A #102 Attempt - First amp

[Vertigo]

Shot in the dark here:

Your speaker jacks; have you got an actual ground wire or are you grounding through the chassis?

I've had earth hum on anodised chassis because of this. I always wire an actual wire connection to ground from the speaker jacks. I Know these chassis are not anodised on the grounding places but might be worth checking.


M

This is an AF chassis that has been powder coated and the jack area was masked off so it's bare aluminum. I'll check it out though. Raw aluminum grows an oxide skin that acts as an insulator so it's possible that they're not making a good ground. Thanks for the suggestion. I suppose that's a possibility for all ground points as well, but anywhere with a serrated contact should easily cut through the oxide layer...something to check out at least.

[Vertigo]

Have you tried disconnecting the input on V1a and injecting a signal to see if that eliminates the hum? I can't quite follow the wiring with the FET/NOR inputs and this would be an easy way to remove that from the equation.

I'll give that a shot. Thanks for the suggestion. Since it hums even without an instrument plugged in, would disconnecting the input at V1a be sufficient? This effectively takes the input jack and FET out of the loop? so if it's quiet, then it's likely a ground problem there?

[ijedouglas]

Have you tried disconnecting the input on V1a and injecting a signal to see if that eliminates the hum? I can't quite follow the wiring with the FET/NOR inputs and this would be an easy way to remove that from the equation.

I'll give that a shot. Thanks for the suggestion. Since it hums even without an instrument plugged in, would disconnecting the input at V1a be sufficient? This effectively takes the input jack and FET out of the loop? so if it's quiet, then it's likely a ground problem there?

That's my thinking. If it still hums after bypassing the input and FET then your issue is likely around V1. If it's quite I would concentrate on the input and FET section.

[Vertigo]

So with the input disconnected from V1a at the grid stopper, it's as loud as ever. With V1 pulled out, it's quiet. So it HAS to be in the V1 circuit eh?

[pompeiisneaks]

Correct, if it was being induced later in the circuit you'd hear it with V1 pulled, that means you can find the culprit in one of the components, or leads in the chain to V1. V1a or V1b. Sometimes moving wires with a chopstick will show more/less hum and point you in the right direction.

~Phil

[ijedouglas]

So with the input disconnected from V1a at the grid stopper, it's as loud as ever. With V1 pulled out, it's quiet. So it HAS to be in the V1 circuit eh?

V1 thru tonestack:) I would leave V1 out and then inject a signal at various points....from the 0.047 V1b coupling cap and work your way back to V1a

[Vertigo]

next new guy question...

Based on Marcus' suggestion, I was checking continuity on the jack sleeves to ground as well as to other things that maybe ought not be grounded. I have an impedence selector switch and was noticing that each of the secondaries is grounded. In looking at the spec sheet, they're all tapped off the same coil so it would make sense that if the common leg was grounded, the rest would be as well. But that means that the jack tip and sleeve are both connected to ground. I have the common wire from the OT attached to the sleeves of the speaker jacks (and the shunt of the main jack) and the common leg of the impedence selector switch attached to the tips of the jack. If I follow the wiring, it's all logical. This is obviously different than the FX jacks or input jack, so I'm wondering why?

9276D003-7080-4373-B151-10B72F2F0189_1_105_c.jpeg

[pompeiisneaks]

The resistance of the output transformer is pretty low, so it will show continuity... nothing to worry about.

~Phil

[ijedouglas]

next new guy question...

Based on Marcus' suggestion, I was checking continuity on the jack sleeves to ground as well as to other things that maybe ought not be grounded. I have an impedence selector switch and was noticing that each of the secondaries is grounded. In looking at the spec sheet, they're all tapped off the same coil so it would make sense that if the common leg was grounded, the rest would be as well. But that means that the jack tip and sleeve are both connected to ground. I have the common wire from the OT attached to the sleeves of the speaker jacks (and the shunt of the main jack) and the common leg of the impedence selector switch attached to the tips of the jack. If I follow the wiring, it's all logical. This is obviously different than the FX jacks or input jack, so I'm wondering why?


