loud , low frequency hum in AB763 clone

[chris shaw]

I have just done my first ever build and my biggest problem is a Moog like low frequency hum when the amp is turned up to about 2 and has no signal input. Below 2 I have an less loud hum which is probably an earth issue
Wth a signal going in the signal comes through v. loud and clear.
. Have tried various ground points for Caps and inputs etc but it makes no difference
I have tried detaching the Preamp stage from the PI and the hum remains. Presumably that means it is in the PI or output stage
All preamp plate and screen voltages are fine, as are plate currents
I dont know about output valve currents as I dont know how to measure them !
Can anybody give me some help please
I am running out of (beginners) ideas

Thanks
Chris

[loudthud]

If the volume controls kill the hum, it's coming from the preamp, not the phase inverter or power amp.

If the hum fades out for a couple of seconds when you put the amp into standby mode, it's probably not a grounding issue. Does the power transformer have a center tap for the heater supply (usually a green wire with a yellow stripe) or is there a virtual centertap consisting of a 100 ohm resistor from each side of the heater supply to ground? If not, the easiest thing to do is install a pair of 100 ohm 1/2W resistors on one of the power tube sockets. Connect one from pin 2 to pin 8 and the other form pin 7 to pin 8.

[chris shaw]

Thanks loudthud

Thanks for the help, it really is appreciated. I am using a hammond transformer and the centre tap is red/ yellow and is grounded on the transformer , along with first 3 filter cap ground,s and bias ground , (preamp and PI ground going to input ground point)
How does it manage to make this noise when I've disconnected the preamp so the only things going thro power amp tubes is the PI (and the heater supplies)-- (sorry if this is a silly question)

Am I missing a point somewhere?, If so am happy to go and but the resistors and install tonight (Its 9 am here and I am at work!)

[chris shaw]

partial success

thanks loudthud, the 1 meg input resistor wasnt connected correctly. This has stopped the oscillation at input (how did it get into power stage when pre amp had been disconnected?!)
nonetheless I am left with the following:-
a hum/ buzzing tone which comes on when amp is turned on(even with volume down )is quite loud , can get louder as amp volume is turned up BUT if i put a cd player through it , even with the cd player internal volume set to zero the buzzing will reduce as I turn up the volume of the built amp (not the CD) to 1 or 2 ,
if I pull the cd player jack plug out the buzz comes immediately back even if the volume of the amp is set at a level which, with the cd player connected has previously reduced the buzz level to very low
I am assuming this is a grounding issue ?!!!,

[loudthud]

There are several amps that Fender designated AB763. Is yours a Deluxe Reverb, a Twin or what?

There is probably more than one thing causing you amp to hum. Based on your description I have tried to help you discover and correct the cause. Go back to post #2, perform the test I ask you to do and report the results. Do you have a DVM? Look at this thread: http://music-electronics-forum.com/t12667/

You might try the checks I suggested in post #14. That will tell us if the input jacks are wired correctly and the jacks are making contact on the switching part.

Also measure the resistance to ground (power off) of each side of the heater supply, pins 2 and 7 on the power tubes.

[chris shaw]

Buzzing ab763

hi Loudthud
Thanks for the help
will do the check as per post 14
Ive only done single channel, single input but the circuit is based on BF deluxe - its on the Weber site
will send the link shortly. The heater wiring is quite thick - I wonder about re-wiring it all , but having said that the sound is good when powered up so maybe no need?
thanks
Chris

[chris shaw]

no centre tap from heater

HI Loudthud
Switching to standby doesnt result in any drop in hum volume
HOWEVER the following link is the hammond transformer;
http://www.hammondmfg.com/pdf/EDB290BEX.pdf

and the following link is the circuit layout i used (excluding channel 1 and vibrato in channel 2 )
https://taweber.powweb.com/store/6a20_layout.jpg
as you can see the centre tap is the 300v , not the heater supply,-- neither have i installed a 100ohm centre tap connection on one of the power tubes . My understanding was if I kept pin 4/5 and 9 wiring seperate (different color wiring) it would be hum cancelling. A little knowledge eh?
I will get some resistors and do the deed tomorrow, , say a little prayer and report back
will also do heater resistance check tonight and also what is a DVM?

