full-wave diode rectifier for non-center tapped transformer. ground issues

[Mr Johnny Birchwood]

Hey guys, I'm experimenting with a huge old GE transformer i found in a junk pile. The tranny puts out 480VAC and is rated at 250VA. No center tap on the secondary. I built a full wave diode bridge rectifier and the first filter cap reads about 650VDC(after an improvised VCR transformer used as a choke). I grounded the amp chassis to earth, and the ground of the circuit of course is the negative side of the diode bridge. The chassis is isolated from the circuit. The input jack is usually grounded to earth grounded chassis when using a half-wave rectifier and a center tapped transformer, but I can't figure out if the input jack should be grounded to earth or to the negative side of the bridge. I've tried it both ways, and the hum just gets louder with the input jack grounded to the -HV, but i get no guitar signal in the output either way. Just curious if anybody has used full-wave rectifiers without an earth-grounded center tap and if somebody can tell me where i should ground the input to before i proceed with troubleshooting the signal path through the preamp tubes. I would like to use this transformer in my next project, a massive push-pull amp with TV sweep tubes running at 600+VDC on the plates, but I need to solve the input ground issue first.

thanx

[Steve Conner]

Simple answer: The negative side of the diode bridge, the negative of the first filter cap, the input jack sleeve and the chassis should all be connected together. Furthermore they should be connected to the green wire of the power cord.

The exact details of those connections will affect the amount of hum you get. In particular, the connection between diode bridge negative and first filter cap negative needs a wire of its own, or you're guaranteed lots of hum. But it shouldn't stop the guitar signal. That points to some more fundamental mistake.

[Mr Johnny Birchwood]

Simple answer: The negative side of the diode bridge, the negative of the first filter cap, the input jack sleeve and the chassis should all be connected together. Furthermore they should be connected to the green wire of the power cord.

The exact details of those connections will affect the amount of hum you get. In particular, the connection between diode bridge negative and first filter cap negative needs a wire of its own, or you're guaranteed lots of hum. But it shouldn't stop the guitar signal. That points to some more fundamental mistake.

Thanx a bunch. I've got the chassis and transformer case grounded to earth(grn wire) but I couldn't figure out if I should ground the neg of the diode bridge to earth as it actually is -340(ish) and earth ground is at 0. Hum is not something i am TOO concerned about right now, hum problems can always be chased down later. I just want to get the input ground issue straightened out before i troubleshoot my preamp section. I'm using a 6sk7 pentode to drive the volume and tone controls and a 6dt8 as a phase splitter driving a pair of 6cg7s(with paralleled triode sections) push pull. This is a true junk box amp, just for the sake of experimentation. I am guessing that the preamp will need a good bit of troubleshooting since I have no previously proven schematic to work from for this arrangement.

[Steve Conner]

If the transformer had a centre tap on the secondary, and you connected it to ground, then yes, the positive output of the bridge would be +340 relative to ground and the negative output would be -340.

But in the absence of that centre tap, the winding is "floating", neither end has any well-defined voltage relative to anything else. So you have to connect the negative end of the bridge to ground, then the positive will go to +680. If you don't connect it, the circuit will be incomplete and you'll get no juice at all.

Keeping the magic smoke in is tricky, but not as hard as getting it into a new untried circuit in the first place.

[Mr Johnny Birchwood]

If the transformer had a centre tap on the secondary, and you connected it to ground, then yes, the positive output of the bridge would be +340 relative to ground and the negative output would be -340.

But in the absence of that centre tap, the winding is "floating", neither end has any well-defined voltage relative to anything else. So you have to connect the negative end of the bridge to ground, then the positive will go to +680. If you don't connect it, the circuit will be incomplete and you'll get no juice at all.

Keeping the magic smoke in is tricky, but not as hard as getting it into a new untried circuit in the first place.

Hmmmm... I had it hooked up with and without the negative side of the bridge grounded to earth, and it had juice in the circuit both ways. Before I switched some leads around I had 630VDC sitting on the plates of the 6cg7's for a couple minutes and the amp produced a loud 120hz hum in both arrangements. Another concern was that hooking the negative of the bridge to earth ground would cook the diodes. I'm gonna rework the filter section anyway with a center tapped 230-0-230 tranny from an industrial ice-machine control unit, as this amp will not pull anything near 250VA that the 30 pound chunk of GE iron delivers and I want the supply-sag tone for the guitar amp. Gonna eventually use the GE tfx to power a push-pull 6LX6(Video sweep pentode-40W plate dissipation) bass amp driven by a _AX7, 6CG7 phase-splitter, and triode-wired 12HG7 video pentodes each driving one of the 6LX6's. With the MASSIVE OPT I sourced I'm hoping to achieve the full 80w at 40hz. Of course, the iron in the transformers alone will weigh at least 50 lbs, but practicality is never one of my main considerations.

[Mr Johnny Birchwood]

I'm gonna rework the filter section anyway with a center tapped 230-0-230 tranny from an industrial ice-machine control unit, as this amp will not pull anything near 250VA that the 30 pound chunk of GE iron delivers and I want the supply-sag tone for the guitar amp.

With the new transformer installed and the filter section rearranged, the amp is working..somewhat. The 600+VDC was WAY too much for this application anyway, and the 230-0-230 yields about 320VDC at the first filter cap thus 320VDC on the ctr tap of OPT and 317VDC on the plates of the parralleled-section 6cg7 power tubes with a half-wave rectifier. I moved the choke so that it is after the first filter cap, and ran it in series with a resistor for my pre-amp supply voltage. Not getting much gain out of the 6sk7 drive/6dt8 splitter sections. The amp produces a weak yet clean guitar sound only with the volume knob above 8. Absolutely no AC hum, which, when considering the general shoddiness and haste of my build is a dead giveaway that I am lacking gain in the pre-amp tubes. The 6dt8 can essentially be treated as a 12AT7, I am told, but I have no experience with the 6sk7 or using pentodes as preamp tubes in general. If anybody has any experience with this tube, please tell me: What are the typical operating points for the various sections of a 6sk7(if one is seeking high-gain operation as a preamp tube). What voltages do I want to see at the plate, screens(screen pin 3 is grounded to earth) and cathode? What kind of cathode current draw on the 6sk7? What kind of voltage/current do I want to see on the cathodes of the 6cg7's?(they are tied together and again: used here as power tubes)