C-lator hum

[martin manning]

Let's review the bidding.

In this configuration you still have hum:

- Cable from amp "preamp out" to C-lator input removed
- Cable from C-lator output to amp "power amp in" in place
- Tube removed from the C-lator
- C-lator main power on, standby on standby

And, the hum is affected by turning the C-lator output level control, but the hum does not go away with the control fully CCW.

Correct?

[ynor]

Let's review the bidding.

In this configuration you still have hum:

- Cable from amp "preamp out" to C-lator input removed
- Cable from C-lator output to amp "power amp in" in place
- Tube removed from the C-lator
- C-lator main power on, standby on standby

And, the hum is affected by turning the C-lator output level control, but the hum does not go away with the control fully CCW.

Correct?

That is correct.

[martin manning]

So it seems the hum is somehow getting into the C-lator output from the heater circuit or the PT primary. With the setup exactly as above, I'd try breaking the heater circuit at the board, pulling one of the red or blue leads, or disconnecting one lead at the PT.

[ynor]

.

[ynor]

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[GAStan]

. Never mind.

[ynor]

. Never mind.

Thanks, I messed up those two previous posts. Question was if my 10k 2W resistors could handle discharging those high voltages that are left in the unplugged C-lator as it doesn't have bleeding resistors. The few amps I've come across before had so little voltage left in the caps that discharging with 2W resistors are OK. But C-lator has > 100 V. Thanks for your suggestion using two in series.

[GAStan]

No problem, you're welcome. As they say about some things-its all in the timing lol.

[martin manning]

If the power supply is like the schematic posted here, then there is no path to bleed the voltage down. Do you have a 220k 0.5W? That would be fine, since you have ~360V, and 360^2/220k = 0.58W. It will just take a bit longer, about a minute.

With two 10k in series you will have initially 6.5W dissipation, probably ok since the voltage will drop quickly.

I would try disconnecting the heater circuit before moving the PT. I think it's more likely that there is something else wrong here since C-tone has sold many of these built just like yours.

[ynor]

If the power supply is like the schematic posted here, then there is no path to bleed the voltage down. Do you have a 220k 0.5W? That would be fine, since you have ~360V, and 360^2/220k = 0.58W. It will just take a bit longer, about a minute.

With two 10k in series you will have initially 6.5W dissipation, probably ok since the voltage will drop quickly.

I would try disconnecting the heater circuit before moving the PT. I think it's more likely that there is something else wrong here since C-tone has sold many of these built just like yours.

Ah, yes I have some 220k 1W laying around. Will build a drain stick (is that a word??).

[martin manning]

I was just about to suggest building a capacitor discharge tool. Here's a simple one with an LED to show when it's safe. https://ampgarage.com/forum/viewtopic.p ... 88#p439088 At the top of the thread is a really fancy one :^)

[ynor]

I was just about to suggest building a capacitor discharge tool. Here's a simple one with an LED to show when it's safe. https://ampgarage.com/forum/viewtopic.p ... 88#p439088 At the top of the thread is a really fancy one :^)

Now that's cool!

[ynor]

I would try disconnecting the heater circuit before moving the PT. I think it's more likely that there is something else wrong here since C-tone has sold many of these built just like yours.

Got that discharge tool to work with 220k 1 W.

Did disconnect one of the heater wires, see photo. Hum is still there. Next step to move the PT?

20240318_192856.jpg

[martin manning]

Just to be sure, can you disconnect one of the heater wires coming from the transformer? You still have a load and chassis connection via the 2x 50Ω balance resistors.

[ynor]

Rewire the blue heater wire to the board and disconnect at PT instead? Sure.

I'm pretty sure those resistors are 100 Ohm. Is that consistent with the 50 Ohm readings I have at the heater pins?