Weird Heater Supply Problem - And other solutions!

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Littlewyan
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Re: Weird Heater Supply Problem - And other solutions!

Post by Littlewyan » Sat Oct 28, 2017 5:13 pm

Ok this is going to sound crazy. Lay out some kitchen foil on your bench and sit the amp on it. This will act as a temporary shield so you can see if the buzz goes away. Also try different 12AX7s as I find some (quite often JJs) buzz like that.

Also, do you have any fluorescent lights around? They cause that.

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Sun Oct 29, 2017 10:14 am

Alright, I covered the bottom of the chassis with tin foil. It did nothing.

Then I started screwing around with grounds. The buzz is there all the time even with the input jack empty. But it does get amplified significantly with a plug in the jack. It responds to volume control.

I have tried about 10 different 12ax7s in there. I tapped all over with a chopstick and discovered a low rumble that is temporary from one of the new 6L6GCBs. It lasts maybe a second or so when you tapped the chassis, board, or tube itself.
I pulled that one (and another from the other side) and the 120hz buzz was still there.

I did some reading and apparently diodes can begin to fail and cause leakage. So I am going to rebuild the diode bridge later this afternoon and see what gives. They are cheap enough anyway.

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Re: Weird Heater Supply Problem - And other solutions!

Post by Littlewyan » Sun Oct 29, 2017 10:39 am

How have you got the grounds hooked up?

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Sun Oct 29, 2017 9:33 pm

Littlewyan wrote:
Sun Oct 29, 2017 10:39 am
How have you got the grounds hooked up?
I am 99% sure I do.

Here are a bunch of pics.
ImageGrounds by Paul Abbott, on Flickr

ImageGrounds by Paul Abbott, on Flickr

ImageGrounds by Paul Abbott, on Flickr

ImageGrounds by Paul Abbott, on Flickr

ImageGrounds by Paul Abbott, on Flickr

ImageGrounds by Paul Abbott, on Flickr

I have continuity between the end of the cable and the ground on the chassis.
ImageGrounds by Paul Abbott, on Flickr

ImageGrounds by Paul Abbott, on Flickr

ImageGrounds by Paul Abbott, on Flickr

ImageGrounds by Paul Abbott, on Flickr

ImageGrounds by Paul Abbott, on Flickr

ImageGrounds by Paul Abbott, on Flickr

ImageGrounds by Paul Abbott, on Flickr

Littlewyan
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Re: Weird Heater Supply Problem - And other solutions!

Post by Littlewyan » Mon Oct 30, 2017 3:36 am

No sorry I meant which ones are attached together? Can't see that well going by the photos.

Also that chassis has a bit of rust, did you sand it a bit where the ground lugs are to ensure they have a good connection? Check them with an ohmmeter.

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Mon Oct 30, 2017 2:20 pm

  • 10K and .68/2.7K are grounded to a lug on its own to the far right of the chassis (short run).
  • Input jack, preamp pot, master volume pot, and 820ohm resistor are all grounded together to a lug below the master volume pot.
  • Mid Pot and 470k voltage divider are grounded to a lug below the mid pot.
  • Power tube grounds are to the screws that hold the sockets in.
  • Output jack, presence cap and 4.7K resistor are all grounded to another lug just below the presence pot.
  • Full Wave Bridge Rectifier is grounded to a lug all its own.
  • 50uf/50uf filter cap for one of the 10K 2w resistors is grounded to a lug on its own right next to the cap.
  • 50uf/50uf filter cap for the preamp 10K 2w resistors and the two 50uf/50uf cans in series with the 56K 3 watt resistors are grounded together at a lug on its own.
  • Two 50uf/50uf cans for the DC B+ from the FWB and the bias trim pot are grounded to an individual lug.
  • 6.3VAC virtual center tap are grounded on a lug on their own.
  • 120VAC line in ground is on its own on the extreme left side of the chassis.

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Re: Weird Heater Supply Problem - And other solutions!

Post by neikeel » Mon Oct 30, 2017 3:29 pm

Tha all sounds good.

Did you Dremel the rust off first?
Neil

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Mon Oct 30, 2017 3:33 pm

neikeel wrote:
Mon Oct 30, 2017 3:29 pm
Tha all sounds good.

Did you Dremel the rust off first?
I didn't because I used self tapping screws that went into fresh metal between the outside and the inside of the chassis. I suppose I can do some continuity tests. Or perhaps the self tapping screws aren't good conductors.

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Re: Weird Heater Supply Problem - And other solutions!

