SB200 oscillation??

[K3WQ]

Next time you have the amp open check the 33 ohm resistors. in the SB-200. If they are drifting
upwards in value they might cause instability problems.

It's not those resistors, because they were replaced when I refurbished the amp four months ago.

Here's a theory: The 572B tubes in the SB-200 amp are flanked by several tuned circuits--the SB-401's pi network in series with the amp’s input circuit at the cathodes, and the amp’s output pi network in series with the antenna tuner and antenna at the plates. These networks are tuned to nearly the same frequency, and if the tube are "hot" (transconductance above spec), the amp functions as a tuned-input/tuned-output oscillator. Oscillation doesn’t occur when I bypass the tuner/antenna combination with a 50-Ohm dummy load because the amp’s pi network is sufficiently “swamped” over all frequencies.

I suppose an attenuator between, say, the transmitter and amp could put enough damping in the configuration to prevent oscillation, but I'd possibly loose the ability to drive the amp to full power. But, hey, my rotate-the-transmitter-load-control fix works well enough.

-Dave

[AG6K]

I recently bought a SB200 that I think is going into self oscillation.

On 80m, when I tune up with a CW signal at low power from my exciter, all is ok and the tune is fine until I wind up the exciter power to anything more than about 50w. At that point my swr/pwr meter suddenly goes full scale on both FWD abd REV. I button off the exciter and it stops ok. If I wind the power from the exciter down, all is well again until I wind it up again. This all happens ONLY on 80m.

Now, I'm fairly new to QRO. Is this typical of an amp "taking off" and how do I stop/fix it?

Dene

 To find out if there was a VHF osc., unsolder one end of each parasitic osc. suppressor R and measure the Ωs. If R is more than c. 50% higher than the marked value, chances are you had a parasitic osc. The fix is to decrease the VHF gain of the 572Bs by decreasing the suppressor's VHF-Q.
- note - 572Bs have 0.6pF of feedback-C. At 100MHz this is nothing to sneeze at since XC = 2800Ωs. By comparison 3-500Zs have 0.15pF of feedback-C.

[W1QJ]

I recently bought a SB200 that I think is going into self oscillation.

On 80m, when I tune up with a CW signal at low power from my exciter, all is ok and the tune is fine until I wind up the exciter power to anything more than about 50w. At that point my swr/pwr meter suddenly goes full scale on both FWD abd REV. I button off the exciter and it stops ok. If I wind the power from the exciter down, all is well again until I wind it up again. This all happens ONLY on 80m.

Now, I'm fairly new to QRO. Is this typical of an amp "taking off" and how do I stop/fix it?

Dene

You said " I tune up with a CW signal at low power from my exciter". That's Ok but, unless you are going to operate at that low power input YOU CAN NOT simply add additional drive from the exciter and keep the loading of the amplifier the same. This makes the amplifier severly under coupled, under coupling can cause all sorts of problems. As you stated a small adjustment of your exciter will stop this problem, but I'll bet if you re tuned the tank circuit of the amplfier to resonate it once again at the highest drive level, you won't see this problem. Furthermore, you a loosing a lot of power output by not tuning the amplfier to the highest drive level you are feeding the amp with. This is mistake #1 many newbies to amplfiers make.

[AG6K]

You said " I tune up with a CW signal at low power from my exciter". That's Ok but, unless you are going to operate at that low power input YOU CAN NOT simply add additional drive from the exciter and keep the loading of the amplifier the same. This makes the amplifier severly under coupled, under coupling can cause all sorts of problems. As you stated a small adjustment of your exciter will stop this problem, but I'll bet if you re tuned the tank circuit of the amplfier to resonate it once again at the highest drive level, you won't see this problem. Furthermore, you a loosing a lot of power output by not tuning the amplfier to the highest drive level you are feeding the amp with. This is mistake #1 many newbies to amplfiers make.

 Sound advice. Amplifiers are designed for a certain RL - and that goes out the window with reduced drive tuneup. The best way to run an amp is to tune it up at 100% - then if you want less power turn down the microphone gain --- but don't re-adjust the Tune and Load controls.

[KM1H]

If we can keep this discussion focused on the OP it would be appreciated!


Did you check what I suggested last week?

The primary reason for the R of the suppressors to change is heat, RF and plain old thermal. Further a parasitic on 80M is a rather rare occurance unless the resistor is effectively no longer there and the coil part turns the amp into an oscillator.
I dont know component availablity in VK land but the Ohmite OY series of resistors are excellent subs for obsolete carbon composition. I would use a pair of 100 Ohm in parallel and the original coil.

Also dont overlook the possibility of a melted switch contact or something else that would cause the amp to no longer see a load. Plus the comments about proper tuning is very valid.

Carl

[AG6K]

If we can keep this discussion focused on the OP it would be appreciated!


Did you check what I suggested last week?

The primary reason for the R of the suppressors to change is heat,

 More R means less Q. In a VHF suppressor less Q means a lower RL on the anode - and that means less VHF amplification - which means better VHF stabilization. In other words the thing a VHF suppressor primarily suppresses is VHF-amplification.

RF and plain old thermal. Further a parasitic on 80M is a rather rare occurance unless the resistor is effectively no longer there and the coil part turns the amp into an oscillator.
I dont know component availablity in VK land but the Ohmite OY series of resistors are excellent subs for obsolete carbon composition. I would use a pair of 100 Ohm in parallel and the original coil.

Also dont overlook the possibility of a melted switch contact or something else that would cause the amp to no longer see a load. Plus the comments about proper tuning is very valid.

Carl

 G-G amps are self-neutralized below the frequency of the grounded-grid's self-resonance. For 572Bs this is reportedly c. 70MHz. For 3-500Zs this is c. 89MHz.

[WX7G]

What happens to the RF output power of the exciter when this "oscillation" occurs? If it jumps up the problem is the exciter and not the amplifier.

[AG6K]

Peter,

Good point. The SB-200 doesn't have high gain, but if internal losses are low enough, self-oscillation could still occur.

... ... ...
-Dave

 A 572B has 0.6pF of feedback-C. A 3-500Z has 0.15PF of feedback-C. Hello!

[KM1H]

Ive yet to see a 6M SB-200/201 that was completely neutralized, even stock ones are at the edge of taking off on 10M.

Rich and others confuse lack of neutralization caused oscillation with actual parasitics.

Carl