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RE: Is this feasible? Rectifying 6.3v for heaters

Hi Audio Pete,

This is the heater supply for my HK Citation 4. The OEM circuit used a selenium bridge feeding a 2 stage cap input filter. The resistor was partly composed of an incandescent lamp to light the on off button. After the selenium had to be replaced I used a silicon bridge and added a three terminal regulator. The heater supply was also used to power the electronics in the regulator I used for the B+ supply. I was getting some intermittent hum. I found that the one of the electrolytics was getting funny but also the heater supply was on the edge of coming out of regulation. The search for a low drop regulator led me to LT3080. Actually an app engineer at LT suggested it to me.

The LT3080 has an input to output minimum differential of ~ 350 mv. The caveat is that the V control pin has to be a bit above that to achieve that spec. Of coarse the input voltage which counts is the bottom of the ripple. Note the extra diodes feeding the V control pin. The V control draws much less current than the tube heaters so there is very little ripple so the V control is a good bit above the input at the bottom of the ripple. I can get down to ~ 90 VAC input to the OEM power transformer before the heater supply comes out of regulation. The 33 Ohm resistor from input to output of the LT3080 is to reduce the dissipation of the regulator at high line (~125 VAC). No problem with ripple from the 33 Ohm as the regulator has a very low output impedance.

The 12AX7 tube heaters are configured for 12.6 volts and there are three pairs of 12AX7s in series with the connection between each pair grounded. It turns out the the heater currents are not exactly matched so the + side did not match the - side exactly. Naturally after this effort I was not going to have that so I built an active circuit to balance the + and - side. The 24 Volts is actually 25.2 Volts so each of my 12AX7s gets very close to 12.6 Volts across its heater. If you want to see that circuit I'll scan it and post it as well.

Phil







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