C-lator hum

[martin manning]

Worth a try.

[ynor]

Worth a try.

I'll keep note on your suggestion in case I get stuck with this chassis. That toroidal seem to be perfect

[martin manning]

It's actually a bit more current than you need (30 mA, where you probably only need 10 mA or so), but that means the voltage will be a bit higher, making it just about right.

[ynor]

I've ordered the toroidal that Martin kindly suggested for the C-lator. And I've ordered new trannys for the OTS as well. To protect that expensive OT from accidentally being played without a speaker or with a malfunctioning cable, and also having the opportunity to use it for silent recording, I thinking of installing some kind of safety load. My brothers Friedman can be played without a speaker cable inserted (for line level silent recording). How can that be installed my amp? I've not seen the schematics or layout of his amp, but I guess it's just a switched output jack that kicks in a resistor going to ground when a cable is not inserted?

[martin manning]

You probably have a shorting contact on the main speaker jack, so if no speaker is plugged in you are safe, but if a speaker cable is plugged in to the amp and open at the other end, or defective as you said, you have a problem. Placing a resistor across the output will protect the OT until you figure out why there is no sound. Something like a 7W 100 ohm would work, being more than 10x the (8 ohm) speaker impedance it won't be noticed. Its power consumption when playing at 90W output with a speaker would be about 6.8W. At 50W, that would drop to about 4W. It will get hot, so keep it away from other components. I would not recommend this for silent recording since the load impedance is >10x what it should be. An attenuator would be the ticket for that. The resistor is only a safety net. Low wattage amps can be fitted with a built-in resistive load or something more sophisticated with the correct impedance because the space required isn't much. The upper limit on that is probably around a 15W-20W amp.

[ynor]

The upper limit on that is probably around a 15W-20W amp.

That would explain why I haven't seen the silent recording option on 50-100 watters.

[ynor]

Still waiting for the toroidal, but got my $70 scopemeter to toy around with. One interesting observation: The C-lator is installed in the bottom of the head cab. I can hear a very slight hum in the speaker cab when only the C-lator is powered on. So I measured the output current of the amp by connecting the DMM to the speaker out jack. When the amp is powered off, and the C-lator is not connected in the amps loop but still powered on, I can read around 150 uA on the speaker out. Voltage is too low to be detected on the oscilloscope I guess, or I have triggering set wrong. This 150 uA seems to be enough to hear a very low hum in the speaker. When moving the C-lator away from the amp, the current drops with increasing distance. I believe this might be caused by the C-lators PT leaking flux into the amps OT, as the only thing connected here is mains power to the C-lator. Or am I mistaken?

Snapchat-732616763.jpg

[martin manning]

...When the amp is powered off, and the C-lator is not connected in the amps loop but still powered on, I can read around 150 uA on the speaker out. Voltage is too low to be detected on the oscilloscope I guess, or I have triggering set wrong. This 150 uA seems to be enough to hear a very low hum in the speaker. When moving the C-lator away from the amp, the current drops with increasing distance. I believe this might be caused by the C-lators PT leaking flux into the amps OT, as the only thing connected here is mains power to the C-lator. Or am I mistaken?

I think that's very likely. The changing magnetic field around the C-lator's PT is inducing a current in the OT coils. The same thing will happen if the amp's PT and OT are not oriented correctly with respect to each other. Your DMM or your scopemeter should be able to pick up a voltage on the OT secondary. What is the maximum vertical sensitivity of the scope in mV/division? Is it set as high as possible?

[ynor]

What is the maximum vertical sensitivity of the scope in mV/division? Is it set as high as possible?

Just tried some manual triggering settings, and got a voltage reading @50 Hz with DC-coupling. The maximum vertical sensitivity is 20 mV/division.

img_06.bmp

[ynor]

Your DMM or your scopemeter should be able to pick up a voltage on the OT secondary.

It just occurred to me that I was measuring without a speaker load as I was measuring tip and ground on the speaker cable without the speaker connected. The amp was in standby and should not hurt the OT. Connecting the speaker and measuring at the terminals will probably change the readings. Will try.

[ynor]

My friends, it works!!

20240405_201558.jpg

I followed Martins suggestion on that toroidal. Got it today and installed it right away. Did not cut any leads yet as I wanted to try it out first. It is dead silent!! No vibrations in the chassis! No leaking flux to the amps OT! No leaking flux into the internal signal path! OK, it is possible to hear a slight humming noise when the amp and C-lator is turned up really loud, but that won't bother me as I only play at those volumes on stage. But now, at living room volumes, there is absolutely no extra hum from the C-lator!

So, how do you suggest I tidy up the wiring? No wires are cut yet. The toroidal is only taped to the chassis at the moment as I wanted to try it out before drilling a new hole in the chassis. Should the toroidal be tuned around so the wires face the bottom of the chassis, not the lid?

I can post some voltage measurements before and after the transplant if you're interested.

[martin manning]

Good to hear it worked out! I would leave the side with the leads on top, and shorten them to eliminate excess, but no more than that. Does it fit within the height you have available, including the mounting hardware? Remember that you must not let the mounting bolt make electrical contact with the top and bottom of the chassis. I would make sure that neither the bolt nor any metal washer on the bolt makes contact with the top.

Voltage measurements would be interesting.

It looks nice in there, BTW!

[ynor]

Does it fit within the height you have available, including the mounting hardware? Remember that you must not let the mounting bolt make electrical contact with the top and bottom of the chassis. I would make sure that neither the bolt nor any metal washer on the bolt makes contact with the top.

Voltage measurements would be interesting.

It looks nice in there, BTW!

Thanks! Yes there's plenty of room, so should not be any problems with mounting inside without the bolt nor the toroidal making contact with the lid.

Some measurements: They are more or less exactly the same before and after on the secondary HV, but the heaters have slightly more voltage on the new trafo (6,04 vs 6,46 VAc).

Measurements:

C-lator voltages.png

Does that look correct?

[martin manning]

Looks great! That PT is an excellent option for D'lator builders in EU.

[ynor]

Looks great! That PT is an excellent option for D'lator builders in EU.

Works great! Also has two 6,3 VACs. I only used one and blinded the other. Maybe both can be useful if building a stereo version with two tubes, where there's a need for more current?