Weird Hum

[Len Rabinowitz]

There is unquestionably an issue. What the issue is hasn't yet been determined. If they don't find anything that amp is gone.

 

[amrose57]

There is unquestionably an issue. What the issue is hasn't yet been determined. If they don't find anything that amp is gone.

I hear you. I'm not there yet, but slowly working my way there.. :lol:

I documented my hiss issues here and on TGP as well with this amp. I've rolled over 30 preamp and power amp tubes, new, and NOS, and finally the Mesa Mesa SPX7a tubes in V1 and V2 got the hiss down some. The thing that stinks for me, is when the amp is at full song (ie-band setting) it sounds awesome and the hiss doesn't matter, but here in the music room it just hisses too much, and now the hum as well. My DC2 hisses to, but much less. I've come to the conclusion that pretty much all high gain EL84 amps hiss more than a 6L6 or 6V6 amp, but I like the sound of EL84's. Actually, I've seriously considered buying a Quilter for home and solo use, and just use the Express for giging with a band. FWIW - listening to the new Mark V:25 on the demo clips I still heard the hiss, but without being there I can't gauge how loud it really it.

As an aside here - I did figure out that 1/2 of the hiss is coming from the power section, by plugging a cable in to the send of the effects loop and leaving the cable hanging. That takes the preamp out of the circuit. When I did that the hiss went down by a few db, but it was still there. That's why I concluded it's a character of an EL84 amp.

Al

 

[Len Rabinowitz]

My issue wasn't hiss- this a little bit noisy of an amp. It was initially a hum, and then an almost total loss of volume. Both of these on both channels, 5 watt setting only.

 

[amrose57]

My issue wasn't hiss- this a little bit noisy of an amp. It was initially a hum, and then an almost total loss of volume. Both of these on both channels, 5 watt setting only.

Understood. The one thing I've not done is put the amp on a power conditioner. And I mean a true, transformer isolated power conditioner. I plan on trying that soon, just haven't gotten around to buying it yet. I want to because I noticed the noise level was less at a friends house, and he's on a different substation than I was. (I'm an ex-power guy who still works in that field, just not for a power company but for a forensic company) When I get around to it (it's not an inexpensive proposal, and I'd be actually buying it for my home hi-fi) I'll post the results.

Al

 

[Len Rabinowitz]

Got my 5:25 Express plus back today. Sounds great!

Here’s what the repair ticket said, as best as I can read the handwriting anyway:

“Replaced leaky 115/400v coupling cap, one side of driver tube. This was killing one side of power amp, giving no signal in 5 watt mode. Replaced R125 (a part location on the circuit board I think) stuffed wrong part with 5k6/7w. Wrong part was producing hum in 5 watt mode.”

I don’t suppose you tech sorts can translate that into English?

How the heck did a wrong part get in there anyways?

Thanks!

 

[jazzphan89]

Got my 5:25 Express plus back today. Sounds great!

Here’s what the repair ticket said, as best as I can read the handwriting anyway:

“Replaced leaky 115/400v coupling cap, one side of driver tube. This was killing one side of power amp, giving no signal in 5 watt mode. Replaced R125 (a part location on the circuit board I think) stuffed wrong part with 5k6/7w. Wrong part was producing hum in 5 watt mode.”

I don’t suppose you tech sorts can translate that into English?

How the heck did a wrong part get in there anyways?

Thanks!

Only one tube is running in 5 watt mode, so it makes sense that the cap was causing the issue. R125 is a circuit board part AFAIK. Maybe they were out of the traditional part, and put an equivalent spec in there? Obviously wasn't tested for durability or noise ration though, It just seems unlikely Mesa would do that though.

Glad you got your Express back!