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PostPosted: 17 Feb 2018, 10:33 
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Location: Prêles, Switzerland
gofar99 wrote:
Hi I believe you are on the right track. Any amplifying device can pick up and amplify noise. I have seen a number of cases similar to yours. The ground side of the AC mains is full of junk. Since an amplifying device senses the difference between ground and signal imput it has no clue if the stuff is signal or not and it gets amplified. The AC mains noise is being mixed with the input signal. What I have found needed in high noise conditions is to first have three wire AC mains power going to everything it can with noise filters on each input. I like the one piece IEC power entry style with combined fuse and line filter built in. Second (seems redundant) I run a separate chassis ground from each piece of equipment. The only exceptions I have in my own system are an OPPO Blue Ray player that only has a two wire mains cord and the final mono-block power amps that do have three wire power but I find they do not need the extra ground. You are correct though in that there are many sources of noise that have crept into our homes. I went through 4 external A to D external boxes to get one that would not containimate the signal going into the PC (most were powered by the PC, and its power supply is really full of crud). In my shop ...regardless of what I do, I have extensive grounding and use APC and Triplet power filters and still can not get the noise as low as I would like. The best I can get is about -100dbv. This is not good enough if you want to measure things that are quieter. There is a way to make the amp more immune to above band noise. Increase the 1K resistor in the grid to a higher value. This will form a RC filter when combined with the tube's internal capacitance. You could start with a value of about 10K and see what happens. If you increase it too much the actual response of the amp at high frequencies will be reduced. I would expect values above about 25 K or so to slightly reduce the gain as well.

Good listening
Bruce


Hello Bruce again !

I have re-boxed my amp, and used a 10K resistor in the grid, and the spikes are all gone. I haven't yet installed a line-filter, as I have trouble to get one that has the house-ground filtered, that's not allowed by my local regulation it seems. But as the spikes are gone, I might skip this step.

I tried to trace the frequency response with my "poor" tools. High-freq seems unaffected by using 10K on the grid. Low freq is about -3 dB at 50 Hz and -7 dB at 20 Hz. Using my Sennheiser HD449, I don't feel I miss basses, but it might be nice to compare my results with someone else's.

Attachment:
freq_response.jpg


I have two remaining problems (noob in asymptotic search for perfection) :

1. Hum.

The level is very low, higher on the left than on the right. Evaluated at 2mV on the scope, hard to be precise, cheap DSO. It doesn't really disturb when music is playing, but with high-sensitivity headphone you can hear it when no signal at input.... and I want silence, no 50Hz hum.

Below the pic of my arrangement:

Attachment:
boxing_metal_1.jpg


There is that 1mm-thick metal sheet with mu-metal foil sticked on both sides. I thought it would be enough to avoid 50Hz going from the PSU transformer to the output transformers. Apparently not. The distance between the transformers is about 4cm (1.6 inch). As the noise is stronger on the left channel and the L-opt is closer to the PSU than the R-opt, I concluded the 50Hz hum comes over from magnetic induction. When I carefully shift another metal sheet in between, the noise drops a bit. So here, my plan is to modify the amp side (low part on pic) to have the OPT's as far away as possible from the PSU and use 3mm steel in between.

Edit : the hum level doesn't change when I change the volume setting.

2. High-frequency noise

The other issue is noise above about 200Khz on the output. Below two graphs, input (yellow) at 400 mV 1KHz sinus, output (blue) loaded with 150 ohms resistor, output amplitude about 60 mV.

The first graph shows the output without any filter on the DSO :

Attachment:
1khz_without_filter.jpg


The second graph shows the output with a 100KHz filter on the DSO :

Attachment:
1khz_with_filter.jpg


Now I guess this is not oscillation of the amp itself, I would guess instead power-line pollution, when I zoom it I see low-level sinus on top of my signal, this is what makes the trace "thick" when unfiltered on the scope. So I will probably still need a filter. Not sure if it is a real issue and if I really need to do something about it. Another option would be another condensator in parallel on B+, something like 100nF.... Not sure it will bring something.

Kind regards,
Charles


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PostPosted: 17 Feb 2018, 11:24 
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Joined: 09 Oct 2012, 19:43
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Location: Vancouver Canada
From the pic you provide it looks to be possible to remove the tube board and the OTP's with their tag strips. Then flip the aluminum bottom plate and reassemble. Only possible if the power supply plate and the amp plate are two separate plates. It would give more room between PWR trans and OTP's. Then you could also put more layers of shielding between PWR and AMP.


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PostPosted: 04 Mar 2018, 16:13 
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Joined: 02 Jan 2018, 22:26
Posts: 22
Hello!! I'm new in the forum, and with tubes!!

I'm reviewing the 50! pages of accumulated information and I learned a lot about this project and tubes!!

I have almost all the part already, and i'm working on the cabinet design, it will have a top aluminium plate, the main chassis, and will sit in a wooden box, some people had some hum problems, and magnetic interaction in between transformers, some suggested to put the power transformer in another box away from the output stages, in this case, its better to move out just the transformer and run AC thru the wires up to the Amp or the full PSU and run DC up to the Amp?

My first approach was to put all over the plate and make a P2P cabling underneath the plate...

I bought my transformers from a guy that makes tube guitar amps, they are a bit big and Ugly, so is no problem to conceal them in black boxes...

another option in my mind is to put the power transformer underneath the plate and OPTs in the upper part with the tubes or vice versa, this will prevent transformer interaction? the power transfomer it's not so ugly...

I'm attaching my first approach of design:

Attachment:
Ampli.JPG


Thank you all for sharing ideas, and specially to Bruce for so nice design!!


