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

Marshall Amp Discussion

Moderators: pompeiisneaks, Colossal

FourT6and2
Posts: 178
Joined: Wed May 31, 2017 11:11 pm

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

Post by FourT6and2 »

martin manning wrote: Mon Aug 21, 2017 10:51 am 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?
With the pot on the back all the way off, the signal is pretty low so it sounds like the hum is gone. But if I max out the master and both gain controls, I can still hear it slightly.

Layout: http://www.ceriatone.com/ceriatone/wp-c ... t-2015.jpg

DC Schematic: https://img02.deviantart.net/2eff/i/201 ... bb05j4.jpg

You can see how the DC supply is set up there. It's created from the extra 5v tap on the PT. Three 4700uF caps, two diodes, and the regulator. The ground of the DC supply is right there at the regulator's ground at a local chassis node. Just like in my other builds (and they are quiet).

You can see it in the upper right corner of this photo: https://orig13.deviantart.net/8c5d/f/20 ... bkpvon.jpg
FourT6and2
Posts: 178
Joined: Wed May 31, 2017 11:11 pm

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

Post by FourT6and2 »

Tried reversing the phase on the heaters. No change.
Tried a snubbing cap across V1a plate-to-grid. No change.
FourT6and2
Posts: 178
Joined: Wed May 31, 2017 11:11 pm

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

Post by FourT6and2 »

Ok, I figured out the source of the problem. But I don't know how to fix it just yet.

It's the cap to ground on V1b. The 0.001uF cap from pin 7 to ground. Not sure why it's causing hum. But removing it totally eliminates the noise. That cap is integral to the circuit. It's in my other builds and it doesn't cause noise. Not sure why. Maybe it's where it's grounded? I might try grounding it to the bus bar on the turret board instead of locally.
FourT6and2
Posts: 178
Joined: Wed May 31, 2017 11:11 pm

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

Post by FourT6and2 »

Now that I've identified the problem, I need to figure out how to solve it haha.

That cap should be in the circuit. It's part of the tone of the amp. But why is it adding noise in this build but not my others?

I tried grounding it at the bus bar on the turret board instead of locally and it reduced a lot of the hum/buzz. But there is a great deal of hiss and radio stations with that cap in the circuit.
User avatar
sonicmojo
Posts: 669
Joined: Thu Mar 03, 2011 3:41 am
Location: Oahu, HI

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

Post by sonicmojo »

Have you reversed the cap orientation to make sure the outer foil is on the ground side? Just a thought.
---------
Bryan
FourT6and2
Posts: 178
Joined: Wed May 31, 2017 11:11 pm

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

Post by FourT6and2 »

sonicmojo wrote: Mon Aug 21, 2017 7:15 pm Have you reversed the cap orientation to make sure the outer foil is on the ground side? Just a thought.
Nope. I'll try that.


Someone else also suggested I split the 150K grid stopper up into two 68K resistors and place the cap at their junction. So a grid stopper is blocking any additional noise from the cap.

So:

Coax -> 68K -> 1000pF to ground -> 68K -> grid
User avatar
martin manning
Posts: 13080
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

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

Post by martin manning »

Reversing the cap is worth a try. Is the radio station noise going up while the hum goes down when you ground to the bus? It's odd that you could kill the hum by grounding the Gain 2 pot... that cap was still there with a high resistance from the grid of V2A to ground.
FourT6and2 wrote: Mon Aug 21, 2017 8:23 pmSomeone else also suggested I split the 150K grid stopper up into two 68K resistors and place the cap at their junction. So a grid stopper is blocking any additional noise from the cap.
So: Coax -> 68K -> 1000pF to ground -> 68K -> grid
You would be raising the -3 dB low-pass cut-off frequency by a factor of two, and in any case a 68k grid stopper will give -3 dB up around 20kHz... no help for 60 or 120 Hz hum, but good for RF attenuation.
Last edited by martin manning on Mon Aug 21, 2017 10:16 pm, edited 1 time in total.
FourT6and2
Posts: 178
Joined: Wed May 31, 2017 11:11 pm

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

Post by FourT6and2 »

martin manning wrote: Mon Aug 21, 2017 8:26 pm Reversing the cap is worth a try. Is the radio station noise going up while the hum goes down when you ground to the bus? It's odd that you could kill the hum by grounding the Gain 2 pot... that cap was still there with a high resistance from the grid of V2A to ground.
This whole thing is weird lol.

