My 50 watt #183 with footswitchable FET build.

[martin manning]

I checked the voltage on the send jack with the master up, came out to about ~3.3VAC. Seems small, but would this amount be cause for concern...

There could be a lot more than that with preamp volume up, PAB on, drive level up. Fully driving the last stage of the preamp might produce as much as 100V peak.

[Matt J]

I checked the voltage on the send jack with the master up, came out to about ~3.3VAC. Seems small, but would this amount be cause for concern...

There could be a lot more than that with preamp volume up, PAB on, drive level up. Fully driving the last stage of the preamp might produce as much as 100V peak.

Thanks, Martin! I'll make sure to test for that this evening!

- Matt J.

[Matt J]

I've just finished doing some revisions to the footswitch and a thought occurred to me, could I have forgotten to use an isolated jack at some point in the amp?

Currently there are no isolated jacks in the amp, they are all Switchcraft brands with toothed washers to help them adhere to the chassis better. Are there any points in the amp where I should considered isolating the jack? I'm wondering if they may be responsible for the hum that gets louder when I turn the Master Volume up.

- Matt J.

[norburybrook]

I've just finished doing some revisions to the footswitch and a thought occurred to me, could I have forgotten to use an isolated jack at some point in the amp?

Currently there are no isolated jacks in the amp, they are all Switchcraft brands with toothed washers to help them adhere to the chassis better. Are there any points in the amp where I should considered isolating the jack? I'm wondering if they may be responsible for the hum that gets louder when I turn the Master Volume up.

- Matt J.

Matt,

I've never used any isolated jacks in any of my builds and not had any hum issues.

So I doubt it's that. Make sure your output jacks are grounded well. Heater center taps or resistors. They're usually quiet amps if you follow the layout grounding scheme.

M

Sent from my Nexus 6 using Tapatalk

[Matt J]

I've just finished doing some revisions to the footswitch and a thought occurred to me, could I have forgotten to use an isolated jack at some point in the amp?

Currently there are no isolated jacks in the amp, they are all Switchcraft brands with toothed washers to help them adhere to the chassis better. Are there any points in the amp where I should considered isolating the jack? I'm wondering if they may be responsible for the hum that gets louder when I turn the Master Volume up.

- Matt J.

Matt,

I've never used any isolated jacks in any of my builds and not had any hum issues.

So I doubt it's that. Make sure your output jacks are grounded well. Heater center taps or resistors. They're usually quiet amps if you follow the layout grounding scheme.

M

Sent from my Nexus 6 using Tapatalk

I'll double check the output jacks when I get back home. I'm using only one input jack to the guitar, a Switchcraft 12a. The FET jack is a dummy that is not wired to anything. With the input jack, I did not run a grounding wire from the sleeve of the jack to the preamp ground, like you see in many Dumble layouts. I was afraid it would create a ground loop since the jack is already grounded to the chassis. There's no powder coating, just bare metal, at that point.

The heaters use a center tap of two 150 ohm resistors off of the heaters to ground. The second set of 6.3vAC leads that feed the relay board do not have a center tap and there's no indication they need one. I'm guessing they work through the ground that comes from the relay board. I grounded every point as the layout recommended.
Here's how I set out the ground points, you can see some of them in the pictures posted above:
- IEC input has a separate ground.
- PT and B+1 electrolytic caps share a ground lug near the PT.
- B+2 electrolytic is grounded to the same point the center tap resistors and cathode 1 ohm resistors ground, along with bias rectifier caps and the 22k bias resistor. This point is also connected to the common ground for your DMM connect and is between the power tubes.
- Relay rectifier, and the ground wire to the footswitch are grounded at a point near the relay rectifier board.
- Negative feedback and phase inverter section are grounded to the return jack of the effects loop.
- Preamp to the preamp ground, along with the B+3, B+4 and B+5 electrolytic capacitors, relay board ground, and bus wire.

- Matt J.

[Matt J]

In case I've wired the send/return jacks incorrectly, here is a drawing of how I wired them, and a close up view of the actual parts in question.
- Matt J.

Send-Return close up.jpg

Send-Return wiring.jpg

[amplifiednation]

If the hum increases with the master volume then it’s something in the preamp. So it would be before those jacks (they are correct). Does it hum on the clean channel? If so then you can eliminate the OD circuit. Does it hum without a guitar?

You don’t need isolated jacks.

Did you try poking around with a chopstick yet? Tug and push on literally everything with the amp on and see if there is any difference. I’ll go back and check out your lead dress but looks like you did a nice job!

[Matt J]

If the hum increases with the master volume then it’s something in the preamp. So it would be before those jacks (they are correct). Does it hum on the clean channel? If so then you can eliminate the OD circuit. Does it hum without a guitar?

You don’t need isolated jacks.

Did you try poking around with a chopstick yet? Tug and push on literally everything with the amp on and see if there is any difference. I’ll go back and check out your lead dress but looks like you did a nice job!

Thanks Taylor!

It hums on the clean channel too, and without a guitar plugged in.

I've poked around some with a chopstick. You can certainly hear tapping sounds though the amp when I touch something in it, especially on V1, but there's no major change by moving anything.

I managed to record two audio clips with my phone to give you an example. It's not the greatest quality and I'm certainly no expert player. I just play some chords and noodle around a bit. First clip is the clean channel, second the OD. My guitar is a Fender Telecaster I've converted into an Esquire with a single Danny Gatton humbucking pickup in the bridge. All the amps controls are at noon, except the OD drive, which is at 6, and the OD level, that is at 4. I start by rolling up the guitar's volume pot, then you can hear me playing the master volume control some.

If this type of hum sounds familiar to anyone feel free to chime in!