9276D003-7080-4373-B151-10B72F2F0189_1_105_c.jpeg

Is your question regarding the secondary's having continuity or the difference in wiring between the speaker and FX jacks?

[Vertigo]

next new guy question...

Based on Marcus' suggestion, I was checking continuity on the jack sleeves to ground as well as to other things that maybe ought not be grounded. I have an impedence selector switch and was noticing that each of the secondaries is grounded. In looking at the spec sheet, they're all tapped off the same coil so it would make sense that if the common leg was grounded, the rest would be as well. But that means that the jack tip and sleeve are both connected to ground. I have the common wire from the OT attached to the sleeves of the speaker jacks (and the shunt of the main jack) and the common leg of the impedence selector switch attached to the tips of the jack. If I follow the wiring, it's all logical. This is obviously different than the FX jacks or input jack, so I'm wondering why?


9276D003-7080-4373-B151-10B72F2F0189_1_105_c.jpeg

Is your question regarding the secondary's having continuity or the difference in wiring between the speaker and FX jacks?

It looks like the secondary is essentially wired like a single coil pickup with two coil taps for three different output levels. In my head, from any tap to any other tap is a half-loop that has continuity when connected. Looking at the way the jack is wired, I completely understand why I get continuity between the tip and sleeve. I guess I'm getting tripped up about the grounding. Why would we want both the tip and sleeve to be connected to ground?

[sluckey]

Why would we want both the tip and sleeve to be connected to ground?

The tip is not connected to ground. It just seems that way because your meter can't read the very low resistance between the tip and sleeve.

[ijedouglas]

next new guy question...

Based on Marcus' suggestion, I was checking continuity on the jack sleeves to ground as well as to other things that maybe ought not be grounded. I have an impedence selector switch and was noticing that each of the secondaries is grounded. In looking at the spec sheet, they're all tapped off the same coil so it would make sense that if the common leg was grounded, the rest would be as well. But that means that the jack tip and sleeve are both connected to ground. I have the common wire from the OT attached to the sleeves of the speaker jacks (and the shunt of the main jack) and the common leg of the impedence selector switch attached to the tips of the jack. If I follow the wiring, it's all logical. This is obviously different than the FX jacks or input jack, so I'm wondering why?


9276D003-7080-4373-B151-10B72F2F0189_1_105_c.jpeg

Is your question regarding the secondary's having continuity or the difference in wiring between the speaker and FX jacks?

It looks like the secondary is essentially wired like a single coil pickup with two coil taps for three different output levels. In my head, from any tap to any other tap is a half-loop that has continuity when connected. Looking at the way the jack is wired, I completely understand why I get continuity between the tip and sleeve. I guess I'm getting tripped up about the grounding. Why would we want both the tip and sleeve to be connected to ground?

The tip is not connected to ground when a plug is inserted into the jack.

[sluckey]

The tip is not connected to ground when a plug is inserted into the jack.

+1

Much better answer than mine.

[Vertigo]

I appreciate you guys taking the time to help me understand this better. My meter does indeed read resistance from the tip to the sleeve whether or not a jack in plugged in. 1.4 Ohms. It's roughly the same resistance I get from one ground to another in the chassis. Apparently it's not enough resistance to break continuity in my meter.

Anyhow, thanks again. This is all very humbling. As a person who's generally extremely mechanically competent, I find myself struggling to understand how and why electricity works they way it does...which is exactly why I wanted to do this in the first place.

Back to searching for the source of my hum.

[martin manning]

My meter does indeed read resistance from the tip to the sleeve whether or not a jack in plugged in. 1.4 Ohms. It's roughly the same resistance I get from one ground to another in the chassis. Apparently it's not enough resistance to break continuity in my meter.

Continuity is usually passed by something like 25Ω or less, and shorting the probes together will usually register as a few ohms.