.  Then connect the grid resistor to the low end of the RFC and meter to ground.  Problem is you need to isolate the tuning cap frame above ground.

With this design the grid resistor is no longer across the coil, it is in series with it which removes any loading on the grid tank circuit.  My HB xmtrs are designed this way and it works perfect every time.

Fred

Thanks Stu and Fred for the detailed explanations, this makes sense to me now. I moved the tank circuit into the oscillator plate and played around with the values a bit. I'm now getting plenty of drive and 3ma on the grid without issue. Problem is that I am using a slug tuned coil and a mica cap... I'll have to wire an air coil of 1" diameter and see how I can isolate a variable cap. I think I have something that might work...

That's correct Fred. After all of this tinkering I still have the original issue. I am confident that the power meter and dummy load are working. I've been using these for a number of years on different gear and all looks good there. The plate coupling cap is the 1nf 6kV coupling capacitor from the Globe Scout which I replaced the old one with when hunting for the source of the problem. I've attached the latest schematic for reference. I'll be working on the correct components for the oscillator tank.

Joel

Joel

Try increasing the 2E26 cathode-to-ground capacitance to 200pF... by tacking on an additional 100pF in parallel with the existing capacitor.
 
This should reduce the feedback... and cause the oscillator to saturate with a smaller plate current RF component when the tank circuit is tuned close to resonance.

That may reduce the heating of the crystal and the 2E26 plate by a significant amount... while still allowing you to obtain sufficient grid drive for the 6146.

Stu

Are you sure the power supply voltage are not dropping under load??  What about the filament voltage??  How did you wire the filament??  Should have one side grounded at the xfmr and only one lead going to the tube with the other filament pin grounded at the tube socket.  Also the filament lead needs a by-pass cap to ground right at the filament pin.  Use at least a .01ufd by-pass for the filament.  This is also needed at the oscillator socket.

Your power loss is starting to get puzzling.  You have to check all the components around the 6146 (this is assuming you're getting a steady 3ma grid drive.

Still wouldn't hurt to try some other coupling cap just to be sure.  Make sure all the connections and solder joints look good.  Checked screws holding the pi-network caps (BTW these screws in the RF tank circuit should be brass).  Your schematic shows a rotary coil in the pi network, make  sure everything is making a good tight connection.  Make sure the 52K screen resistor is good.  Loss of screen voltage level will cause the power to drop.

Check the parasitic coil and resistor.  Your problem could be a parasitic but I don't think it would cause the power to drop after a few minutes of operation.

Only other thing left is the plate RFC.

Fred

Stu,  Do you mean the cathode to ground capacitance??

Fred

Yes... I meant the cathode to ground capacitance. I will correct this in my earlier post.

Thanks
Stu

Removed duplicate

Hi Joel,

You have received a lot of good info from Stu and others.

Here is my $0.02 in schematic form.

Regards,

Phil - AC0OB

I had the same problem with a homebrew project some years ago. Timtron and Steve helped me troubleshoot it while we talked on the air. The symptom was the same.  After transmitting, power would drop off dramatically after maybe 10 seconds on my dual 3-500 linear amplifier.  I would have to re-tune the plate cap to keep power up while I transmitted.  

The problem was the plate coupling cap.  I had used a cheap, TV type one and the heat would cause it to change value during key down periods.  I replaced it with a good doorknob type and the problem cleared.

I know you're using a known good one borrowed from another rig, but I thought I would still just throw the idea out there.

If re-adjusting the plate cap brings your power back up, this might be the problem.
 
Jon

I like Phil's (DMOD's) changes... including the changes in design of the oscillator tank circuit to allow the variable capacitor to have the stator side grounded, and the separation of the oscillator screen voltage dropping circuit from the oscillator plate voltage dropping circuit (i.e. both connected directly to the HV supply).

It is important that the 2E26 tuning inductor (shown in Phil's diagram as 10.5uH) not be too large or too small. Phil's 10.5uH value is a good choice... with around 482 ohms of reactance at 7.3 MHz. This will resonate (at 7.3 MHz) with a capacitance of around 45pF. That includes the input capacitance of the 6146 + the output capacitance of the 2E26 + stray capacitance. So the required value of the variable capacitor... to achieve resonance... will be around 25pF. The inductor should be rated to handle at least 100mA of DC, and it should have a self resonant frequency that is well above 7MHz. A fixed, air-would inductor would be a good choice. I.e. no problem with saturation of a ferrite core. With an air-would inductor, you can always reduce the 10.5uH inductor's value by connecting one side to a tap, rather than to the end. Reducing the inductance would allow you to use a higher capacitance value for the variable capacitor... to achieve resonance.

