DIY Audio Projects Forum

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.



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:



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 :



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



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

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.

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:



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

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

_________________
Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

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!

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.

Thank you laurie54 for your thoughts!!

I will will work around!!

Best Regrads!

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.

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.

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 !

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!!