Mark III Power Trans Hum

I have a set of cloth-lead Mk III's, they have SDS cap boards and EH KT-88's, other tubes circa original. They sound fine to me, but the PT's hum, at least that's where it appears to be coming from. One of the multi-caps is burnt out, but of course bypassed.

I run them a bit low bias, 1.4v right now. The pots are little twitchy, but they tend to hold a setting pretty well.

Just loosened the transformer base screws and tightened the bell screws then tightened the bases, same hum. Doesn't seem to matter which way the non-polarised plugs are inserted as far as hum. I have felt feet on the little bumps.

Very low hum from my speakers, they are 89db, have to put my ear up to them at idle to hear it. But the amps themselves are humming away. Power here is 124.5vac today, which is high average.

Any suggestions for what to look into? Thanks for any help.

yeah, most likely the power transformer humming (not in tune with the music
you can add rubber washers in between the transformer and chassis, it won't stop the humming, but might minimize it being worse by transferring vibration to the chassis.

B + cap going bad................or ck this MK3 info out.....I have 3Pr of MK3s...all stock
An I run all of them on a Varic....
.Drop your the B+ to about 480 I bet you lose the hum.....an youll get better sound.. An tubes well last a longer time..your B+ now is way out of spc...like over 500....I seen 550...B+.....
40 years was AC was 110 .....now. to day is 125-7...
stock B+ was 480-90

....good luck





http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0CDMQtwIwAg&url=http%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3Dx2HJYtO3D1Y&ei=yDW0VK3EIselNtWIgNgK&usg=AFQjCNHgfudd-C0v2rSlvqyNe4WZT9bC9Q&sig2=VUoI4aXE0F5CDhKAlZl3Mg

t4hifi, I'll dig around for rubber washers or something.

zx, the SDS boards seem to be well-regarded. Is one of the sets on that board for B+ ?


Have also thought of building a bucker. If I made one in a power strip, how many amps would I need to be safe? And since I have an idler Thorens, I'd probably run that off it as well. Might be easier to make a time machine and go back to 1968.

The power transformers may not like the modern ~123v line voltage found in many areas of the US. If you have a variac, try lowering the voltage to 117v or so to test whether that reduces the transformer buzz.

speedyk wrote:t4hifi, I'll dig around for rubber washers or something.

zx, the SDS boards seem to be well-regarded. Is one of the sets on that board for B+ ?


Have also thought of building a bucker. If I made one in a power strip, how many amps would I need to be safe? And since I have an idler Thorens, I'd probably run that off it as well. Might be easier to make a time machine and go back to 1968.

In my experience rubber washers will not help. What has worked for me is soft rubber grommets. I enlarged the holes in the transformer tabs so I could insert the grommets. This is important as you need to make sure that you aren't coupling the transformer hum through the mounting hardware. As T4HiFi said you cannot stop the humming but you can prevent the chassis from humming along with the transformer--if you isolate the transformer carefully.

Thanks j beede, grommets we got plenty of around here. Agree about coupling. But that puts me perilously close to removing everything, polishing the chassis, and repainted the bells. Dangerous territory once entered. Wondering what would happen if a grommet were placed on each side and the bolt kept centered so it didn't contact the hole.

We have a great old time electronics shop in town, I might go see what they have for Variacs this week, I imagine they aren't cheap to ship so need to find local ones. I have a suspicion that if I built a bucker it might wreak havoc in some way. A Variac would give me an idea of the true range of the issue as well, I could find the target voltage for my amps.

20 year ago we put rubber under our output trans to get better bass.....but to day I have all my tran. bolts lose on the chassis......give the best sound so far for me......
All things.... parts that have audio signal runing though them ring some.....so most parts sound better lose... than tied down......one of the biggest sound killers are ZIP ties.....Never use them ...an pull them out of any amp ss are tube ....all the old great sound amps never used them an people love there sound an pay big Cash.........
If we were talking SS sand amps......we would only use the varic for repires....thay pull to much current to run on a varic..................but tubes on pull  MV....so I run 2 MK3s with eze...........on the same varic
Yes there info on the site here about a Bucking tran.........Vary ezey to bild...........
you can get varics off the web....for about $50.
I do some tube work .... sale amps an preamps......any thing over 20 years old get a varic with the sale................an I have not lost tubes in years.....
ARC an others... has found the sweet spot for B+ to be about for amps 480-90....sound wise Any thing above an the sound gets a little hard..... an your amp can start eating tubes.....one tube can be the $$ of a varic......
I think there a must have for tube diyers..
like always..... this is just 40 years of one man finding...........have fun with tubes sound....