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PostPosted: 06 Mar 2018, 16:32 
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Joined: 04 Jun 2008, 20:59
Posts: 3687
Location: Arizona, USA
Hi, If you are reasonably sure the power transformer doesn't radiate too much EMI then it ought to be OK. Ones with either full steel covers or bells are best. If they are open windings then I would put the outputs inside the chassis. A steel chassis is IMO better than an aluminum one at shielding. The tubes themselves are pretty immune to picking up hum in this design and can go on top if you want.

Good listening
Bruce

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PostPosted: 06 Mar 2018, 18:58 
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Joined: 02 Jan 2018, 22:26
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Thank you Bruce, unfortunately the shells are plastic...

So, I will make an steel plate as a chassis (or maybe keep the aluminium but add Mu metal sheets), and put the OPTs behind it.

The other option is to put the power transformer in a steel case out of the main cabinet, in this approach, do you recommends me that I run AC or DC to the HeadAmp? I mean, I put the rectification and filtering stage in the separate box, or just the power transformer and run a meter or so of cable with the AC secondaries to the Hedamp box?

Another doubt arise since my last post, I bought an stereo Alpha 100k tapper (a blue velvet it's a bit out of my reach) for the volume control, and the resistance of each channel its a bit different, there are about 3k of difference in between, this difference will not introduce an imbalance in the HeadAmp output for a given input?

Thank you again for your support!


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PostPosted: 06 Mar 2018, 21:42 
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Joined: 09 Oct 2012, 19:43
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Location: Vancouver Canada
You would be better off putting the power trans inside the cabinet (at the same location) then leave the tubes and output trans on the top. Still keep your pwr trans 90 deg from the outputs as you have in your drawing.
Put shielding around the power trans underneath to keep it's emissions from the rest of the cct parts. It will get warm so ventalate in the back side. Experiment a little with different orientation of the power trans if there is hum. It is always preferable to not have the power supply (in any part) in a separate box due to the high voltage on the line between pwr and amp. If you must do it make the lines AC rather than the rectified DC. Get zapped once from DC and you will know what i say. Even with only 40mA supply.
The pot you have for volume has/should have, a logarithmic taper. Therefore i would place the shaft at the 50 percent position and re-measure the difference between them. The largest portion of the resistance difference is at the end of it's travel so that is where the most difference would come into play. That equates to 70 to 100 percent power. I bet you will never get close to that value, and if you ever do, the difference in volume at that level is guaranteed NOT to be what your brain is concerned about! Besides, the hole for the pot is made and you can change it at any time you like.


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PostPosted: 07 Mar 2018, 05:33 
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Joined: 02 Jan 2018, 22:26
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Thank you laurie54 for your thoughts!!

I will will work around!!

Best Regrads!


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PostPosted: 07 Mar 2018, 11:41 
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Location: Vancouver Canada
Bruce suggested putting the output trans under the cab and that will work to as the output trans are what are going to be picking up any noise. They are very sensitive and will even pick up the opening and closing of a pair of pliers if done close to the outs. Since they are open frame trans and are lacking any kind of shielding they are often put in cans. (Faraday cage). One person in this form even used double sided copper PCB's to make a cage for them. Great idea and they can be painted any color or texture. Have a look at: DIY Single-Ended (SE) 6L6 / 5881 Tube Amplifier.


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PostPosted: 08 Mar 2018, 15:31 
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Joined: 17 Mar 2016, 07:26
Posts: 27
Location: Prêles, Switzerland
laurie54 wrote:
From the pic you provide it looks to be possible to remove the tube board and the OTP's with their tag strips. Then flip the aluminum bottom plate and reassemble. Only possible if the power supply plate and the amp plate are two separate plates. It would give more room between PWR trans and OTP's. Then you could also put more layers of shielding between PWR and AMP.


Laurie thanks for the hint. It took a while, it was easier to swap the positions of the OPTs and the wire strip. I moved the OPTs to the front as much as possible, hum is almost gone, at least probably not noticeable if you are not instructed to "listen for it".

Pic of the boxed amp. After one hour powered, I have measured about 45°C (113°F) right in the middle hole of the back, 3cm over the 6V DC regulator heat-sink, and about 55°C (131°F) between the tubes. I'm aware it's a bit high, I will probably re-do a top plate with more ventilation holes, and to make the tubes more visible.
Attachment:
hp1.jpg

The sound is excellent, even if the yellow tablecloth ruins the look of it :-)
Thanks to all who helped and especially to Bruce for providing the schema and the instructions !


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PostPosted: 08 Mar 2018, 19:00 
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Joined: 02 Jan 2018, 22:26
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laurie54 wrote:
Bruce suggested putting the output trans under the cab and that will work to as the output trans are what are going to be picking up any noise. They are very sensitive and will even pick up the opening and closing of a pair of pliers if done close to the outs. Since they are open frame trans and are lacking any kind of shielding they are often put in cans. (Faraday cage). One person in this form even used double sided copper PCB's to make a cage for them. Great idea and they can be painted any color or texture. Have a look at: DIY Single-Ended (SE) 6L6 / 5881 Tube Amplifier.


laurie54, I had the same feeling, so I decided to conceal the three transformers in Faraday cages, and paint it black. I will take both ideas, put the PT underneath the chassis inside a Faraday cage (all the AC parts will be there including the IEC connector +EMI filter, fuse and main power switch), and the OPT's on top in metallic boxes, I saw many tube amps like that and now I have clear why!!

I'm amazed how an almost 6 years old post is still active!! I'd never saw it in internet!!

Thanks for the hints!!


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