Carvin power transformer - am I really seeing this?

[pjd3]

Heyo all,

I could really use a sanity check here if you could.

I'm using a donated Carvin V3M combo am as a donor for a JCM800 2204 build that a friend gave me. When I got the amp (his amp tech refused to work on it) the first thing I did after gutting it was to check the voltages on the power transformer. So, this is what I found (at first).

2 red wires - 610 vac (figured that must be the HV coil)
1 Yellow wire - (thought this might be the HV center tap and did infact measure around 300vac from this wire to each of the red wires. Per usual HV with centertap I figured.
2 green wires - measure 6.3 or so, typical heater coil, no surprise there.
2 brown wires - measured close to 20vac - figured that is the coil for the effects or relay switch (or both).

Thats all the wires from the secondary. BUT!!! Then later on, I noticed that the yellow wire is measuring common to all the rest of the wires. This is while curiously measuring the ohms of the various secondary coils. Then, I did more AC measurements on the secondary and found that this Yellow wire is acting like the centertap for all the other wire color pairs. It splits the 600vac in half, splits the 6.3vac in half, and finally splits the 20vac in half. Ha. I am really a baffled little boy over here now.

Is this even possible? I mean, I've only build 2 complete amps but normally assumed that each coil had its own private centertap. And now this.

Does anyone have any explaination for this? I was really hoping to use this amp for a JCM800 2204 type build but, now I don't even know what I'm working with with this strange behaving power transformer.

Has anyone ever seen something like this?

Thanks everyone. This was just very unexpected!
Best,
Phil D

[xtian]

The V3M schematic is available (though they greyed out the tone stack values)--did you check it out?

[wpaulvogel]

I’ve never heard of anything like this before but it’s possible. It would have to be ganged together in the final construction of the transformer though. Each of the windings are separated and insulated from each other when winding a transformer. I don’t agree with placing all the center taps together. I think it’s a bad idea to locate the grounding point of all the center taps at a single point because of noise but it appears that they did it here. I’d spend the less than $200 on a proper transformer for your build to ensure a better outcome.

[brewdude]

If all the center taps go to ground…. Do they have to be separated?

[wpaulvogel]

If all the center taps go to ground…. Do they have to be separated?

It’s generally a good idea to place the separate center taps at different locations in the ground plane because they have different current draw. The HT winding has significant current through it and should be connected to the HT filter-. The heater if using AC heater has no current on the center tap and is quietest referenced to ground away from the HT center tap.

[sluckey]

I have a couple PTs that have the HV CT and filament CT connected together internally with a single wire exiting the bells. Works fine. You can use that PT for your 2204 build if the HV and filament windings have the proper voltage/current ratings.

[pjd3]

Thanks everyone for the generous responses here:

I did look at a schematic for the VM3 (but, not since the internal common ground discovery). The PT was drawn out more like a diagram than a schematic but will have to go back and look more closely. A normal schematic would have probably made the PT orientation much more clear.

I believe the voltage/current ratings are fine to run a "safe" amp, its just going to be a matter of whether 400 approximate volts will suffice for the tone/feel of the amp. I was willing to give it a go as I've heard numerous report of guitarists preferring a lower plate voltage on their EL34's. I have a pair of essentially new Winged C EL34's and about 4 Mullard xf2 Blackburn EL34's. I have no idea how well the Mullards are up the the task (old as hell and pulled out of a Leak hi fi tube amp).

The 20 volt tap is yet a mystery in terms of current rating. I was going to use that as a source for 12 volt relay power and maybe a source for DC filament current for V1 and V2. That might be a bit optimistic (the wire gauge is pretty thick, though) - I've looked for specs on this transformer with no luck so far.

So, Sluckey gives me a bit of hope that this could possibly work. The PT originally drove a quad of EL84's and 4/5 12ax7's so, I did some math and it looks like its at least up to the heater and plate current requirements. Just don't know about the 20vac. I'll keep hunting for that.

Thanks again all for your comments on the "Red headed step-child". Ha, I hate that analogy. Don't think I'll use it anymore. And I will be ready to vie for a new PT if that's what needs to happen.

Best,
Phil D.

[bepone]

noise wise it is not a good idea to ground heater central tap where are the highest currents in the amp (high voltage winding and center tap-highest current and noise)
should be opposite, heater grounding to be where are the lowes currents, so input jack grounding, some preamp gnd spot, chassis close to preamp , etc etc

mentioned PT from topic has probably all the windigs wound separately, and then to save some job and wiring they connect all inside the transformer. nothing "good" and "best" , "usable" category only. for low gain amps ok.

[martin manning]

I’ve grounded the heater CT with the HV secondary CT many times with no issues. Just be sure to ground the reservoir cap there too.