Wem dominator mk3 need help re hum

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Wem dominator mk3 need help re hum

Postby bobster on Thu Dec 03, 2015 11:45 pm

hi guys ,
im just looking for any suggestions as to get the hum down on a Wem dominator [ 1974 vintage ]
http://www.vintagehofner.co.uk/britamps ... ommk3.html
im hoping someone may have some experience with these and some pointers as to sorting a deep mains hum [ not buzz ]
ive had it 2 weeks and have searched here and google high and low and tried a number of things and have made some progress.....
the hum is a bit too loud for recording but ive got it ok for gigs

so far i have -
cleaned and tightened all grounds and also input jacks
replaced all electrolytic caps
separated heater string from signal wires and used twisted wiring
replaced all anode resistors
checked EL84 grids for and dc volts and only read around 0.005v
put in good old glass into pre amps and tried numerous pairs of healthy old stock EL84s with no difference in hum level
resoldered inputs from PI to pin 2 on EL84s so i can use modern production if need be
reflowed all pots and valve bases on circuit card

im getting great tone and power from the amp but hum is there all the time even with all pots at zero...

interestingly , when i plug into left channel and play , if i turn the volume pot on the unused right channel up to around half it seems to reduce the hum a bit.....
also , when i switch the treble boost on there is a sharp pop through the speaker but the little 680pf cap feels securely soldered on to the board...

thanks in advance!
bob
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Re: Wem dominator mk3 need help re hum

Postby stratele52 on Fri Dec 04, 2015 8:18 pm

1- Did you check if center tap for heater is connected inside the power transformer ?
No heater center tap = hum .

2- Did you check shorting input jack's contact to ground ? Bad contact to ground = hum.

Often those contact are broken or "dirty " , they did not work .
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Re: Wem dominator mk3 need help re hum

Postby trobbins on Sat Dec 05, 2015 2:50 am

I suggest you record the output waveform, and also do a spectrum of the output. That can assist in identifying the dominant contributor, and hence focus further effort.
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Re: Wem dominator mk3 need help re hum

Postby bobster on Sat Dec 05, 2015 3:39 pm

hi guys , thanks for replying

the i/p jacks are making good contact and i cleaned them thoroughly
im just a player / hack so dont have access to scopes etc..
i was thinking of trying an artificial center tap with two 100r resistors but im not sure where the present centre tap is attched in the amp and i dont want to make a boob and let out the magic smoke :(

next small job [ which likely wont help hum ] is to try a 150r cathode resistor as it has a 100r just now and the el84s smell quite toasty

hum seems to be inherent in these amps but some guys on the interweb seem to be able to get them super quiet at idle so there must be some tricks out there that will help....
i dont want to rip out all the old mustard caps in a shotgunning exercise as they are usually reported to be pretty reliable and i dont know how to test them in circuit to see if they are all ok
i may have to seek help from a tech but i like to try and sort things myself :cheers:
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Re: Wem dominator mk3 need help re hum

Postby stratele52 on Sat Dec 05, 2015 3:48 pm

bobster wrote:hi guys , thanks for replying

the i/p jacks are making good contact and i cleaned them thoroughly
im just a player / hack so dont have access to scopes etc..
I'm sure you will answer that , people who have these equipments don't ask question on forum

i was thinking of trying an artificial center tap with two 100r resistors but im not sure where the present centre tap is attched in the amp and i dont want to make a boob and let out the magic smoke :(
Heater center tap is attached / solder to ground / chassis, green-yellow wire . Do you have a meter ?

next small job [ which likely wont help hum ] is to try a 150r cathode resistor as it has a 100r just now and the el84s smell quite toasty

hum seems to be inherent in these amps but some guys on the interweb seem to be able to get them super quiet at idle so there must be some tricks out there that will help....
I have the same amp and is dead quiet with my good tune up , you must see a qualified tech

i dont want to rip out all the old mustard caps in a shotgunning exercise as they are usually reported to be pretty reliable and i dont know how to test them in circuit to see if they are all ok
i may have to seek help from a tech but i like to try and sort things myself
You may do that

:cheers:
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Re: Wem dominator mk3 need help re hum

Postby bobster on Sat Dec 05, 2015 4:28 pm

hi , yes i have a good fluke meter ........
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Re: Wem dominator mk3 need help re hum

Postby trobbins on Sat Dec 05, 2015 10:58 pm

If you have android phone then there are free spectrum analyser apps. Simple to use.
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Re: Wem dominator mk3 need help re hum

Postby stratele52 on Sat Dec 05, 2015 11:58 pm

bobster wrote:hi , yes i have a good fluke meter ........


Do you know how to use ohms scale ?
One test probe to the chassis ground, other to any tube heater pin, you must read few ohms if heater center tap is good .
Amp OFF and disconnect from wall outlet .
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Re: Wem dominator mk3 need help re hum

Postby bobster on Sun Dec 06, 2015 1:34 am

thanks guys ,
yes i can measure ohms volts ok will check on that valve pin to ground tomorrow

re spectrum analyser i do have an android phone but have a lot more reading to do :lol:

wouldnt know where to start with the spectrum !

