Transformer transplant question

[glebert]

I'm dipping my toe into fixing a Crate Vintage Club VC60 that had shorted out power transformer. Apparently this is not that uncommon for that model. Only direct replacement I could find is Mercury Magnetics and is way more $$$ than the amp is worth. The amp was given to me for free, so I'm not super invested in it, but I do love the sound of the other VC amps I've played. If I can make it work basically with what I have I will, otherwise probably not.

I have a set of iron from a Laney Pro Tube 50 (pre AOR) sitting here and was hoping to use this for the high voltage and 6.3V filaments. That model didn't have the +/-12V so I found a small 120V/12V tranformer to take care of that.
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Here's the issue.. The 12V and filament power supplies seem to be working fine. The high voltage stuff, not so much. I don't know if the original Crate PT had an internally grounded center tap for the high voltage winding (original PT is gone so I can't check anything from that). The diagram in the schematic implies there is a CT there but I'm not sure that is what they mean. The Laney PT does not have a CT on the high voltage taps. I've only taken the primary voltage up to about 60V (so half of full voltage), but the voltage before the standby switch is only at +13VDC, but at J38/J39 I'm getting -86VDC. I absolutely suck at visualizing how rectifier circuits work, so I'm struggling with what is going on. The only thing I can think is that this rectifier design assumes the secondary of the PT gives the ground reference, but without that the bias circuitry is back charging it.

Here's the schematic for the Crate.
Crate_VC60_VC120_07S271-01_sch.pdf

Can't find a schematic that really matches the donor Laney, but this is the AOR 50 model that is supposed to be very similar.
Laney-AOR50-Schematic.pdf

[Helmholtz]

Yes, the VC 60 uses/needs a HV secondary with a grounded CT as it uses a 2-phase/full wave rectifier.
You can't use this type of rectifier without a CT.

The Laney schematic shows a bridge rectifier without a CT.

[glebert]

Realized I still had the original Crate PT (recycling hadn't gone out yet) and if I measure across the secondaries I get continuity to the tap that goes to D35 and resistance from each leg to the CT is about 1/2 of the total tap resistance.

Assuming this is why my approach didn't work, the next question is if there is a reasonable way to work around this. I was thinking converting the high voltage rectification to something closer to the Laney AOR schematic, using a panel mount bridge rectifier and grounding the negative side. I would then just bypass the rectifier diodes on the PCB. I don't know why these rectifiers use 2 or 3 diodes in each leg though, more of my rectifier ignorance.

Crate


Laney

[The Dude]

What's wrong with the original transformer. I ask because if it's an open primary, there may be an internal thermal fuse that could be replaced.

[glebert]

What's wrong with the original transformer. I ask because if it's an open primary, there may be an internal thermal fuse that could be replaced.

It was shorted, not open. It was making my LBL glow bright with all secondaries disconnected.