Is this heater supply OK?

[Nuuk]

Is this heater suply OK?

I am building a DC heater supply for my valve pre-amp and wanted to check that it was OK before putting the valves in and powering up.

I understand that wired in series, the heaters require 12.6 volts but I get around 21-22 when I test the circuit without the valves. I presume that with the valves in place, the voltage will be pulled down but I don't know by how much.

Here is the circuit. Is it OK to install the valves and apply power?

An externally hosted image should be here but it was not working when we last tested it.

 

[wa2ise]

Looks like a "constant current" supply. Should work fine with the tubes installed, just measure the voltage across each tube heater once things are warmed up. Voltage sould start out low and slowly increase until warmup is done. Don't get real worried if the voltage is excessive for a short period of time ( several seconds to a minute); tubes in manufacture are fed excessive heater voltage when first evacuated anyway.

 

[Nuuk]

Thanks, it's 21:39 here now so I'll power it up tomorrow morning and measure those voltages!

 

[EC8010]

Add a 10n capacitor from each valve heater pin to chassis and it'll be even better.

 

[Nuuk]

Add a 10n capacitor from each valve heater pin to chassis and it'll be even better.

I just had time for a very quick check but I get 6.5 volts across the supply for each valve so that looks OK.

As regards the 100n caps from the pins, should they go to chassis ground or signal ground star? The two are already separated by a 100n cap.

 

[EC8010]

Directly to the nearest available point on the chassis (probably a solder tag on one of the screws securing the valve socket). Do this at each valve, and for each heater pin.

 

[Nuuk]

Thanks, I'll try that next.

 

[jarthel]

Add a 10n capacitor from each valve heater pin to chassis and it'll be even better.

say for a common 9pin tube, heater pins are usually 4 and 5.

are you suggesting that a cap on each pin going to chassis (which I would assume is then connected to the wall socket ground)?

any reason?

thank you.

 

[Nuuk]

And do we put them on both pins 4 (or 5) and 8?

 

[SY]

say for a common 9pin tube, heater pins are usually 4 and 5.

are you suggesting that a cap on each pin going to chassis (which I would assume is then connected to the wall socket ground)?

Since EC8010 may be a bit tied up, I'll take the liberty of answering. Yes, that's exactly the idea, pin 4 bypassed to chassis and pin 5 separately bypassed to chassis. That arrangement will bypass common-mode noise travelling up the heater line.

 

[Nuuk]

If I may ask one more question could somebody answer me this please?

I have included a delay circuit on my pre to short the output until the heaters have warmed up and the DC offset 'settled'. I can get a maximum delay of 27 seconds, but from cold, the heaters take 37 seconds to reach 6,3 volts. Does this mean there is still a potential danger to an SS power amp connected to the pre?

 

[jarthel]

Since EC8010 may be a bit tied up, I'll take the liberty of answering. Yes, that's exactly the idea, pin 4 bypassed to chassis and pin 5 separately bypassed to chassis. That arrangement will bypass common-mode noise travelling up the heater line.

and the chassis is connected to wall socket ground?

 

[jarthel]

If I may ask one more question could somebody answer me this please?

I have included a delay circuit on my pre to short the output until the heaters have warmed up and the DC offset 'settled'. I can get a maximum delay of 27 seconds, but from cold, the heaters take 37 seconds to reach 6,3 volts. Does this mean there is still a potential danger to an SS power amp connected to the pre?

you could put a simple ntc thermistor on the primary side of the transformer of both equipment and both will have a "soft start".

 

[Nuuk]

you could put a simple ntc thermistor on the primary side of the transformer of both equipment and both will have a "soft start".

Is it really that easy? What about all those more complex 'sort start' circuits out there?

I understood from this that it is important to protect an SS power amp from possible damage while the valve pre is warming up.

 

[jarthel]

Is it really that easy? What about all those more complex 'sort start' circuits out there?

I understood from this that it is important to protect an SS power amp from possible damage while the valve pre is warming up.

the reply I got in AA and another thread I found in another forum seems to suggest that an NTC thermistor is the simplest soft-start. just 1 part and that is it.

ps. be sure to connect the thermistor on the "line" (or is it also called "hot"?) AC wire.

ps. the part that was suggested to me is the "CL" series from GE. I will be getting a CL-80.

 

[Nuuk]

Here is the delay circuit that I am using.

An externally hosted image should be here but it was not working when we last tested it.

I wondered if increasing the size of the cap from 47 uF to say 100 uF would increase the possible delay.

 

[jarthel]

Here is the delay circuit that I am using.

An externally hosted image should be here but it was not working when we last tested it.

I wondered if increasing the size of the cap from 47 uF to say 100 uF would increase the possible delay.

that was the same circuit that was given to me in another forum.

it's quite simple but I would like to use the least complicated/expensive circuit and it seems it's a simple ntc thermistor.

 

[SY]

and the chassis is connected to wall socket ground?

Yes!

 

[jarthel]

Since EC8010 may be a bit tied up, I'll take the liberty of answering. Yes, that's exactly the idea, pin 4 bypassed to chassis and pin 5 separately bypassed to chassis. That arrangement will bypass common-mode noise travelling up the heater line.

just another clarification: and this applies to AC or DC heaters?

 

[SY]

just another clarification: and this applies to AC or DC heaters?

Yes.