First time fire up problem: PT getting hot...

[smurf]

Hi folks,
I just finished a 36watt marwatt, and I'm using the hammond 370hx PT (http://www.hammondmfg.com/300series.htm)
The HT & 6.3V secondaries seem to work just fine (reading 550V & 5.9V on my meter), but the 5V secondary (CT) reads out 0V between the filament leads, and about 2-2.2V with respect to ground. The wires get awfully hot aswell as the PT itself.
I've hooked the wires up the way it should: yellow(f)-yellow/black(ground)-yellow(f)
I haven't got any tubes installed yet.
Anyone knows what could cause this ?
Thanks !

[57chevy]

I've hooked the wires up the way it should: yellow(f)-yellow/black(ground)-yellow(f)
I haven't got any tubes installed yet.
Anyone knows what could cause this ?

DANGER!!! Do not connect the 5 volt CT (yellow/black) to ground. Just tape it up and tuck out of the way.

[smurf]

Oops, didnt know that
It seems to be ok now
thanks!

*edit: just put the GZ34 in its place, and after about a minute, both fuses popped . Guess i'll have to do some searching..

[smurf]

I'm still getting the blown fuses after i put in the GZ34.
It goes well for less than a minute, then i hear a short buzzing sound from the PT it seems, and then the mains fuse blows out.
Does anyone have an idea what could cause this ? Faulty tube ?
thanks!

[what_wires]

You didn't merntion if the rest of the tubes are installed. Have you tried powering it up with just the rectifier tube installed?

[smurf]

yes, it happens with just the rectifier installed, no other tubes and with only the power switch turned on, not standby. Without the rectifier installed, the fuse-popping doesn't happen.

[smurf]

A few days ago i went and got myself another rectifier (TAD RT515, SS direct replacement for GZ34 and other octal rectifiers), and before inserting it i powered on the amp to recheck if all the voltages were ok. I was rather dissapointed when i found out that the PT had died...
Now before i install a new PT, i'd very much like to know what went wrong.
I based the powersection on the 36watt TMB v2.0 layout in the downloads section. Only difference is that I had a 6.3V secondary with CT, so i didnt use the two 100 Ohm resistors but the CT to ground.
Any input as to how this could have happened is greatly appreciated!
thanks!

[zaphod_phil]

Usually means you had a short somewhere. Possibly either a short to ground/chassis in the wiring somewhere, or maybe even a short inside the PT itself. The fact that you were seeing 0V across the 5V heater winding sets a lot of alarm bells ringing.

[57chevy]

I've hooked the wires up the way it should: yellow(f)-yellow/black(ground)-yellow(f)

Connecting that 5VAC CT to ground is a likely suspect for killing the PT, especially if you turned it on for any length of time with a rectifier plugged in.

[smurf]

I didnt have the rectifier in it when the 5V CT was connected, but i won't be making that mistake again now
thanks for the help ppl!

[chabby]

Why can't the 5V CT be hooked to ground? The reason I ask is because I want to learn...and I've seen them hooked to ground before.

[57chevy]

The filament in most 5 series rectifier tubes is also connected to the cathode. The cathode will have 300, 400, 500 volts B+ on it. The 5 volt filament winding probably only has 1 or 2 ohms resistance. So connecting the 5 volt CT to ground is like putting a dead short on your B+ line to ground.

You may have seen a 6 volt CT connected to ground, but I doubt you've seen a 5 volt CT connected to ground.

[chabby]

Thank you - I wonder if thats what I've mistakenly done with my hammond AO43 PT? I connected a wire that was (or seemed to be) connected to ground in the amp it came from so I did the same (5u4GB) in this case. But that wire has been getting hot too. But looking at pix of another guys project he seemed to connect it ground without issue.

Just similarities that are making me wonder - why else would a ground wire get hot like that? I just built the exact amp no more than a week ago, so I know I hooked the rest up alright, but that was a heyboer tranny made for the 18 watt.

[mcrracer]

Thank you - I wonder if thats what I've mistakenly done with my hammond AO43 PT? I connected a wire that was (or seemed to be) connected to ground in the amp it came from so I did the same (5u4GB) in this case. But that wire has been getting hot too. But looking at pix of another guys project he seemed to connect it ground without issue.

Just similarities that are making me wonder - why else would a ground wire get hot like that? I just built the exact amp no more than a week ago, so I know I hooked the rest up alright, but that was a heyboer tranny made for the 18 watt.

Chabby, I don't mean to sound harsh But this is the difference between reading a schematic and knowing what you are doing and looking at a layout and trying to "paint by numbers" and hoping everything is OK. You really need to learn alittle bit about this stuff first. It is agreat hobby but like any new endeavor you do need to learn a little bit before enjoying it.

[chabby]

Thanks for the advice - I did read a schematic of what I built and downloaded several schematics of the AO43 too. But the configuration of the AO43 was different than what I'm used to seeing and somewhat incomplete. A tranny is a tranny I suppose, but this one didn't come with specs like my heyboer did. I looked at the schematic on the AO43 but only to try and reference what the PT was about. It does have 5V windings but I couldn't find whether or not that CT was hooked to ground or a resisitor. I can tell you from loking at the old amp it came from it was indeed hooked to ground on the Recto tube frame just like the CT of the heaters. I guess I know just enough to be dangerous, but that's also why I'm here: to learn

[mcrracer]

OK Man, Just concerned about your well being. Stay safe and have fun building. Man I love this hobby!!!

[chabby]

Me too and thanks for the wel wishes - same to you!