Noise on startup

[Snake5150]

Jtm 45 new build

Was fine on limiter, installed power tubes. Went off limiter. Bias at -45vdc

The amp after 10-15 seconds develops a significant constant hum and reaches 20mv across the cathode resistor before forcing shutdown.

I’m very confused. I’ve checked wiring, nfb, bias and socket wiring.

Only differences I’ve made from original is the cap layout attached. Any help would be awesome .

[Latole]

20 milivolts across the 1 ohm resistors on each EL34 ?
Please be more specific.

Schematic of your build will be more helpfull than a layout

Your mod is the issue IMO
Why don't you build like the layout ?

[Leevi]

A constant (loud?) hum indicates that there is some bad/broken contact in the circuit.
Try to isolate the problem to see if it comes from pre or power amp.
For instance short the treble pot wiper to the ground.


/Leevi

[Latole]

In my opinion, it's a mistake to modify a new construction without making sure that the original version works properly.
Especially if you're not a “pro” in the field.

Then you can modify it.

[Snake5150]

20 milivolts across the 1 ohm resistors on each EL34 ?
Please be more specific.

Schematic of your build will be more helpfull than a layout

Your mod is the issue IMO
Why don't you build like the layout ?

I was told I can be problematic to have the cap after the standby switch. At this point I’ll prolly go back to original.

[Latole]

I was told I can be problematic to have the cap after the standby switch. At this point I’ll prolly go back to original.

You are right .
You may have a mistake somewhere. On your mod or elsewhere ?

Build like the original circuit, check all you wiring.
Make the amp playing first.

[Snake5150]

I was told I can be problematic to have the cap after the standby switch. At this point I’ll prolly go back to original.

You are right .
You may have a mistake somewhere. On your mod or elsewhere ?

Build like the original circuit, check all you wiring.
Make the amp playing first.

I’m hoping it’s something to do with the fact my standby switch is not connected to the cap directly but to the rectifier dc out.

[Willabe]

I’m hoping it’s something to do with the fact my standby switch is not connected to the cap directly but to the rectifier dc out.

This makes no sense. 

It should be; rectifier > 1st B+ filter cap > stand by switch > 2nd B+ filter cap.......

[Leevi]

https://www.valvewizard.co.uk/standby.html


/Leevi

[Snake5150]

I’m hoping it’s something to do with the fact my standby switch is not connected to the cap directly but to the rectifier dc out.

This makes no sense. 

It should be; rectifier > 1st B+ filter cap > stand by switch > 2nd B+ filter cap.......

Which is how the original basically layout has it. So basically all the internet advice led me astray from common sense.

The og layout shows rec to standby- out of standby to cap , otct, and choke , this then feeds the other cap correct?

[Willabe]

The og layout shows rec to standby- out of standby to cap , otct, and choke, this then feeds the other cap correct?

No.

Not rectifier to stand by.

It should be; rectifier > 1st B+ filter cap > stand by switch > after stand by switch take B+ to feed OT CT > choke > 2nd B+ filter cap.......

[Snake5150]

The og layout shows rec to standby- out of standby to cap , otct, and choke, this then feeds the other cap correct?

No.

Not rectifier to stand by.

It should be; rectifier > 1st B+ filter cap > stand by switch > after stand by switch take B+ to feed OT CT > choke > 2nd B+ filter cap.......

Like this?

[mresistor]

Like this

[Snake5150]

Like this

Totally on me but can’t really decipher what the schematic is saying in relationship to my situation. There’s multiple parts labeled on the schematic that my rookie ness has no idea what they are.

I’m sure it’s annoying to some but I’m a super novice and am just trying to finish this thing.


I understand how the circuit chains together but when b+ goes to the 2nd cap, is this were otct and choke as well as the power tube pin 6 feeds come from?

[Willabe]

In the schematic mresistor posted, see where it shows TR2? That's the choke, just to the right, before the choke, just above, see +440dcv, that goes to the OT CT, and just to the left of the choke, where it shows B, there's the 2nd B+ filter cap and that's the screen grid B+ feed.

You really need to be able to read a basic schematic. And Fenders are about as easy as they get to read. And with that you really need to understand how a basic tube amp power supply works.

What this means is you need to stop taking short cuts and invest the time it takes to read up on these things if you want to be able to build and/or repair a tube amp.

I don't see how we can help you until you do some studying. You don't understand what we're saying and you don't understand the schematic. 

Start with this, here's the link to Rob Robinette, much of what you need to read is in here;

https://robrobinette.com/Amp_Stuff.htm

Scroll down on any page in this forum and you'll see links that our host Doug posted that have lots of great information. Great information in here too;

https://el34world.com/schematics.htm

And this on servicing Fender amps is in the above link;

https://el34world.com/schematics.htm#Servicing%20fender%20amps

And this is the link to Merlin's web page;

https://www.valvewizard.co.uk/

[Willabe]

Here's a full Fender schematic with a solid state diode rectifier. Maybe you can understand this power supply with stand by switch a little better.

The 1st filter caps, right after the rectifier and before the stand by switch, are stacked to get a higher voltage rating. Depending on your PT, you might only need a single cap there.

https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_ab763_schem.pdf

Or, if you have a tube rectifier;

https://el34world.com/charts/Schematics/files/Fender/Fender_super_reverb_ab763_schem.pdf

[Snake5150]

Here's a full Fender schematic with a solid state diode rectifier. Maybe you can understand this power supply with stand by switch a little better.

The 1st filter caps, right after the rectifier and before the stand by switch, are stacked to get a higher voltage rating. Depending on your PT, you might only need a single cap there.

https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_ab763_schem.pdf

I’ll deff work on my schematic reading.

However after reading the valve wizard site

I’m hella dumb. It’s all right there


That’s also cool cuz I have a 74 super reverb never even thought about looking at a schematic for it