thanks for the site Bob.....................

zx wrote:
All things.... parts that have audio signal runing though them ring some.....so most parts sound better lose... than tied down......one of the biggest sound killers are ZIP ties.....Never use them ...an pull them out of any amp ss are tube ....all the old great sound amps

thanks for the site Bob.....................

Are you suggesting to unbundle all carefully routed cables in any amp?

I only Speaker for my work...........Look at Most any older Tube Amps preamps...point to point...
As for what I hear....A nice neat look layout to me means nothing if it changes the sound!
I have had Sand amps I have pulled 148 zips out of......other have said...the amp sound like a diff amp.....so I know it not just these old ears.....
So when we talk Tube Amps.....were in the 4-500V.....tie a buch of diff wires to gather as a zip dose...it cases a frequency peak......Caps..wire ...tubes...all parts...expand & contract..it changes the sound...
.Now some well say ...he like noise...well that's all well got!...........
To me an my work here are some sound killers....Zip ties....heat srink...Chokes.....less there 5h.......there should be air between All parts...
For over 40 years I never questions chokes......changed ever thing...wire, caps,tubes tranfourmers feet..........
You name it.....I was will to try anything to get the sound I wonted...
An always thought that Dynacos ....were fun. but vary low on the food chang of tube amps.. liked Eicos tube amps better.... thay used NO choke in some of there amps ....
But then I pulled my MK3s chokes out....Wow.... I did it to see if I could here the Big diff in rectifiers....that I had that were going for $100......not so much diff...with the choke in....yes I hear diff...not good.......what i hear is that a small choke sounds like a big zip tie...
now I see Dynaco in a league of ther own.......Biger sound  diff than any cap,wire,anything...gofig.....
Less the chokes is as big as my output tranfourmers I never use one again in any tube peaces I own!
All just one mans finding...............




thanks for the site Bob.......

Interesting zx. Wire binding goes back to the Western Electric days anyway, they were mental about sound quality, but voice and not hi-fi. Although Ellington and Strayhorn composed stuff over the phone supposedly.

I have a 6V6 amp, dead brand, Boulevard. I think it was cheaply spec'ed, no chokes, and the power trans was replaced with something else which might not be right. But: It has a certain presence, I hate to mess with it but it hums also. Replaced likely bad caps and leave it alone, no ties but twisting of signal pairs. Perhaps because my parents had a VOM mono player growing up, and I recently found it also had 6V6, so that "first sound" maybe is part of my comfort with it.

When you remove chokes what do you put in place? Caps in parallel? Wouldn't be hard for me to try it with a bypass. But I am ignorant of electronic theory for the most part. If it isn't too hard I'd like to try it.

We have schematics thanks to Bob an others.... on the tube amps MK3
https://2img.net/h/i9.photobucket.com/albums/a78/Bob01605/DynacoMarkIIIschematic.jpg

we have on pin 8... 30mf.....I put a 1.mf polly cap right on pin 8 of the rectifier....but others like res. inpace of the choke....as you know............look at MK2......the only Dynaco I have never owned...look No choke..............as I said over 40.....was better.............right...it was a given...
https://2img.net/h/i9.photobucket.com/albums/a78/Bob01605/DynacoMarkIIschematic_2.jpg
...yes I seen a lot of ties...cotton,rubber,u name it..Zip wins .... like you were saying
I only say what I hear........... an wont to hear.....
I have speakers like B&W Thiel  JM Labs Focal
But most all my sound work.... too hear what going on with sound it speakers like Apogee fullrang ribbons an ESL,Magnepan........the panels don't lie!..an it ezer to hear these diff...
..that can be good an bad....like life..hehe
Look Life is short....like a min...have fun with sound...tubes...



Thanks for the site Bob....