UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > General Vintage Technology Discussions

Notices

General Vintage Technology Discussions For general discussions about vintage radio and other vintage electronics etc.

Closed Thread
 
Thread Tools
Old 12th Jan 2023, 7:51 pm   #21
Trigon.
Hexode
 
Trigon.'s Avatar
 
Join Date: Jun 2018
Location: Buckinghamshire, UK.
Posts: 382
Default Re: Hum in homebuilt valve amp

Speaking of headphones, here's a rather extreme example of aligning transformers to avoid unwanted coupling:-

Click image for larger version

Name:	DSCF0221.jpg
Views:	170
Size:	84.9 KB
ID:	271374

That's a modified Koss Electrostatic Headphone supply with two mains transformers on top and two output transformers below. Both mains transformers are rotated in two planes, and optimum orientations were eventually found experimentally. Positioning was surprisingly critical - a degree or two of twist in any plane would be enough to make an measurable difference. (Hence the sealed adjustment screws...)

Having reduced hum to below (my) audibility, it was dissappointing to find that a small amount returned when the aluminium cover was re-fitted. I assume eddy currents in the aluminuim changed the shape of the field slightly. A new wooden cover solved that one!

Cheers
Trigon. is online now  
Old 12th Jan 2023, 8:01 pm   #22
Jan Zodiac
Pentode
 
Join Date: Sep 2021
Location: Aabenraa, South Jutland, Denmark.
Posts: 217
Default Re: Hum in homebuilt valve amp

Quote:
Originally Posted by Trigon. View Post
Speaking of headphones, here's a rather extreme example of aligning transformers to avoid unwanted coupling:-

Attachment 271374

That's a modified Koss Electrostatic Headphone supply with two mains transformers on top and two output transformers below. Both mains transformers are rotated in two planes, and optimum orientations were eventually found experimentally. Positioning was surprisingly critical - a degree or two of twist in any plane would be enough to make an measurable difference. (Hence the sealed adjustment screws...)

Having reduced hum to below (my) audibility, it was dissappointing to find that a small amount returned when the aluminium cover was re-fitted. I assume eddy currents in the aluminuim changed the shape of the field slightly. A new wooden cover solved that one!

Cheers
That looks, well, different I once read an article in a danish hifi magazine from the 80's, about an external RIAA phono preamp. Here the small mains transformer was also rotated by experimenting, to cancel hum.

Jan
Jan Zodiac is offline  
Old 12th Jan 2023, 8:05 pm   #23
GrimJosef
Dekatron
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
Default Re: Hum in homebuilt valve amp

Quote:
Originally Posted by stevehertz View Post
I've seen commercial designs where one of the transformers is at a jaunty 45 degrees wrt the other.
I once machined a 2-axis gimbal assembly on which I mounted a commercial amp's phase-splitter transformer to minimise the hum that it was picking up from the mains transformer. I was able to null the overall hum down by nearly 20dB as I recall.

In a slightly off-topic diversion, it's worth remembering that odd-order hum can come from many sources and can have a range of phases - at it's simplest both 'positive' and 'negative'. In some amps the designer has done his best to balance these against one another. You can look at the thing, spot an 'obvious' source of hum and spend a deal of effort fixing that design flaw only to find that your improvement has actually made the overall hum worse ! It turns out that there's another and even larger source of trouble somewhere else which was being partially nulled by the one you spotted .

Cheers,

GJ
__________________
http://www.ampregen.com

Last edited by GrimJosef; 12th Jan 2023 at 8:11 pm.
GrimJosef is offline  
Old 13th Jan 2023, 2:04 am   #24
trobbins
Heptode
 
trobbins's Avatar
 
Join Date: Oct 2009
Location: Melbourne Australia
Posts: 898
Default Re: Hum in homebuilt valve amp

Using fibre washers to break electrical loops formed by the clamping bolts needs to be done with some awareness of the loops being broken, and a continuing need to maintain a safety earth connection between the chassis and the transformer core and any exposed bell-ends.

Perhaps more so for OPT's, it was not uncommon to find that the clamping bolts also had insulation tubing added to avoid inadvertent touching of the bolt with laminations. That can sometimes be difficult to retrofit due to lack of uniform clearance between a bolt and all the laminations and end metalwork, and the thickness of available tubing (even for heatshrinked tubes).
trobbins is online now  
Old 13th Jan 2023, 11:45 am   #25
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,526
Default Re: Hum in homebuilt valve amp

Quote:
Originally Posted by Rich Woods View Post
Quote:
Originally Posted by Restoration73 View Post
What I was thinking was, that if there is no inter-winding screen 0n the heater supply transformer, hum could be capacitively coupled to the heater/regulator circuit.

