Strategies for reducing noise/hiss

I'm working on a Laney GH50L for a client that wants less noise.

There's nothing wrong with the amp, he just wants less noise in an amp that is inherently noisy.
IMO, these just have a somewhat high noise floor. Especially with drive engaged.

I'm going to replace all of the preamp tubes with high performance JJs from tube depot with the low noise/microphonics option.
Replace all of the preamp plate resistors with 3W metal film, and all of the resistors in the drive circuit with the highest wattage that will fit. Maybe the coupling caps, I dunno. I've read that doing the coupling caps works on hum more than hiss, and the result is minimal.
I considered maybe replacing all of the wires going to V1 and V2 from the PCB with shielded coax. Alot of work though, and may do more harm than good. Would swapping the cathode resistors to a higher wattage metal film help?

There's no hum, Just hiss. Would converting it to DC heaters help?

Anybody have any tips, tricks, or advice for taming the hiss?

Buy a boogie. That's my tip.

My Boogies have zero hiss, zero hum and so much gain they can rip small rodents into pieces of pie.

You're on the right track.

1. Replace input grid resistors with 1W or higher metal film if it has it
2. Replace the 1M ground reference resistor on the input jack if it has it
3. Replace all plate resistors with 1W or higher metal film
4. Shielded coax

Right on. What about the bypass caps, and the cathode resistors?
Anything to be had there?

Some electrolytics can leak DC as they age and ceramic caps can also leak DC and contribute to harshness and possible hiss, if you replace ceramics with a high quality Silver Mica it will smooth the tone some and possibly reduce noise.

I had a hissy 72 Marshall here's the link you mind find some insight pertaining to your application. viewtopic.php?f=3&t=201626

I have also heard reducing the input grid resistor from the input jack from 68K to 33K will reduce noise/hiss.

kilgoretrout5447":3h2epps7 said:

Right on. What about the bypass caps, and the cathode resistors?
Anything to be had there?

Click to expand...

I would leave the signal caps alone (bypass and coupling), unless you know they are faulty. Cathode resistors... you can leave those as they are for now.

As harddriver also mentioned, reducing the value of the input grid resistor from the input jack to the first stage, along with changing it to a 1W or higher metal film, will certainly reduce some thermal noise. You want to use the lowest value you can while still providing some RF attenuation. I would need to look at a schematic of the amp in question. But for a Marshall, for example, you can go down to like 27K without any real issues and it will definitely reduce noise as long as you're not right next to a radio tower.

Another option is to take it out completely and use ferrite beads instead. But I think a resistor provides better protection from RF.

kilgoretrout5447":3nmtmwg1 said:

I'm working on a Laney GH50L for a client that wants less noise.

There's nothing wrong with the amp, he just wants less noise in an amp that is inherently noisy.
IMO, these just have a somewhat high noise floor. Especially with drive engaged.

I'm going to replace all of the preamp tubes with high performance JJs from tube depot with the low noise/microphonics option.
Replace all of the preamp plate resistors with 3W metal film, and all of the resistors in the drive circuit with the highest wattage that will fit. Maybe the coupling caps, I dunno. I've read that doing the coupling caps works on hum more than hiss, and the result is minimal.
I considered maybe replacing all of the wires going to V1 and V2 from the PCB with shielded coax. Alot of work though, and may do more harm than good. Would swapping the cathode resistors to a higher wattage metal film help?

There's no hum, Just hiss. Would converting it to DC heaters help?

Anybody have any tips, tricks, or advice for taming the hiss?

Click to expand...

All techs will tell you capacitors should be checked out of circuit (removed).

BUT...

DC flows in one direction. The capacitor will pass DC until it is fully charged and there is no more current flow. With amp on if you are getting "any" DC readings "out" of the capacitor this is an indication of failure. You must then remove the capacitor and test it or just replace it.

Many ways to properly test caps out of circuit. Refer to Google lol.

Just a quick test to isolate a suspected culprit. BE CAREFUL AS CAPS CAN HOLD LETHAL DC VOLTAGES!

