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Bias Supply Frying Resistors Like Bacon!

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8/19/1999 12:52 PM

Bias Supply Frying Resistors Like Bacon!

It's a Homebrew (2xEL84) using a Hammond 1620 OT and a

Hammond 272FX PT (300-0-300 at 150mA) .

I took a bias supply "tap" from the Rectifier pin #4

(which is direct from the Main PT secondary). I used

the Fender DR bias supply circuit.

The first bias resistor off the "tap" (which is a 2W 470ohm)

keeps frying.

Any ideas?

Added info:

I'm using a 4007 diode (which is installed so it "points" to the PT "tap")

The e-lytic is a 50/50 (I have this installed backwards, so the "+" goes to ground)

Thanks,

Mook

8/19/1999 2:57 PM

Seems like 470 is way too low for this. Looks

like at 300v you'd be dropping about 1/2 an amp

through this. I am successful using a 150k to

220k. Try the bias circuit type I got off the

Kevin O'Connor web site (I believe) which has

the correct values for what you are trying to do.

If you can't find it, let me know and I'll send

it to you...

You need a high value resistor to tap some E

for a bias circuit from the AC output.

Steve K

8/19/1999 3:00 PM
Carl B.	Hey Mook. 470 ohm right off of an 300-0-300 outer tap? Sounds way too low, unless I'm misinterpreting your setup. I'd think that first resisitor going to a grid-biasing setup would be in the neighborhood of 100k give or take. Ah, I've probably got your setup wrong ...

8/19/1999 5:22 PM
John Stokes	Mook, look at the Princeton Reverb bias circuit. The resistor you use off the HV secondary needs to be like a 100k 2 Watt unit.

8/19/1999 11:10 PM

AAAAHHHH!!! Could I have FRIED my PT???

I seem to have gotten the bias supply working correctly, thanks. I now have -14 to -36 V.

But, My rectifier tube keeps lighting up like an X-mas tree. I disconnected the B+ line, so I only have the Red wires hooked to pins #4 and #6

and the Yellow wires on pins #2 and #8.

I get about 332 VAC on pins #4 and #6, which is good, but nothing (0V DC) on pins 2 and 8. The rectifier glows red, orange, white, and starts blue flashes!!! This, then, pops the fuse.

My recto is a 5Y3, the PT is a Hammond 272BX (300-0-300 at 150mA).

Could I have done something to the secondary winding with the yellow wires???

By the way, the winding with the yellow wires has a Center Tap, I have that running to ground.

Time for a new PT?

How do I check my existing PT to make sure it's OK??

Thanks,

Mook

8/20/1999 12:16 AM
SpeedRacer	pull out the rectifier tube and see if you have the 5V.. sounds like a short in the tube from this quick glance..

8/20/1999 12:29 AM
moocow	Unhook the yellow center tap. It is grounding out the B+.

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