You could try grounding the other side of the heater supply to see if hum changed.
It’s a shame you didn’t go for an earthed centre tap on the heater winding.

If that's not possible, you could try a "humdinger" instead- a low value (50-100R?) WW pot across the heater winding with earth taken to its wiper rather than either end of the winding. Tweak for minimum hum. This could well work well enough to obviate the need for dc heaters at the front end.
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is online now  
Old 13th Jan 2023, 1:26 pm   #26
Richardgr
Heptode
 
Join Date: Oct 2019
Location: Täby, Sweden
Posts: 692
Default Re: Hum in homebuilt valve amp

This was a useful discussion. I was wondering whether a better location for this thread would be the 'Vintage Audio (record players, hi-fi etc)' board? I think that is where I would have first looked if I had an amplifier with a hum issue.
Richardgr is offline  
Old 13th Jan 2023, 2:06 pm   #27
Rich Woods
Pentode
 
Rich Woods's Avatar
 
Join Date: Sep 2017
Location: Ceredigion, Wales, UK.
Posts: 243
Default Re: Hum in homebuilt valve amp

Quote:
Originally Posted by Herald1360 View Post
Quote:
Originally Posted by Rich Woods View Post
Quote:
Originally Posted by Restoration73 View Post
What I was thinking was, that if there is no inter-winding screen 0n the heater supply transformer, hum could be capacitively coupled to the heater/regulator circuit.

You could try grounding the other side of the heater supply to see if hum changed.
It’s a shame you didn’t go for an earthed centre tap on the heater winding.

If that's not possible, you could try a "humdinger" instead- a low value (50-100R?) WW pot across the heater winding with earth taken to its wiper rather than either end of the winding. Tweak for minimum hum. This could well work well enough to obviate the need for dc heaters at the front end.
Good idea, like the Rogers twin EL84 stereo power amps.
__________________
It’s all about the music…….
Rich Woods is offline  
Old 13th Jan 2023, 6:53 pm   #28
Jan Zodiac
Pentode
 
Join Date: Sep 2021
Location: Aabenraa, South Jutland, Denmark.
Posts: 217
Default Re: Hum in homebuilt valve amp

I agree that this problem could occur when working on any valve amp, but I was a bit confused where to place the thread.

So, I am a bit confused on the heater connections. I am fully aware of the pot across the heaters, with the middle going to earth. I have seen that in several amplifiers, and it works just fine.

But in my case where the hum is present as soon as the mains are switched on, and caused by magnetic flux, what will it help to either connect the heaters floating, or switching the ground connection from one side to the other?
The hum is obviously not caused by one side of the heater winding connected to chassis, since the hum is present even before the valves has warmed up.

So am I wrong here, or are we now talking about two different hum problems? One from the magnetic flux due to poor chassis layout I am struggling with, and another source of hum coming from how the heater winding is connected to chassis.
On the amps I worked on that had the "humdinger" pot, it was when the amp was fully warmed up, the hum would be eliminated by finding the right setting of the potmeter. During warmup, they were dead quiet.

So in my case, should I worry about the ground connection of the heater winding, when this apparantly doesn't cause hum? Only the output valves are fed with AC on the heaters, and I believe they are not really sensitive to which end is grounded or not. Or does it has something to do with what happens in the mains transformer itself? All preamp valves are fed with regulated DC, and I am certain no hum will be generated in this path.

When I turn volume fully up on both input channels (open inputs) there is not the slightest sign of increasing hum to be noticed. Only valve hiss.
The hum I am experiencing is present as soon as mains are switched on, and there is no difference during and after it has warmed up, or by turning any of the controls on the preamp.

I hope someone can explain if I should worry about the heater connection too, and why, if that's the case.

So far I am planning to lift the transformers from the chassis with spacers, and mount a heavy steel shield beside the mains transformer. I will keep in mind to have it electrically grounded for safety reasons. If it is not necessary to do anything about the heater connections, I will leave it be. If I can avoid changing the wiring, I would prefer that.

