Lowering Noise and Microphonics

[JammyDodger]

To All,

Another thought.... I do not (yet) have the ability to determine the inner / outer foil on the capacitors. Could this be a significant contributor?

Cheers, Mike

[Lonely Raven]

Briane,

Hey - I used the carbon film resistors that came in the kit from Nik. No Carbon Comps in the circuit at all.

Mike

Same here.

I'm thinking about switching all to metal film or something else quieter.

[nickt]

Briane,

Hey - I used the carbon film resistors that came in the kit from Nik. No Carbon Comps in the circuit at all.

Mike

Same here.

I'm thinking about switching all to metal film or something else quieter.

Almost all your hiss will be from stage 1 so you don't need to change out everything. This isn't specific to the express but generic to all amplifiers.

Note that your minimum hiss should be when the guitar is not plugged in - when the input is shorted to ground. Pluging in your guitar adds all sorts of stuff that's nothing to do with the amp and can't be fixed at the amp. Get your hiss minimized without the guitar or you'll be chasing your tail (IME anyway )

[Lonely Raven]

I understand. I was referencing the hiss without the guitar plugged in. In fact, this amp has also pointed out to me that I need a new volume pot on my guitar.

Briane,

Hey - I used the carbon film resistors that came in the kit from Nik. No Carbon Comps in the circuit at all.

Mike

Same here.

I'm thinking about switching all to metal film or something else quieter.

Almost all your hiss will be from stage 1 so you don't need to change out everything. This isn't specific to the express but generic to all amplifiers.

Note that your minimum hiss should be when the guitar is not plugged in - when the input is shorted to ground. Pluging in your guitar adds all sorts of stuff that's nothing to do with the amp and can't be fixed at the amp. Get your hiss minimized without the guitar or you'll be chasing your tail (IME anyway )

[nickt]

I understand. I was referencing the hiss without the guitar plugged in. In fact, this amp has also pointed out to me that I need a new volume pot on my guitar.

LOL - yeah all that lovely silence then in goes the jack and it all goes to hell - I'll shield the wretched thing one day...

[JammyDodger]

Well, after hours of changing this and that, moving wires caps, etc. I got feed up. I just wanted to play the amp noisy or not. So I put it in it's cabinet and moved it into my music room. Flicked her on and ..... quiet as anything! I then realized that on my work bench I have a solder station close by, my cell phone and florescent lights overhead. This accounted for all my noise!

I swapped a few tubes back and forth (mostly V1) and for a NOS GS 7025 that was quiet and non-microphonic. Sounds great, still burning it in.

Now, if I could just find a way to tame that volume control????


Cheers, Mike

[muchxs]

What worked for me was to wire a 33k grid stopper resistor directly on pin 2 of V1 AND take the solid copper wire leading from the volume pot to pin 7 of V1 and replacing it with this...

I got by with shielded cable only from the input, but a lot of folks have used it on the volume control, as well. I do like your choice of solid core shielded cable. I'll have to get me some of that

Somewhere in my stash I have half a dozen packets of teflon shielded cable originally intended as a mod kit for Ampex VPR-3 videotape machines. We had 16 of them, Ampex would send us upgrade kits. The shielded cable was supposed to be used on the audio heads. Ampex sent us 16 kits, we dutifully installed them then a week later 16 more kits showed up!

THAT'S the good stuff!

[briane]

yeah, dont go swapping out carbon films for metal, probably not worth the effort.

I had a lot of lead dress and shielding issues when I brought up my express, and as stated above, there can be a lot of exterior sources of noise. I used a fair amount of shielded wire, and also had to use the wire wrap gig on the hard/soft switch I added.

I like to take the amp out to the woods and play guitar naked to test.... just kidding....

I have always found an amp on the bench, not yet in a cab to be a bit noisy...ussually fixed once in a cab and off the bench. There can also be a lot of issues with small pf caps early on in the signal chain....I specifically noticed a certain brand of ceramics squealing and humming in another amp, but once I swapped those to silver mica it was clean as can be....And bias settings can also really affect the noise floor.

