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Old Tube Amp causing fregeuncy drift on SSB??

B

bgosper

Member
Jul 23, 2010
30
0
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Ok, here is an issue I am having with my linear amp. It is an old 9 tube base amp (a PAL 750B). Wattage is normal at 750-800 watts with 2 watt drive.

But recently, what is happening is that it is somehow causing frequency drift on my radio on SSB only. What is unusual is that the degree and consistency of the drift seems to depend on the tone of my voice and also where the mike gain is set. As I change the inflections in my voice it will cause this frequency drift to take place. The frequency always goes down, not up. The amount of drift from 27.385 for example would be to go down to as low as 27.3830 but generally no more than 27.3840.

Again it ONLY does it on SSB, not AM. It also is not dependent on band or antenna either so I believe it is a local problem with the amp or radio. It does not happen with the amp off or with my smaller 200 watt amp so I believe it is the large amp causing it.

The problem does seem to self correct for the most part after the amp warms up but does seem to reoccur.

Anybody have any ideas? Perhaps a problem with the power supply in the amp or the filter cap circuit? Let me know, and thanks!
 

Doesn't make much sense that the amp is drifting. That is usually a problem defined by the radio; as it is the element that can shift freq's by design.

Did you try it w/o the amp?
What make/model/year radio is it?
 
Robb said:
Doesn't make much sense that the amp is drifting. That is usually a problem defined by the radio; as it is the element that can shift freq's by design.

Did you try it w/o the amp?
What make/model/year radio is it?
Click to expand...

Hi, the radio is fine without the amp in use-no problem at all. the radio is a cobra 2000. I know, does not make much sense at all...
 
bgosper said:
Hi, the radio is fine without the amp in use-no problem at all. the radio is a cobra 2000. I know, does not make much sense at all...
Click to expand...
CLARIFICATION:
The amp is not drifting. It is the radio that is drifting. The amp seems to be causing my radio to shift off frequency while TX'ing on SSB. the Cobra 2000 has a built in freq counter which is obviously behind the amp. That is what is changing on TX with the amp.
 
Are you running the radio and Amp off of the same Plug?
Voltage drop on the 110 when the radio and amp are drawing max current?
RF getting into the power supply in radio, or into the radio itself maybe?
All the jumpers good?
Good Grounds on all the equipment?

73'
Jeff
 
AudioShockwav said:
Are you running the radio and Amp off of the same Plug?
Voltage drop on the 110 when the radio and amp are drawing max current?
RF getting into the power supply in radio, or into the radio itself maybe?
All the jumpers good?
Good Grounds on all the equipment?

73'
Jeff
Click to expand...
Jeff,
All those sound legit. The radio and amp are on different circuits so I dont think it is that. I also thought RF into the radio but why only the problem on SSB and not AM? How would I fix that even if so? the amp and radio are very close together. Maybe separate them more? I will check jumpers and grounds as well....
Thanks..
 
Since it is a tube amplifier - you already answered your own question.

The tubes requires at least one half of an hour before you can operate the amplifier - so that is what is causing the drift.
Click to expand...


If you had said that the radio needs to warm up to stabilize, I could agree with that.
But I do not see how the tubes being cold in the Amp are the problem in the radio.
The rest of the post is on another topic, and of no use to help the gentleman figure out what the problem is.

________________________________________________________
OK
If the Amp is sitting right next to the Radio, yes I would try moving things around...might be that simple.
But recently, what is happening is that it is somehow causing frequency drift on my radio on SSB only.
Click to expand...
I take it from this that it was working fine before and this started suddenly, can you think of anything that changed?


73
Jeff
 
bgosper said:
Jeff,
All those sound legit. The radio and amp are on different circuits so I dont think it is that. I also thought RF into the radio but why only the problem on SSB and not AM? How would I fix that even if so? the amp and radio are very close together. Maybe separate them more? I will check jumpers and grounds as well....
Thanks..
Click to expand...

Possibilities:

*A fair fluctuation in the dc voltage feeding the radio can cause drift. If it doesn't drift when the amp is off; then it goes to prove that the radio is getting enough voltage. If it happens only when the amp is on; then it goes to prove that a voltage drop is happening. Overloading an AC line is now the probable cause.