[chris shaw]

input resistances

loudhud
with single input as per my version the resistances with and without guitar cable all make sense as per your posting although pin 7 varies depending on vol control - with it completely closed it goes straight to ground (as should be ?)
am optimistic re the centre tap installation for heater supply so will report back after installation
presumably dvm is digital voltmeter - yes I have one! am slowly learning to use it !

[loudthud]

That little trick with the standby switch tells me that the hum is coming from the heater supply or an issue with a signal wire being too close to the heater wiring (or a bad tube). When you switch the B+ off, all the hum from grounding problems should stop immediately. It's a pretty neat test because it doesn't require any test equipment except your ears.

Thanks for the link to that transformer. I've never seen a Hammond with a single 240V 50Hz primary. You are right, there is no center tap on the 6.3V winding. The two 100 ohm resisitors will get rid of most of the low Moog note hum. There still may be a 100Hz buzz that comes from grounding issues. The 4-5 and 9 wiring cancels hum but it depends on the 6.3V wires to be balanced so that they cancel. The 100 ohm resistors force the green wires to balance. On some amps there is a pot called a "Hum Balance" adjustment. It takes the place of the two 100 ohm resistors and allows you dial in the exact amount of balance it takes to cancel the hum.

DVM is short for Digital Volt Meter. They are really cheap these days, less than the price of a 12AX7. Anyone who builds an amp should have one. If you measure the resistance of the heater wires to ground you should get an open circuit as the amp is now or 50 ohms when you install the 100 ohm resistors. If you measure a low resistance on both sides to ground, there is a short somewhere that needs to be found.

Re: Resistance checks, The input to the first tube could be on pin 2 or 7 depending on your layout. I was trying to make the instructions generic for those instances where things were a little different like a tweed Bassman. In your amp the side that doesn't go to the input jacks comes from the volume control so you are ok.

[chris shaw]

thank you

loudthud
I dont know how big texan bass players react to big kisses but if you were here I would give you one!!! The centre tap on the heater supply was all that was needed
the hum is gone , the amp is biased at about 22ma (the two tubes are a couple of m.a. different but I dont think that is a big deal (is it?)
the amp sound sweeet even with the weber speaker on the bench !
next job is to fix grille cloth , load with speaker, fasten reverb tank and put chassis into cab and off we go

I started this over 6 months ago knowing nothing. Now i reckon I can understand a lot of webers books and I;ve made a few friends into the bargain. I work in a well paid professional role as a director of health but it could never be as rewarding as doing this !!
I am totally in your debt and you've made one brit very happy
what to build next (I am a jazz player with a love of sweet joe pass and lambchop-- any suggestions?)
thanks again
peace
Chris

[loudthud]

Chris, just curious what country you are in where you need a 240V 50Hz transformer. Can't be US or Canada. If the Deluxe meets your needs for volume you might like a Champ or Princeton for practice or if you ever do larger rooms, a Super or Twin.

[chris shaw]

sheffield - steel city

hi loudthud
Im uk based- sheffield - steel city- a steel/ mining city which thanks to Mrs Thatcher in the 80's now has neither!

Dude- can you advise me with a couple of minor niggles
I need help solving
the first is a 'farting' noise under the bottom notes(this appeared after about 1/2 hour of first playing) -
the noise appears only between e to about c . when I play a bottom E it runs underneath for a couple of seconds , whereas by the time I get to c it last less than a second. It isnt vibration, I can hear it in the speaker. Ive tried varying bias but no help, happens even at low volume, never on any notes above the low c of an electric guitar-- any ideas?
the second is that the reverb starts to 'screech/ high pitched whistle if I take it above about 25% of full-- almost like high feedback
I have fastened the reverb can onto the bottom of my home made sapele cabinet -- no isolation sponge or anything -- should i try and isolate it or is it something more 'electrical ?
I know youll solve it!

[loudthud]

Farting issue: You must try the amp with a different speaker AND try the speaker with a different amp. That's the only way to say for sure what is at fault.

Reverb issue: Try isolating the tank in some way from the speaker. Most people use a bag with some kind of foam inside. Simply turning the tank upside down works in some cases.