5:50 clean channel hissing.

Despite the success of my proof of concept experiment (shorting the 1.5 M Ohm clean mode input resistor), I've decided to leave the amp stock for now and make up for lost input level with a clean, quiet boost pedal. Pedal of choice right now is the Xotic RC Booster.

As a test, I set the RC Booster pedal to 2.5 times gain (set with an o-scope) to make up for the amps 60% reduction and the EQ knobs at noon. At that setting the output of the pedal remains clean. There is ZERO detectable noise added by the pedal when in the clean mode on the amp. Yes, the pedal must be adding some noise, but it is so little that the amp's noise floor completely consumes it.

Bumping up the signal with "no added noise" seems to bring the amp back to a somewhat reasonable overall signal to noise ratio for a clean channel/amp. Not as quiet as with my mod or most other amps, but it will do.

- John

JKos said:

Don,
I want to experiment a bit before I spell it out but it's basically taking the 1.5M Ohm resistor between the input and the grid of V2A out of the circuit when the clean mode is active. If you look at the schematic of the Caliber 50, you'll see that they use a 1.5M and 1M resistor to form a voltage divider at the input. That same scheme is used on the Express.

Click to expand...

I think I know the resistor that you're talking about. R1? Did you just jumper it?

I've had some real problems with this issue myself (I own a 5:50 Express 2 x 12 combo) but a solution that has worked reasonably well for me is adding a Boss NS-2 Noise Suppressor into the effects loop which has made a significant difference. Also keeping the Master down has helped along with the recommended tubes from Doug's Tubes as several earlier posts have suggested. The hiss is a bit of a pain but nothing I can't live with. For me, as the tone is really great on the clean channel this seems the best compromise.

Pete

JKos said:

Despite the success of my proof of concept experiment (shorting the 1.5 M Ohm clean mode input resistor), I've decided to leave the amp stock for now and make up for lost input level with a clean, quiet boost pedal. Pedal of choice right now is the Xotic RC Booster.

As a test, I set the RC Booster pedal to 2.5 times gain (set with an o-scope) to make up for the amps 60% reduction and the EQ knobs at noon. At that setting the output of the pedal remains clean. There is ZERO detectable noise added by the pedal when in the clean mode on the amp. Yes, the pedal must be adding some noise, but it is so little that the amp's noise floor completely consumes it.

Bumping up the signal with "no added noise" seems to bring the amp back to a somewhat reasonable overall signal to noise ratio for a clean channel/amp. Not as quiet as with my mod or most other amps, but it will do.

- John

Click to expand...

Hi John,

I understand, why you prefer to add an external device to mount the level.
It's much easier indeed.

Anyway,
your idea is good with the 1m5 resistor.
if i look on any other known amps like fender, the input from guitar goes directly to the grid of the first tube.

in case of the f-30 this resistor is in the circuit also when lead is selected.
may be because of muting crackle when switching from clean to lead.

don't know.

so just shorting the resistor would change the circuit for the lead, too.
but in the schema you can see the relais where two pins are free.

So, on the relais RY1 I could make a bridge from RY1/L NO to RY1/R NO.
Then the 1M5 resistor and 180pF condensor would be shortened when in CLEAN mode.

Wonder why mesa did it not like this.
Did the want to protect the first tube for distortion?

Anyway,
proceeding this idea means to demount the mainboard completely to get access under the PCB.

i sent them a mail with this idea,
wonna hear what the say befor i try this modification.

so long

cheers

JKos said:

I talked to Mesa today and was simply told the hiss is part of the design and it's not going away. He said they tried to reduce it but doing so degraded the tone.

I tried some known good, low noise tubes in it and it didn't help. Which exact tubes does Doug's recommend?

Thanks,
John

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It seems some people expect tubes amp to be as quiet as solid state - none of em are.
All tube amps have a little hiss and the 5:50's hiss is less than the 5:25s.
And the 5:25s is about average for tube amps overall imo.
Some tube amps have less, some have more.
Most high gain distortion pedals have more hiss than the 5:50's burn channel imo.

The hiss on the 5 different 5:50s I've played is almost inaudible on any clean channel voice I've dialed in.
Hiss is audible (but not bad) on the burn channel only when you crank the gain past 2 oclock and you stop playing.
But this hiss is never audible once you start playing.

Someone mentioned quiet clean jazz solos - you shouldn't be hearing any noise while playing.
Stop playing and you'll hear a very low hiss/humm that is quickly forgotten in your bedroom.
Play live gigs and even an extremely noisy rig is never noticed due to room background noise.
Even my friends extremely noisy Peavey 5150 is not heard over room noise in a bar.
He thinks my 5:50 is silent by comparison.

Change tubes to quiet one's and you knock off 60-70% of the hiss.
Stick a good noise suppressor in your loop and it'll eat 99% even when cranked on the burn channel.

I've never seen so much complaint about a little insignificant hiss :mrgreen:

JKos said:

After tracing the circuit I identified the design "feature" which causes the noise to be so bad on the clean channel. They cut the guitar level by 60% before the first stage which means preamp noise is about 2.5 times more than it needs to be! Testing a very simple mod proved extremely successful and the noise was cut way down without "special" tubes or control settings.

"Tone?" you ask. To me it sounded every bit as good on the clean channel. Actually a little better but that could be because I can now hear details that the hiss was covering up. The mod brings the input stage of the clean channel in-line with most other amps, including those deemed to be clean tone monsters.

- John

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If the cause of the excessive hiss on the clean channel is from within the 1st gain stage of the pre-amp why is it generally accepted the 5.25 has more hiss than the 5.50 when they use the same pre-amp circuit?

assat012 said:

JKos said:

After tracing the circuit I identified the design "feature" which causes the noise to be so bad on the clean channel. They cut the guitar level by 60% before the first stage which means preamp noise is about 2.5 times more than it needs to be! Testing a very simple mod proved extremely successful and the noise was cut way down without "special" tubes or control settings.

"Tone?" you ask. To me it sounded every bit as good on the clean channel. Actually a little better but that could be because I can now hear details that the hiss was covering up. The mod brings the input stage of the clean channel in-line with most other amps, including those deemed to be clean tone monsters.

- John

Click to expand...

If the cause of the excessive hiss on the clean channel is from within the 1st gain stage of the pre-amp why is it generally accepted the 5.25 has more hiss than the 5.50 when they use the same pre-amp circuit?

Click to expand...

Got no idea.
Having played lots of both 5:25s and 5:50s I've just noticed it is all.
Having played many EL84 and 6L6 amps over the years I've also noticed that most (but not all) EL84 amps tend to have more hiss.
This happens on my Egnater Rebel 30 amp also which has both EL84 and 6V6 tubes.
It has a tube mix dial and you can switch or blend both tube types.
The EL84s are noisier on this amp also :mrgreen:

I'm new to mesa having had several disappointments with fender blues juniors - so I decided to spend the money and buy a quality amp I.e 3 year old ex demo 5:25. I loved it straight away. The thing I've found about the hiss is you only hear it when you listen for it and tune into it. I've taken it to jams and small gigs and not once has anyone commented on any hiss ( loads of comments on my playing though a couple of the comments were almost complimentary - I think !! ). At home it is not apparent in the mix when playing - my pc fan is louder. I honestly believe only us perfectionist guitarists hear it when we stop playing in between noodling - so my advice is play songs and riffs and don't waste time noodling !