Peavey 6505+ PT Replacement

[Justin Thomas]

Hi All,

I picked up some used tubes on FB Marketplace recently & the seller threw in the blown amp head (he chopped the combo) for cheap so I took it. A prior diagnosis by a tech had the PT listed as open primary, which I did verify. I'll have to go through & check for other blown stuff; I've read a little about heater rectifiers failing in these & taking out PTs...

The original PT seems either unobtainium (I do have one possible contact) or stupid expensive. So I'm wondering about using an off-the-shelf Hammond. The original has 4 secondaries - HV, AC Heaters, DC Heaters, & the bias/relay supply.

My thinking is, I can get a PT with a 6.3VAC supply & can put half the tubes on AC & then rectify it halfway downstream for DC or just rectify & put all of the tubes on DC, use a ~50VAC winding for the bias & relays, & it'll have a HVAC winding.

I'll obviously have to work out new values & stuff, but there's a good bit of room inside the chassis to build a new power supply & I can just bypass all the stock bits that are bad or unneeded. This is definitely a "step up" in complexity for me. Any downside switching from the HVAC being bridge rectified to being regular full-wave rectified? Any reason to not go with something a bit more "conventional"? I think I can simplify it a little bit... I know he loved the amp & so I agreed to sell it back to him for my purchase price plus parts cost if I can revive it.

Thanks for any insights, cautions, etc.!

Jusrin

[Jon Snell]

Wouldn't this suit ... https://www.tubeampdoctor.com/en/mai...6505-112-combo
This won't fit the 120W version!
Unfortunately, manufacturers only need to supply specific spares for up to 7 years from end of production and the Peavey spares supplier in the UK doesn't even list these tramsformers.
No idea where you are in the World.

I would get it rewound.

[Justin Thomas]

I'm in the States. I just saw it's a 240V primary; I only found results for 120V when I searched & all of them were discontinued. I could look into getting it rewound. Thanks for the options! Could throw in a step-down...

Jusrin

[tedmich]

Antek has the cheapest transformers usually Jusrin, if you can deal with a toroid!

You can always get the bias tap with a voltage doubler from a 24VAC tap, like a JSX does.

Or at those prices add a tiny dedicated bias transformer!

[g1]

If it was a combo originally, you need the combo schematic (attached).
Rather than calling it a bias winding, I'll call it a LV winding, which is used for the +/-15V circuits. The bias uses a voltage doubler to get bias voltage from the LV winding.

[Justin Thomas]

@g1 I've got the combo schematic. I'm mainly just trying to do it on the cheap, maybe practice my chops a bit; I guess technically I own it to fool around with. I just noticed there's lots of trannies available for cheap where I could "kludge" something together that "works" (diodes & resistors & caps are really cheap) & if it works I can get it back to its previous owner cheap too. I'll look at finding some trannies with LV windings. Thanks for the tip!

tedmich I have no issues whatsoever with toroids. If I find one that works & is reasonably priced, I have no problem using one. Thanks for the vendor suggestion!

Jusrin

[drewl]

Open primary, did you check to see if the power trans has a thermal fuse under the windings?

I've repaired several transformers that way.

[DrGonz78]

Open primary, did you check to see if the power trans has a thermal fuse under the windings?

I've repaired several transformers that way.

Heck yeah!! I fixed a friend’s transformer and I reinstalled a thermal fuse. Cost like $10 for parts! Time to dissect the problem.

[Justin Thomas]

Update.

drewl & DrGonz78 y'all called it. I opened up the amp & the PT says right on top "Thermal Fuse Protected." So I opened it up & did some dissection. Found the fuse under a few layers of insulation. Looks like DigiKey has a suitable replacement.

But beyond that, one of the spade connectors for the heater winding (DC heater supply) is brown. The metal connector itself looks clean as does the terminal at the PCB; board also looks good on a surface inspection... The winding has continuity (0.5R across the green winding). I'll go about checking related components on the board in the circuit & I'll go over solder joints with a magnifier to see what might have cooked that terminal.

How about getting the transformer insulation back together? Any recommendations on a product to patch it back together? Fish paper? Electrical tape? Nothing too thick so I can get the bell end back on. This is like a thin plastic material. I'll also have to get the copper band back on it & around it... Might just use some of my copper shielding tape I got years ago...

Any other stuff to check out? Definitely a much more complex amp than I'm used to... I'd like to make Resident Peavey Wizard Enzo smile. This amp really seems well-built for what it is & at the price point. Once I found a few tricks out it was easy to disassemble & put back together. I'm impressed.

Thanks again everyone!

Jusrin

[Tom Phillips]

Good work on diagnosing the thermal fuse issue. I'd be tempted to just bypass the thermal fuse myself but that's not a recommendation for others. I'm guessing that their modern use is due to some regulatory agency requirement.

As to the insulation restoration, the repair may depend upon how the original insulation was disturbed during your surgery. Maybe, just a layer of insulation material cut to size and laid loose between the core and the end bell would be sufficient.

[Justin Thomas]

Thanks Tom Phillips. That'll definitely do the trick. It was just a basic "cut, peel back, cut, peel back." Any ideas on materials to use? I could easily get some small pieces of electrical tape in there to just patch up the original but if there's something better out there I'm gsme for buying some.

Jusrin

[DrGonz78]

I got some kapton tape during a more recent repair. It works pretty good and it is good to have on hand. I can use the tape to cover over heat sensitive areas of a circuit board when doing surface mount work.

[Tom Phillips]

Thanks Tom Phillips. That'll definitely do the trick. It was just a basic "cut, peel back, cut, peel back." Any ideas on materials to use? I could easily get some small pieces of electrical tape in there to just patch up the original but if there's something better out there I'm gsme for buying some.
Jusrin

In my experience, standard electrical tape will not stay in place when the transformer heats up when the tape is put on as a "patch' rather than being wrapped around onto itself. Kapton tape, as mentioned my DrGonz78 is good. The sell site only lists narrow versions and, if it's the real deal, the wide tape can be high cost. It is also usually wrapped around onto itself to stay secure.

I have used super glue to tack a coil in place and to attach an insulating layer. Hi pot tests showed no issues with the super glue breaking down the insulation coating on magnet wire if the glue contacts the winding layer.

For a patch, I have used flexible PVC sheet or other materials salvaged during old equipment tear down. Some transformers also have a loose piece of fish paper under the bell cover. Attached is a gut shot picture of one example showing what was under the end bells.

[Justin Thomas]

IT LIVES!

Thanks to everyone for the input & suggestions. I ordered some thermal fuses (minimum was 25 dagnabbit) that were 3A 250V but only 115°C instead of the 150°C that was in it. Everything else DigiKey had was 185°C or above so I went underrated rather than over. I let it cook for about an hour tonight; fun amp, but not my thing, & it made me miss my Prosonic - it had better reverb & I think even more ridiculous gain. I'll cook the Peavey more on my next day off. Then figure out what to do for tubes.

I wish I'd taken some pics, but a new thermal fuse & a few strips of Kapton tape to secure the layers I had opened & it seems good to go. I'm wondering if a loose connection on one of the spade connectors on the board (for the DC heater supply) could have been what cooked it initially? There's no charring or signs of obvious damage besides the browned plastic cover on one of the winding terminals, which I removed. I crimped all of the spade connectors a bit tighter & that's pretty much it.

Learned a bit about modern amp assembly too. And I think this one is pretty good for the price range - they were about $600 new, I think?

Jusrin

[catalin gramada]

That called " stretch tape" - thermal resistant and have elastic propriety to add pressure tension over the coil with each layer you add.