Tube heaters in series.

[nigelwright7557]

I have built a pre amp using an ECC83 and wired the heaters in series.

I have noticed on power up from cold one heater glows considerably brighter than the other for about a second and then things settle down.

Will this damage the valve or could it cause unreliability ?

[Geek]

Hi,

Heater "flash" is common in the 12A_7 valves (especially nasty in many 12AT7's).

You do need a controlled heater warmup for series stringing though - denoted by an "A" suffix in the US numbering system. Don't know what it is for european types.

Cheers!

[Mickeystan]

Hi,

This problem is typical of series AC or DC heaters operated without a constant current source. The one and only way I know to eliminate this problem is to feed the heater series string with a DC constant current source set for the specific current of the heaters.

I have this set up in my current amplifier project and it works great. My implementation was with an LM317 wired as a constant current source and it is feeding just the front end voltage amp stage and the differential LTP phase inverter tubes.

MickeyStan

[nigelwright7557]

Many thanks for your help.

Nigel

[Jim McShane]

Originally posted by Mickeystan
Hi,

This problem is typical of series AC or DC heaters operated without a constant current source. The one and only way I know to eliminate this problem is to feed the heater series string with a DC constant current source set for the specific current of the heaters.

I have this set up in my current amplifier project and it works great. My implementation was with an LM317 wired as a constant current source and it is feeding just the front end voltage amp stage and the differential LTP phase inverter tubes.

MickeyStan

Not that there is anything wrong with using a CCS or a current regulator of some sort, but the fact is certain tubes "flash" whether they are in a series-string or not. It's part and parcel of the heater wire material used by some makers.

BTW, a slow start voltage regulator will work well too - the circuit in the great 1982 National Semiconductor Voltage Regulator Handbook works quite well. IIRC that book has been reprinted, I think the Audio Express/Old Colony people did it but I'm not sure. They do show up on eBay from time to time.

[nigelwright7557]

Originally posted by Jim McShane


Not that there is anything wrong with using a CCS or a current regulator of some sort, but the fact is certain tubes "flash" whether they are in a series-string or not. It's part and parcel of the heater wire material used by some makers.

BTW, a slow start voltage regulator will work well too - the circuit in the great 1982 National Semiconductor Voltage Regulator Handbook works quite well. IIRC that book has been reprinted, I think the Audio Express/Old Colony people did it but I'm not sure. They do show up on eBay from time to time.

Due to some hum problems I changed from AC heater supply to a DC supply. Its didnt make very much difference to the hum but the flashing has now stopped ! All I use is a half wave rectifier into a 100uF cap to feed the heaters. Its possible the heater voltage is slightly lower and that has stopped the flashing.

Thanks for the advice.
Nigel