question about "immortal amp mod"

[norstroms]

I searched around various threads and FAQs, sorry if this is a redundant topic.

I recently smoked a PT in my AB763-ish circuit. The PT was a replacement for the 125P5D/022798 (for Super/Pro Reverb). Somehow I had unknowingly shattered the enclosure of the 5V4 recto and powered on without realizing it. Saw the the amp was mysteriously blowing fuses and it took me a little while of testing to determine the PT was RIP. So I just installed a new one and this time I followed the advice from R.G. Keen's Immortal Amp Mod as well as this thread on another forum, and specifically Bruce from Mission Amp's additional advice "Just put a 47 ohm 2w resistor from the rectifier tube to the 16uF-33uF filter cap... that's in front of the filter cap's + lead. The inrush current will pass through the resistor, momentarily limiting the current a bit and tube will last many times longer." I wanted to do this because I would like to (if possible) use a 5R4 for the similar sag to the 5U4 but without the higher current rating (the 5R4 wants to see a very low input capacitance).

It is my earnest intention to protect my PT's from facing a similar catastrophe. So I installed the 1N4007's, made sure to put the cathode (bar) side to the rectifier and the anode side to the PT. I also installed the 47 ohm/2W resistor off pin 8 of the rectifier to place it in series with the circuit, attached the line leading to the filter caps to the other end of the resistor.

According to the Immortal Amp Mod article and many other sources (if I understand them correctly), I should be seeing a drop of 0.7 volts across the diodes on the PT leads feeding the rectifier. However, it appears that I am now observing a drop of 163 volts across those diodes. On the PT side of the diode I measure 353 vdc, and on the other side I measure 190 vdc. I am seeing  425 vdc off of pin 8 of the rectifier, and the voltages across the amp appear to be within the normal ranges. I'm using a 5U4 at this time, in anticipation of the arrival of my replacement 5R4 (and a 5V4 to boot just in case the 5R4 doesn't work out so well).

Did I somehow install the diodes backwards? I don't think it's possible, I would think that I would see obvious signs of error if I had done so. Are the diodes bad? I hope that this is a "normal" observation and I'm just misunderstanding the material I've read about this safety mod. Thanks for any thoughts on this...

[norstroms]

Hmmmm... I'll bet I installed that resistor in the wrong spot. I should have put it in series with just the positive lead of the filter caps, not the entire circuit. Okay so that's fix #1...

[John]

The voltage coming off the PT will be AC, not DC. The rectifier "turns it into" DC. (more involved than that, but I'm  not yet capable of explainging it) Set your meter to read AC volts and see what it reads.

Also, part of the Immortal Mod is putting a fuse on each wire going from the PT to the rect. tube. That way if something goes wrong on the B+ side of things, the fuse should blow protecting your PT.

Hope this helps!

[norstroms]

Oh of course, yes I wrote vdc but I did in fact measure vac. Gah!

Original post should have read: "On the PT side of the diode I measure 353 vac, and on the other side I measure 190 vac"

My bad, thanks for pointing this out.

[John]

Hey, I just figured you did just what I did the first time I was measuring voltages. 

[sluckey]

According to the Immortal Amp Mod article and many other sources (if I understand them correctly), I should be seeing a drop of 0.7 volts across the diodes on the PT leads feeding the rectifier. However, it appears that I am now observing a drop of 163 volts across those diodes. On the PT side of the diode I measure 353 vdc, and on the other side I measure 190 vdc. I am seeing  425 vdc off of pin 8 of the rectifier, and the voltages across the amp appear to be within the normal ranges. I'm using a 5U4 at this time, in anticipation of the arrival of my replacement 5R4 (and a 5V4 to boot just in case the 5R4 doesn't work out so well).

There will be 0.7v dropped across a typical forward biased silicon diode. But that's a dc voltage drop and a better way to measure it would be directly across the diode. But in your case, you're putting AC voltage on one side of the diode and the voltage on the other side is still ac, although it is only half cycles. Your meter doesn't know whether to treat the half cycles as AC or DC. That's why the numbers don't add up properly.
 

Did I somehow install the diodes backwards? I don't think it's possible, I would think that I would see obvious signs of error if I had done so. Are the diodes bad? I hope that this is a "normal" observation and I'm just misunderstanding the material I've read about this safety mod. Thanks for any thoughts on this...

You didn't do anything wrong. The diodes are working properly. If they were installed backwards you would not have any B+ voltage out of pin 8 of the tube.

[norstroms]

Ah! Perfectly explained. Thanks! What a relief.