Final problem to solve on the new AB763...buzzing on the low E string

[Planobilly]

Hi,

I thought I had everything working well...not...lol

The amp buzzes only on the low E string when it is played hard. Does the same thing when a less than 120hz signal is injected and as I increase the gain . Does this on both channels. Does this when plugged into different speaker cabinets. Tried different tubes, resoldered all grounds and have been through the amp pretty thoroughly.

Not sure what to do next.

Ideas?

Thanks,

Billy

EDIT: I got tired of trying to figure this out and changed all the components on V4A...plate resistor. cathode resistor, cathode cap, and coupling cap. Not sure what was defective.

There should be something to learn here...reverb hum plus low frequency buzz...which of those components would cause that kind of fault??

[shooter]

The amp buzzes

Try putting your scope probe on the PA tube grid side of the coupling caps coming from the PI.  look at a 1khz signal, then switch to 100 - 200hz, see if you can "spot" the buzz.

[eleventeen]

Are you saying that the shotgun replacement of all the parts hanging on V4 solved the issue?

[Planobilly]

Hi Guys,

Are you saying that the shotgun replacement of all the parts hanging on V4 solved the issue?

Here is what happened. Originally there were two issues.  Large amount of hum as the reverb control was turned up. Crackling/static/buzz sound in lower notes.

After I changed the Plate resistor, cathode resistor, cathode cap, and the .0033 coupling cap  on V4A, the reverb recovery circuit, the amp played super well. With no signal I turned all controls to maximum and there was almost no audible hum or noise of any kind. I tested some new speakers which sounded great. Really good as a matter of fact.

After playing the amp for about two hours something changed. I now have a serious buzzing sound only on the low E string or at least I can not hear it on  higher  notes. This sounds almost identical to how a blown speaker would sound. The other condition is at the maximum rotation of the tremolo intensity control the amp motorboats ( only at the very max setting). I am not sure if these two things are related. There is no buzz at low gain settings and starts when the guitar volume control or the amp volume control is increased. Turning the bass control up on the amp makes this happen at lower volume settings.

Due to the fact the amp played and sounded really good for two hours I assume some component has failed. I pulled V1, V3, and V5 tubes and the conditions do not change. When the buzzing occurs I don't see any change on the scope with the signal generator set at any frequency.

I have replaced several coupling caps, replaced all the tubes, Tested ground connections, measured resistance on jumper wires on the back of the board. Re measured all tube voltages which are normal.

As the bias supply flows through the intensity pot so I am going to take a look at that part of the circuit this morning.

Obviously the amp is unstable and oscillating. I assume some sort of positive feedback is occurring due to a failed component.

Any Ideas a super welcome at this point.

Thanks,

Billy

EDIT: the motorboating does not happen with all the tubes in the amp.

[sluckey]

If this is a Hoffman AB763 the trem motorboating issue can likely be fixed by tweaking the bias pot.

[Planobilly]

Hi Steve,

After I reinstalled the tubes the motorboating issue associated with the tremolo intensity control went away.

The only current issue is low frequency buzz when  the E or A string is played. At a certain low level of gain the amp starts to oscillate and continues until the note almost decays. The issue is made worst with turning the bass control up.

The speaker is away from the amp a few feet but I guess it is possible some wire could be picking up the vibration from the speaker.

I am going to try to make a video of the issue....not sure how well the audio will show the issue.

Thanks,

Billy