Perfect, thank you. Should adding the additional filter cap and choke have any negative effects on an amplifier? Or other areas that I would need to compensate for in terms of perhaps, lower B+? (still new to a lot of this)
Nope the filtering will just stiffen up the power supply a bit, and the choke is an equivalent replacement to the resistor for the voltage drop but you also get the hum rejection of the choke added to the B+ supply.
Win win The B+ might go up a touch just due to that added filtering giving a bit more available storage in that capacitor, less 'dips' on heavy current usage in the amp.
~Phil
Thank you! I ordered a choke and additional 100uF and they will be here Thursday. I'll provide photos and status as to how everything progresses!
sluckey wrote: ↑Tue Mar 03, 2020 2:59 pm
Refer to the service manual you posted.
1. Break the connection between the junction of the rectifier diodes and point B+.
2. Install a 250Ω/10W resistor between the junction of the diodes an point B+.
3. Connect a 100µF cap between the junction of the diodes and ground.
For even better hum filtering, use a choke rather than a 250Ω resistor.
I very well could have done this wrong. Pics below are just initial hookup to make sure its getting rid of the hum (its not).
I disconnected the value from the original 100uF to the rectifier diodes. Connected the postive end of the new 100uF capacitor in its place and connected the negative end to the negative end of the original 100uF. Then I placed the choke between the two positive ends of the 100uF's. No change in hum. moving the choke around also doesn't change anything. Any ideas?
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xtian wrote: ↑Fri Mar 06, 2020 10:16 pm
Wow. OK, let's back up to the top. If you pull the EL84 power tube out of the circuit, does the amp hum?
Yes it does!
Damn! This is a production amp--I thought they would have worked out the basics!
The relative position of the PT versus the OT is all about avoiding the hum induced by the PT into the OT. Can you remove the two screws holding the OT and then rotate the OT a bit? Listen to the hum as you do this, and see if you can position the OT so that the hum is minimized.
I build and repair tube amps. http://amps.monkeymatic.com
xtian wrote: ↑Fri Mar 06, 2020 10:16 pm
Wow. OK, let's back up to the top. If you pull the EL84 power tube out of the circuit, does the amp hum?
Yes it does!
Damn! This is a production amp--I thought they would have worked out the basics!
The relative position of the PT versus the OT is all about avoiding the hum induced by the PT into the OT. Can you remove the two screws holding the OT and then rotate the OT a bit? Listen to the hum as you do this, and see if you can position the OT so that the hum is minimized.
I left the choke and cap in... but also move the OT over about 4 inches and the hum of SIGNIFICANTLY lowered. Any other tips on getting rid of the last little bit?
peckjed wrote: ↑Sat Mar 07, 2020 1:41 amI left the choke and cap in... but also move the OT over about 4 inches and the hum of SIGNIFICANTLY lowered. Any other tips on getting rid of the last little bit?
Moving the OT farther away from the PT will diminish the EM coupling, but also ROTATING the OT will have a big effect. Try it out.
I build and repair tube amps. http://amps.monkeymatic.com
peckjed wrote: ↑Sat Mar 07, 2020 1:41 amI left the choke and cap in... but also move the OT over about 4 inches and the hum of SIGNIFICANTLY lowered. Any other tips on getting rid of the last little bit?
Moving the OT farther away from the PT will diminish the EM coupling, but also ROTATING the OT will have a big effect. Try it out.
When you say rotate do you mean 180 degrees or just swivel it a few degrees on direction or the other? Swiveling didn’t really do much. But at this point if this amp is on and I’m standing next to it i can’t hear the hum. I need to put my ear to the speaker... which is probably normal.
peckjed wrote: ↑Sat Mar 07, 2020 1:41 amI left the choke and cap in... but also move the OT over about 4 inches and the hum of SIGNIFICANTLY lowered. Any other tips on getting rid of the last little bit?
Moving the OT farther away from the PT will diminish the EM coupling, but also ROTATING the OT will have a big effect. Try it out.
When you say rotate do you mean 180 degrees or just swivel it a few degrees on direction or the other? Swiveling didn’t really do much. But at this point if this amp is on and I’m standing next to it i can’t hear the hum. I need to put my ear to the speaker... which is probably normal.
That sounds like the best you can expect.
I build and repair tube amps. http://amps.monkeymatic.com
Just as an update - I moved the output transformer further away than before. Essentially it’s at the other end of the chassis now (I needed to extend a few wires). The choke is pretty close to the power transformer and oriented in the same direction as the OT. Moving the OT further helped a bit more. The hum is pretty quiet.
My question: anything else that may mitigate more hum? Would a higher quality OT eliminate more hum? Does proximity/orientation of the choke impact hum?
As someone who loves tinkering I can’t help myself!
I can't see in your photos where the CT tap is fused, and where that wire attaches to ground. Can you find it and get a closeup shot of where this 'fuse wire' attaches?
chief mushroom cloud wrote: ↑Mon Mar 16, 2020 4:29 am
I can't see in your photos where the CT tap is fused, and where that wire attaches to ground. Can you find it and get a closeup shot of where this 'fuse wire' attaches?
The fuse is attaching at the 2 100R resistors all the way to the right on the turret board (blue wires). Its a little easier to see in the un-modified picture I added as well. Based on the schematic it looks like thats the heater ground and goes to the PCB for the footswitch...
The B+ might go up a touch just due to that added filtering giving a bit more available storage in that capacitor, less 'dips' on heavy current usage in the amp.