Jan
Jan Zodiac is offline  
Old 13th Jan 2023, 9:56 pm   #29
Ian - G4JQT
Octode
 
Ian - G4JQT's Avatar
 
Join Date: Jun 2004
Location: Reading/Fakenham, UK.
Posts: 1,320
Default Re: Hum in homebuilt valve amp

When I built my Wadar amp, I did the headphone test as Gabe describes in post 20. I also added a 470 ohm humdinger pot (wiper to earth) across the 6.3V heater feed of V1, V2 and V3. When dipped the hum was much reduced compared to a Wadar I had for comparison that just earthed one side of the heaters.

(Don't forget to twist the heater wiring fairly tightly and keep it away from the input circuitry!)

Ian
Ian - G4JQT is offline  
Old 13th Jan 2023, 10:01 pm   #30
joebog1
Banned
 
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 2,704
Default Re: Hum in homebuilt valve amp

Lifting the transformers may help the mechanical hum a bit, but rotating one or the other of them will be a better idea I think.

Joe
joebog1 is offline  
Old 13th Jan 2023, 11:08 pm   #31
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,799
Default Re: Hum in homebuilt valve amp

You're getting help from lots of people, and all the help is different.

Each focuses on a different hum mechanism.

All these mechanisms are real. And in the end result they can all interact.

No-one knows what their relative magnitudes are. Interaction includes partial cancellation. Cancellation between several different routes means that you can fix one route and the overall hum gets worse. This isn't just counter-intuitive, it will drive you in wrong directions and nothing will seem to add up.

Scary?

There is a route through the maze which avoids the iteractions.

Remove all the valves. Switch on. Got hum? Well, there is no heater current, there is no amplification going on. This is a basic problem, it has to be fixed first. Trying to go further without fixing it will get you lost.. So this will be the sum of transformer to transformer mag field coupling, plus the transformer making large circulating currents in the chassis, and then however much of those currents get to circulate through the underside of the output transformer.

Rotating transformers will let you try to null the transformer to transformer mag coupling.

Insulated screws and a rise above the deck will reduce the chassis circulating cuttent effect.

When you've got these done, you can start introducing the electronics, stage by stage, but that's another instalment.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 14th Jan 2023, 9:34 am   #32
Jan Zodiac
Pentode
 
Join Date: Sep 2021
Location: Aabenraa, South Jutland, Denmark.
Posts: 217
Default Re: Hum in homebuilt valve amp

Well yes it's a bit scary and confusing, but I'm happy for all the replies, and I'm sure it will be possible to get to the bottom of the problem I am experiencing.

So, I removed the valves, and the hum is still present as soon as the amp is switched on. The hum comes from the loudspeaker only. It is not mechanical hum. The mechanical hum/vibration is well below my hearing limit, even with the ear close to the transformer. So that is not an issue at all.

I still don't think the heaters or heater connection has anything to do with my problem. V1, V2 and V3 is in a closed box as can be seen in the attached pictures. Except for the signal path, no AC at all enters the enclosure. B+ is very well filtered with no visible (on the scope) or audible ripple. Heaters are fed from a regulated supply. No need for hum canceling pots for this.

The hum in my amp obviously comes from stray flux from the mains transformer. I don't really want to rearrange the transformer if it can be avoided. I tried lifting the mains tranformer like 1cm with the amp on, and the hum was reduced a lot. I will mount it on spacers, and also put a heavy steel shield next to it, as well as mount the output transformer on spacers. This way, I don't have to fiddle all too much with the chassis. It appears the most of the hum travels directly through the chassis, since lifting the mains transformer reduced the hum level by a lot. The chassis is made of 1,2mm steel.

Adding a heavy shield between the transformers will help further in reducing hum, since I tried to put a piece of steel sheet between them while running, and this helped too.

I hope that by doing the above mentioned, I can reduce the hum to an acceptable level, if not completely remove it. I am not repositioning the transformers if I can avoid it. Especially when there is not much space at all inside the amp, as can be seen in the last picture.

Jan
Attached Thumbnails
Click image for larger version

Name:	119.jpg
Views:	85
Size:	96.9 KB
ID:	271457   Click image for larger version

Name:	120.jpg
Views:	78
Size:	83.3 KB
ID:	271458   Click image for larger version

Name:	IMG_2209.jpg
Views:	80
Size:	81.1 KB
ID:	271459  
Jan Zodiac is offline  
Old 14th Jan 2023, 12:17 pm   #33
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,059
Default Re: Hum in homebuilt valve amp

Quote:
Originally Posted by Radio Wrangler View Post
Remove all the valves. Switch on. Got hum? Well, there is no heater current, there is no amplification going on. This is a basic problem, it has to be fixed first. Trying to go further without fixing it will get you lost.. So this will be the sum of transformer to transformer mag field coupling, plus the transformer making large circulating currents in the chassis, and then however much of those currents get to circulate through the underside of the output transformer.

