Tubes buzzing with UA Ox

[Stopping-by432]

Has anyone experienced power tubes pulsing and making an audible sound when the output is plugged into a UA Ox attenuator? The amp works fine when plugged into a speaker cabinet. I have tried two separate Ox attenuators and speaker cables and the same issue. I have multiple amps that work with the Ox and this is first time I have had this issue. Any ideas on what could be causing this issue?

[xtian]

I observe this with ALL tube amps loaded into attenuators with in-room volume turned low. This is normal, but we don't notice it when the amp is LOUD in the room.

[Stopping-by432]

I was able to record what is happening in the amp which I have not found with any other amp when using an attenuator. The clip is at https://youtu.be/B04BjqXIs0Y. Any thoughts on what is happening or help would be appreciated.

[xtian]

Blue glow is normal for some tubes, not a problem. We can often see it pulsing like this when the amp has a tremolo feature. But yours is oscillating (or motorboating) when it should not be. Is the load provided by your OX matched to the amp?

[Stopping-by432]

Yes the amp and the Ox are both set to 8 ohms. I changed tubes and had the same result. I will try to use a different tap of the output transformer (4ohm) to see if get the same result. What is strange is the amp works correctly when connected to a speaker directly. Not sure why the Ox is causing the oscillation when a direct speaker load does not.

[Stopping-by432]

I used the 4 ohm tap and set the attenuator to 4 ohms as well and its is still doing the same. Is it possible that the Output transformer is not working correctly even if it sounds good through a regular speaker? Very strange.

[pdf64]

That amp is certainly not playing nicely with the Ox. If the amp (details eg make, model, age, maintenance history?) and Ox work happily with other attenuators and amps, then perhaps it’s just one of those things.
It may be that there’s some speaker cabs your amp would do that with too, eg low power speakers in a smaller sealed cab can have quite an extreme bass resonance impedance spike.
I know it’s the first thing folk fixate on when there’s any kind of issue that isn’t cured by replacing valves, but don’t pick on OTs, there’s no good reason to suspect it of failing and causing this
Rather that type of motorboating is typically caused by weak, failing electrolytic HT caps.

[Stopping-by432]

The amp is a 5f6A bassman that I recently built from Rob Robinette's layout with some of his recommended mods. The amp has four F&T 22uf 500v caps for the power filtering. Is there a way to test to see if the B+1 caps are not working correctly? Or do you recommend I use a different type of filter cap to see if this issue is related to the filter caps.

Thanks,
Scott

[xtian]

Is there a way to test to see if the B+1 caps are not working correctly?

Easy, just clip in (seriously, alligator clips are fine for this test) a 47uF/500v cap across the existing reservoir (first) filter cap. This will raise the total capacitance of the first node to ~69uF. (After the test, remember to discharge the new cap before handling!)

[Stopping-by432]

I just tested the 47uf 500v cap across the first filter cap and it is still doing the same. I replaced the rectifier tube and power tubes and it still occurs as well. Thanks

[pdf64]

The amp is a 5f6A bassman that I recently built from Rob Robinette's layout with some of his recommended mods...

Is the negative feedback arrangement still stock, eg 2ohm OT, 2ohm load, 27k and 5k feedback resistors?

[TUBEDUDE]

What does the input of the Ox look like? Does the load impedance, even though 8R, have a large capacitave reactance, rather than inductive or resistive?

[Stopping-by432]

I am using a 56k resistor for the negative feedback since I am using the 8ohm tap. I also have the ability to switch the negative feedback to 20k for more of a JTM45 sound. I am not using a 5k resister on lug 3 to ground on the presence pot rather lug 3 is connected to the 56k resistor.

[Stopping-by432]

What does the input of the Ox look like? Does the load impedance, even though 8R, have a large capacitave reactance, rather than inductive or resistive?

I am not sure as UA does not disclose that info. It only shows that the ground input goes to ground across a 10k resister and the tip goes to reactive load.

[pdf64]

... I also have the ability to switch the negative feedback to 20k for more of a JTM45 sound...

How does that switch affect the motorboating?