The comments Jon and others have made about the choice of 6146 plate blocking capacitor are (to me) compelling... because they are based on actual past experience that others have had with the problem of slowly decreasing output power on key down.

Stu

I moved the tank circuit to the oscillator plate. I'm using an isolated 17-214pf air variable and a coil wound on a 3/4" form. I lowered the inductance to around 5uh as I wanted a little more play room on the variable. The 75pf variable value is what I'm reading in circuit. Out of circuit it's around 50pf. I'm now able to get well over 3ma as I tune through resonance. It's tuned on the high side right now. Thanks Phil for your schematic. I've applied some of the modifications. For some of the others I don't have the appropriate components right now to implement them. I have a 220pf cap on the cathode-to-ground of the 2E26 and a 47ohm resistor in series with the crystal. I'm able to get accurate grid bias readings now and the values match perfectly with the calculations. Overall I feel that the rig is dialed in nicely. Except that I'm still loosing power! My grid current might drop .25ma during the loss of power, but I can easily bring it back up without any improvement to output power. Per Jon's comment I also can increase power a little bit by adjusting the PA tune capacitor, but it ends up fully open and that's end of story at that point.

I have an Apache waiting to be restored on the bench so I borrowed the 500pf 5kv door knob from it and have it now on the HB rig. I'm still loosing power! What the heck!?!

Fred, as the output power goes down I drop about ~5vdc of B+ voltage. My plate current goes up about 10ma so I guess that makes some sense... My filament voltage is steady at 6vac. I do have the filaments wired as per your description. I added a filament bypass capacitor to both the 2E26 and 6146.  

Could I be having an issue with my PA efficiency? Could my Pi network not be coupling enough power out of the PA and causing a heat build up? The best I've done so far is 66%. 30W in & 20W out. Then down to 10W out... If my Pi tank inductor is not up to the job, could it cause this problem? I was using a rotary coil from an old tuner and taping it, but then wound my own coil with a similar design as the Globe Scout. It's about 20 close wound turns of 22 gauge on a 3/4" form. I was having the same issue before on the larger coil as well.  

When I was building the PA I had a parasitic oscillation around 117MHz. Since adding the suppressor that issue has been resolved.  

My power supply uses a Tektronix transformer 120-0140 from an old Oscope, two 1N5399 diodes as full wave rectifiers, and for the filter a 150uf 450v/choke/150uf 450v combination. I'm truly out of ideas at this point. I really appreciate all the help with this issue.

I've attached the latest schematic with all changes made.

Joel

Joel

Please post a version of the schematic that shows the values of the same voltages and currents at a point in time where the power output has dropped to 10W (without changing the tuning capacitor or the output loading capacitor values).

Of particular interest (to me) are the post-power-drop values of 6146 screen voltage, 6146 cathode voltage, and 6146 grid voltage (measured at the bottom of the grid choke).

Stu

A few volt drop in B+ doesn't mean much, that's not the problem.  This probably also doesn't mean much but the cathode, G3 and internal shield are connected to pins 1,4,6. At the socket pins, wire all three of these pins together with a short wire going right over the socket.  Next, pin 8 is the base sleeve. Do you have this pin grounded??  If not, ground it.

Seems the problem is not the blocking cap.  You're increasing the loading to max when the loading cap plates are open.  Jon said that he would readjust the PA plate tuning cap, not the loading cap.  Try the test over and leave the loading cap as set for the highest power out.  When the power drops readjust the plate tuning cap.

The plate RFC is about the only part left that, if it's breaking down, could cause a loss of power.

Lastly, are you sure your watt meter and dummy load are working correctly.  If the dummy load goes up in resistance you would have to increase the loading to maintain power output.

Fred

Joel

If the PA loading capacitor is not fully open... when it is adjusted for maximum power output and the transmitter is first turned on...

And if the PA loading capacitor is fully open... when (re) adjusted for maximum power output after the output power drops...

