Old Tube Amp causing fregeuncy drift on SSB??

[bgosper]

My vote is for RF getting back into the radio somehow.

A slight drift on AM will not be noticed plus the constant carrier means less dynamic power out.

I agree...!
Any ideas on how to diagnose/stop that? Again, it was never an issue until recently...

 

[Kamikaze]

I agree...!
Any ideas on how to diagnose/stop that? Again, it was never an issue until recently...

Snap on ferrites on everything going into the radio. Power lead, antenna cable, mic wire, etc.

Or try it one at a time. Depends if you like a good expelliment

 

[bgosper]

Jeff,
I will check that. One thing I notice is that the output from the amp when it is totally turned off is about 1/2 of what is going into it from the radio. Another words, I use a about a 2 watt carrier out of the radio and into the amp. If I check the wattage after the amp, it is down to 1 watt so something is eating up power. I would imagine that it should pass it through with little or no change. Not sure if that is related but I will check the SWR at the input as well.

UPDATE:
The input SWR (between the radio and Amp) IS high-around 2.5 that includes with the amp turned off. I changed the jumpers but that made no difference. SWR between amp and antenna is normal--1.3 or so. Perhaps then this is the issue? Not sure what would be causing that though. Perhaps that is also what is causing the carrier wattage from the radio to be eaten up as it passes through the amp even when turned off...
Ideas??

 

[Captain Kilowatt]

My vote is for RF getting back into the radio somehow.

A slight drift on AM will not be noticed plus the constant carrier means less dynamic power out.

BINGO!! but not how you think nor the effect that you think.

LSB mainly...

There is your answer.

I suspect that RF is getting back into the radio and affecting the frequency display and NOT the actual frequency of the radio. I suspect that the freq display is reading what the TX frequency SHOULD be based on the center carrier frequency. When you go to SSB that freq changes up about 3 KHZ on USB and down 3 KHz on LSB hence you seeing about a 2 or 3 KHz shift. This also changes with the modulating frequency of your voice.The extra power from the tube amp is getting back into the freq counter and you are seeing the true NORMAL transmit freq and not what the counter is set to indicate. The actual frequency of the radio is NOT changing is what I suspect. Try listening to another radio and see if it actually is changing. I suspect not.

 

[bgosper]

BINGO!! but not how you think nor the effect that you think.





There is your answer.

I suspect that RF is getting back into the radio and affecting the frequency display and NOT the actual frequency of the radio. I suspect that the freq display is reading what the TX frequency SHOULD be based on the center carrier frequency. When you go to SSB that freq changes up about 3 KHZ on USB and down 3 KHz on LSB hence you seeing about a 2 or 3 KHz shift. This also changes with the modulating frequency of your voice.The extra power from the tube amp is getting back into the freq counter and you are seeing the true NORMAL transmit freq and not what the counter is set to indicate. The actual frequency of the radio is NOT changing is what I suspect. Try listening to another radio and see if it actually is changing. I suspect not.

Ok, that also sounds like a viable reason though I do not have another radio to confirm with. Let's assume it is, what would be the cause of the high SWR between the amp and radio and the wattage loss caused by having the amp in line even if turned off?

 

[Captain Kilowatt]

As I don't think you have told us what kind of amp you have I can't say for sure but my best guess is that the stray inductance/capacitance of the input circuit and associated bypass/changeover relay is presenting something far from a perfect 50 ohm input impedance. Without a tuned input on the amp the only thing you can do is either insert an antenna tuner between the radio and amp or play with the length of jumper cable between the radio and amp. Various lengths of coax will transform whatever the input impedance of the amp is to something nearer to 50 ohms that the radio would like to see. This will lower the SWR and most likely restore a great portion of the lost power you are seeing. try using different lengths and see how it affects the SWR. You may find that when the SWR goes down to less than 1.5:1 the display problem may possibly go away but I can't be certain about that.

 

[bgosper]

As I don't think you have told us what kind of amp you have I can't say for sure but my best guess is that the stray inductance/capacitance of the input circuit and associated bypass/changeover relay is presenting something far from a perfect 50 ohm input impedance. Without a tuned input on the amp the only thing you can do is either insert an antenna tuner between the radio and amp or play with the length of jumper cable between the radio and amp. Various lengths of coax will transform whatever the input impedance of the amp is to something nearer to 50 ohms that the radio would like to see. This will lower the SWR and most likely restore a great portion of the lost power you are seeing.

