SLO-50 clone 100hz hum on OD channel?

[Spenzer Lee Horton]

Hi All,

I’ve built a clone of the SLO-50, I’ve bought the pcbs and chassis and followed Rob’s thread as best I can.
The only differences are the output txf is out of a 100watt amp, works well though and I’ve changed the tubes to EL34 and changed the 470R resistors to 1k 5W and the screen resistors from 2.2k to 5.6k.
I’ve also changed the fx loop resistors from 2.2k to 3.3k on cathode and added the 22uf cap to the other 2.2k cathode, changed the 1uf cap To 2.2uf and added two resistors to send jack, 750R to gnd and 1.2k as voltage divider.
I use a quad cortex with 4 cable method.
Getting a fair bit of 100hz hum.
I do have a scope and have taken a photo of it.
I will try taking out all the preamp tubes and finding exactly where it’s coming from but putting them back one by one.
I suspect it’s V2?

 

[Spenzer Lee Horton]

I’ve read some threads about the diodes possibly being the source of the 100hz noise problem?

 

[Thatbastarddon]

Welcome @Spenzer Lee Horton !!!
A member here, that may have some of the insight you seek, would possibly be @syscokid . IIRC he has played with some of the SLO circuits before. Perhaps he will have something to add soon.

 

[CoyotesGator]

A couple of questions before the smart people arrive:

Did it have the hum before changing the resistors and caps?

What does it do in a shielded enclosure?

 

[fitz]

I use a quad cortex with 4 cable method.
Getting a fair bit of 100hz hum.

Eliminate the peripherals before attempting to diagnose the amp.
Are you getting this hum with the guitar straight in?
Loop send to another amp to isolate the pre-amp.
Loop return from another amp to isolate the power section.

 

[Robb]

I'll be damned if I know anything about this

 

[Spenzer Lee Horton]

i will remove all fx and just test the amp for noise with guitar in input first.
if noise still there (which im pretty sure it is!!) i will take out all preamp tubes and put back from each on V4, v3, v2, and test.
i will detail some pictures and a diagram of what mod i have done for the fx loop (its a version off this forum which works apprently)
it could be coming from the filter cap board, but hopefully these tests will prove otherwise.

 

[Spenzer Lee Horton]

right, ive taken off the fx loop mod and put R24 2.2k cathode resistor back for V3B, taken off the 1.2k / 750R on the send jack but left the 2.2uf cap inplace of the c12 1uf cap and the 22uf (2 x10uf which measure at 25uf) on R27 2.2k.
i put a jack from send to return and tested amp, apart from some windy noises (which i assume is a 12AX7 preamp tube) there seems to be no 100hz hum? on either channel.
The EL34s i was using started to get a bit smelly, so i changed them for a pair of KT77 and readjusted the bias a little, all sounds good so far, now lets test the fx loop.
when my pedals and quad cortex is involced through the fx loop, 100hz hum is introduced.
I will now go through each pedal and see how each one sounds and see if its a PSU on them which is bring in the hum.

 

[Spenzer Lee Horton]

this is the fx loop mod that i had
is this a good mod for using quad cortex and a few pedals, guitar into wah wah, into MXR flanger, into Phase 90, into Tremolo pedal into MXR Dyna Comp into Quad Cortex.

 

[Spenzer Lee Horton]

I have ordered some UF5408 to replace the 1N5408, got no idea whether this will improve?

Also ordered some better filter caps, rubycons low ESR, should make a little bit of a difference.

after changing the EL34s to the KT77 i did notice on the OD channel turning up the gain i get squeals now about 10 o'clock on the gain, the further i move away from the amp it does improve a little bit its always really on the edge of squealing!!!

 

[syscokid]

First of all: Welcome to TTR…

when my pedals and quad cortex is involced through the fx loop, 100hz hum is introduced.

This only happens on the Overdrive channel?

Don’t know if the following applies, but have you tried the Quad Cortex’s Ground Lift feature in the menu’s I/O settings?

I don’t know jack-crap about modelers, but your described amp gremlins sounds very similar to what I experienced when I first tried my Fryette PS-2 attenuator with an amp: Nasty hum and squeal! Flipped the PS-2’s ground lift switch to resolve the issue!

 

[67plexi]

Looks like a wiring oscillation issue get out the chopsticks and move signal wires around
maybe Greg could post a photo of his SLO build and you could rewire your build it gives me PTSD looking at it

 

[Spenzer Lee Horton]

I did check if the hum was on the normal channel and it was but much lower level, will try the gnd lift

 

[Spenzer Lee Horton]

I have also spotted a few connections to grids which could use a screened cable, I’ll change those and get a chopstick in there and start tapping!!

 

[Spenzer Lee Horton]

not the progress i wanted to see but the Quad cortex gnd lift just makes it worse?
could the quad cortex PSU be damaged?

if not then it seems the 100hz is coming from somewhere else?
im going to double check the fx loop again and just plug in a patch lead and listen and test at all volumes and channels

update

when putting in patch lead in fx loop just have acceptable hiss, no hum, when i turn up OD channel master is squeals!!
so need to look at wiring with chop stick etc first, this is problem A, problem B is the fx loop.

 

[67plexi]

This may help lead dress on a real SLO 100

 

[Spenzer Lee Horton]

well, chopstick tapping does reveal some problems, master at 9am, gain at 1pm, tapping pin 7 which has 33k resistor from input results in squeal city!!!
each time i tap that pin it squeals?
it is a screened cable up unitl the 33k resistor, also tapping pin 6 and pin 1 on V1 i can hear the tapping being amplified, C2 / 22uf is also noisy when tapped.
V2 pin1 and pin 2 are noisy
im wondering whether its the output txf primaries that need swapping? but i remember this being a lower sound before?

voltages are as follows:
V1
pin1= 255v
pin3=1.87v
pin6=179.3v
pin8=1.3v

V2
pin1= 247.9v
pin3=1.76v
pin6=334v
pin8=3.81v

V3
pin1= 362v
pin3=193v
pin6=190v
pin8=1.39v

V4
pin1= 198.2v
pin3=198v
pin6=195.6v
pin8=1.66v

V5
pin1= 235.6v
pin3=41.7v
pin6=239.4v
pin8=41.5v

 

[Spenzer Lee Horton]

Jack input socket and fx loop socket should be isolated from chassis right?

also could this be the negative feedback? and everything needing either more NFB or less? pretty sure i have 39k in there at the moment

 

[syscokid]

not the progress i wanted to see but the Quad cortex gnd lift just makes it worse?

Not surprised. It was an easy process to check on though. You’re going to have to connect your Quad Cortex into another amp or audio source and verify that is not damaged.

if not then it seems the 100hz is coming from somewhere else?

I wonder if your 100Hz noise is related to AC, or is it DC? In the USA, 60Hz hum in an amp is mostly related to AC and the heater filament’s circuit. 120Hz amp hum is DC related, and it could be a filter cap issue as an example. A poor heater wiring layout could easily inject AC hum into the sensitive grid wires, with V1 being the most sensitive.

I have also spotted a few connections to grids which could use a screened cable, I’ll change those and get a chopstick in there and start tapping!!

When you say “screened cable”, I’m as(s)uming this is another term to describe “shielded cabling”, correct? And, only one end of the shielding is connected to a ground source while the other end of the shielding is not connected to anything, correct?

 

[Spenzer Lee Horton]

Yes sorry screened cable / shielded cable same thing.
Yes I’ll test the cortex with my other amp and see if I can hear anything, although I think it’s this fx loop and n the SLO-50 which is the problem but the squealing problem I believe it to be a separate issue probably something to do with the NFB