...

Rotating transformers will let you try to null the transformer to transformer mag coupling.
All good advice! But even more fundamental first ...

Remove loudspeaker. Switch on. Got hum? It's direct from the mains transformer, core magnetostriction, no amount of re-orientating will fix it, you'll have to mount it on cushioned mountings; vacuum impregnate it (if not already done); or at an extreme, get another transformer!
kalee20 is offline  
Old 14th Jan 2023, 1:35 pm   #34
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,799
Default Re: Hum in homebuilt valve amp

Aye. He's already said that acoustic clatter from the transformer isn't audible.

The dominant improvement from lifting the transformer says that transformer-to-transformer magnetic coupling isn't the biggest issue. So it's likely that we have large circulating currents in the chassis. Partly this will be the partial-turn effect of throughbolts, partly the result of having an outer lam hard against the chassis.

Lifting the transformer on non-conductive (electrically!) spacers, and insulating the through bolts looks a promising first move. Doing the same to the output transformer should help a bit more, Then it's time to try the steel shield again. With luck, it might not be necessary, or it might do the job. And don't forget a safety earth from core to chassis. One single connection can't pass circulating currents, there are no loops.

Progress is being made.

For people building classic Mullard designs, there is frequent enough advice in the forum to stick to their layout. This is usually meant as the electrical aspects took a lot of work to optimise, but there were also magnetic aspects which aren't so obvious.

Building an amplifier from scratch, not as a copy of a successful commercial design or something a valve manufacturer published as a sales tool bypasses some important but invisible parts of the development process. They still need doing in order to get a good amplifier.

This is a great thread. It's showing up some things many amp builders aren't aware of, and Jan's doing a very good job.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 15th Jan 2023, 1:29 pm   #35
Jan Zodiac
Pentode
 
Join Date: Sep 2021
Location: Aabenraa, South Jutland, Denmark.
Posts: 217
Default Re: Hum in homebuilt valve amp

So for now I managed to lift the output transformer. I had a piece of phenolic (don't know if that is the correct english term for this) with a thickness of 4mm.

I cut some smaller pieces, so the transformer is supported along all four sides. I also insulated the throughbolts with heatshrink and a fiber washer at the top, as can be seen in the pictures.

Now I am waiting for the longer bolts I ordered for the mains transformer. As I am waiting for the bolts, I will make a heavy shield to be mounted beside the transformer.

It is indeed important to take a lot of things into consideration, when building an amp from scratch. I admit that I was more focused on ground loops, DC for the heaters in the preamp, and stiffness of the chassis by mounting the transformers the way I did. I was aware that I should not put them right next to each other, so I figured at each side of the chassis was far good enough.

I had no idea that the flux produced by the mains transformer, could actually travel that far from one transformer to the other, as well as through the chassis. At least not creating that amount of hum in the speaker, even with the valves removed.

So I believe I have learned an important lesson with this, and hopefully it can shed some light on the importance of proper layout.

Another thing I noticed is with my Leak TL25+. I use it with its matching preamp for my mono system. It is connected to a large horn enclosure with a 15" duplex driver. It is a very efficient speaker.
When I switch on the amplifier, immediately I have the same type of hum as in the guitar amp I made. It is not very loud, but certainly at a level where you would think something is not right here.
As soon as the amp is warmed up, the hum disappears completely, and only a faint hiss is to be heard. Even with the ear next to the speaker, no hum is present. The amplifier is pretty compact, and the two transformers are close to each other.
I wonder how this hum can disappear as soon as current is drawn from the valves.
They must have done some experimenting when developing this series of amps, since they are so compact, and yet are very silent (when warmed up).

Jan
Attached Thumbnails
Click image for larger version

Name:	IMG_2220.jpg
Views:	48
Size:	97.1 KB
ID:	271540   Click image for larger version

Name:	IMG_2221.jpg
Views:	48
Size:	73.1 KB
ID:	271541  
Jan Zodiac is offline  
Old 15th Jan 2023, 3:14 pm   #36
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,799
Default Re: Hum in homebuilt valve amp

The disappearance of hum may well be that in development, in fiddling things around to reduce hum, what may have been going on was some cancellation. Before the valves get going you get hum via transformet to transformer (both direct and via induced circulating currents) Then, as the valves get going, they add hum but in antiphase.