Hi-thanks for the info.
It is a PAL 750B base amp. Uses 9, 8950 tubes-I believe 1 drives 2&3 and they drive 4-9. . About 800 watts out on high with 2 watts in.
I will try the coax lengths and see if that helps...

 

[JustinDePolis]

A lot of misinformation in this thread so far....

1: The amplifier tubes do not need to "stabilize for 30 minutes" to prevent the radio from drifting, this is absurd, no offense to the person who stated that.

2: The power supply voltage coming from the 120 VAC outlet dropping should in no way cause this either because the radio will maintain a constant 13.8 volts to the circuits, or 8 volts, or 5 volts depending upon the section of the radio due to internal regulation.

3: The frequency counter should not change frequency with your voice on this radio because of the way it samples the operating frequency.

4: My guess is that RF is getting into the freq counter itself, and if the radio in fact itself is drifting, I would suspect RF is getting into the board and affecting the regulation, VCO, or PLL circuitry. Better RF bypassing on the amplifier, spacing the amplifier farther from the radio, and RF ferrite chokes can help this.

 

[bgosper]

A lot of misinformation in this thread so far....

1: The amplifier tubes do not need to "stabilize for 30 minutes" to prevent the radio from drifting, this is absurd, no offense to the person who stated that.

2: The power supply voltage coming from the 120 VAC outlet dropping should in no way cause this either because the radio will maintain a constant 13.8 volts to the circuits, or 8 volts, or 5 volts depending upon the section of the radio due to internal regulation.

3: The frequency counter should not change frequency with your voice on this radio because of the way it samples the operating frequency.

4: My guess is that RF is getting into the freq counter itself, and if the radio in fact itself is drifting, I would suspect RF is getting into the board and affecting the regulation, VCO, or PLL circuitry. Better RF bypassing on the amplifier, spacing the amplifier farther from the radio, and RF ferrite chokes can help this.

I lot of different ideas on this thread, all are speculation. I will try the chokes and spacing issue first since those are the "easy fixes"...thanks for the feedback.

 

[bgosper]

A lot of misinformation in this thread so far....

1: The amplifier tubes do not need to "stabilize for 30 minutes" to prevent the radio from drifting, this is absurd, no offense to the person who stated that.

2: The power supply voltage coming from the 120 VAC outlet dropping should in no way cause this either because the radio will maintain a constant 13.8 volts to the circuits, or 8 volts, or 5 volts depending upon the section of the radio due to internal regulation.

3: The frequency counter should not change frequency with your voice on this radio because of the way it samples the operating frequency.

4: My guess is that RF is getting into the freq counter itself, and if the radio in fact itself is drifting, I would suspect RF is getting into the board and affecting the regulation, VCO, or PLL circuitry. Better RF bypassing on the amplifier, spacing the amplifier farther from the radio, and RF ferrite chokes can help this.

Also, what do you mean by better RF bypassing? Thanks!

 

[JustinDePolis]

What this means, is usually RF bypass capacitors .01uf ceramic disk caps 500v or so from power leads inside of the amp to chassis ground, and also perhaps several points in the power supply although the voltage of the caps would depend on the part of the supply. (screen if there is one, plate supply, etc) Also in the radio this can be helpful as well to prevent RF from getting back into parts of the radio you don't want it. Another thought is, on the Cobra 2000 there is supposed to be a "metal shield" under the wooden case on the top, I have seen radios where it was missing. Do you have one in there? Try thr easy stuff first, and let us know how it goes.

 

[bgosper]

What this means, is usually RF bypass capacitors .01uf ceramic disk caps 500v or so from power leads inside of the amp to chassis ground, and also perhaps several points in the power supply although the voltage of the caps would depend on the part of the supply. (screen if there is one, plate supply, etc) Also in the radio this can be helpful as well to prevent RF from getting back into parts of the radio you don't want it. Another thought is, on the Cobra 2000 there is supposed to be a "metal shield" under the wooden case on the top, I have seen radios where it was missing. Do you have one in there? Try thr easy stuff first, and let us know how it goes.

