ac ripple on screen grids Ultralinear

[trobbins]

In addition to LoudThud's post - is the B+ supply voltage to the OT CT stable and what is the level in DC and AC? What meter do you have?

[Rattler66]

Grounding

The schematic shows the HV CT (the noisiest ground)(1) connecting to the preamp ground (1). Never a good idea. The HV CT should go to (2) where the reservoir cap grounds. If possible the HV CT should go directly to the cap and a separate wire from the cap to the chassis. The (C) node filter cap should ground to the preamp ground (1). The bias supply filter (3) and the power tube cathodes (5) should go to (2) or close by. The ground connection for the heater CT might cause noise in the preamp if grounded there. This depends on parasitic coupling within the transformer. Try it at (1) and (2) and use the one that is lowest in noise. Ground for the output transformer should go the the speaker jacks with a separate wire to the chassis.

I'm going to redo the grounds. Thanks

[Rattler66]

In addition to Gordon's post - is the B+ supply voltage to the OT CT stable and what is the level in DC and AC? What meter do you have?

I have to check this out when i have a minute. Thanks for your help

[Rattler66]

okay I incorporated the dual bias and the audible hum goes down but the voltage at the plate/screen noise is still causing the meter to not be able to lock on plate voltage (the meter just beeps and jumps )as soon as I ac ground the grid, it's all good. so I think I have no choice but to add the choke. if that doesn't do it, then I'm done with this amp.

[frus]

I'd say it's a wrong conclusion
If the amp generates buzz in the preamp (for whatever reason), it will be amplified by the power tubes and of course the plate voltage will jump too (plate voltage is actually the output of your amp!)
If you can eliminate it by grounding the power tube grids, the problem is not in power amp section at all

[Rattler66]

I'd say it's a wrong conclusion
If the amp generates buzz in the preamp (for whatever reason), it will be amplified by the power tubes and of course the plate voltage will jump too (plate voltage is actually the output of your amp!)
If you can eliminate it by grounding the power tube grids, the problem is not in power amp section at all

I completely separated the power amp from the rest of the circuit, removed b+ and disconnected the coupling caps right before the grids, just left the bias intact. it still hums, it only quiets down when I ground the screen grids ( not even the grids grounded quiet it down, only screens)through a capacitor. then I can adjust bias and there's no hum

[Rob's Radio-Active]

I think the simplest way to add dual bias and keep them from interacting with each other is to just duplicate the bias circuit you have right from the transformer's bias voltage winding. Won't cost much in parts, really, and would be pretty much independent. You don't want the bias to change on Tube A when adjusting Tube B.
cheers
Rob

[Rattler66]

I think the simplest way to add dual bias and keep them from interacting with each other is to just duplicate the bias circuit you have right from the transformer's bias voltage winding. Won't cost much in parts, really, and would be pretty much independent. You don't want the bias to change on Tube A when adjusting Tube B.
cheers
Rob

That's exactly how i did it. They don't interact if they do it's super minimal. But even balancing the bias the hum remains. Only grounding the screen grid quiets it down. I dont know what else to do except add the additional lc filter

[Rob's Radio-Active]

Well, you've proven by testing it (AC grounding the screens) that doing so will fix it...so I agree, it needs more filtering, at least at the screen grids themselves.
cheers
Rob

[Rattler66]

Yep I'll add the choke. Thanks for your help

[g1]

So if you just ground the power tube control grids but leave the screens alone, the hum is still there?
I think frus and I both thought you meant control grids in post #34 'ground the grid'.
Generally we call the control grid 'grid' and call the screen grids 'screens'.

[Rattler66]

Yes if i just ground the grids the hum is still there. But if i ground the screens it quiets down

[frus]

I think frus and I both thought you meant control grids in post #34 'ground the grid'.

yes, and I haven't read the whole thread carefully my bad
have you checked the tubes?
because something is fishy, it shouldn't behave like this

[Rattler66]

yes, and I haven't read the whole thread carefully my bad
have you checked the tubes?
because something is fishy, it shouldn't behave like this

yeah I agree with the weird behavior. yes I have gone through about 3 sets of tubes and I've mixed and match and still the same thing. I built this amp back in 2007 and it's been built and rebuilt since. I always has the same problem no matter what I do. Grounding the screens was the only way to eliminate the hum. since its ul the only place for the hum to show up on the screen (with the rest of the ckt disconnected) is from the first node of the PS, which comes straight out the rect.



disregard the grounding points, they have been redone

[trobbins]

Just to be sure - you can't actually ground the input grids (pin 5) as that would force the bias voltage seen by the output tube to 0V. You could AC short each pin 5, via a cap, to cross check that there was no ripple getting to the input grids. You certainly need to confirm that the bias voltage is not the hum injection point - it often is.

You could temporarily add an RC filter to 'A' supply going to OT CT - a means of checking whether the hum was mainly related to B+ supply.