Then, that implies that either a) the Q of the LC output circuit (i.e. the Q of the coil) has increased (not likely)... or there is something in parallel with the adjustable loading capacitor that is adding additional loading capacitance after key down...or there is something across the output that is acting like a resistor whose value is decreasing.

What could that be? Perhaps the RF output connector is becoming leaky when it heats up.

The RF load impedance that the 6146 sees is Q x Q / R-effective... where

Q is the Q of the tuned circuit consisting of the tuning capacitor and the inductor of the pi network.

R-effective is [the transmitter output load impedance that is connected to the output of the pi network (i.e. ideally 50 ohms for a dummy load), taking into account any parallel resistance associated with a leaky connector or a bad piece of coaxial cable] / k

and

k is {1 + [2pi x f x C-loading x the actual antenna output load impedance] x [2pi x f x C-loading x the actual antenna output load impedance]}

Normally, when tuning up... you increase the output loading capacitance in order to reduce R-effective. Reducing R-effective increases the RF load that the tube sees.

If Q increases or if R-effective decreases... then the RF load that the tube sees will get too large... causing the output current to drop.

Stu

Joel,

According to my MatLab simulation using the Wingfield Pi-Net equations, and using your Vp and Ip, and F= 7.05 to 7.3 MHz,here are the results:

Qi = 12
Cplatetune = 111 to 109 pF
Lpi = 5 uH (stayed around 5 uH)
Cantune = 588 to 568 pF
Final RF plate choke = 210 uh.
Rplate = 2.14k
Efficiency = 68%

I am with Stu, check the voltages across the screen grid dropping resistors and other dropping resistors and so we can see what stage is dropping.

For power drops, I too have seen the output coupling cap cause this as well (as Jon noted) and if the 6146 screen overheats this can cause this as well.

It would also help if you can notate the components.


Phil  

Joel

Adding to my most recent prior post:

Could it be that the resistance of the dummy load that you are using is getting significantly smaller as it heats up?

Stu

Stu,

I questioned his dummy load.  More likely the resistance may be increasing.  

Question, if the load resistance increases would the loading have to be increased or decreased to maintain a certain power??  I think increase the loading.

2nd question,  for an increase in loading the loading cap would have less capacitance or do I have this backwards.  I'm trying to remember which way it works in my xmtr.  I can not see my loading cap and it has turns reduction unit on the shaft.  The loading cap is a four gang 525pfd (each) cap.  Two sections used for 40M and all four sections used on 75M.

Fred

Fred

If the resistance of the dummy load that is connected to the antenna port increases, then the value of the loading capacitance must increase... in order to produce the same RF load impedance on the tube side of the pi network.

Stu

Sorry guys, when I said PA loading cap I meant PA tuning. The tuning cap is the one that ends up fully open (10pf) as I try to increase power. I've correct my previous post.

I've attached the schematic with pre and post power loss voltage and current values. The bottom one is pre and the top one is post. So for the 6146 plate, the voltage went down and current went up as power was lost. The voltages were taken in respect to the cathode. Let me know if you rather me take the measurements differently. The cathode measurement is to ground. The 2E26 grid measurement caused oscillation to stop so I wasn't able to get an accurate reading there. Currents and voltages stayed pretty constant in the oscillator stage.

Thanks for running that simulation Phil. So looks like I need more antenna tune capacitance. I had a feeling that was the case. Overall that doesn't look too bad right? I've noted all components.

Regarding the dummy load. I'm having the same problem on the air using my Delta Loop so this "should" rule out the dummy load. I made the connection between 1-4-6 on the 6146. Pin 8 was grounded already. 

Joel

Q1: Do you have a Forward/Reverse Power meter between the transmitter and the dummy load? Maybe reflected power is increasing after the dummy load heats up?

Q2: What kind or type of dummy load do you have?

Q3: Have you bypassed the mod transformer T1 and placed your 370V directly to the RF section?

Comment: If the B+ is really 370V, then you have a rather large voltage drop of 70V across the T1 mod transformer.


Suggestion: Add a 220 pF 1 kV ceramic cap across the Cantuner.



Phil - AC0OB

H Phil,

Q1: Yes, I'm using an MFJ 962D tuner. Reflected power is staying low throughout key down.

Q2: I'm using a cantenna type dummy load with the large resistor immersed in mineral oil.