Cancellation tricks are always a bit unreliable, small errors produce disproportionate impairment, so you want to get things as low as you can before doing things which can cause cancellation effects. The end result in more stable.

So that's why I recommended you got the valves-out hum sorted before going any further. This way you can go through a development process which converges on a good outcome. People doing this sort of work don't usually talk about it much, largely through a lack of interested listeners. It's all part of taking an orderly approach. You seem to be going slowly, you seem to be doing unnecessary things. You're doing things your boss won't understand most likely so you don't get credit for it. You won't do the control experiment of showing what happens otherwise. One of those cases where the non-dramatic good guy usually gets undervalued.

Career-wise, it's better if someone else got it wrong and you got brought in to fix it. You're a hero.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 15th Jan 2023, 6:56 pm   #37
GrimJosef
Dekatron
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
Default Re: Hum in homebuilt valve amp

Quote:
Originally Posted by Radio Wrangler View Post
... Before the valves get going you get hum via transformet to transformer (both direct and via induced circulating currents) Then, as the valves get going, they add hum but in antiphase ...
This may well be true, but the main cause of things going quiet when the amp warms up is the appearance of the negative feedback. The transformer-to-transformer hum is introduced inside the feedback loop, so it will be suppressed (by 26dB if I remember rightly in the Leak) when the amp's gain turns on.

If you are hunting for hum it's often worth disconnecting the NFB while you do it. It makes the problem much easier to hear and it doesn't cause its source to be confusingly relocated by the action of the feedback loop.

Cheers,

GJ
__________________
http://www.ampregen.com
GrimJosef is offline  
Old 22nd Jan 2023, 10:10 am   #38
Jan Zodiac
Pentode
 
Join Date: Sep 2021
Location: Aabenraa, South Jutland, Denmark.
Posts: 217
Default Re: Hum in homebuilt valve amp

Just a quick update.

I have now lifted the mains transformer from the chassis too, like I did with the output transformer. I also made a shield of heavy 3mm steel, and mounted it right next to it as can be seen in the pictures.

The overall result from these simple things I did, is a very effective damping of the hum I struggled with.
When the amp is switched on, a very faint hum is again immediately present, but at a very very low level. It is barely noticeable.
Like on my Leak amp, this hum disappears completely when the valves has warmed up, and is replaced by a very faint his.

So this thread has been very helpful, and the problem is now completely solved, and on top of all, an important lesson is learned.

Just wanted to share the end result and thanks for all the great inputs.



Jan
Attached Thumbnails
Click image for larger version

Name:	IMG_2247.jpg
Views:	79
Size:	97.9 KB
ID:	271970   Click image for larger version

Name:	IMG_2248.jpg
Views:	70
Size:	90.8 KB
ID:	271971   Click image for larger version

Name:	IMG_2249.jpg
Views:	71
Size:	84.9 KB
ID:	271972   Click image for larger version

Name:	IMG_2250.jpg
Views:	70
Size:	99.1 KB
ID:	271973  
Jan Zodiac is offline  
Old 22nd Jan 2023, 11:57 am   #39
Rich Woods
Pentode
 
Rich Woods's Avatar
 
Join Date: Sep 2017
Location: Ceredigion, Wales, UK.
Posts: 243
Default Re: Hum in homebuilt valve amp

Nice job & well done sticking with it until the problem was resolved
__________________
It’s all about the music…….
Rich Woods is offline  
Old 22nd Jan 2023, 4:32 pm   #40
AdrianH
Octode
 
AdrianH's Avatar
 
Join Date: Mar 2020
Location: Blackburn with Darwen, Lancashire, UK.
Posts: 1,566
Default Re: Hum in homebuilt valve amp

Can I just ask a question on mains induced hum. Some of the old mains transformers I have saved, have a copper band wrapped around the outside of the laminations passing over the core, this band is soldered to it self providing a closed loop/shorted turn. Does this reduces leakage magmatism? if not what is it's purpose please and I wonder if this may have helped rather than a steel screen being fitted?

Adrian
__________________
Asking questions and learning, or trying to!
Youtube EF91Valve
AdrianH is online now  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 12:39 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.