David...♫

Thanks all. Much appreciated. I changed all of the electrolytics along the B+ supply, all of the plate, grid, and cathode resistors to 2W or better (Some were a tight fit), used jj ecc803s, with the low noise/microphonics option from tube depot, moved the ground on the power tubes to share the same points as the power caps, and cleaned all pots, sockets, jacks, and switches. This thing still hisses like a sonovabitch. Maddening. the caps were all out of comfortable ESR range for sure, so that needed done anyways.


Edit:
I took out some stuff here where my head was up my ass.


Then it's on to coupling, and bypass caps just because I don't know where else to go with it.

Let me know if there's anything else that might help.
The schematic is online if anybody feels like taking a look.

No dice. This is F**king ridiculous. Also, V1 was insanely microphonic. I swapped it with a normal JJ ecc83, and it's better than it was microphonics wise, but did nothing for the hiss.
Low noise and microphonics my ass. I have no idea where else to go with this. After the pre and power tubes, caps, resistors, and all of the work done, the client's tab is 250+, and it's not much better than it was when it came in.

Link to the schematic. Can't seem to find one.

Okay, Disclaimer I have the flue and am high as i kite about now but I do have this to offer. Basic trouble shooting 101.

1.) I would pull the preamp tubes accept for the last one as it is you PI. Plug your guitar straight into the loop return. If you have loud clean no hiss signal then you know the problem is in the preamp as most of us suspect.

.2) If the hiss stops put the preamp tubes back in.
Plug into High input first and turn on. We know you will get signal and or hiss.

3.) Place on standby then pull VI. see if there is any signal as each 12AX7 contains two triodes (Stages). If it generates a signal with no /less hiss V1a may be the culprit.

4.) If not, remain plugged into low input place on standby and pull V2 at this point it will have to go silent providing data that the issue is in V2 somewhere unless the PI is a mess and that is rare.

My $.02 Will check back in later.

David a.k.a., New Religion...♫

Okay, Disclaimer I have the flue and am high as i kite about now but I do have this to offer. Basic trouble shooting 101.

1.) I would pull the preamp tubes accept for the last one as it is you PI. Plug your guitar straight into the loop return. If you have loud clean no hiss signal then you know the problem is in the preamp as most of us suspect.

.2) If the hiss stops put the preamp tubes back in.
Plug into High input first and turn on. We know you will get signal and or hiss.

3.) Place on standby then pull VI. see if there is any signal as each 12AX7 contains two triodes (Stages). If it generates a signal with no /less hiss V1a may be the culprit.

4.) If not, remain plugged into low input place on standby and pull V2 at this point it will have to go silent providing data that the issue is in V2 somewhere unless the PI is a mess and that is rare.

If you are uncertain you can always pull the power tubed of the AO and plug into it. run its send loop to the return loop of a known good amp this can tell you a lot too.

My $.02 Will check back in later.

David a.k.a. Dr Feelgood, New Religion...♫

http://www.thetubestore.com/lib/thetube ... ematic.pdf

There's the schem.

I havn't pulled any tubes to see whisch stage it is in, (forehead slap) but I may have found the issue. It's definitely in the pre tho, as ot responds to the MV. I gotta dig in more, but I scoped the plate supplies last night, and for a few seconds before the capacitance in the probe ate it up, there was definitely an overlay of fine crinkly low amplitude noise on there. It couldn't detect it in the power tube plates, but ot's definitely on the plates of v2. Can't remember on v1, but I don't think so. Now, I'm thinking the 10k 4w fireproofs along the HV supply rail are picking it up.
Any thoughts on this theory before I blow more cash on parts and shipping? Gaaahh! The shipping! I hate blowin 7 bucks shipping on 50 cents worth of parts. Haha!

Guh. This amp sucks. The lead dress is jive, there's noise everywhere, and now, the whole damn chassis is microphonic. It's not the preamp tubes. I tested them all in a valve jr chassis that i keep around just for testing "good tubes" for microphonics. I tried prolly 20 different tubes in v1 and v2, and it solves nothing None of them ring in the valve jr. I feel like these issues are related. Ground loop or something. The microphonics are loudest around v1, but nothing on the pcb seems to ring like the chassis. I'm gonna try grounding the power tubes, and filter caps all to one spot. Then find where the preamp pcb makes its way to ground, and ground it to the same spot. Also gonna replace all caps in the first two tubes. Already did all of the resistors. Then replace all of the heater wiring with sheilded twisted pair cable. Desperation.