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toufu 31st January 2012 11:03 PM

AC Heater reference to B+
I am using AC for heater on my tube phono amp. How should the heater be referenced to B+? Do I have to do anything? Thanks.

DF96 31st January 2012 11:13 PM

B+ may be too high, and could exceed heater-cathode voltage limits. The heater must be referenced to something. Try ground. If this works, fine. If not, try somewhere around +40V (use a potential divider).

toufu 1st February 2012 12:20 AM

Do I just take one of the ac lead and tie it to ground or +40v?

jjman 1st February 2012 12:24 AM

Yes, or use the center tap (if if exists) or make an artificial CT with 2 resistors or a pot.

gingertube 1st February 2012 02:57 AM

I see this used quite often (referencing the heater to a +ve voltage with respect to the cathode). Typical values I've seen are +30 to +40 Volts. I used +50V in both my latest HiFi Amp and Guitar Amp.
Last night I looked up the topic in RDH and I note that they recommended +70V.
Has anyone used this sort of "stand-off" voltage. Does that seem a bit high?

Johan Potgieter 1st February 2012 03:16 AM

One can use any voltage which is safe for the lowest heater-cathode rating. I am not aware of anything less than 100V. But I am not sure of a high voltage advantage; as far as I have read, some +30V to +40V is enough to cope with any heater-cathode leakage. Only if there is e.g. a cathodyne phase splitter or Schmitt (the LTP) where a signal carrying cathode is positive, a little consideration is necessary. The 'bias' voltage can then go up to a safe 80V, so as not to leave the signal cathode (at perhaps some +100V to +120V plus peak signal voltage) too positive w.r.t. the heater.

trobbins 1st February 2012 06:22 AM

Heater-cathode leakage resistance increases with voltage away from 0V for most tubes it seems, with most change occuring by 10-15V difference. However the resistance even at 0V has been meassured for a few common valves of different ages to be pretty bloody high, and effectively negligible in terms of noticing any improvement, as the coupling is effectively swamped by heater/cathode capacitance (which is not changed by DC elevation). And if you don't bypass the DC elevation correctly then the hum/noise may get worse from ripple injection via the HV supply. It seems best initial aim is to do all standard hum reduction techniques wrt cabling, and use a humdinger pot to tune out gate/heater residual hum.

DF96 1st February 2012 10:35 AM

In "Getting the most out of vacuum tubes" the author (I forget his name - you can Google) recommends +40V as giving the best life IIRC. This may or may not coincide with lowest noise/hum.

SY 1st February 2012 10:46 AM


It's important to note that you would want that 40V (or whatever you choose) to be with respect to the cathode voltage, not ground!

Johan Potgieter 1st February 2012 11:16 PM

SY!! Don't do that .... ;)

As SY said, but that is not always possible e.g. when max. heater-cathode voltage is 100V and a cathode sits at +120V. One then does as well as one can.

This brings me to the point that the problem is not with heater-cathode (leakage) resistance as such, but the fact that sometimes a positive cathode can pick up heater "hum" by acting like the anode of a heater-cathode diode (depending on circuit circumstances obviously). I have had a few occasions where the heater-positive state has cured hum-pickup. This is possibly a rare occurrence, but still it is easy to get the heater chain positive, just in case a rogue tube comes along acting up sometime.

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