Power Trans wiring

[MacGwyn]

Having trouble getting suitable B+ out of this classic tone 40-18029.  It's for a Fender Brown Super clone I'm building.  It has dual voltages, so...
On the primary side, I've got black and brown wired to the hot/fuse/switch side of AC... and brown/white and black/white to the common AC, for 120 volts.
On the secondary side, I've tried Red/Red to GZ34 pins 4 and 5... and the two yellows to pins 2 and 8... which should give me around 700 volts off pin 8... but I'm showing only around 50 volts.  Center tap red/yellow is not connected.  Any ideas on what to try?

[shooter]

I'm showing only around 50 volts

start simple, leave the secondary un-wired and measure Volts AC red to red.  Make sure power is OFF, clip meter leads, tape leads, meter set to VAC......Then power up, I would guess you'd be getting close to 900vac unloaded!!!!!  so make sure your meter can deal with 1000 volts AC!!!

[sluckey]

I've tried Red/Red to GZ34 pins 4 and 5...

Connect to pins 4 and 6!

Center tap red/yellow is not connected.  Any ideas on what to try?

Yes. The center tap must be connected to ground! Then you will get either 500VDC or 430VDC (unloaded numbers) at the first filter cap.

[MacGwyn]

I've tried Red/Red to GZ34 pins 4 and 5...

Connect to pins 4 and 6!

Mis typed - they are connected to 4 and 6.  I'll connect CT to ground now - thanks!

Center tap red/yellow is not connected.  Any ideas on what to try?

Yes. The center tap must be connected to ground! Then you will get either 500VDC or 430VDC (unloaded numbers) at the first filter cap.

I've tried Red/Red to GZ34 pins 4 and 5...

Connect to pins 4 and 6!

Center tap red/yellow is not connected.  Any ideas on what to try?

Yes. The center tap must be connected to ground! Then you will get either 500VDC or 430VDC (unloaded numbers) at the first filter cap.

[eleventeen]

Sluckey has it: pins 4 & 6 [are the correct pins for the AC HV feed wires] AND the CT has to connect to "ground".


It is also true that if you are powering this up in an incomplete state, eg; before the rest of the amp has been built and wired in...perhaps you have not yet connected the negatives of the filter caps to "ground" (where you now know the CT has to go) That connection has to be there.


Incidentally, if you have an old 5Y3 laying around, this is a much cheaper tube to replace should you do something that could blow up the rectifier tube and has the same basing diagram as a GZ34. I happen to have piles of them, others may not. But in a new build, I usually keep the 5Y3 in there as long as possible just to avoid smoking a GZ34.

[MacGwyn]

Center tap to ground was obviously my main problem.  Switched to the lower powered secondary for fun... 2 red/white wires to the GZ34.  Now getting 312 volts dc at pin 8.  That's much higher than before... and now the bias wire is putting out the required 50 volts (it was around 20 volts before). 

[Willabe]

but I'm showing only around 50 volts.  Center tap red/yellow is not connected. 

You need to hook up the CT to ground connect a filter cap with a high enough DC voltage rating to pin 8 of the rectifier tube before you'll get a stable DCV reading.

With no CT going to ground the PT secondary can't pull/develop any current/voltage through the secondary B+ wind.

You also need to build and use a lamp limiter to test for shorts on new builds. Here's a link;

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

[MacGwyn]

Good word on the lamp limiter... I always use one when working on projects!