Hi-yes the shield is there on the freq counter box. I tried moving the amp further away but that did not change it. I had to order some toroid cores so I will have to wait on that. What concerns me is that I had the amp for almost a year with everything in the same spot and essentially unchanged with no problem. This has developed just in the last several weeks so that is why I suspect there is more going on inside the amp rather than the quick fix stuff. Of course I would rather it be that but somehow I doubt it...On another forum someone suggested checking/replacing the coil/resistors on the tube plate cap connection as he stated if they break down RF could be leaked. There are just to many possibilities in the amp. It is to bad because it works great otherwise. I hooked up my smaller box (250 watts) and the problem was not there...all normal.

 

[Shockwave]

Forget the amp, that's not causing frequency drift. The reality is the Cobra 2000 has an exceptionally poor power supply that has no headroom to maintain regulation under full load. Peak this radio or open up the ALC on SSB and watch the power supply struggle to maintain voltage. The input to the regulator is barley high enough to keep the output constant. Now add a 15 amp load on the 110 line with that amp and watch the DC voltage drop on the 2000. If the meter lights dim at all when modulating the 2000 on SSB, that's it for sure.

I understand they are on separate circuits but it's still possible the amp is pulling the line voltage down just a little. All you need is a few volts to drop on the AC line and the radios power supply no longer has enough. If the air conditioners are running, that contributes too. The frequency counter in the 2000 is well shielded and would be very difficult to influence from outside RF. Try running the 2000 on a decent external DC supply and I bet that will reveal your problem.

 

[wavrider]

Forget the amp, that's not causing frequency drift. The reality is the Cobra 2000 has an exceptionally poor power supply that has no headroom to maintain regulation under full load. Peak this radio or open up the ALC on SSB and watch the power supply struggle to maintain voltage. The input to the regulator is barley high enough to keep the output constant. Now add a 15 amp load on the 110 line with that amp and watch the DC voltage drop on the 2000. If the meter lights dim at all when modulating the 2000 on SSB, that's it for sure.

I understand they are on separate circuits but it's still possible the amp is pulling the line voltage down just a little. All you need is a few volts to drop on the AC line and the radios power supply no longer has enough. If the air conditioners are running, that contributes too. The frequency counter in the 2000 is well shielded and would be very difficult to influence from outside RF. Try running the 2000 on a decent external DC supply and I bet that will reveal your problem.

BINGO, I chased the same issue around on a junk pile cobra 2000, worked great on AM unstable on SSB, Power supply was not original in the Cobra 2000, a hack job, put a decent external PS on the radio and has worked fantastic every since.

The dimming lights on the meters while modulating in SSB mode will surely let you see if the PS is being strained.

Changing amps cures the problem?
Input tune on the Pal is the first issue that needs to be corrected, or as stated before put a match box in between rig and amp to get the VSWR input down and this may cure your problem.

Been years since I have been in a PAL last I remember they do not have tuned inputs, just let it fall where it may as far as VSWR.

 

[bgosper]

BINGO, I chased the same issue around on a junk pile cobra 2000, worked great on AM unstable on SSB, Power supply was not original in the Cobra 2000, a hack job, put a decent external PS on the radio and has worked fantastic every since.

The dimming lights on the meters while modulating in SSB mode will surely let you see if the PS is being strained.

Changing amps cures the problem?
Input tune on the Pal is the first issue that needs to be corrected, or as stated before put a match box in between rig and amp to get the VSWR input down and this may cure your problem.

Been years since I have been in a PAL last I remember they do not have tuned inputs, just let it fall where it may as far as VSWR.

Well like I said before, all the suggestions on here are all good and worth investigating. As far as the PS in this radio goes, should not be an issue. I rebuilt it myself several months ago with beefed up parts. Actually had a bit to much coming off the board and had to scale it back. And again, this problem only developed in the last few weeks after a year with no issue so makes me suspicious about it being anything other than the amp. But all are worthy of investigation as I try to figure it out...
thanks again for the suggestions...I will post updates and other questions as I go...