Heater biasing

[johnheath]

I have read here and there about the "need" for biasing the heater supply for longer lasting tubes…

But… I cannot seem to find a proper schematic for this and before I start digging into it I wonder how much longer the tubes will survive or how quickly the tubes will die if using "non-biasing" heater supply?

And the obvious… If somebody have a decent schematic? Feel free to post it =)

It is mostly around the  "Slow-blow" preamp discussion that the talk about the floating biased heater supply occur… why?

Thanks

/John

 

[ruffrecords]

The reason the heaters are elevated in the slow blow preamp is that it uses a pair of SRPP stages. The top triode of each of these stages has its cathode at about half the HT supply voltage. Now, all tubes have a specification for the maximum voltage that is allowed between the heater and cathode known as Vhk. If the heaters in the SRPP were referenced to HT 0V, as on most normal designs, the the voltage between the cathode and heater of the top triodes would be about half the HT voltage. When the circuit passes signal, the cathode voltage goes up and down so the peak heater to cathode voltage is half the supply volts PLUS the peak signal level. If the slow blow is used with a step down output transformer then the signal voltage at the top cathode could easily reach 50V. I don't know what the slow blow HT voltage is but if it were 300V then the top cathodes could peak at 200V in normal operation.

The bottom line is that most tubes will not withstand a heater cathode voltage of 200V. A few have a Vhk spec of 200V but even then there is  no margin in the design if the heaters are referenced to 0V. The solution is to elevate the heater voltage. It is typical for them to be elevated to around +75V or 80V above HT 0V. This gives an adequate margin for most tubes that will allow the heater to be 150V above the cathode. For some reason, the Vhk spec for heater voltage above the cathode tends to be lower, 100V being a typical figure. Elevating to  80V or so keeps comfortably within this limit.

There is some evidence to suggest that, even where the heaters are referenced to HT 0V, there is some improvement in reliability by elevating heaters. I think it was Crowthorne who wrote about this in one of his books  many many moons ago. Also, where heaters are +ve with respect to cathodes, the heater/cathode looks like a reversed biased diode so heater hum is less likely to be injected into the cathode than if they were both referenced to 0V.

Cheers

Ian

 

[RuudNL]

Just a remark: when the switch is in the "1/4 input" position, the grid is floating. (No ground reference).
I think it is a good idea to put a resistor with a high value (1M or so) between the grid and ground.

 

[johnheath]

Thanks both of you

Ian… to get this "elevated heater voltage" it seems like I should get it from the HT…. am I right? And… I suppose there should be, in this case, a 12 v difference between the separate heaters (pin 4 and pin 5)… coupled in series.

And to keep it this way and "floating"… no grounding of this certain circuit?

Best regards

/John

 

[johnheath]

After reading a lot more about there there seems to be more questions to be asked.

If I use 12 regulated dc for the heathers and the HT supply is 245 vdc… is there still a need for elevating the heater voltage?

What I have seen is schematics were they use the centre tap for biasing (or two resistors to create a artificial centre tap)… But
what if I use toroids with 15vac secondary… to feed a second toroid secondary 15vac. (this can be found in the Gyraf G9 schematic).

Thanks

John

 

[johnheath]

And this one

 

[johnheath]

Hmm… I think I have it figured out now.

/John

 

[ruffrecords]

To elevate the heaters above HT 0V all you need is a potential divider across the HT to create the necessary voltage. If you look at the top of this schematic, you can see how I did it for the poor man's tube gain make up amp:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/PMEQP1A/PMTGMUcctjpeg

The 220K and 75K form the potential divider which creates about 75V from the 300V HT supply. The 75K is decoupled by a 47uF electrolytic so the heaters are still at AC ground.  You can see how one side of the heater supply is connected to this point. That is all you need to do to elevate the heaters. If you have a dc heater supply you still do the same thing. Just connect one side of the heater dc supply to the 75V point. The dc supply must be quite separate from the HT  and any other supply (like phantom) and so must use its own separate transformer winding

I have since modified this circuit slightly because some tube types have a spec for the maximum resistance between heater and cathode. So in the latest design, the 75K is now a 22K 2W resistor and the 220K is replaced by two 33K 2W resistors in series (to make 66K). The 2W rating is necessary because of the higher current in the pot divider. It also provides a useful secondary function of discharging the HT supply when it is turned off.

Cheers

Ian

 

[johnheath]

Thank you Ian… a clean good answer and I like it

Cheers

/John