60 Cycle Hum in '74 50watt JMP

[maxwedge572]

New here and I guess most forum newbys start with a problem??

I'm having a hum problem with my old 74 Marshall and I hope someone can lead me in the right direction. Don't ask when the problem started because I don't know. I use my other amps more often because this one is just so friggin loud. I'm using a 1960a cab btw and the hum is quite obnoxious into the 4-12's.

It has no other issues other than the hum and the only mod in the circuit is for the AC input, and an added AC extension socket and a rotary impedance selector. It used to have a master volume mod but I took it out.

Situation:
It has a 60 cycle hum at idle, both volumes turned all the away down and no guitar cord plugged in. It's fairly loud and annoying but sounds fine otherwise. The hum is as loud as normal TV watching.
No change pulling V1
Pulling V2 lessens the hum but it is still there.
pulling V3 lessens it even more but it is still there. Now this is really quiet and if you weren't paying attention you'd say 'what hum?', but it's there if you listen closely.

Putting into standby I still hear the hum and if I pull the power tubes, it is the same. This hum level is the same as pulling V3.
So I'm thinking that this is being amplified when all the tubes are in.

I've tried different tubes in all positions including another set of el34s.
I've tried lead dress adjustments of the wiring.
I've cleaned all the pots.
It has new F&T can caps (10 months ago).

I'm thinking at this point that the power supply is causing this or possibly the heater circuit.
It's plugged in to an AC socket that has it's own dedicated 6 foot ground stake.

Here's a global pic of the chassis, if any others are needed let me know.
PS: The transformers are all original and used 6550s but has been re-biased for el-34s.

[Mars Hall]

How long ago was the ohm selector changed? Is that an AC receptacle on the back of the amp?

[maxwedge572]

How long ago was the ohm selector changed? Is that an AC receptacle on the back of the amp?

Both have been on it since I got the amp in the early 80's. I've gone over the selector and it is clearly wired correctly but I may replace it anyway with a new Marshall 'modern style' selector.
Yes that's the AC receptacle and the earth grounds to the chassis through the receptacle body. I just rewired that yesterday, it's all good. It uses a plain old extension cord for power.

I'm thinking of lifting the center tap for the heaters and attaching it to a voltage divider tapped off of the pre B+.
I've checked all the grounds and they are all good. The last thing I want to do is replace transformer's but if I have to I will. I have a 61 Bandmaster that is dead quiet through the same cab.

Thanks for the reply.

[Mars Hall]

If the problem has coincided with the filter cap change, it is possible one of the new filter caps are bad. Those electrolytic caps use a chemical reaction much like a battery does. It's not entirely impossible that you got a dud. Other than that your symptoms sound like a bad tube, which you've ruled out. Hope it's not a Tranny.

[maxwedge572]

I've measured 1.7v ac on the 1st can and I bet that this is somewhat normal. As a test I jumper-ed another F&T 47uf axial cap to the 1st cap and although the ripple was reduced the hum was still there. I just don't recall when this problem started happening but most likely creeped up on me, lol.

I'm going to try that heater mod in a little bit.

Thanks again,
Scott

[maxwedge572]

Its Fixed!

Lifted the heater center tap from ground and referenced it at 23v.
Took the B+ for V1 plates from the the last can cap to a 470k resistor. On the other side of the 470k resistor, the heater center tap, a 82k resistor and a 100uf/100v cap are attached. The 82k resistor and cap go to ground. No more hum!
PS: I tried moving the heater wires around before but nothing worked.

Hard to explain in words, a picture is much better.



I know that the logo is incorrect. It never had one when I got it and I didn't know any better back in the 80's.

[beaulieu]

I have that same problem with a 73 all original 50 watt lead. Was told to lift the brown wire and then use a res to ground.Seems to work for you. I should try it!!

[neikeel]

Interesting board wiring to say the least

[maxwedge572]

Interesting board wiring to say the least

Why? This is actually a very common mod, especially with cathode biased amps where the heater center tap can be tied in with the cathode of the power tube.
You need a 20-50v source for this mod so with a fixed bias amp you're going to need to 'create' this voltage source by adding a circuit.
I was posting here to see if other people had had this problem with this amp, but I hadn't tried this mod yet.

[maxwedge572]

I have that same problem with a 73 all original 50 watt lead. Was told to lift the brown wire and then use a res to ground.Seems to work for you. I should try it!!

Yes, it's really easy. Just loosen the can for the preamp and pull it up. Snake a wire under the board and solder it to the can and then use a terminal tree bolted to the power trans bolt to attach the resistors and cap. In my pic, the red wire is the one I added coming from the pre can and the brown is the center tap from the transformer. It's all grounded to the 2nd can ground.

[neikeel]

Interesting board wiring to say the least

Why? This is actually a very common mod, especially with cathode biased amps where the heater center tap can be tied in with the cathode of the power tube.
You need a 20-50v source for this mod so with a fixed bias amp you're going to need to 'create' this voltage source by adding a circuit.
I was posting here to see if other people had had this problem with this amp, but I hadn't tried this mod yet.

I was not referring to the mod. I was referring tho the sloppy board installation with the wires flying in mid air coming onto the board with excessive length, not looped through the board holes and laid out in neat fashion to avoid parasitics, wires having induced hum through poor layout etc, etc.

The board has obviously been out and reinstalled in a very lazy amateurish fashion and I am not surprised that it is noisy and needs modding to cover up errors.

Sorry if it seems harsh but the truth sometimes is

[demonufo]

Well, somebody has installed small turrets all over the board, which will make it a little more secure, but granted, the lead dress does leave a bit to be desired. (okay, quite a lot in fact )

[maxwedge572]

Interesting board wiring to say the least

Why? This is actually a very common mod, especially with cathode biased amps where the heater center tap can be tied in with the cathode of the power tube.
You need a 20-50v source for this mod so with a fixed bias amp you're going to need to 'create' this voltage source by adding a circuit.
I was posting here to see if other people had had this problem with this amp, but I hadn't tried this mod yet.

I was not referring to the mod. I was referring tho the sloppy board installation with the wires flying in mid air coming onto the board with excessive length, not looped through the board holes and laid out in neat fashion to avoid parasitics, wires having induced hum through poor layout etc, etc.

The board has obviously been out and reinstalled in a very lazy amateurish fashion and I am not surprised that it is noisy and needs modding to cover up errors.

Sorry if it seems harsh but the truth sometimes is

Well, look at this picture here and see how that the red dye on the board mounting posts have not been torn up. How would someone install these supposed turrets, which are in fact not turrets but snap on terminals, without removing the board and damaging the dye? I have some pictures of one other marshall that has the same type of terminals and the wires don't run through the holes. I've had this amp since about 1980 and if anything major was done to it, like you say, then it was done before I bought it. I'm fairly certain it's a 1974. The serial # is 5024F

[maxwedge572]

This is supposedly a 73 JMP 50 watt.
It is not mine but you can see how the wiring is done like mine and it has the same terminals.
So the same guy modded this one too.

[neikeel]

This is supposedly a 73 JMP 50 watt.
It is not mine but you can see how the wiring is done like mine and it has the same terminals.
So the same guy modded this one too.

Maybe as some of the wire insulation is melted with an iron and there are blobby bits of solder