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Reducing Hissssssss

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Lonely Raven, Dec 13, 2007.

  1. Lonely Raven

    Lonely Raven Member

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    Trainwreck style amp, I built it myself from a Ceriatone kit with upgraded caps and I had to change some of the resistors I had in stock since some of the supplied ones were more then 25% out of spec per my calibrated Fluke.

    [​IMG]



    Not super pretty, but she runs perfectly (loud as hell!), no performance issues at all...but she's got more hiss then I'm happy with. No hum assuming I put a bottom plate on, just hiss.

    I figure the best place to start is replacing the resistors with better quality ones...but where do I start and what's the *best* to put in there that won't sterilize the amp?

    I'm going to add externaly accessable bias pot and test points, and a half power switch, as well as building an Air-Brake. I'll take the time to clean up the wiring a bit, so I figure I might as well work on the hiss while I'm in there.

    Suggestions?
     
  2. JimmyR

    JimmyR Member

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    Better tubes? It IS a hissy kind of amp with all that distortion built in. I believe metal film resistors are supposed to be quieter, but don't think they'll make a huge amount of difference. This circuit is notorious for being finicky re. lead dress so I'd probably try to tidy things up a bit but doubt that will help with hiss. The main things I would do is tidy up the filaments and keep the signal path as short as possible while making sure that the grid and plate wires crossed at 90 degrees or so - it looks good in that regard anyway. Basic lead dress stuff.
     
  3. Lonely Raven

    Lonely Raven Member

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    Yeah, I know the mess around my power tubes could use some work, but it sounds so good now, I'm thinking component changes is what I need. I'm not thinking lead dress will fix hiss at all, and from what I've read, cleaning it up too much might actually make the amp too tame!

    That's why I figured resistors are a good, safe swap.

    I'm looking for a specific line of resistors to try out. Hopefull others will chime in on this??
     
  4. mwoeppel

    mwoeppel Member

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    Lead dress affects the sound? that defies my understanding of electricity. But it's been a long time since I took a class or built an amp.
     
  5. Trout

    Trout Member

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    In some cases it makes a huge difference. I have on several occasions while working on a couple vintage Marshall SL 100W amps had scenarios where just moving a lead 1/4 inch with a chop stick had a huge impact on noise.
    I would rather see an amp amplify the signal instead of noise.:D Especially if the noise starts near the front end.
     
  6. PRNDL

    PRNDL Member

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    Trainwrecks have three preamp gain stages, which means that a little noise in the first stage is amplified.

    Better quality resistors will certainly help. Carbon comps have better tone. Look for 5% tolerance.

    You might also want to replace the first two cathode bypass caps with high quality electrolytics, since they have a major impact on noise. Plus, if the resistors were low quality, you can expect everything else to be the same.

    Lead dress is critical for lowering noise. I'd begin with the heater wires. Twist them tighter and shorten them. On the other hand, I've built amps without twisting, and little noise.

    In my experience, grounding issues are a major source of noise. Bolting to the chassis is not reliable, especially in the long term. Leo solders to the chassis right next to the chassis bolt.

    Ground schemes are the next issue, but that's a very complicated topic.

    Adding a choke is probably the easiest way to reduce noise, although the amp will be less dynamic.
    Check out this thread
    https://www.thegearpage.net/board/index.php?threads/325809
     
  7. JammyDodger

    JammyDodger Silver Supporting Member

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    Hey,

    I went thru the same issue. Here is what I posted on the Ampgarage forum.

    ....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????

    By the way, I had previously added a 33k resistor in line with pin 7 of V1 and changed my solid core wire to stranded from Pin 2 of V1 to the center lug of the volume pot.

    Cheers, Mike
     
  8. Lonely Raven

    Lonely Raven Member

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    Hey Mike, I read that over at Amp Garage.