Q3: The 6146 voltages are taken in reference to the cathode. If I connect to ground then the plate voltage is much closer to 370vdc. I'll have to take this measurement again and report back, but it didn't look alarming to me when I took it last. The power supply idles at around 440v and drops to 370vdc under load with all tubes plugged in. I was having the same issue before adding the mod transformer if I recall. I can bypass it and confirm though.

Joel  

Joel

If the pi network tuning capacitor is fully open (when maximizing the output power) after the power drops.... but not fully open when you first "key down"....

Then, the inductor in the pi network has too much inductance when the power drops (due to heating) ... and probably has more inductance than it should, even when it isn't hot.

If the pi network inductance is too high (on key down, when it is not hot)... you may be able to peak the output power (i.e. resonate the LC tuned circuit)... but the Q will be higher than it should be. This will make the pi-network very sensitive to changes (due to heating) in the inductance of the pi-network inductor.

Stu

Thanks Stu, interesting. So looks like the inductance should be more in the order of 5uh for the coil. I've never built a large Pi network coil before. What are the best dimensions to avoid this heating business? I'm thinking a trip to Home Depot to get some bare copper wire is in order.

Joel

Joel

If you have room (making sure the coil isn't too close to the chassis or the front panel of the transmitter)... I would suggest a target diameter of approximately 1 inch. The copper wire size you use should be #14 or #12... just to be conservative on skin effect resistive losses.

There are calculators on line that you can use to calculate the inductance of a simple solenoid... given the wire diameter, the coil diameter, the total length of the coil, and the number of turns.

If the coil that you wind turns out to have too much inductance... you can reduce the inductance by stretching out the coil to increase the spacing between turns.

You can wind the coil around a piece of PVC pipe... but make sure to remove the PVC pipe after the coil has been wound. Many plastic materials (including PVC) will absorb RF ... and cause losses.

Stu

Thanks Stu. Let me work on the coil build then so that we can cross this component off the list. I'll report back.

Joel

To calculate the values for a pi-network you need to figure the plate load.  Plate load is the plate voltage divided by twice the plate current.  370v/200ma= about 1850 ohms.  From the chart in the Radio Handbook your plate tuning cap needs to be about 150pfd, so you need about a 200-250pfd cap.  The inductor needs to be about 4uh.  The loading cap needs to be about 1000pfd.  So you need a cap with about 1500pfd for the loading.

The coil, (with 20 watts) should be about 1.5' to even 2" in diameter.  14ga bare wire is fine,  the coil should not be close spaced.  The coil length should not be more than 1.5 times its diameter to get the right Q.  With 4uh a 2" coil probably would not be even an inch long.

The coil should be away from other metal as much as possible but it can be close to the tuning cap.  The axis of the coil should be parallel with the shaft of the plate tuning cap.  The loading cap can be anywhere.

Another thing,  as the plate load gets lower the value of the inductor goes down and values for the caps go up.

Fred

Thanks Fred. Hmm, a 1500pf load capacitor... That's going to be a problem. I don't have a variable with that much capacitance right now. What are my options? Can I put a capacitor in parallel with it maybe?

Joel

Hang in there Joel, when this is all over you'll be a HB xmtr expert.

Well, at least at the 20 watt level. ;D

Fred

At a low power level you can use some fixed caps to get that much capacitance.  Add some micas if you have them.  Voltage rating does not have to be high.  1000v caps is more than enough.  Better to use a number of lower value caps than just a single cap.  This will divide the current through a number of caps rather than just one.  Keep in mind that the loading cap has very low voltage on it.  Most of the time a common BC radio tuning cap will work fine, even at much higher power levels.

Fred

Joel,

For a Plate current of 90 mA, your Pi-Net values are as follows:

Cplatetune = 120 pF

L = 4.4 uH

Cantload = 613 pF;


For coil calculations I use:

http://hamwaves.com/antennas/inductance.html

which is one of the more accurate online coil calculators. Here is the basic data:
 
D   = 25 mm   Mean diameter of the air core coil, measured from wire center to wire center

N = 20            Number of turns

ℓ = 40 mm   Length of the coil, measured from the connecting wires center to center

d = 1.29 mm   Wire diameter (#16; Most Low power rigs such as the HT-40 and DX-60 used this diameter)

from: https://www.tedpella.com/company_html/wire-gauge-vs-dia.htm

f = 7.2  MHz   Design frequency

L = 4.55 uH     Inductance




Phil - AC0OB

Thanks Phil, Fred and Stu! I'll get to work on this and report back.

