[R-390] B+ Dropping Resistance (was Drill a Hole...)

[Drew Papanek]

Gord Hayward wrote:

>Is drilling holes permitted or is this an unforgivable heresy?  I'm still 
>debating a couple of 9/64 hole in the rear panel to locate a
>220 ohm Dale resistor near the B+ fuse if and when one of the 26Z5 tubes 
>dies and I have to go to silicon. I think that will be
>better (and easier) than putting it under the AF deck.

<snip>

For the non-heretical who wanto use chassis mounted poweresistor to drop B+, 
said resistor can be mounted to a flat piece having holes for resistor 
mounting plus holes matching diameter/spacing of some exisiting component's 
mountings.  The adaptor would then be sandwiched between component and 
chassis, and perhaps use longer screws.  Flathead screws for resistor 
countersunk into adaptor would permit flush mounting to chassis.  Pigeon 
Poop (heatsink compound) could be used for improved heatransfer.

A poweresistor (lead mounted, no holes required) can be installed on the 
power supply unit from transformer HV center tap to ground.  It can be 
spaced away from rectifier tube sockets to provide free air circulation.  A 
disadvantage is proximity to PTO and resultant heating although the 
rectifier tubes do that anyway.

Locating resistor under audio module chassis worsens the already existing 
heat problem there.

Under chassis location of poweresistors cooks components in a lot of tube 
gear.  I've pondered (but not tried) disumpuckying above chassis filter 
caps, moving poweresistors to inside capacitor cans (perhaps with thermally 
conductive potting), and mounting new leaded caps under chassis.

>BTW would a high power Zener be better than a resistor to keep the B+ 
>voltage down with the solid state mod?

<snip>

Vacuum rectifiers have a forward voltage drop characteristic which is 
roughly approximated by a constant voltage in series with a resistance. 
Using silicon (with its lower voltage drop) plus resistance to replace tube 
rectifiers would result in degraded regulation.  One claimed that he could 
detect FMing of PTO at high audio level when doing this.  I've not noticed 
any untoward effect on PTO stability from use of series resistance to drop 
silicon rectified B+.   Silicon diodes plus resistance is an official gov't 
approved modification.

Using a power zener for constant B+ drop has been discussed here.  Power 
zeners tend to be expensive and difficulto find.  A small zener and power 
transistor can be connected to perform same function.  Note that either of 
these arrangements would dissipate same power as a resistor but must 
dissipate it at a lower temperature probably necessitating a sizable 
heatsink (or sunk to chassis).

If using power transistor plus zener, you have a pass element and voltage 
reference so you could add error amplification and make it a regulator...ad 
nauseum.

At r-390a.net you will find Wei Li's brilliantly conceived "Pearls of 
Wisdom" where postings of this topic have been neatly distilled for 
convenient reference.

Drew

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