Uk amp mods?

[witeter]

Hiya guys, just wondered if you knew of any amp mod people/sites based in the uk? i keep on seeing a lot of options over the USA,etc but  nothing over here. I wouldn't be after a drastic mod but probably something to reduce background hiss on a Peavey. Thanks

[Dmoney]

Where are you based?
Any competent tech should be able to help you out. It's always better to find one who is keen on working on something like a Peavey.

Which Peavey is it?

[witeter]

Im based in Cardiff-havent got the Peavey yet but most likely to be the 3120.

[Dmoney]

I'd see how you get on with it before you start thinking of modding it.

For example, the 5150 is hissy partly due to the fact it has SO much gain and partly because of how the PCB tracks are all laid out, which you can't really do anything to fix. You can do a few things though. Add some caps here and there and also look at using the noise reduction circuit from the Peavey JSX. I think the 3120 has a Noise Reduction circuit it in that is non variable, unlike the JSX.

If the amp has BAGS of gain you can try using a 5751 valve in V1. It'll have a little less gain than a 12AX7. If you use one, it will reduce the amount of gain you get at V1 ever so slightly, and you should hear the hiss drop too. I quite like this approach.

[JacksonRR]

Fitting it with 1/2W or 1W resistors in the preamp section will also reduce hiss. The 1/4W ones that are in the signal path would be the ones to swap out. The little "noise gate" circuit from the JSX/XXX series does work OK. I've done both of these to my 5150 and it's a LOT quieter at idle than stock.

[Dmoney]

I've read the 3120 is 95% the same as the XXX or something?
If it already has the noise coring circuit in it, but it isn't adjustable, then that doesn't sound like a big mod. Would need to compare schematics.

are they going to be carbon films in there too? I guess you could go to metal film on the plates and increase the wattage like you say.

I wouldn't worry about it before you actually have the amp in your hands though. I mean, the 5150 is notoriously hissy, but enough people use them still.

Jackson. You've increase the resistor power rating in a 5150 and noticed it cut noise? have you ever changed the caps in one? I've got a mate bringing me another 5150 for an adjustable bias mod and screen resistor 'upgrade'. I might ask him if he's up for letting do some extra bits.

[HTH AMPS]

if the amp doesn't have one, a choke is always a worthy mod, as it elevating the heater circuit's CT to a DC reference.

above all else, a noise gate in the loop will work wonders.

[Dmoney]

I don't think the 3120 has choke. Adding one wouldn't really alter the levels of hiss though, right? That's a cool mod but its altering the operation of the amp a bit. Might not be ideal if you're totally happy with the amp but all you want to do is reduce hiss. And if its like the 5150 choke mod is a little involved isn't it? because of the where the standby switch is you can end up with some wacky flyback voltage through the choke unless you do a bit of rewiring, or move the location of the standby switch in the circuit.

That stereotypical hiss wouldn't really be cut down vastly be elevating the filaments either would it? Though it's worth doing because of certain elements in the circuit anyway.  

What about adding a cap from the filament wiring to ground to shunt off noise?

As for a noise gate in the loop, I'm not sure where the noise gate circuit in the JSX/XXX lives but if its towards the loop then you'd kind of end up with two noise gates in series right? though in the 3120's case, the built in noise reduction circuit isn't variable. In the manual there is a mention of it: 'Special noise gate circuitry for lead & rhythm'.

I've had less then happy results from running at least an NS-2 in the loop of a 5150. I tried it once and decided it wasn't for me.


EDIT: ah! the noise gate in the JSX is in the middle of the preamp before the driver stage for the lead and rhythm tone stacks. So not near the loop. I imagine its in the same place in the XXX and 3120.

[JacksonRR]

Yes, increasing the wattage rating will result in the reduction of thermal noise ie resistor hiss. It makes sense if you think about it with the term "thermal noise" in your head to up the wattage. All the resistors that are directly in the signal path I've managed to swap them to 1/2W units. Not a pretty mod, but that's OK. Just bend the leads under and down. Needle nose pliers helped me with that. I really don't even think I needed to do all of them, you can probably just get by with the first 3 gain stages.