Joel

I know there are many approaches to circuit design, but the schematic you showed appears to be a prototype development schematic due to the non-standard values.


So FWIW, below is a complete set of schematics for your consideration.

Most of all, have fun, which you appear to be doing. :)


Phil - AC0OB

Thanks for the schematics Phil. They are most appreciated. That is correct on the prototype dev schematic. I built this rig on an old mobile vhf transmitter chassis I got on eBay for $10 +shipping. The rest of the parts came from my junk drawer. Also, I used the actual values of the components as I measured them. Probably not a good idea, but...

Good news, I found the problem! It was the wire I used between the plate coupling capacitor and the PA tune variable (pic attached). Oops...  Let's just chalk this up to my learning curve ok. :-[

I replaced the bad wire with a short run of enamel copper wire. I'm putting out a steady 24 watt signal after a minute plus of keying down with no loss of power. My efficiency is up to around 76% also.

I built the Pi network coil using #12 copper wire as well. Its D=2.4", N=11, ℓ=2.8" and L=4.6uh on the meter. The Pi variables are both about 50% open so plenty of room for adjustments.

I want to thank Stan, Fred, Phil and all the rest of you for helping me with this problem. I now have a much better working RF stage thanks to your advice and guidance. We started at the crystal and worked our way to the Pi network where the problem was, awesome!

Thanks again!
Joel

Ok great,  you found the problem.  What exactly was wrong with the piece of wire??

I like the size of the coil you made for the pi-network.  Just slightly longer than the diameter.  2.4" diameter is good, much better than a small 1" coil.

Fred

A two tube set and it only took us a week to get it working.  Not too bad, good thing it wasn't a five tube set. ;D

Tell me about it! And I had been fighting this thing for weeks before finally giving up and asking for help.  :)

The wire is about 7" long and it is reading 6.5 ohms of resistance and 35pf of capacitance. Really?!?

I have one more question regarding the secondary impedance of the Mod transformer. I read that to calculate the modulating impedance you take the PA DC pate voltage and divide by the DC plate current. That's 370/.090=4100. Is this correct? If so then I'm leaving the secondary at 4200 ohms.

Thanks
Joel

Awesome Joel. What kind of cable/wire was it?

A secondary load impedance of 4200 is correct, but I question the Plate to Plate primary impedance.

Are you going for Class B or  AB2?


If you are targeting Class AB1 then see the last schematic.

Phil - AC0OB

So it looks like the coax (or audio cable?) had its shield grounded?  Did we have a 50 ohm cable in a high impedance RF plate circuit with a massive swr?

Did the cable act like a padding capacitor with a lossy dielectric that heated up and caused the power loss?  The 10 watts of heat had to go somewhere.  The center conductor was big enough to handle the current.

Yes, a single wire or strap is all you need to connect any of the pi network components together, including the plate  circuit.

That was a fine piece of troubleshooting and helpful suggestions, guys!  In hindsight, I'll bet it would have gone quicker with a few close up pictures of the rig...

T

What is the wire, a piece of shielded wire??  A seven inch long wire in the PA plate circuit is way too long.  Better placement of parts is important in the PA.  Leads should be short.

As for the mod xfmr impedance, that is correct  PAv/PAa and 4200 ohms is close enough.

Oh No,  I forgot about the modulator.

Fred

Yes a few pictures, just what I was thinking.  Probably would have taken us two minutes to spot the problem.  Piece of shielded wire in the PA plate circuit.  I always told him that the problem is a part breaking down under load. Who would think that there would be a piece of shielded wire in the plate circuit.

And so we shift over to autopsy mode:

Hmmm... I wonder why that shielded cable had a 6.5 ohm center conductor?  Strip it and find out.

Actually, normal shielded wire cable would have worked OK IF the shield was left ungrounded. It would act like just another piece of wire. But grounding the shield made it a padding cap and a lossy transmission line at huge swr, I suppose.  

Usually there is not a big demand put on the path between the tube plate and the first tuning capacitor of the pi-network. The big current is inside the pi-net resonant circuit. If that wire was there instead, it may have caught fire and made it easy to find.

