New head amp help

[DougUSMC]

Bias is the control that is adjusted to make sure the power tubes receive the voltage that the manufacturer thinks will sound best with the amp. On the back of PRS amps are three jacks for testing the bias, one black (common) and two red (one for each tube). This is a VERY easy thing to adjust if you have a multimeter that reads millivolts (every one I’m aware of reads millivolts).

Set the multimeter to millivolts.

You stick the black probe into the common jack, and the red probe into one red jack, adjust the bias for that tube, then do the other tube.

There’s a hole in the chassis for the screw that adjusts bias; you’ll need a small screwdriver for the purpose.

No two tubes are perfectly matched, but with the screw adjustment, you bias the tubes to an average that’s close to the manufacturer’s spec.

I don’t know PRS’ bias recommendation for the Dallas, because I don’t have one. Ask Customer Service.

Here’s a video on the process.

Ta da! Done.

Awesome, thanks!!

 

[DougUSMC]

Definitely not quiet when first turned on.

When turned on, on standby, no instrument cable plugged in: dead quiet.
Switch standby to on, buzz/hum.
Back to standby (dead quiet again), plug in guitar off standby : same buzz/ hum (No noticeable difference)

It seems obvious to me that it’s not the instrument. Either a ground loop or maybe tube? Any suggestions for continued troubleshooting? FYI: The Dallas is plugged directly into an outlet

 

[Alnus Rubra]

Make sure you’re wearing your rubber wellies!

 

[DougUSMC]

Any thoughts on using something like THIS?

I watched a few videos, and I'm thinking it's an easy first try, but IDK if these things are legit or fools gold? Most of the reviews I found are solid, except for some that seem like they're trying to apply thm against the wrong problem?

 

[bodia]

Any thoughts on using something like THIS?

I watched a few videos, and I'm thinking it's an easy first try, but IDK if these things are legit or fools gold? Most of the reviews I found are solid, except for some that seem like they're trying to apply thm against the wrong problem?

It's a solid product. I have never used, but there was a "noise" thread some time back, about the MT-15 I believe, and more than one person said they used these on all of their tube amps.

 

[DougUSMC]

It's a solid product. I have never used, but there was a "noise" thread some time back, about the MT-15 I believe, and more than one person said they used these on all of their tube amps.

Awesome, thanks, I just ordered one.

 

[László]

Definitely not quiet when first turned on.

When turned on, on standby, no instrument cable plugged in: dead quiet.
Switch standby to on, buzz/hum.
Back to standby (dead quiet again), plug in guitar off standby : same buzz/ hum (No noticeable difference)

It seems obvious to me that it’s not the instrument. Either a ground loop or maybe tube? Any suggestions for continued troubleshooting? FYI: The Dallas is plugged directly into an outlet

Since Standby shuts off the audio, yes, one always expects no hum! Nothing’s getting to the speaker.

I’m not clear on whether you disconnected the amp from your switching system, and from any pedals.

If the amp, all by itself, is plugged directly into an outlet, and directly into the speaker with a single cable, nothing at all connected to the amp except one cab directly, no cab switchers, no pedalboard or pedals, no speaker patchbay, and it makes noise, it isn’t a ground loop, since there isn’t a second signal path to ground connected to the amp.

The Ebtech only solves a problem if there’s a ground loop, I.e., two paths to ground, which isn’t possible if every other path to ground is disconnected.

Another path to ground would be something connected to the amp with an AC plug. That can include a powered pedalboard, a switcher or patch bay connected to a second amp, etc.

No diagnosis can be made unless the amp is connected only to the speaker directly.

 

[DougUSMC]

Since Standby shuts off the audio, yes, one always expects no hum! Nothing’s getting to the speaker.

I’m not clear on whether you disconnected the amp from your switching system, and from any pedals.

If the amp, all by itself, is plugged directly into an outlet, and directly into the speaker with a single cable, nothing at all connected to the amp except one cab directly, no cab switchers, no pedalboard or pedals, no speaker patchbay, and it makes noise, it isn’t a ground loop, since there isn’t a second signal path to ground connected to the amp.

The Ebtech only solves a problem if there’s a ground loop, I.e., two paths to ground, which isn’t possible if every other path to ground is disconnected.

Another path to ground would be something connected to the amp with an AC plug. That can include a powered pedalboard, a switcher or patch bay connected to a second amp, etc.

No diagnosis can be made unless the amp is connected only to the speaker directly.

Ah, ok. I thought it was possible to get that 60Hz hum directly from the power lines, if something else in the circuit was badly wired up? It should be here today, so we'll see what happens...

To clarify:
The amp is sitting on top of the 4x12
There is a speaker cable connecting the two.
The amp is plugged into the wall, the other outlet of the plate is empty.
Nothing else is connected to anything.
Getting the hum.

THEN
I connect the guitar to the amp DIRECTLY with an instrument cable
Getting the same hum

For clarity's sake: the rack o'amps and switching that I built is on the other side of the room. It's still up and running, but the Dallas and 4x12 are their completely own setup.

 

[László]

Ah, ok. I thought it was possible to get that 60Hz hum directly from the power lines, if something else in the circuit was badly wired up? It should be here today, so we'll see what happens...

To clarify:
The amp is sitting on top of the 4x12
There is a speaker cable connecting the two.
The amp is plugged into the wall, the other outlet of the plate is empty.
Nothing else is connected to anything.
Getting the hum.

THEN
I connect the guitar to the amp DIRECTLY with an instrument cable
Getting the same hum

For clarity's sake: the rack o'amps and switching that I built is on the other side of the room. It's still up and running, but the Dallas and 4x12 are their completely own setup.

OK, it’s not a ground loop. I can’t imagine the Ebtech solving it, though it’s a useful tool to have around.