I didn't replace any caps for noise issues. After the resistor swaps and the XXX "gate", I was happy that it was less like one of those white noise sleep aids. I'd say it knocked down about 80% of the hiss. I can show you where I stuck the XXX circuit if you'd like. I did change caps for tonal reasons though. Things between gain stages and coupling caps... Didn't notice any more or less noise, but then again I wasn't listening for that change so maybe that's even better than if I was.

edit: Yes, in the middle of the preamp. I believe I stuck mine between the 2nd and third stages.

[Dmoney]

Yeah I get the thermal noise thingy. I just wondered if you'd tried it yourself or if all you did was add the XXX circuit.
I might ask my mate if I can try swapping in some high rated resistor in his 5150 then, he has two. So it would be good to compare them afterwards.

i wouldn't have thought changing caps would do much with noise unless there was an issue with one. I've seen photos where people have put orange drops into a 5150. I just wondered if you'd maybe tried that too. Not so much for noise reduction, but just for general tonal improvement.

Did the the noise reduction circuit make a huge difference? is yours adjustable or is it fixed like the XXX?

[JacksonRR]

Yeah, it was a substantial reduction with the XXX circuit. Actually I lifted it from the Rockmaster preamp, but I think it's the same. Two diodes facing opposite with a 1M resistor in parallel. Fixed operation. My experience with the JSX and the Bugera 333XL gave me reason to believe that drilling a hole for a pot would be cool, but useless. If it was for someone else, I might have gone with the pot as they can play with it and remember how much noise used to be there at idle.

The caps I used were polypropylene, but not Sprague brand. I just bought them at a local store, same with the resistors. Didn't notice any changes other than what I assumed would happen with the value changes. At the time, I really really really hated searching through Mouser and Digikey.... Navigating like a pro now though.  :lol:

Perhaps just throw in the gate and swap R22, R82, R6 and R7 for a 1/2W or 1W unit. Now that I understand things a bit better, that may have been the bulk of the improvement. I also jumpered R27 and R30 completely...

[JacksonRR]

Rob, I e-mailed you the pdf that Peavey gave me for the 5150. Contains scheme, layout and BOM.
 
For the OP, FYI, the noise gate that FJA adds to the 5150 IS the JSX circuit, but he has his so it's only active on the lead channel. It does add artifacts that are not wanted in a clean sound.

[Frank]

I'd probably start by replacing the factory preamp tubes with a decent set, experiment to find which brand give you a good sound with a low amount of noise.

[Dmoney]

Rob, I e-mailed you the pdf that Peavey gave me for the 5150. Contains scheme, layout and BOM.
 
For the OP, FYI, the noise gate that FJA adds to the 5150 IS the JSX circuit, but he has his so it's only active on the lead channel. It does add artifacts that are not wanted in a clean sound.

Cheers!

I'd probably start by replacing the factory preamp tubes with a decent set, experiment to find which brand give you a good sound with a low amount of noise.

To be fair, I think peavey use JJ's ecc83s. I might be wrong though. If its the short plate JJ's though, they may not have the greatest sound ever, but they are low noise and very robust when it comes not going microphonic and so on. a 5751 is lower noise still, but its also lower gain, so its cheating!

[jpfamps]

Yes, increasing the wattage rating will result in the reduction of thermal noise ie resistor hiss. It makes sense if you think about it with the term "thermal noise" in your head to up the wattage. All the resistors that are directly in the signal path I've managed to swap them to 1/2W units.

The reduction thermal (Johnson) noise it minimal when increasing the wattage of the resistors in an amp. Yes the resistors run at a slightly lower temperature, so the noise is less, but not by much.

Thermal noise is proportional to the square root of temperature in Kelvin, so going from 20degC (293degK) to 50degC (323degK and which would represent a very significant increase in temperature) only results in a 5% increase in noise.

The main effect is the reduction in current noise. The main culprits for this are the plate load resistors, and changing these to higher wattage metal film resistors will give you the most bang for buck.

High gain amps frequently have high value grid stoppers, which is a bad idea for noise performance and these can also be replaced by lower noise resistor types to good effect.