Reversing the cap didn't do anything.
Yes, radio station noise goes up but hum goes down when grounding to the bus.

Doesn't grounding Gain 2 effectively kill all signal through the amp? So it's not surprising the hum would go away even with the cap still to ground. There is no signal getting to the speakers if you ground out the gain pot, no?
User avatar
martin manning
Posts: 13080
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

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

Post by martin manning »

Grounding gain 2 eliminates any noise coming from upstream of that point. Here's a picture. Does this sum it up at this point? I would want to associate the grounds as shown by the dashed lines.
You do not have the required permissions to view the files attached to this post.
FourT6and2
Posts: 178
Joined: Wed May 31, 2017 11:11 pm

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

Post by FourT6and2 »

martin manning wrote: Mon Aug 21, 2017 10:46 pm Grounding gain 2 eliminates any noise coming from upstream of that point. Here's a picture. Does this sum it up at this point? I would want to associate the grounds as shown by the dashed lines.
Yeah, that makes sense. :)


But... I've fixed it! :D

A new standoff at V1b, with an isolated lug for that cap worked. Instead of grounding locally, I ran a wire under the board and grounded the cap on the preamp ground bus.

Thanks for all the suggestions and help tracking it down. The amp is now as quiet as my others. With all the EQ and gains and master volume at 100%, there is only a tiny amount of hiss. You can barely tell the amp is even on.
User avatar
martin manning
Posts: 13080
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

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

Post by martin manning »

Great! Where did you ground the pot that is on V2B's grid?
FourT6and2
Posts: 178
Joined: Wed May 31, 2017 11:11 pm

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

Post by FourT6and2 »

martin manning wrote: Mon Aug 21, 2017 11:39 pm Great! Where did you ground the pot that is on V2B's grid?
Kept it in the same place—grounded locally at that standoff. Ideally, I guess I should move that to the bus bar too. I might just throw a 470K resistor on the board to replace the pot at some point.

The amp does sound a bit different. Not sure if it's my imagination or the solar eclipse...
User avatar
martin manning
Posts: 13080
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

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

Post by martin manning »

Are the other amps you mentioned that are quiet really identical to this one? It's hard to believe the same problem wouldn't appear with that remote ground. I see that the C-tone layout shows it that way. Does this amp follow that layout exactly?
FourT6and2
Posts: 178
Joined: Wed May 31, 2017 11:11 pm

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

Post by FourT6and2 »

martin manning wrote: Wed Aug 23, 2017 10:07 am Are the other amps you mentioned that are quiet really identical to this one? It's hard to believe the same problem wouldn't appear with that remote ground. I see that the C-tone layout shows it that way. Does this amp follow that layout exactly?
They are all the same circuit. Some slight voicing changes here and there (lie different slope resistor or different grid leak/bias splitters in the PI or different treble peaker cap, or different choke (3H vs 5H). But otherwise, they are the same amp.

The grounding in this one is different. I listened to the "experts" on here and on another forum, and "improved" the grounding compared to how Ceriatone does it. I used the first filter cap's negative terminal as a floating star ground point. And then sent that to chassis ground with one wire, rather than using the chassis star ground as the main ground node that all the individual floating nodes go to.

So instead of preamp bus bar going to chassis star ground, and PT center tap going to chassis star ground, and so on... I sent preamp bus bar to screen cap ground. Then that to firs cap ground, and PT center tap to first cap ground. And then first cap ground to actual chassis ground.

I'm guessing the new ground scheme is the real culprit.
User avatar
martin manning
Posts: 13080
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

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

Post by martin manning »

Yes, probably. Your new scheme sounds like good practice, but then you have some other grounds that are "rouges" as I call them messing with the electronic fengshui.
Post Reply