*The radio should see ~13.8v in receive, and it shouldn't drop more than 1/2v - if that - in transmit mode.

*If you should see a change in the voltage between transmit and receive; then I would say that the radio's power supply is inefficient and needs to be replaced. That radio shouldn't draw any more than 2 1/2 to 3 amps in transmit; so any voltage drop would indicate a weak power supply - IMO. If you are using the internal power supply; then use the external 12v plug instead. An external 5 amp power supply to run the radio should be enough.

*SSB will draw more current than AM operation. In effect, the voltage will drop more. So, whatever weakness is in the radio's power supply is going to be made much worse in SSB mode.

*The voltage regulator in the radio's circuit itself might be getting weak. I would exhaust all other mentioned possibilities first before considering replacing it.

*I would put a volt meter across the power into the radio when the amp is on. See if the radio doesn't drop a volt or two when the amp is on - when you are transmitting. If that is the case; then I would run the radio and power supply on a different circuit than the amp. Make sure that they are all grounded too . . .
 
Last edited:
AudioShockwav said:
If you had said that the radio needs to warm up to stabilize, I could agree with that.
But I do not see how the tubes being cold in the Amp are the problem in the radio.
The rest of the post is on another topic, and of no use to help the gentleman figure out what the problem is.

________________________________________________________
OK
If the Amp is sitting right next to the Radio, yes I would try moving things around...might be that simple.

I take it from this that it was working fine before and this started suddenly, can you think of anything that changed?





73
Jeff
Click to expand...

Jeff,
Nothing has changed in terms of equipment or otherwise. This problem started without any apparent reason in recent weeks. I will try to move things around a bit and try to eliminate or confirm that any external parts (coax jumpers Low pass filters, grounds,etc) are causing the issue and try spacing it out some as well. Certainly thinking there is some RF leakage happening. Someone suggested the resistor/coils on each tube at the plate connection should be checked as one or more might be old/damaged and could be causing leakage.
 
Robb said:
Possibilities:

*A fair fluctuation in the dc voltage feeding the radio can cause drift. If it doesn't drift when the amp is off; then it goes to prove that the radio is getting enough voltage. If it happens only when the amp is on; then it goes to prove that a voltage drop is happening. Overloading an AC line is now the probable cause.

*The radio should see ~13.8v in receive, and it shouldn't drop more than 1/2v - if that - in transmit mode.

*If you should see a change in the voltage between transmit and receive; then I would say that the radio's power supply is inefficient and needs to be replaced. That radio shouldn't draw any more than 2 1/2 to 3 amps in transmit; so any voltage drop would indicate a weak power supply - IMO. If you are using the internal power supply; then use the external 12v plug instead. An external 5 amp power supply to run the radio should be enough.

*SSB will draw more current than AM operation. In effect, the voltage will drop more. So, whatever weakness is in the radio's power supply is going to be made much worse in SSB mode.

*The voltage regulator in the radio's circuit itself might be getting weak. I would exhaust all other mentioned possibilities first before considering replacing it.

*I would put a volt meter across the power into the radio when the amp is on. See if the radio doesn't drop a volt or two when the amp is on - when you are transmitting. If that is the case; then I would run the radio and power supply on a different circuit than the amp. Make sure that they are all grounded too . . .
Click to expand...

The problem does NOT occur in the radio with the amp off or in stand by, only when it is kicked in. The radio works fine otherwise. The amp and radio are on separate circuits as well. I am leaning towards some kind of RF leak via some kind of defective part in the amp or bad jumper, ground, etc...
 
Have you ever checked the match going into the Amp? ( input swr)
Maybe there is something showing it`s age, and there is high input swr causing a problem feeding rf back to the radio until the Amp components warm up and stabilize, another good thing to look at...that was a good idea.

73
Jeff
 
AudioShockwav said:
Have you ever checked the match going into the Amp? ( input swr)
Maybe there is something showing it`s age, and there is high input swr causing a problem feeding rf back to the radio until the Amp components warm up and stabilize, another good thing to look at...that was a good idea.

73
Jeff
Click to expand...
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.
 
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.
 

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