Help with my Divided by 13 SJT 10/20

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jdsgilmour, Feb 21, 2005.


  1. jdsgilmour

    jdsgilmour Suffering from chronic gearitis Gold Supporting Member

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    I just got my 10/20 head and it rocks! Looks great, too. However, I'm having two issues and I'm hoping somebody out there can help me (I've already spoken to Fredric, and he's offered to do an overhaul if I ship it back to him, but I'd prefer to see if anyone else has some thoughts first).

    First, it broadcasts radio stations. Not when I turn it on. Not when I turn up the volume. But as soon as I plug in an instrument cable, voila! I have Tejano coming loud and clear through the speaker. This, of course, is amplified by any sort of gain. When I spoke to Fredric, he said certain locations around the country were prone to this issue -- that the cable was acting as an antenna for certain super high frequencies. He suggested I try different cables. I have. George L's, Whirlwind, old cheap-o curly cue cables. Even a 6" effects patch cable. All result in perfect broadcasts of unwanted radio. Now I have not tried Evidence Audio, yet, which Fredric suggested. That is next on my list. I have also tried lifting the ground plug and changing speaker cables. To no avail. Just for further information, once I start playing, you can't hear the radio. But as soon as I pause, or palm mute, or hold a note just a little too long, back it comes.

    Second, it hisses quite a bit. Now I'm no amp guru, and truth be told, this is the first non-ss rectified amp I've owned (I've played plenty, just never bought one until now), but there is a significant difference in hiss between my 10/20 and any of my other amps (Fargen DC, Marshall DSL-401, Fender Blues Junior and even my ZVex Nano Amp). Hiss definitely increases with the 5879 channel and with volume. Again, once I start playing, you cannot hear it, except at low volume clean settings. However, as soon as I pause, palm mute, etc., there it is, loud and clear. Could this be a tube issue? Perhaps break in? The amp is only 1 week old. I have noticed that if you use an attenuator and set it at more than 1/2 way on the attenuation level, both the radio and hiss disappear. I am guessing this is because the attenuator is cutting out the high frequencies?

    Any help y'all could offer would be greatly appreciated. Thanks much.

    John
     
  2. Neon

    Neon Member

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    John,

    With the volume control set low, I also find the hiss level on my SJT 10/20 is higher than expected when compared to other amps. Even with the volume set to zero, the hiss is very noticeable.

    I have an ERT33 too. With the 33's reverb channel at noon, it's much quieter than the 10/20. When the 33's click channel is past noon, the hiss is about the same as the 10/20 at low volume. I expect hiss on the higher gain settings.

    With the damping switch on (up), the low-volume hiss drops about 8dB SPL which is noticeably quieter and more like my other amps. I measured this with a bargin SPL meter.

    I sent the amp back to Fred. He made a minor adjustment, but he said the low-volume hiss is normal for the 10/20.

    I would be interested in what you find out. Try the damping switch too, and see if it's to your liking.

    I just recorded some tracks for our cover band and got some really fun '60s grind out of the 10/20 in the class A mode.
     
  3. jdsgilmour

    jdsgilmour Suffering from chronic gearitis Gold Supporting Member

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    Thanks, Tom. I will give the dampening switch a go. How did you like your 10/20 when you got it back from Fredric? Did you hear a noticeable difference after the mod? As I said, he's offered to do the same for me. I'm just trying any home remedies anyone has first.

    John

    P.S. Are your clips with the 10/20 up on your band's website? I'd love to hear them.
     
  4. Neon

    Neon Member

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    John,
    Fred only changed a single preamp tube in my 10/20. It didn't lower the hiss all that much. Although, the hum dropped a bit. I'm really curious about the mod you mention, and wonder if he has a new way to lower the hiss.

    I'm working on a clip with the 10/20, but haven't posted it on our band site. The 10/20 track is only a rhythm part on Kinks' song we play. There are some ERT 33 clips on the site. I'm a bit reluctant to mention the clips since the playing isn't as spectacular as the amp! If anyone is curious, perhaps I can post the amp/effects setup somewhere.
     
  5. KLB

    KLB Member

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    Try connecting a piece of wire from the barrel (ground) of your input jack to the chassis. If the chassis is metal near the input jack, you can use a gem clip. This trick will often shunt to ground the incoming RF riding on the outer shield. The reason this works is that many amps don't ground the input signal where it enters the chassis. The tradeoff with this trick is that it may introduce a small bit of hum.

    I had to rewire a Marshall once, using a star ground near the input jack to get rid of both RF and hum.

    A buddy brought over an ERT33 a couple of years ago. It had the same RF issue you describe.

    As for the hiss, some of that could be RF.

    Cheers,
    Ken
     
  6. Neon

    Neon Member

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    Ken,
    Thanks for the helpful info. I wondered about the hiss being partially from RFI too. Occasionally, there is a very faint sputtering noise buried in the hiss. After listening closely, I realized it was a radio station.
     
  7. jdsgilmour

    jdsgilmour Suffering from chronic gearitis Gold Supporting Member

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    Thanks, Ken. Much appreciated. I plan on spending some time trying to work on these issues this weekend. I'll let y'all know what happens.

    John
     
  8. dstengineer

    dstengineer Member

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    There's a couple of quick things you can try to minimize radio pickup (this includes the treaded CELL phone noise). Try placing an RF bead on your guitar cable near the amps input (sold at Radio-Shack, these are the clip on ferrites that you see on computer printer and monito cables). Even though the cable is shielded sometimes this will help. The best place to add these "filters" is inside the amp right at the input jack, if you are inclinded to open it up.

    Also, a filtered power outlet (here again like you see for computers) may help a bit too. It's not unusual for radio signals to come in the AC power line as well as through signal cables.

    Bob Dettorre
    DST Engineerning.com
     
  9. jdsgilmour

    jdsgilmour Suffering from chronic gearitis Gold Supporting Member

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    Thanks Bob. By "filtered power outlet" do you mean like a Furman power conditioner? Or do you just mean like a power strip? Please forgive my ignorance.

    UPDATE: So I took my rig over to a buddy's house this morning to jam for a couple of hours. I set up exactly as I do in my place (same speaker cable, same instrument cable, same power cable, etc.). Flipped on power, then flipped on standby and guess what? No radio whatsoever. Even dimed. Not a blip. Fredric did tell me that it could be location, but I didn't think it could be that simple. But I guess it is. That said, as soon as I got back to my apartment and plugged in, it was back. Very frustrating. But a relief to know that my 10/20 is not to blame.

    Also, not sure if my amp is "breaking in" or what, but the hiss seemed greatly diminished at my buddy's place, too. He was playing a Victoria tweed and both amps made about the same amount of noise. I can say that for about 3 hours today, he and I were all smiles. Sam Evans told me to just keep playing, that it got better with time. So far, he's dead on right! Thanks, Sam.

    John
     
  10. dstengineer

    dstengineer Member

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    Not surprised at all about the radio issue being location sensitive. Cell phones (such as the one I carry in my poket for Seimens are worse. I spent many years designing Radio Frequency equipment and believe me it's always an issue as to how much RF can blast into audio systems. And radio waves being the way they are, shielding doesn't often work that well. As for the power strip, I use a $39 APC multi strip that just has good line filtering built in to it. I've used the Furman rigs in large PA systems (which are notorious fo rhtis problem) and had great luck.

    Bob D.
     

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