Yup, Another One... Kraken 50 Build: Round II

Marshall Amp Discussion

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FourT6and2
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Re: Yup, Another One... Kraken 50 Build: Round II

Post by FourT6and2 »

martin manning wrote: Sun Aug 20, 2017 6:56 pm Ok, thanks. So, if you ground the top or wiper of the Gain 2 pot, does the hum go away? The h-k insulation in the CF triode (V2B) might be under a lot of stress (don't know the voltage there), but if you can kill the hum by grounding upstream of that it's ok, at least for now. You said you swapped tubes? which ones, and you swapped for the ones in one of the quiet amps?
Ok I'll give that a try.

But yes, the cathode follower is indeed under stress like in most Marshalls. If I remember correctly the differential between the cathode and heater in that stage is about 187v so not terribly high but still over 180. It's a bit higher in my other builds. But still no hum in those.
FourT6and2
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Re: Yup, Another One... Kraken 50 Build: Round II

Post by FourT6and2 »

Ok yeah, grounding the wiper of Gain 2 stops the hum (same goes for grounding the input lug).
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martin manning
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Re: Yup, Another One... Kraken 50 Build: Round II

Post by martin manning »

Ok, work your way upstream by grounding the wiper of Gain 1. If the hum is still there then the source is V1B or perhaps a filament lead too close to its grid lead.
FourT6and2
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Re: Yup, Another One... Kraken 50 Build: Round II

Post by FourT6and2 »

martin manning wrote: Sun Aug 20, 2017 9:41 pm Ok, work your way upstream by grounding the wiper of Gain 1. If the hum is still there then the source is V1B or perhaps a filament lead too close to its grid lead.
Grounding the wiper or input of Gain 1 doesn't reduce the hum. In fact, it actually creates a slightly different hum frequency on top of the existing hum.

Here are a photos of V1 and V2:

https://pre07.deviantart.net/3955/th/pr ... bkpvlz.jpg
https://orig15.deviantart.net/2a3b/f/20 ... bkpvnx.jpg
https://pre15.deviantart.net/a0f9/th/pr ... bkpvo8.jpg
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martin manning
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Re: Yup, Another One... Kraken 50 Build: Round II

Post by martin manning »

The hum you are chasing is 60 Hz, correct?
FourT6and2
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Re: Yup, Another One... Kraken 50 Build: Round II

Post by FourT6and2 »

martin manning wrote: Sun Aug 20, 2017 10:49 pm The hum you are chasing is 60 Hz, correct?
No idea what its frequency is; don't have an o-scope.
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martin manning
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Re: Yup, Another One... Kraken 50 Build: Round II

Post by martin manning »

What if you ground the terminal strip lug where the 150k grid resistor on V1B is connected? Does that make it silent? If so then that would say it's coming in through the coax. That terminal strip is a rogue ground point too, but if your other amps are quiet it must be okay.
FourT6and2
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Re: Yup, Another One... Kraken 50 Build: Round II

Post by FourT6and2 »

martin manning wrote: Sun Aug 20, 2017 11:07 pm What if you ground the terminal strip lug where the 150k grid resistor on V1B is connected? Does that make it silent? If so then that would say it's coming in through the coax. That terminal strip is a rogue ground point too, but if your other amps are quiet it must be okay.
Nope. That amplifies the hum. Just like grounding Gain 1 wiper.
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martin manning
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Re: Yup, Another One... Kraken 50 Build: Round II

Post by martin manning »

Hmm. So where are you grounding for these tests? And is that the same place that the input or wiper of gain 2 was grounded when the hum stopped?
FourT6and2
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Re: Yup, Another One... Kraken 50 Build: Round II

Post by FourT6and2 »

martin manning wrote: Mon Aug 21, 2017 12:00 am Hmm. So where are you grounding for these tests? And is that the same place that the input or wiper of gain 2 was grounded when the hum stopped?
Doesn't matter where I ground. The result is the same. I have a test probe with an alligator clip on the other end that I can clip to the chassis anywhere. I can also just touch it to any point on the chassis. Result is the same no matter the location.
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martin manning
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Re: Yup, Another One... Kraken 50 Build: Round II

Post by martin manning »

Does the hum go away if you turn the pot on the back panel all the way up? That should be the same as grounding V2A's grid. I don't know what to think about that pot on V2A's grid and the 1n cap from V1B's grid being grounded at the same point and way off the bus... doesn't seem like a good idea.

Is there a dropping resistor missing from your schematic? There should be one between the PI plate resistors and the CF anode, I believe.
FourT6and2
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Re: Yup, Another One... Kraken 50 Build: Round II

Post by FourT6and2 »

martin manning wrote: Mon Aug 21, 2017 1:10 am Does the hum go away if you turn the pot on the back panel all the way up?
All the way up? As in maxed out? No, the hum doesn't go away. But it's not like that pos is causing it. With that pot all the off, there is no hum. But that makes sense since there would be no signal with it off. Same thing in my other builds.
That should be the same as grounding V2A's grid.
No. With the pot all the way off, it's like grounding the grid. With the pot all the way up, the pot is at 100% and fully on, so to speak.
I don't know what to think about that pot on V2A's grid and the 1n cap from V1B's grid being grounded at the same point and way off the bus... doesn't seem like a good idea.
It's like that in my other builds and there's no hum.
Is there a dropping resistor missing from your schematic? There should be one between the PI plate resistors and the CF anode, I believe.
Nope. Not in this design apparently. Only two droppers in the amp. One on the screen filter cap (between each positive lug) and one between the lugs on the 50+50 cap on the turret board. Screen cap (after dropper), PI node (before plate resistors), first half of 50+50 preamp cap, and CF anode all have the same B+. It's not like a regular 2204 or JTM45 where there are additional droppers. Unless I made a mistake on the schematic. Maybe I put a filter cap in the wrong spot?

Take a look at the layout. Maybe I did the schematic wrong: http://www.ceriatone.com/ceriatone/wp-c ... t-2015.jpg
FourT6and2
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Re: Yup, Another One... Kraken 50 Build: Round II

Post by FourT6and2 »

Ugh... I've been over every single connection in the amp. Everything appears correct. No idea what's causing this noise haha. Maybe a bad pot or switch?

Would a short between one of the DC heater wires and ground and/or a short between DC filament on pin 9 and pin 4/5 of V1 or V2 cause noise? Or would the filament not even light up?

For example, I have the filament wires routed pretty tightly to the socket. And I have the pin 9 wire running between pins 4/5 to get to pin 9. Maybe one of them has shorted to pin 4/5.
FourT6and2
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Re: Yup, Another One... Kraken 50 Build: Round II

Post by FourT6and2 »

Some folks on another forum have me on a wild goose chase that I'm not really going to do lol. They think it's heater hum. But I'm running DC on V1 and V2. And my other builds don't have hum. They want me to try reversing the phase on the filaments. Not sure why. That would require ripping out all the filament wiring and redoing it. Not gonna happen lol.
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martin manning
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Re: Yup, Another One... Kraken 50 Build: Round II

Post by martin manning »

Sorry, on the back panel pot I meant "up" on the schematic, grounding the grid. The behavior is as expected there.
Since the hum is stopped by grounding the input to Gain 2 that pot is okay, and that pinpoints the source as V1B. That leaves the tube, socket (poor contact perhaps), and the filament and socket wiring. If you have DC on the filaments it could have some residual ripple, but a short would upset the bias. What does the filament DC supply look like, and where is its ground reference?
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