Still, it showed up when the tuning had to be "chased" as the heat increased showing something was happening AFTER the tube plate.  I wonder if the shielded wire actually got warm?  Touchy-feely troubleshooting.

T

It appeared to be a shielded, single conductor audio cable (could be wrong) and I too would like to know exactly what it was.

If it was, I suspect (as you) the dielectric heated up, melted, and partially shorted the shield to the inner conductor.

A shielded cable will only possess a measured, specific impedance when terminated in it's characteristic impedance:

http://www.standard-wire.com/coax_cable_theory_and_application.html

And usually a shielded wire is shown as thus on a schematic and nothing was shown as one:

Same thing I thought, the shielded cable wasn't shown on the schematics

A short* length of coaxial cable, having a characteristic impedance of Z₀ ohms, can be represented by the following, simple, lumped element circuit:

1. A capacitor between the center conductor and the shield, having value C (farads) = C₁ x D... where C₁ is the capacitance per unit length specified for the coaxial cable, and D is the length of the piece of coaxial cable

2, An inductor in series with the center conductor, having inductance, L, equal to Z₀ x Z₀ x C (henrys)

For example:

If Z₀ = 50 ohms, D = 0.25m, and C₁ = 80pF per meter

Then C= 20pF, and L= 50 x 50 x 20 picohenrys =  0.05uH

Stu

*I.e. where 2pi x f x C x Z₀ <0.3


 

Duplicate deleted

Phil, I'm using Class AB1 on the modulators so will study your schematic. I went with 9k p-p impedance on the primary as I tried to follow the values on the datasheet for AB1 (attached). If this is not correct, I can change the primary impedance easy enough since I'm using a UTC S-19 transformer. I don't see a secondary of 4K though so might be a bit off there if I change.

Yes, the wire was a piece of shielded audio wire from the old VHF transmitter. I had it grounded on both sides. I'm also using the same type of cable in the audio stage. Let me try to find a shielded cable symbol so that I can add to the schematic. I'll take it apart later to see if it was breaking down or melting. When I was checking for warm components I didn't think to check the wires also so not sure if that one was getting hot, although I'm sure it was. The wire was between the plate coupling cap and the Pi tuning capacitor. It was that long because I have the choke, parasitic suppressor and coupling cap under the chassis and the Pi network on top. Since the radio is wide open on top I didn't want to have those high voltages so exposed... I'll work on the placement of the parts in that stage more when I start working on the enclosure. As it stands now the piece of copper wire in use now is about 6" long still, but everything is working fine now...

In my defense I did make the statement that I had a feeling that it was something I did during the build process, just didn't know what it was.  ;D

Joel 

For class AB1 approx. 9k plate-to-Plate on the primary and approx. 4k on the secondary should work.

Hey, we all learn by doing.  8)

One other note with ref. to schematic 1. Depending on the voltage of the Keybias line, you may have to experiment with R7 in order to get the voltage up around 50V in Keyup in order to make sure the Mod. and RF stages are shut off during Keyup.

In my view, there is no need to control the speech amp or phase splitter in the audio section since their power dissipation is minimal.

Glad to see it is operating as expected.


Phil - AC0OB

The PA plate RFC should be on top of the chassis.  Coupling cap should be on top near the plate tuning cap and RFC.  The parasitic choke and resistor should be on a short lead right off the plate terminal ceramic cap to the RFC.  The by-pass cap on the RFC should also be on top of the chassis.  The RFC should be standing upright bringing the top end closer the tube plate terminal and other connections etc.  Only thing that should be coming through the chassis from below is the modulated B+.  The screen resistor for your low power rig can be below the chassis.

Fred

Thanks Phil. I debated as to not bothering to key the phase inverter. I compared my design with a Lettine 240, among others, as I was building and that one had it being keyed so that's why mine is like that. I think I'll just ground the cathode for that stage as well. So R7 is used to keep the keyup voltages from soaring correct? That's a concern I have in my current design...

Thanks for the placement details Fred. I'll be moving those parts above the chassis once I make this rig safer to operate.

Joel

Easiest way to key the modulator is to key the 6L6 screen voltage.  The 6L6s will shut off with no screen voltage.

Correct. It has been my experience that you need at least 50 volts on Keyup in order to completely cut off those stages.

