Hum on blackface deluxe that goes when normal channel Vol 2/3 up???

[robertamp]

Hi,im currently building a Blackface Deluxe AB763 with Mojotone chassis/board/transformers etc.All done but have a nasty hum at turn on which dissapears when the Normal channel volume is turned up to a point approx 2/3 on?
This build has a buss bar ground that picks up the mains earth at the IEC inlet and with supports (off the chassis),makes its way through to the input sockets where it grounds to the chassis only here.All ground points are connected to the ground buss as it passes them.Any ideas as to why this humm is there...Ground loop??

[MWJB]

Where does your preamp filter cap ground? Seperate the grounds in the cap pan so that main & screen supplies ground at the PT, preamp caps ground to buss bar/input jack.

Ground the IEC socket to it's own PT bolt, not with the rest of the grounds.

[robertamp]

Hi MWJB,and thanks for replying.There is no cap pan as the psu caps are situated in the amp itself.The two main smoothing caps by the PT and the PI cap nearby to these,then the preamp cap down the board directly on the eyelet where the dropping resistor is,to the ground buss.I will put some photos of the circuit on shortly and try grounding the earth IEC point to the PT bolt.

[robertamp]

Hi,here are some pictures of the circuit build and earth buss connections.I thought this would work well,but.....

[guitician]

You may be amplifying an out-of-phase hum that is canceling out what is at the PI/OT. Those 68uf 400v caps seem to have very short leads. You may have damaged them soldering them.

[robertamp]

Hi guitician,thanks for replying.Those 68uf 400v caps replaced a 30uf 500v as the standby voltage was over 500v and i didnt want to stress the cap.The humm was there before i changed to the 68uf's.This hum is non existent with all pre-amp tubes exept the PI in so i think it must come from there.I shifted the input IEC earth to the chassis and this made no difference.Ho hum im racking my grey cells to trace the source of it!

[robertamp]

Ah,and finally the problem,it is solved!It WAS the centre tap ground of the HT secondary which on the ground buss system i used needed to be soldered directly with the first PSU smoothing cap at the same solder point.(It was soldered to a lug on the PT bolt as would normally be in fenders).Thank you MWJB and guitician for replying and incidentally this way of grounding is very effective.I saw it once in a custom shop Pro amp where the buss picked up the IEC inlet earth and ran all the way around the PT & Bias and parallel to the eyelet board grounded to the chassis only near the input skts and then went through a chassis hole to the pre-amp part of the cap can.(The main and screen grid cap earths wired back to the buss at the PT soldered with the CT of the HT secs.).I only run the buss to the input skts and solder via a brass stand-off bolted to the chassis.With this it is possible to solder in the PSU caps point to point directly from dropping resistor to earth buss.(assuming the caps are not to large!).All the other earth connections connect direct off board to the passing buss.Anyhoo seems to work as long as you connect the main PSU caps in the right place.:Ps;buss wire is 16swg solid tinned copper.

[booj]

I dig the pics. NIce example of buss bar groounding!

[booj]

uuhh. you might check the solder connection to the ground buss on the big filter cap with the gold letters rubbed off. Looks like the wire is looped aroound ok, but I don't see a solder flow between the two wires.

[robertamp]

Hi booj,thanks for that observation,during my attempts to solve the humm prob i resoldered a few ground joints including that one and also took the under board HT wiring to the top of the board like old fenders(though i probably didnt need to),however they got it right and i love the old eyelet board spaghetti wired tweed,blackface and silverface(when blackface modded)tone monsters!
I've now a few small probs to sort out namely Vibrato thump/tick noise and the reverb is a little noisy but nothing to serious.This little blackface mojotone kit amp has a professionaly built pine cabinet and a lovely Weber 12" Alnico speaker to compliment it,i cant wait.Any ideas on effective Vibrato and Reverb noise quelling would be appreciated.Thanks.

[MWJB]

You have grounded the amp much like an old Selmer...many of which hum until the volumes are turned up! Nevertheless, glad you sorted it.

[booj]

Trem tick solutions can be found I'm sure on an archive thread. I've tried a .02 cap across the 10 meg. I've read of using a .01 from where the 10 meg and the LDR assy meet to ground also.

[robertamp]

Hi booj,thanks for that i tried the .01uf 600v polyester from 10M & neon to ground and it has got rid of the ticking but at certain intensity settings the pulse of the vibrato as a thump is audible but overall accepable.I'm going to try and pull the gain of the 12at7 Reverb tube down by liftng the cathode bypass cap as the reverb is full on and too much at 1 even!Have tried the amp so far and it sounds fantastic,just need to burn it in for a couple of hours in the garage to check for any vibration problems etc.

[booj]

Trem thump is going to be audible at times, but once you reach actual playing levels it shouldn't be a problem. The ticking problem is also one of those things that becomes buried in the mix once you crank it.

[robertamp]

Hi booj,the vibrato is perfectly acceptable as you say now with the .01uf cap,and i finally substituted a 1uf 63v electrolytic instaed of the 22uf cathode bypass cap on the reverb driver 12AT7.Works a treat to tame the reverb intensity in the first part of the reverb volume control.Today is test day for the amp and then it will be used live by a blues player.Thanks for your help and i'm found the whole building thing so rewarding i'm going to build a blackface super reverb head soon!