RC / power supply question

[tdvt]

I'm laying out a DeArmond R5T SE amp & have noticed that in the power supply, there is a 500/5W resistor straight out of the rectifier tube (6X4) instead of the usual reservoir capacitor first. Looked at a bunch of schematics & I think I only saw one other with a resistor first.

What would be the reason they chose this design? Is it due to the particular rectifier tube or just too high a B+ on the pile of in-stock transformers?

I had thought I would use a full-wave bridge on this one & I'm wondering if the R-C sequence should be the same?

[sluckey]

What would be the reason they chose this design? Is it due to the particular rectifier tube or just too high a B+ on the pile of in-stock transformers?

That resistor just knocks the B+ down some. If you add a filter cap directly to the cathode of the rectifier tube the power supply hum will be much improved.

I had thought I would use a full-wave bridge on this one & I'm wondering if the R-C sequence should be the same?

If you put a FWB on that PT then your B+ voltage will be doubled! The amp will not be happy and the filter caps will be at risk.

[tdvt]

Thanks & thanks for the heads up on the double B+.

Since this will not be an exact clone (1-tube reverb maybe?), I thought I would use the Weber GP PT which has no CT but a bunch of lower voltage combinations to fool with.

Any other insights welcome.

Best, TD

[jjasilli]

Also, per tube spec sheets, typical operation of 6X4 feeding a filter cap directly tends to limit the 1st filter cap to 10uF or less.  That's not much filtering.  The resistor isolates the rect. tube from the cap and allows use of the larger value filter cap.  Didn't do the math, but as a side benefit the resistor might also limit in-rush current at turn on.


Per sluckey's post:  you might try adding a "pre-filter cap" of 4uF - 10uF after the rect. tube & before the resistor and see if it makes a difference that you like.  (If a pre-filter stage changes B+ a bit, it can be restored by a different value resistor.)

[PRR]

This amp is probably an odd-parts design.

The PT was for a bigger radio/amp, has high voltage and low winding resistance. The 6X4 is not-as-big a rectifier as the old-type 5v4 etc. If you try this in Duncan PSD with default values (no 500r), the rectifier "blows up" at turn-on due to high surge current.

The "right" answer is to take a few pennies worth of copper -out- of the PT by using smaller winding wire. Get the resistance up from <100r to several hundred Ohms, the peak current is acceptable to 6X4. The DC voltage may be higher than you want, so you also change the number of turns.

But what if your distributor has a close-out on 5,000 units of this too-good PT? So low that the price of the crate of PTs +and+ a box of 500r 10W is lower than the cost of custom-wound PTs?

Still, added resistance was *not* uncommon. My Fisher 2-24 had about 47r 2W from 5U4 to first cap. It smoked. I used a smaller resistor. I had repeated 5U4 failures. It was happy with 2*100R 5W parallel- original plan up-rated. (We believed my unit was a prototype not a production unit.)

If you have that HIGH-current many-tap PT...

First, you can't use a 6X4/5V3/etc, you need the Bridge. This will be Silicon, and the peak current is less of an issue. The amp is class A, so "sag" won't happen. Wire without resistor, change tap until voltage is in the ballpark. (But that is a BIG transformer for a Champ.)

[tdvt]

I kind of imagined that scenario myself. An offer they couldn't refuse...

I had thought about the bigger Weber PT because I'm cheap & it looked like you could try out a few configuration. The schematic shows 360V right at the rectifier tube. But it was my understanding that sag wasn't a consideration with SE amps, hence SS rectifier.

I scaled the PT bolt circle from an R5T gut-shot & it looks like 2" X 2 1/2", same as a Champ. 

Very cold weekend here (-24F) so I've spent way to much time on this.......

Thanks, 

TD