- Matt J.

Clean[1].m4a

Dirty[1].m4a

[norburybrook]

If the hum increases with the master volume then it’s something in the preamp. So it would be before those jacks (they are correct). Does it hum on the clean channel? If so then you can eliminate the OD circuit. Does it hum without a guitar?

You don’t need isolated jacks.

Did you try poking around with a chopstick yet? Tug and push on literally everything with the amp on and see if there is any difference. I’ll go back and check out your lead dress but looks like you did a nice job!

Thanks Taylor!

It hums on the clean channel too, and without a guitar plugged in.

I've poked around some with a chopstick. You can certainly hear tapping sounds though the amp when I touch something in it, especially on V1, but there's no major change by moving anything.

I managed to record two audio clips with my phone to give you an example. It's not the greatest quality and I'm certainly no expert player. I just play some chords and noodle around a bit. First clip is the clean channel, second the OD. My guitar is a Fender Telecaster I've converted into an Esquire with a single Danny Gatton humbucking pickup in the bridge. All the amps controls are at noon, except the OD drive, which is at 6, and the OD level, that is at 4. I start by rolling up the guitar's volume pot, then you can hear me playing the master volume control some.

If this type of hum sounds familiar to anyone feel free to chime in!

- Matt J.

Clean[1].m4a
Dirty[1].m4a

that sounds like you're really close to the amp/transformer interference noise.

is the amp in it's head case? are you a reasonable distance from it? my 100w bluesmaster will hum like that out of it's head shell if I'm on top of it.

In it's case and a normal distance away it's quiet.


M

[Matt J]

If the hum increases with the master volume then it’s something in the preamp. So it would be before those jacks (they are correct). Does it hum on the clean channel? If so then you can eliminate the OD circuit. Does it hum without a guitar?

You don’t need isolated jacks.

Did you try poking around with a chopstick yet? Tug and push on literally everything with the amp on and see if there is any difference. I’ll go back and check out your lead dress but looks like you did a nice job!

Thanks Taylor!

It hums on the clean channel too, and without a guitar plugged in.

I've poked around some with a chopstick. You can certainly hear tapping sounds though the amp when I touch something in it, especially on V1, but there's no major change by moving anything.

I managed to record two audio clips with my phone to give you an example. It's not the greatest quality and I'm certainly no expert player. I just play some chords and noodle around a bit. First clip is the clean channel, second the OD. My guitar is a Fender Telecaster I've converted into an Esquire with a single Danny Gatton humbucking pickup in the bridge. All the amps controls are at noon, except the OD drive, which is at 6, and the OD level, that is at 4. I start by rolling up the guitar's volume pot, then you can hear me playing the master volume control some.

If this type of hum sounds familiar to anyone feel free to chime in!

- Matt J.

Clean[1].m4a
Dirty[1].m4a

that sounds like you're really close to the amp/transformer interference noise.

is the amp in it's head case? are you a reasonable distance from it? my 100w bluesmaster will hum like that out of it's head shell if I'm on top of it.

In it's case and a normal distance away it's quiet.


M

The chassis was sitting on the bench with a speaker cable running down to the speaker cabinet. I should have mentioned that in my original post!

I played sitting in a chair about 5 feet from the amp, so that's probably got something to do with it. I just want to make sure there's no noticeable AC or DC hum I may be overlooking, or a ground loop.

I going to load the head in the cabinet tonight and see how things go. I'm adding some shielding tape to the inner top of the cabinet too to make sure there's as much shielded as possible. I prefer to run it with the preamp volume fairly low and the master up some to keep things fairly clean when the OD is off. You can really drive the preamp on this thing if you turn the volume past one o'clock!

I'll keep everyone posted!
- Matt J.

[norburybrook]

Matt,

5 feet away it should be quieter than that. You say you can drive the clean channel easily, do you have a high output guitar in use? I've not built a #183 but it's just a high plate skyliner isn't it? with single coils or vintage output humbuckers I've never been able to drive the clean that much on any #102 Ive built. I wonder if you have too much gain on V1 for some reason, that might explain the extra hum as well.


Just thinking out loud here.

M

[Matt J]

Matt,

5 feet away it should be quieter than that. You say you can drive the clean channel easily, do you have a high output guitar in use? I've not built a #183 but it's just a high plate skyliner isn't it? with single coils or vintage output humbuckers I've never been able to drive the clean that much on any #102 Ive built. I wonder if you have too much gain on V1 for some reason, that might explain the extra hum as well.


Just thinking out loud here.

M

Yep, it's a high plate skyliner. 220/150K on the plates, 3.3/2.2K on the cathodes.
The pickup in my Tele is a Joe Barden Danny Gatton model - It's a twin blade humbucker in a Tele bridge casing, but the output is pretty low at only ~4.3K.
I may adjust the plate and cathode resistors to low plate values and see how it goes. I may even add back in the LNFB section.
- Matt J.

[norburybrook]

Actually the #102 has LNFB so that would be a reason it doesn't gain up like you're saying.

put it on a switch



M

[amplifiednation]

Does the hum increase/decrease with the preamp volume pot?

[Matt J]

Does the hum increase/decrease with the preamp volume pot?

The hum does go up with the preamp volume pot, not as much as it does with the master volume pot though.

I've also noticed when I first tried adding the LNFB to V1B I erred and used just one 10M resistor on each side of the .47 cap, instead of two 10Ms or a 22M. That might be why it sounded a little off to me. I'm going to try to reinstall it today with the proper values.

I may also play with the snubber caps on V2. I have some 100, 230, and 500pf to try.

Just all part of the fun of tweaking an amp!

- Matt J.