Biasing mti svt, am I doing this wrong?

[poorbassist15]

Go back to post 96 (?) where ferrite beads are used instead of plate resistors on the power tubes. Every modern SVT uses these for a reason. It cures oscillation issues but doesn’t solve the cause of the problem.

A spectrum analyzer will identify offending frequencies. It helps with the troubleshooting process. Remember, a capacitor might test good at low frequencies but changes at high frequencies.

This may not help solve your problem but this multi-part video series describes design measures that are taken to ensure stability in simple RF amplifiers. Very interesting stuff.

would it be worth replacing the resistors with beads? I’m not opposed to trying it and I haven’t had this issue with my newer Svt, likely for that reason. As I understand though the resistors act like fuses for the tubes in case they go nuclear, or did I misunderstand that part?

 

[Wasnex]

I have tried my other av212 cab, and it still does it in that one too, so I can rule the speakers themselves out as a cause. Yes, the cab is a 4 ohm, and I’ve completely bypassed the preamp and have performed all my tests into the power amp in.

Can you describe the oscillation and exactly when it occurs?

AFAIK, the fact the output tubes make good clean power is not necessarily proof they are not unstable. Back in my radio days, we performed a neutralization process after replacing the output tube in various transmitters.

Wire dress can be a factor.

If the problem becomes worse when the feedback line is disconnected, I assume the polarity of the output transformer is correct. Otherwise, AFAIK connecting the loop would cause feedback.


Are the output taps connected properly...4 ohm tap to 4 ohm position on switch. Notice the connections of the dual winding secondary: Orange to Blue (4 ohms), Red to Green (2 ohms), Brown to Yellow (common)

Perhaps the feedback applied to the cathode circuit of V1a is not strong enough.

The feedback loop is connected between R4 and R5. If the value of R5 is too low, perhaps it's attenuating the feedback too much.

Also, C2 may have something to do with stabilizing the amp. AFAIK this shunts some high frequencies to ground.

 

[poorbassist15]

Can you describe the oscillation and exactly when it occurs?

AFAIK, the fact the output tubes make good clean power is not necessarily proof they are not unstable. Back in my radio days, we performed a neutralization process after replacing the output tube in various transmitters.

Wire dress can be a factor.

If the problem becomes worse when the feedback line is disconnected, I assume the polarity of the output transformer is correct. Otherwise, AFAIK connecting the loop would cause feedback.


Are the output taps connected properly...4 ohm tap to 4 ohm position on switch. Notice the connections of the dual winding secondary: Orange to Blue (4 ohms), Red to Green (2 ohms), Brown to Yellow (common)

View attachment 5099596

Perhaps the feedback applied to the cathode circuit of V1a is not strong enough.
View attachment 5099591
The feedback loop is connected between R4 and R5. If the value of R5 is too low, perhaps it's attenuating the feedback too much.

Also, C2 may have something to do with stabilizing the amp. AFAIK this shunts some high frequencies to ground.

It occurs when going lower than C on the A string, any note on the E string causes what I can describe as a fuzzy fart. I can’t get it to do it with a signal generator or bass into a resistive load, it’s only happening with a cab and bass plugged in. I may have an oscilloscope picture of it I can post to help visualize it.

I did also check r5, thinking something similar, it measures 218 ohms so that should be fine. The amp doesn’t appear to have been modified substantially so I wouldn’t think the transformer wires have been screwed with but you never know, so I’ll look tomorrow

 

[beans-on-toast]

would it be worth replacing the resistors with beads? I’m not opposed to trying it and I haven’t had this issue with my newer Svt, likely for that reason. As I understand though the resistors act like fuses for the tubes in case they go nuclear, or did I misunderstand that part?

Keep the plate resistor. You can put each bead in series with it. The bead on the tube socket terminal with an attached wire going to the plate resistor.

 

[beans-on-toast]

Can you describe the oscillation and exactly when it occurs?

AFAIK, the fact the output tubes make good clean power is not necessarily proof they are not unstable. Back in my radio days, we performed a neutralization process after replacing the output tube in various transmitters.

Wire dress can be a factor.

If the problem becomes worse when the feedback line is disconnected, I assume the polarity of the output transformer is correct. Otherwise, AFAIK connecting the loop would cause feedback.


Are the output taps connected properly...4 ohm tap to 4 ohm position on switch. Notice the connections of the dual winding secondary: Orange to Blue (4 ohms), Red to Green (2 ohms), Brown to Yellow (common)

View attachment 5099596

Perhaps the feedback applied to the cathode circuit of V1a is not strong enough.
View attachment 5099591
The feedback loop is connected between R4 and R5. If the value of R5 is too low, perhaps it's attenuating the feedback too much.

Also, C2 may have something to do with stabilizing the amp. AFAIK this shunts some high frequencies to ground.

C2, the 120pF capacitor can be a source of noise if it is out of spec.

 

[poorbassist15]

Can you describe the oscillation and exactly when it occurs?

AFAIK, the fact the output tubes make good clean power is not necessarily proof they are not unstable. Back in my radio days, we performed a neutralization process after replacing the output tube in various transmitters.

Wire dress can be a factor.

If the problem becomes worse when the feedback line is disconnected, I assume the polarity of the output transformer is correct. Otherwise, AFAIK connecting the loop would cause feedback.


Are the output taps connected properly...4 ohm tap to 4 ohm position on switch. Notice the connections of the dual winding secondary: Orange to Blue (4 ohms), Red to Green (2 ohms), Brown to Yellow (common)

View attachment 5099596

Perhaps the feedback applied to the cathode circuit of V1a is not strong enough.
View attachment 5099591
The feedback loop is connected between R4 and R5. If the value of R5 is too low, perhaps it's attenuating the feedback too much.

Also, C2 may have something to do with stabilizing the amp. AFAIK this shunts some high frequencies to ground.

C2, the 120pF capacitor can be a source of noise if it is out of spec.

I ordered more of those caps last night, I can’t test it thoroughly but they’re cheap to replace so the end result is the same anyway.