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PostPosted: Tue, Sep 18, 2018 9:03pm 
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Hi all! Could use some help here...I bought a diesel Herbert mk1 with external bias and 6550 tubes. No issues with it but I decided to change to kt77 to try something different. I biased each pair to 70mA and fired it up. Ch1 and 2 has no issues but channel 3 has a loud low frequency hum...guitar still plays thru and has appropriate gain but the hum is there without or without guitar plugged in...any thoughts!?


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PostPosted: Tue, Sep 18, 2018 10:11pm 
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Well, if it's just ch 3, my first guess would be the preamp tubes associated with that channel. You can find that info in the service guide on the Diezel site. I would think that, if it was one of the power tubes, the hum would be on all three channels.

I would try replacing each of the 12AX7 tubes involved with ch 3 with known good tubes and see if that helps.

Also, make sure everything other than the guitar, the guitar to amp cable, and the amp to cab cable is disconnected first. That way, you can rule out most other possibilities. Make sure you use known good cables and a known good guitar, too. Still, my first instinct is that it's a preamp tube.

Let us know how it goes, ok? Should be an easy thing to get lined out. :)

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PostPosted: Wed, Sep 19, 2018 6:04pm 
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So I switched out v2 which according to the website is the preamp tube for ch3 and the loud low frequency hum is gone! Phew! Now just curious, I’m hawing a mogami cable and a prs custom 22 and in all three channels there is background noise...when nothing is plugged in, there’s no noise...that means either my cable or guitar is the source? In which case, is there any hope to reduce that!? Regardless so glad to get rid of that ch3 hum! Thx!


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PostPosted: Wed, Sep 19, 2018 7:43pm 
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Glad that tube fixed your problem!!

As for the background noise... is it the same level regardless of how loud the amp is set? Or does it go up and down in relation to the master volume?

It could be a number of things. The phase invertor tube might be going bad (since the noise is in all three channels). Could be related to the cable... is it still there even when the guitar volume is turned down? Could be in the guitar's wiring, possibly a loose ground connection or the volume pot could be getting worn.

Does it get better when you touch the strings? Does it change at all when moving the cable (crackling noise) or when you move the guitar between being perpendicular and being parallel to the amp? Is there any static or crackling when you turn either of the pots on the guitar?

Finally, all amps are going to have some background noise, so there may not be a problem at all. However, my first guess would be that you may have a dirty connection somewhere or a bad ground in either (or both) the guitar (I'd suspect the volume pot or the jack, but could be in several places) or in the cable. If you have other cables and/or guitars, you could see if it gets better or worse when you use a different cable or guitar.

Before investing in new cables or rewiring your guitar, I'd highly recommend getting some Deoxit contact cleaner and cleaning the pots and jack on the guitar. Wouldn't hurt cleaning the jack on the amp and the tube sockets, too. Deoxit has saved my butt more times than I can count (or care to remember). :)

FWIW, I use Mogami cables (among others) and have a PRS CU24. Pretty close to your setup. :rock: Great guitar and a great amp!!

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PostPosted: Wed, Sep 19, 2018 8:02pm 
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Thanks for the tips! I’ll do more diagnostics and get back to you. Btw despite the noise the kt77 was killer! Never really dug the 6550s In there!


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PostPosted: Wed, Sep 19, 2018 8:33pm 
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Yeah, I'm really happy with the KT77s, too. Honestly, I've never been a huge fan of 6550s, although I understand why some people like them. I've got KT88s in a 20th Shiva and love those in that amp, but that's about as close as I get. But, that's just me. :)

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PostPosted: Wed, Sep 19, 2018 8:43pm 
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Someone wants to trade me a shiva 20th for my Friedman small box...what did you like about the Shiva?

Alright, Sonu switched the phase inverter tube and no big difference...if guitar volume is all the way down or all the way up it is still there and no big change with guitar columns change. It gets louder when amp master is turned up and is gone when turned down...I’ve tried two different good cables that don’t have nearly that much noise on my vh4 and two guitars (les Paul and prs) and the noise is def more in my Herbert than my vh4, not even a small difference...


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PostPosted: Wed, Sep 19, 2018 9:46pm 
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Hmmm... since it's going away when the master is down, it's got to be something before that point in the circuit. Since it's occurring on all channels, it's got to be something that they all have in common. I'll take a look at the tube chart, but maybe one of the tubes associated with the effects loop, or maybe there's a common input stage.

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PostPosted: Wed, Sep 19, 2018 10:05pm 
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Looks like V1 is a common input stage for all channels. V3 is also common between all channels. V4 & V5 mix the channels together and handle the sends and returns for the effects loops. Any one of those tubes could being causing extraneous noise. Since replacing V2 helped with the issue you were having with channel 3, I'd be tempted to think that the other preamp tubes could stand a replacement, too.

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PostPosted: Thu, Sep 20, 2018 6:11am 
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I pulled out and replaced one tube at a time and still no big improvement. Next step: I’m just gonna buy a new preamp tube set of Peter recommended tubes and see if that helps. Update to come!


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