[DerStever]

I just converted an Express to a Liverpool - the Express had some hiss to it before the conversion - the 'pool after conversion is dead quiet! My other 'pool is mostly Ceriatone ('Cept the trannies and main board parts) - it hisses a lot - I changed out almost all of the resistors to metal film and it quieted it only a bit. It still uses Nik's caps.

The difference between my two LPs; is the conversion amp has the EL84s spaced out farther apart than the Ceriatone. It is using sprague atoms instead of Niks caps. Presence is SP6 OJD and the OT is Heybour 6600 tap (doesn't have a selection) See attached photos. INADYANN is the conversion amp and the other is the Ceriatone (modified).

[Lonely Raven]

Wow, nice lookin' build!


Question for you all (keeping somewhat on topic).


I have a Ceriatone TW, what would be hands down the best resistors to put into it to clean up the hiss?

Do I replace all, or just on the input?

Thanks!

[paulster]

I have a Ceriatone TW, what would be hands down the best resistors to put into it to clean up the hiss?

Metal film types are the quietest but I don't think they've been too well received by those that have tried them in a wreck.

If I were to try any as an experiment I'd probably go for the 1W PRP PR9372 series (see www.soniccraft.com amongst others for them).

I've used their GP1 series and found them to be great in the amps I've used them in - very musical. The PR9372 is designed as a more specific audio resistor, albeit very similar to the GP1.

Do I replace all, or just on the input?

The thermal noise a resistor produces is proportional to the square root of the resistor value, so the 1M at the input jack and the first stage plate resistor would be good candidates for first replacement.

The second and third stages are after the volume control, so if it's too hissy with the volume down then you'll want to look at their plate resistors.

If the hiss increases too much with the volume control then look at the tone stack 100K or your choice of V1 tube, because you'll be running out of options at this point.

I'd try to do it incrementally and see what the trade-off (if any) in noise versus sound quality is until you reach a happy medium.

Paul

[Lonely Raven]

Thanks Paul, that clarified a lot that I didn't understand in the posts above!

~Eric~

[mumford]

Today I replaced the volume to V1 wire with shielded and sat the chassis on top of a cookie sheet to see if I lost any noise-- it made a huge difference. All oscillation seems to be gone. Thanks for the tips!

[DerStever]

I emailed Kevin Oconnor about the hiss and he suggested the metal films - he has no use for carbon anythings in his builds. I seem to remember it did clean up a little with these vs the carbons.

I'm frankly suspicious of the output tubes being so close might have some impact on hiss. I've had tubes that were close to each other interact strangely before - swap one out with a different manufacturer and problem goes away. Weird. If someone else does a build with the power tubes spaced out a little more and doesn't get hiss that'll be two amps and maybe a pattern. In any event they will stay cooler and KO also makes note of how the power tubes line up can make a difference on how they operate, so there obviously can be some unwanted interaction with these tubes.

I did not notice any loss of tone going to metal film resistors.

[skyboltone]

Somewhere on here there's a post about bypassing the stage two grid resistor with an .02 film cap. That does kick the noise level waaaaay down but also rips the heart right out of the amp IMHO. I'm sure it's acting as a low pass filter with Rl, Rp, and Rk.


I'm up against the same stuff as the rest of you. For awhile I was backing up big time until I found one of the OT primaries ARCING through the insulation. Sometimes smoke tells us alot. I had all the boards out of the chassis twice before that little detail showed itself.

Now here's a question, on the liverpool scheme the grid resistor into stage 3 is shown as anywhere from 33k to 150k and the coupling cap as anywhere between .001 and .01.

If you run a normal gain equation you get a wide difference in stage 2 gain between 33k and 150k. I know that Dana and Allyn both like .002 and 150k but can the noise floor be brought down a bit by experimenting with these values? Without tone suck? Also, Plexi's commentary on Ginger mentioned a high cut network in parallel with the Rl of stage 2. I don't know what that means. If it's an RC network in parallel with V1a Rl it also lowers Rl and lowers gain as well. Has anyone any thoughts about how I'm reading that?

I will go to a shielded stage two grid and a cookie sheet and see if I can bring this thing down a bit.

Oh, and buy the way. I'm using RN65Ds throughout except for the 39k on the PI network and the 4.7k presence ground, and then only because I didn't have RN65s in those values.

Has anybody seen the need to shield the NFB loop?
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