My next step to begin diagnosing is with the tubes. Mesa’s manuals all come with very good instructions on how to check for tube issues, and solve them, and they can be downloaded from their website.

Tubes can develop hums and noises over time, and shipment causes parts in the tube to fail, etc.

If you follow the Mesa instructions exactly, and it’s important to be patient and proceed methodically and slowly, you’ll probably find the bad tube, if there is one.

If it’s not a tube, I’d have PRS check it over. I know it’s a pain to ship, etc., but there aren’t a lot of techs who have experience with these amps, and I’d be inclined to trust the folks who made it.

Remember, the CAD amps ran in the three grand range new, and they’re valuable pieces! It’s bad enough when a local tech screws up a $500 amp.

My old Mesa Tremoverb was literally turned into junk by a poor tech. Never again.

 

[DougUSMC]

Ok, good news / bad news:
Good news: We can continue to worship Les - he was (duh) right, the Ebtech didn’t solve anything
Bad news: I still have a big hum

I also noticed something while playing around today: The more reverb I dial in, the more hum/buzz. Could this mean anything?

 

[Deleted member 5962]

Ok, good news / bad news:
Good news: We can continue to worship Les - he was (duh) right, the Ebtech didn’t solve anything
Bad news: I still have a big hum

I also noticed something while playing around today: The more reverb I dial in, the more hum/buzz. Could this mean anything?

Yeah, the Ebtech is good but for a certain thing and not the thing you have. Have you played this amp anywhere else? Just trying to rule out dirty power, which sounds naughtier than it is.

Edit: Actually, I've gotten way behind on this thread and should read back to what all you've done before making suggestions. I popped in when you were talking about speakers but haven't read the whole thing about the amp hum

 

[DougUSMC]

Yeah, the Ebtech is good but for a certain thing and not the thing you have. Have you played this amp anywhere else? Just trying to rule out dirty power, which sounds naughtier than it is.

Edit: Actually, I've gotten way behind on this thread and should read back to what all you've done before making suggestions. I popped in when you were talking about speakers but haven't read the whole thing about the amp hum

Nope, only in the house. I guess the next step is to try another head in the exact same setup, and see if I get the same results...

 

[László]

Ok, good news / bad news:
Good news: We can continue to worship Les - he was (duh) right, the Ebtech didn’t solve anything
Bad news: I still have a big hum

I also noticed something while playing around today: The more reverb I dial in, the more hum/buzz. Could this mean anything?

30 years in the studio biz, a person picks up a lot of info, no worship necessary, though of course any worship at all is appreciated, please send donations to...uh, ok.

If you’re getting more hum dialing in the reverb, maybe it’s a bad reverb driver tube. But go through the entire procedure of checking the tubes. Because it could be something else.

Also...check the connections between the reverb tank and the amp. Sometimes that could be the source of hum if it’s not a solid connection.

But with a tube amp, the tubes are where to begin checking for sources of noise. Remember, tubes are like light bulbs; they wear out and go bad over time. Power tubes are more subject to wear by far than preamp tubes, but check the preamp tubes too.

Check the tubes one by one, as Mesa suggests in their troubleshooting part of any of their manuals. I’m going to guess it’s a tube until further review.

Also, this is possible, try swapping the speaker cable between the amp head and cab with a known good one. Bad cables can cause lots of noise.

 

[Alnus Rubra]

30 years in the studio biz, a person picks up a lot of info, no worship necessary, though of course any worship at all is appreciated, please send donations to...uh, ok.

Donations to:

[email protected]

 

[László]

Donations to:

[email protected]

Let’s set a minimum donation price, then, K?

Like...oh...maybe $100. Because you know, it wouldn’t do to insult Les (whoever he is) with small, nickel and dime donations.

 

[Alnus Rubra]

Webpage set up and donation made

 

[DougUSMC]

30 years in the studio biz, a person picks up a lot of info, no worship necessary, though of course any worship at all is appreciated, please send donations to...uh, ok.

If you’re getting more hum dialing in the reverb, maybe it’s a bad reverb driver tube. But go through the entire procedure of checking the tubes. Because it could be something else.

Also...check the connections between the reverb tank and the amp. Sometimes that could be the source of hum if it’s not a solid connection.

But with a tube amp, the tubes are where to begin checking for sources of noise. Remember, tubes are like light bulbs; they wear out and go bad over time. Power tubes are more subject to wear by far than preamp tubes, but check the preamp tubes too.

Check the tubes one by one, as Mesa suggests in their troubleshooting part of any of their manuals. I’m going to guess it’s a tube until further review.

Also, this is possible, try swapping the speaker cable between the amp head and cab with a known good one. Bad cables can cause lots of noise.

Thanks again, I have to track down that doc, and start working my way thru it.

I played thru the amp for an hour last night, master and volume 1/2 way up. W/o reverb it's pretty doggone quiet, with even a tiny bit, the hum comes back strong.

 

[László]

Thanks again, I have to track down that doc, and start working my way thru it.

I played thru the amp for an hour last night, master and volume 1/2 way up. W/o reverb it's pretty doggone quiet, with even a tiny bit, the hum comes back strong.

I’m thinking reverb driver 12AX7, or cables to reverb tank. But check everything, per the Mesa suggestions.

On some amps, the reverb tank is connected with RCA jacks (like hi fi cables). Check that they’re pushed in all the way as well, if your amp has them. In fact, a tube that isn’t seated well can happen in shipping, and that can create noise.

Some folks think tubes are a pain, but since 90% of tube amp issues are the tubes, it means fixing problems is easy as replacing a light bulb. If something goes bad on a solid state amp, the fixing process is a lot more difficult!