DC Filament hookup

[didit]

Hello --

What is the DC voltage on the input side of your regulator? Ideally this should at most be 4V or so higher than your desired filament voltage. If it's significantly higher, that regulator will be dissipating all the excess as heat.

Best ..

[liddokun]

I'm wondering if the low resistance of the cold heaters is pulling so much current that the regulator just can't recover. You might try putting a resistor across the output that is equal to the normal heater current and see if it can handle that. Each 12AX7 with heaters wired for 6.3V will have 6.3/0.3 = 21 ohms resistance once it's warmed up.

I'll give that a shot and see. I'm going to alternatively try it with 12.6v to see if the lower current needs might help here.

[liddokun]

Hello --

What is the DC voltage on the input side of your regulator? Ideally this should at most be 4V or so higher than your desired filament voltage. If it's significantly higher, that regulator will be dissipating all the excess as heat.

Best ..

The thought had crossed my mind as well.

It is an 8v AC secondary, so I don't think that's so excessive as to cause thermal shutdown.

As above. I have a 14v ac transformer arriving soon so I can try series filaments as well.

[Stevem]

Do that resistor load test, but I think your wasting your time .

There's flat out no reason why a good 1085 can't power up the filaments on only three AX7s, unless the brand / make of your chosen 1085 is substandard.

Like in the schematic I posted yesterday. A LM1084 not even a 1085 as in your case is powering 4 tubes one of which is big 12BH7 and also A lamp in the meter of the unit.

[B Ingram]

The diagram includes the recommended heatsink for your regulator.

I assumed bolting to the chassis would be a far better heatsink as it's much larger than that heatsink.

Unfortunately, that's not true. Otherwise, we wouldn't have an industry that makes heatsinks of special material/shape that gets heat out of the Sand and into the Air.

[Stevem]

It's true many times when you see enough bad gear designs land on your work bench .

I can tell you that many times there's regulator or a output transistor bolted to a vertical sperate heatsink and there's a vertically mounted 5 watt resistor mounted so close that it's just about leaning on the heat sink.

It's better in a case like this where a regulator is bolted to the chassis and has far more sq inches of cool chassis to disappate into.

[Kagliostro]

When your DC Output voltage drops what happens to the DC Input ?

Franco

[liddokun]

Do that resistor load test, but I think your wasting your time .

There's flat out no reason why a good 1085 can't power up the filaments on only three AX7s, unless the brand / make of your chosen 1085 is substandard.

Like in the schematic I posted yesterday. A LM1084 not even a 1085 as in your case is powering 4 tubes one of which is big 12BH7 and also A lamp in the meter of the unit.

Your hypothesis was correct, with added 21ohm resistor across the DC output, the filaments had enough time to heat up and I was able to adjust the voltage as the circuit was designed. I will wire in the resistors to the DC regulator pcb.