150r resistor arrived today welwyn w22 so that may help a little its a cool little amp
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Re: Wem dominator mk3 need help re hum

Postby Racing on Sun Dec 06, 2015 12:47 pm

100 Ohms cathode for the pair is on the lusty side of things.
150 Ohms will most likely help as far as mellowing things down a bit at least.
However,do the math and be done with it. Install that 150 Ohm resistor and physically check number of Ohms to chassis when done. Decimals ain´t needed but a reasonably exact number is tho.
Fire amp up and let it settle. Divide the voltage drop across that resistor with the exact number you just measured. Now check B+...and multiply with that...and divide by two. That´ll be your anode loss in idle. Aka bias.

Hum then.
Sounds like you´ve got yourself some sort of groundloop. This is rather common with especially older amps and luckily enough rather easy to remedy.
Localize the wiring bundle that is the secondary side of the powertransformer. You´ll most likely be able to follow colour.
You´ll notice that a few of them heads for chassis ground. Unsolder those.

Now put your DMM in Ohms mode and "ring" the wires heading for ground vs the pair of wires that are the HT and heads for rectification...and in turn the centertap for the heaters.
Get a 500 Ohm 1/4w trimpot. Set the wire that is the centertap for the heaters aside,cut it down so it becomes clean and dress it with a bit of shrink sleeve.
Now solder that 500 Ohm pot to each side of the heater wiring. One leg for each side,and in turn solder a wire to the wiper leg of it. Head this wire to chassis ground.
What you´ve just installed is what´s known as a hum dinger. By turning the trim pot you will alter the heater windings "zero". Trim for minimum hum in short.

You unsoldered the centertap for the HT too right (or bridge negative,doesn´t matter). Head this wire (elongate if need be) for the electrolyte that takes the first hit after rectification. NOTHING is to hit the grounding of that e-lyte before that wire. I emphasize,nothing. The first thing the ground of that electrolyte is to see is said wire. Period.
This is known as "the golden rule" and if it doesn´t...the amp will hum.

That done then.
From what you´re telling the amp grounds via its inputjacks? In that case unbolt them and fold them aside-mid air. Take an Ohm reading between the groundpath of the preamp and chassis. If you get any form of reading that even resembles 5k or less you´ve got yourself a groundloop. That simple. Find out where and why and remedy this.
It is sound engineering in many instances to have the preamp especially ground through the inputjacks. Not needed tho..but sound engineering.

Take it from there and need be we´ll just push forwards as you´ve handled the above.
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Re: Wem dominator mk3 need help re hum

Postby bobster on Sun Dec 06, 2015 5:10 pm

great stuff racing !
i've got a couple of those high wattage 500r pots around somewhere and will try what you suggest and hopefully not screw anything...
im fotunate enough to have some other amps to use so i'll take my time plus have the usual famiy stuff going on but will defo get back to the thread in the next few days and report on progress
:cheers:
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Re: Wem dominator mk3 need help re hum

Postby stratele52 on Sun Dec 06, 2015 7:34 pm

I don't understand why you are looking about these resistor for a artificial heater center tap before cheking if the power transformer heater center tap is working ?
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Re: Wem dominator mk3 need help re hum

Postby Racing on Sun Dec 06, 2015 9:54 pm

As much as i hear what you´re saying Stratele...there´s a reason for everything.

Perfect balance of the two sides of the heater wiring is rare and hence a hum dinger,where you take the CT for that winding out of the equation,is thus always a good thing.
Or put another way it never makes things worse.

Many older European made amps actually sports them stock. Many a such amp even carries more then one heater winding and..thus sports several such hum dingers.
Are they the remedy for all ills? Of course not. They never hurt though,which is the main point in all of this. Various amps are various susceptible to interference though and..yeah well..

The more one fools around with this i guess the more anal one gets. I for one like my amps to be dead friggin silent until called upon and that brings sort of a "doctrine" when setting them up if you wish.
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Re: Wem dominator mk3 need help re hum

Postby bobster on Wed Dec 09, 2015 9:26 pm

ok guys , took some readings etc

chassis ground to-

mains plug earth pin - 1.2 ohm
o/t ground lug - 1.1 ohm
heater pins - 1.1 ohm
main filter cap neg - 1.1 ohm
mains trafo ground - 1.1 ohm

haven't done the artificial centre tap yet because im not sure where the present heater centre tap is situated and on my JMI ac30 it is a little wire coming off heaters on one of the pre amp valves to chassis tucked away and hard to see so i dont want to do the wrong thing and screw the amp up.

i put in the 150r cathode resistor , replaced the PI coupling caps with 22n 400v mustards [ supposed to be 10n stock but thats all i had to hand ] , placed a metal shield around the mains switch to blank it off from the circuit , put in an ECC82 / 12AU7 in the PI and hum is a little bit lower again
definitely making progress!
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