speaker clip/amp cutting out

[esandes]

i have two guitars with somewhat similar pickups-alnico 5 based humbuckers with 8.5-10 DCR. one of the pickups is using a 500k volume pot and the other is using a 1M volume pot. no tone pots in either guitar.

when i use the 1M guitar and play higher single notes or trebley chords with alot of attack, it sounds like the speaker clips or the amp kind of cuts out like it's being overloaded or going to ground alot or something. it sounds clipped and then the volume cuts out temporarily. with the 500k guitar it doesn't.

this is only during rehearsal volumes with my 100W amp volume around 2 going through a 100W cab. could it be the speakers or the amp. the amp is around two years old and still has stock power tubes. maybe the 1M guitar has the pickup height too close to the strings? what could it be?

 

[Hank-]

Re: speaker clip/amp cutting out

Time to change the power tubes, 2yrs is a lot of time with current production stuff if ya playing with decent volume. While at getting power tubes it's time to get new preamp spares too. A fresh tube in V1 will help bring it back to life, the PI spot is also due for a change, so get two preamp tubes if possible. Also check the speaker cable, it may be time for a fresh one if the old one has taken a lot of abuse.

 

[esandes]

Re: speaker clip/amp cutting out

i think you're right about the power tubes. i put in a fresh batch of preamp tubes a couple of months ago due to a hiss/buzz issue on the cascaded channel at high volumes. it helped. i also switched the speaker cables.

when i bought the head, the back of it had a big scuff. maybe the amp was used and returned so it may have been abused and the power tubes are worn out.

 

[Hank-]

Re: speaker clip/amp cutting out

I think it's mostly the power tubes, but it can also be a preamp tube, if you have spares around then it should be easier to troubleshoot the preamp section. I've had my sound cutting out before at any volume but it turned out to be a loosely fitted preamp tube or one on it's way out. You could try removing & putting the tubes back in the sockets, it will help clean up some of the contacts. Worth trying that out first, but it's definitely time for a power section retube, that may require re-biasing the amp

 

[glassman]

Re: speaker clip/amp cutting out

I wouldn't suspect power tubes (or preamp tubes) at all. This sounds like an amp design meets your ultra high output guitar problem to me.

Quite a few high gain amps have high frequency snubbers at the input of preamp stages that are susceptible to squeal...they put a capacitor in there from the grid to ground to attenuate the extremely high frequencies. With a guitar like yours, with ultra low loading and no tone control at all, you can be throwing several volts in the really high frequencies at the first grid and a lot higher further down the chain. Depending on the value of the snubber, this high input signal in the high frequencies can charge the snubber and temporarily bias that preamp stage at or near cutoff...effectively killing its ability to amplify. As soon as it recovers (bleeds down) you are making sound again.

If the extra output you get from the 1M pot is important to your sound, consider either a tone control or a snubber (50pF to 100pF capacitor across you volume pot in the guitar); this will probably trim enough highs to stop the cut-off event...you can always make the lost treble up with amp settings.

 

[esandes]

Re: speaker clip/amp cutting out

hmm...****, so it's possibly the guitar. would switching the 1M pot to a 500k do the trick too?

i really like the clear tones from the 1M pot guitar at man cave sound levels. i might just use the 1M pot guitar for practice. i will see if i can replicate the problem with another amp too. as i recall, it has only happened with this one particular head (1959rr). then again the cascaded channel i use on the 1959rr has 36 db of boost on its own-which could really amplify the issue.

 

[glassman]

Re: speaker clip/amp cutting out

hmm...****, so it's possibly the guitar. would switching the 1M pot to a 500k do the trick too?

It most likely would...I'd consider the 50pF or 100pF across the volume pot first though...it will only attenuate the ultra high frequencies that are causing your problem...frequencies that are too high to hear anyway so you won't really be losing anything.