    My noise issue is completely hiss, and I'm not sure messing with heater lines or shielding is going to fix that. I've swapped out a dozen different tubes in v1 and a few others in v2 and v3, this hissssssss is independant of tubes used. I also *don't* have a problem with microphonics as some of the other trainwreck builders do. As sloppy as my heater layout is, the amp is very well behaved except for the hiss. I might pickup a hint of radio stations, but I'm almost positive that's just a shielding issue on the guitar (70's vintage Japanese Strat Clone). It's not a big issue as it's only when I stand a certain way...so I don't stand there. :)

    Now, aren't Carbon Comp resistors *more* noise prone? I have a nice collection of vintage Carbon Comp (garage sale find), but I hesitate to use them as I thought they were *more* noise prone. I could go with Audiophile metal film resistors...I don't mind spending $1 each if I have to in order to clean this amp up...I just don't want to kill it's inherent wild side...which might be the double edge of the hiss.
     
  9. Lonely Raven

    Lonely Raven Member

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    Good info on the Aiken page listed above:

    A few practical notes:
    Since high-quality metal film resistors are more expensive than cheaper carbon films, a player may want a quieter amp without having to change all the resistors to metal film. From the above information, a few general rules can be determined, and a compromise can be reached.
    • Since noise is proportional to resistor value, the 1Meg to ground resistor on the first stage of a guitar amplifier will create much more noise than the 68K grid resistor, because the value is 14.7 times larger. This means that carbon comp vs. metal film is more noticeable and important for the 1Meg resistor than the 68K input resistor. However, when a guitar is plugged in to the amplifier, the pickup resistance/inductance and cable capacitance is in parallel with the 1Meg grid resistor, so its effect on noise is greatly reduced. When the guitar is unplugged or turned all the way down, the 68K series grid resistor becomes the predominant noise source. Depending on the tube type and input stage topology, the resistor noise may be greater than the tube's referred input noise. To reduce the noise to a minimum, use the smallest possible input grid resistor value that still provides RF suppression.
    • Also, since resistor noise is proportional to current flow, a 100K grid resistor is going to be quieter than a 100K used as a plate resistor. There is around 1 to 2 mA of current flow in a typical plate circuit, but the grid current flow is practically negligible. This means that it is better to use metal film for plate resistors. The exception to this rule comes when two resistors are used as a voltage divider from the plate of one tube to the grid of the other. There is no grid current flowing, but there is current flowing in the voltage divider string, so metal films should be used in these positions for lowest noise.
    • Lastly, the noise contribution is greatest at low-level stages, such as input stages, reverb recovery stages, and effects loop recovery stages, so the plate resistors, grid-to-ground resistors, and grid divider attenuation resistors in these locations should be metal films for lowest noise, while locations where there is little gain from that point to the output can use noisier resistors without adding too much to the overall noise level of the amplifier, because the signal level at that point is many times greater than the noise level produced by the resistors.
    One more consideration for resistors: it is sometimes overlooked that resistors have a max voltage rating. The 1/2 watters and some 1 watters usually are only rated for 250-350V. Be sure to get a resistor rated for the appropriate voltage in the amplifier. I use only 1W, 500V min (continuous, 1000V surge) or 2W, 750V rated resistors.


     
  10. Jon Silberman

    Jon Silberman 10Q Jerry & Dickey Silver Supporting Member

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    Does your amp have negative feedback? None always adds to hiss. For amps with neg. feedback, increasing the amount of it often significantly reduces the hiss, though you may not like the tonal/feel tradeoff.
     
  11. PRNDL

    PRNDL Member

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    Quality is a far more important issue than type for resistors, especially since you found that many were way out of spec. Look for 5% tolerance resistors, and choose a good supplier. The garage sale find might be good. Check the fourth band. Gold = 5%, Silver = 10%. I've noticed that resistors on eBay, however, are higher priced and lower tolerance than those from Hoffman or Antique!

    The other place to check is the bias circuit. Low quality or bad resistors and pots will inject tons of noise. I had a hiss problem that went away when I touched the bias pot.
     
  12. Lonely Raven

    Lonely Raven Member

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    I even scored some 2W Carbon Comp, gold band at the garage sale.

    But I think I'm going to try to clean the amp up with audiophile resistors, either metal film or tants. I'll look over the bias circuit. My kit was missing the bias pot, and Nik is sending me a new one...so in the mean time I used a radio shack one that I'm sure is cheap shyte.

    Good Customer Service from Ceriatone by the way. I'm happy with the product...I just wish shipping from Malaysia wasn't so much...otherwise I'd be trying out other kits from him over these long winter months!
     