Post by Littlewyan » Mon Oct 30, 2017 9:11 pm

Definitely do some continuity tests. Make sure there isn't any resistance between the chassis and the ground connections (measure from the bottom of the 2.7k/.68uf resistor/cap to chassis for instance). Check all of them.

Those grounds are a bit mixed up. Have you read Merlin's article on grounding? Really good read. You want to keep stages grounded together.

1. High Sens Input Jack, 1Meg Grid Leak, 2.7k/.68uF
2. Low Sens Input Jack, Preamp gain control, 10K, 50uf+50uf cap can for preamp
3. 470K, 820Ohm, 100K
4. Mid pot, master volume, presence control, cap, 4.7K resistor and speaker jacks

First try connecting the FWB Rectifier ground directly to the main filter cap ground, or vice versa. Then swap the input jack and 10k grounds around so the input jack is grounded with the 2.7k/.68uf cathode and the 10K cathode is grounded with the preamp gain control. If you can't seperate the input jack grounds (high and low) then keep them together.
Last edited by Littlewyan on Tue Oct 31, 2017 4:26 am, edited 1 time in total.

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Mon Oct 30, 2017 9:55 pm

Good stuff! I will do continuity checks everywhere. And then I will rearrange stuff as spec'd.

Thanks for the homework! I will say this, I LOVE the tone and touch sensitivity of the amp right now.

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Re: Weird Heater Supply Problem - And other solutions!

Post by Littlewyan » Tue Oct 31, 2017 4:27 am

Yeah check first then if all checks out ok, start moving the grounds. Change just the two I've mentioned first to see what difference that makes.

Edit: What definitely won't help is you've got a HT wire resting on the speaker jack. Move that away from it. Reroute it next to the cap cans.

Edit2: Or better, make it go the same route as the other wire on that fuse holder. Wrap it around the other wire (Twist it basically). That will cancel out the magnetic field to stop stray AC from getting into other parts of the amp. Or at least lower it a lot! You might also want to invest in some cap clamps like these https://www.modulusamplification.com/Ca ... ?sid=12886
If that glue gives and the cap moves about in the amp it could be very bad news next time you turn it on :shock:
Last edited by Littlewyan on Tue Oct 31, 2017 6:54 am, edited 1 time in total.

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Re: Weird Heater Supply Problem - And other solutions!

Post by Littlewyan » Tue Oct 31, 2017 6:48 am

You might also want to reroute your blue and orange grid wires to the power valves underneath the board so they're away from the AC wires coming from the standby switch. It would shorten them a lot which would help with noise. Also maybe have the grid resistors going towards the chassis so you can keep the grid wires against the chassis.

It's good practice to keep wires pushed down against the chassis if you can. It's also best to keep wires carrying high voltage AC away from any wires carrying audio signals (AC).

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Wed Nov 01, 2017 2:36 pm

Littlewyan wrote:
Tue Oct 31, 2017 4:27 am
Yeah check first then if all checks out ok, start moving the grounds. Change just the two I've mentioned first to see what difference that makes.

Edit: What definitely won't help is you've got a HT wire resting on the speaker jack. Move that away from it. Reroute it next to the cap cans.

Edit2: Or better, make it go the same route as the other wire on that fuse holder. Wrap it around the other wire (Twist it basically). That will cancel out the magnetic field to stop stray AC from getting into other parts of the amp. Or at least lower it a lot! You might also want to invest in some cap clamps like these https://www.modulusamplification.com/Ca ... ?sid=12886
If that glue gives and the cap moves about in the amp it could be very bad news next time you turn it on :shock:
I will do the rerouting tonight when I get off work. I will purchase some clamps. I was meaning to do that anyway. Honestly thinking about doing a cap board instead.

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Re: Weird Heater Supply Problem - And other solutions!

Post by marshallnoise » Wed Nov 01, 2017 2:37 pm

Littlewyan wrote:
Tue Oct 31, 2017 6:48 am
You might also want to reroute your blue and orange grid wires to the power valves underneath the board so they're away from the AC wires coming from the standby switch. It would shorten them a lot which would help with noise. Also maybe have the grid resistors going towards the chassis so you can keep the grid wires against the chassis.

It's good practice to keep wires pushed down against the chassis if you can. It's also best to keep wires carrying high voltage AC away from any wires carrying audio signals (AC).
I see what you are saying about the grid wires. That should be easy to do.

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Re: Weird Heater Supply Problem - And other solutions!

Post by Littlewyan » Wed Nov 01, 2017 3:06 pm

Cap board would be cool but with those cap cans you'd have to tie them down. Up to you really :)

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