Vox AC30 reissue ( Marshall made ) noise reported

[Arbutt]

Hi All
Hope we are all good
Well this is doing my head in. I local recording studio bought this used
It is clear to me that this amp has had this fault and an a repairs have been made to sort

When it arrived i can see all new JJ tubes about 2-3 months earlier. When you turn the volumes down its quite
but when you turn up the normally channel you get a noise ( to me it sounds like a ground issue is it 60Hz.
picture below 50mv 1ms on the anodes of PI
when you turn all the volumes down its quite with a slight background noise.
when you turn the normal up to about NO 2 is quite then the noise gets louder from about NO 4 until the max where is way to much and un usable
the other channels are better but not perfect
all of the vol pots have been changed prior to me using the Solder tag alpha types with short connection wires
removed the PI tube all quite, you can filter some of the noise out with tone cut and other controls
this amp has from stock all of the grounds going back to the same point, By the Power tx
This amp does show signs of someone trying to fix this same fault ( Maybe)

AC3093PA-Reissue.pdf


this is what ive done
1. Checked and changed all the tubes for new inc GZ34
2. Made the ac heater wires a little tighter twists
3. Removed the CT for heater and checked with 2x 100R to ground
4. all of the filter caps changed 10uf caps now 22uf and the 33uf caps now 47uf
5. R1 47k on PI lifted and grounded elsewhere
6. Tact on extra caps for the filter stage still the same
7. all input jack sockets are good


its doing my head in , but the only job i got to do really as the UK are in lockdown

This is way too much noise, I have another Dallas arbitter version in workshop waiting collection which is so quite in comparison

Any help would be grateful

[pdf64]

My experience with those Marshall design/built Voxs is that the pcb will have numerous failing solder joints.
As you’ve tried all the basic stuff to no avail, I suggest to reflow the entire board and reasses.
Another thing may be that those pot brackets may have been providing circuit 0V to chassis connections that the design relied on?

[Helmholtz]

Just to carify: As you're in the UK your line frequency is 50Hz (not 60Hz).
Your scope picture shows a repetition frequency of around 100Hz (time base is somewhat out of calibration). So the noise might be related to HT ripple. But it could also come from an outside EMI source like a fluorescent lamp or a dimmer. Try shielding the chassis.

Maybe a humdinger pot for the heater supply as used in vintage AC30s could help.

Are the input jacks shorting as they should?

[Jon Snell]

A 'humdinger' will have no effect due to the frequency of the hum/noise being 100HZ. It is HT hum.
Check exactly where it comes from, possibly remove V7 and does the hum reduce or increase when the Vol controls are at minimum?

[Helmholtz]

A 'humdinger' will have no effect due to the frequency of the hum/noise being 100HZ. It is HT hum.

Well, a humdinger can compensate noise spill-over from the HT winding to the heater winding caused by mutual capacitance. That noise contains ripple harmonics.

[Arbutt]

I ain't a trained expert guy like you guys. So I ain't sure on the freq. It's a sort of ticking sound. Not a hum

Pdf64. I have inspected the pcb all looks good might have to resolder the whole thing. The pots are the citec type and can't find them any where. Not made anymore. The alpha ones are working. I will look into the chassis mounts

Jon snell if I remove pi tube its quite. If I remove the normal channel tube its much better but still there.
I will have to check tomorrow as I'm done for the day now. I know I have lifted one end of the cap from and that reduced it, but still there.

Thanks for your help I will look at this tomorrow

[Jon Snell]

Does the tick coinside with the tremelo speed?
If it does, reduce the vib/trem level and check the decoupling on the HT supply C38.

[Arbutt]

Hi Jon. During the testing I fit the foot sw to remove the tremolo. Tremolo channel is the better channel. Norm and bright channel are the worst. Bloody thing is doing my head in.

How's business on the jurassic Coast? Lockdown affected you much

[pdf64]

... I have inspected the pcb all looks good...

Needs to be done using visual magnification and super bright lighting?

[Arbutt]

Ive got all that. Will do it tomorrow. Resolder the complete board. As all the grounds are too the same point... Is it worth splitting the grounds.??

[Jon Snell]

Hi Jon. During the testing I fit the foot sw to remove the tremolo. Tremolo channel is the better channel. Norm and bright channel are the worst. Bloody thing is doing my head in.

How's business on the jurassic Coast? Lockdown affected you much

Suits me fine, not too busy, the odd guitar set up and amp repair plus the usual Fender Blues Junior mod kits of mine to fit, a couple of 1962 AC30s just arrived from Kent that need new electrolytics any out of tollerance resistors changed and some TLC. I am in the process of a new build small amp, based loosely around a GU32 output valve, with ears, and a Hi Watt front end. Should be nice to hear a beam tetrode in push pull UL class AB acheiving around 14 Watts for a customer in Surrey and his studio.
I will post the build when I am happy with it.
The secret of a good valve amp, is the amount of iron in the transformers.
From the Ulanov factory. Lovely little bottles.
Keep safe out there!

[Arbutt]

Hello again
Ok. I have removed the board from the chassis and gone over every solder joint. Didn't find or see anything
The 3 x vol pots that have been replaced prior to me from chassis mount for Tag mounted, Ive checked the solder mounts and none are used for any ground reference etc, just there to hold the pots. I have at one time fitted screened wire from pot to board ,

https://vimeo.com/501195598

Link to a short video/sound clip. turning up and down
Recorded from a 5" speaker that i use on my bench, if used too the 2 x 12" in the cabinet or test much much louder
1st Tremolo channel
2nd Normal "" ""
3rd Bright "" ""


All new tubes inc GZ34 prior to me ( checked and changed again
Had been recapped at some point. C45-C46 have been replaced for 100Uf 450v
I have replaced all the 10uf's with 22uf's and the 32uf's with 47uf's
If i remove all pre tubes the amp is quite
If i only have the PI in i hear the background noise and can see noise on the signal line to the pi
Temp fitted 6 x 1n4007 diodes instead of Gz34 no change
Fitted extra caps between stages no change
if i remove V8 or and V7 the noise is there but reduced
320v on F2..... 5v ac on GZ34 heaters unloaded 3.2v loaded
Moved the grounds points around as they are all to the same point as standard
Disconnected heaters from pre amp
Temp fitted a different choke

Noise is present on all channels but usable on the normal channel A part from that the amp sounds great
I'm almost 100% sure that this fault has been attempted to be repaired before , There is a extra ground point fitted below the GZ34 used and whatever connected too it removed and with the caps being changed and when it arrived there was 2 x diodes on GZ34, it had been tubed by another firm 3 months earlier , Im guessing then sold to the music studio

many thanks any help is welcome

[Helmholtz]

Did you try a humdinger?
Your scoped signal looks EXACTLY like the picture of heater noise in Merlin's book, page 76 fig. 3.7.

[Arbutt]

I have tried 2 x 100r connected to heaters and other end to ground. With the CT on the tx lifted. Made zero difference. With the heaters wires disconnected from the pre amp tubes. Noise still be seen at pi. I will try on Monday. Just before I was told I needed to stop I found another post on this site. With the same issue.. Will look at that on Monday too

Thanks

[Helmholtz]

You might even try to connect the wiper of the humdinger trimpot to the EL84 cathodes for some heater elevation.
Of course the CT of the heater winding must be disconnected.