  13. Steve Dallas

    Steve Dallas Member

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    Don't go anywhere near carbon comp resistors if you want quiet ones. Go for metal film. They are the quietest resistors you can get short of metal oxide (which don't sound so good).

    I'm seeing (or think I am) a couple of potential issues:

    1. Your input wire does not appear to be shielded
    2. The wire going from the volume pot the 2nd triode on V1 is not shielded
    3. A lot of Wrecks don't have a grid stopper on the 1st triode. Add 68K metal film resistor to the wire going from the input jack to V1 and put it as close to the tube as possible. Or you can solder a 47pF cap from the input jack's hot lug to ground right under the input jack.
    4. I can't tell what kind of pull-down resistor you are using on the input jack, but make sure it is metal film.

    Beyond that, I troubleshoot hiss following these steps:

    1. Pull the PI tube (V3). Does it go away? If not the problem is between the PI tube and the power tubes. Check for lead dress to and from the PI--especially the grid wires on the EL34. If the hiss does go away when you pull the tube, the problem is either the tube itself or it lies before the PI tube so put it back in.
    2. Pull V2 and repeat for that triode. Check the tube and lead dress around the tube. Once you have ruled it out and/or fixed any problems there, replace V2.
    3. Pull V1. Run the same procedure here.

    If the hiss is pretty loud, it is usually a tube problem with V1 or something else around V1. Wrecks are notorious for being finicky about tubes, so you may have to go through 10 or 15 of them to find 3 good ones that will be both quiet and good sounding.
     
  14. JimmyR

    JimmyR Member

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    Oh yeah - as PRNDL noted - I forgot that I added a choke to my 'wreck and yes it did help make it quiter. Though that's not why I used it. Maybe you could use a Dale 10-15W power resistor? I find they are a fair bit quieter than the white Xicon ones. I used a Dale in my 5E3 and it's super quiet.
     
  15. mooreamps

    mooreamps Senior Member

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    Aiken is sort of right as this article is just an echo from a text book. The real answer is : it's the first gain stage in the pre-amp that sets the noise floor of the amplifier. If you are using resistors for bias in the first gain stage of the pre-amp, this is one of the major contributors to white noise, or hiss, in the loud speaker. As such, I do not use resistive biasing in my pre-amps.

    -g




     
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  16. Lonely Raven

    Lonely Raven Member

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    Interesting.

    I don't know enough to redesign the biasing of the pre-amps. And as I said, I'm just following what the original Trainwreck looked like.

    Thank you for the info though...it's something to keep in mind when I'm working on something more original.
     
  17. vibroverbus

    vibroverbus Member

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    especially as everyone is saying, in the earliest stages. you can't (easily) measure noise in caps so I'd keep the 'cool' vintage parts for little projects and less demanding applications myself...
     
  18. donnyjaguar

    donnyjaguar Member

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    During my amplifier tutelage I was asked which component in a power amplifier, if defective, will add the most amount of hiss. The answer is the resistor in the feedback loop.

    Other tactics for reducing hiss are to put capacitors across the diodes in the rectifier circuit. I believe 250pF should be fine here. Also, some hi-value electrolytics have sh*tty ESR so you can bypass with 0.68-600V metalized film.

    If its an excessive gain issue then just reduce the gain between stages with a resistive divider. In my experience amplifiers with built-in distortion often have w-a-y more gain then you'll ever need, regardless of the type of music you play.

    I think someone else suggested checing the plate resistors and lead dress on the low-level signal portions of the circuit.

    Other thoughts on hiss. I can be induced into the audio portion of the circuit by the proximity of the rectifier (tube or solid state) to it.

    Hope tihs helps!
    DJ
     
  19. Lonely Raven

    Lonely Raven Member

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    Good info. I didn't realize the feedback resistor could cause havok. I'll look into replacing that one as part of my "go over".

    Thanks!
     
  20. JJman

    JJman Member

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    Is there hiss with no guitar plugged in and the amp volume all the way down? If not, all the hiss is coming from v1a. Plate resistor, tube, socket, cathode resistor. The tone stack could also be a culprit but DC does not normally flow thru it so hiss would normally not be possible there. This all assumes we have the same definition of hiss which I think we do.

    Are you on http://ampgarage.com/forum/index.php ? Checkout the Trainwreck forum there.:BEER
     

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