I use a 25k 5W WW pot. between the Keybias line and ground to set the resistance needed for the cut off voltage.    

In some cases I have had to use a pullup resistor of about 220-270k from B+ to that line in order to get around 50V. It just depends on how much leakage current is available through the tubes .


Phil

Actually. the Lettine 240 only switched the B+ to the phase inverter, not the cathode circuit.

The only circuits keyed are the Osc. and the Final.


Phil - AC0OB

Thanks Phil. That's right, the Lettine does switch the B+. I didn't want to switch such high voltages and only wanted the one switch to key all the tubes anyway. I'll work on implementing R7 as this is needed on my rig for sure.

BTW, I took the bad wire apart and didn't notice anything wrong with it. No melting or damage of any kind. Must have just been me grounding the shield that messed everything up!  ;)

Thanks again guys!
Joel   

Hi Joel,

Consider using a 12 volt multi-pole relay for the switching job. You will keep the voltages away from you and the switch. A tiny toggle switch can control the T/R. Better yet, a hand-held push button switch will make a better T/R control with the relay.

BTW, where did you measure 6.5 ohms the other day on the bad wire? Was it between the shield and center conductor OR from end to end of the center conductor?

T

Thanks for the tip Tom. I measured the 6.5 ohms from end to end. After I took it apart I measured again and it was around 1 ohm. I personally never want to see that piece of wire again!

Joel

Talked to Joel tonight on 40 meters (7.290) (> 505 miles) and although band conditions were poor, his signal came through 7-9, and his modulation was really good.

Good job on getting it on the air.


Phil

Joel:

A few thoughts.  Some may have been mentioned.

1) I would not use a 2E26.  My first choice would be a 6AG7.  6V6 might be OK.  A 6L6 has too much soup for modern crystals....even if you put a pilot lamp in series with the crystal.

2) Parallel the rock with 50 or 100 K.

3) Clamp tube a good idea.  If used, the cathode resistor can go.

4) Screen resistor likely a bit high.  See discussions by Tom, W8JI.  Your best  result will likely be between 7500 and 20K.  It will take some experimenting.

5) Plate block cap between driver and 6146 would be fine in the range of 100 to 25O pF.  1000 pf a bit much.

6) HV ceramic disc only for bypassing.  No orange drops.

7)  Plate block cap between 6146 and tank should be a proper, transmit rated cap.  Think Centralab doorknob, or 2000 VDC minimum Mica Tx cap.

8)  Use old '50's, '60's FT-243 rock.  Better yet, an old Bliley from before World War Twice.  Those things can take lots of current.
Good luck!

Joel:

One more thing:

PVC is an abomination for tank coil forms.

Use G10 or ceramic.

Even large pill bottles make decent coil forms as they are polystyrene.

Nice job with the rig!

Thanks Phil! It was a pleasant surprise to be able to make the contact with you on this rig which you helped me get on the air. There ended up being a bit of an AM pileup there after our QSO.  :)

I appreciate the tips Matthew. Yep, I've been reading through W8JI's site. The PA screen resistor still needs to be adjusted some. I need to place an order for additional components for sure. I can now justify spending a bit on this rig since it looks like it's working. There are no PVC forms being used, only air coils.  ;D   

Joel

Hi Matt -

Always curious, I've always wanted to run some tests on ABS, PVC and ceramic material - and did so tonight.

I used a 4" PVC pipe, a 4" ABS pipe and a ceramic standoff insulator.  Each item was tested separately in a 1500 watt microwave oven for 1 minute.

The results as follows -  a laser temperature gun was used for measurements:

The room temperature was 73.8F.

Ceramic insulator: 73.8F after one minute

Black ABS pipe:  92.3F after one minute

White PVC pipe: 102.6 after one minute.

The ceramic was stone cold to the touch. The ABS was hardly lukewarm.  The PVC pipe was warm to the touch.

My question is, this was a test at microwave freqs. Has anyone tested PVC and ABS at 160-10M, especially 75M?  There would be SOME degree of heating at HF as shown by the microwave test, of course, but just how much?  I wonder if the heating would decline linearly from microwave to HF and become insignificant?  For example, 2,450 MHz (microwave oven freq) is 600 times higher than 4 MHz. So would the heating be 1/600th on 4 MHz?

Opinions?

T