1972 OR80 - Restored on Turret Board - My First Build

Orange Amps Technical Q&A's

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thedudear
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1972 OR80 - Restored on Turret Board - My First Build

Post by thedudear » Wed Apr 21, 2021 2:14 am

Hello All!

A few years ago, I picked up a "working" OR80 off Reverb. With my luck, it was junk. Had a horrible parasitic oscillation, and when I opened it up, I discovered it was a 72 OR80 converted halfway into a 74+ circuit. I say halfway because it was really poorly done and hacked together. Permanent damage was done to the PCB, so I decided to gut it and start fresh with turret board construction which I've finally begun.

My question today is regarding the rectifier section. The amp came with a separate connection on the PT labeled "SC-"(It appears it could be SCR, the last letter was hit with an iron) which I assume stands for Silicon Controlled Rectifier and was wired to the DC "negative" side of the rectifier bridge, which on the schematic goes to the ground. This is questionable to me. The center tap of the 190/0/190 secondary is correctly wired to the middle of the two larger filter caps.

Ultimately, my question is what is this "SCR/N" connection? A dedicated ground for the rectifier? Or a ground for the transformer? Should I just ground the rectifier connection the way it is on the schematic? The SCR terminal is the closest to the faceplate of the chassis. I haven't seen any photos online where the labeling was visible. It's definitely the original Parmeko transformer.

I will post pictures of the rig when it's done.

EDIT: I'm beginning to think it's a transformer ground. Let me know if anyone sees this and agrees!

EDIT 2: Final build pics below!

Cheers!
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Last edited by thedudear on Wed Apr 28, 2021 3:53 am, edited 3 times in total.

Jondog
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Re: 1972 OR80 PT Wiring "SCR"

Post by Jondog » Thu Apr 22, 2021 11:32 pm

Never worked on one of those amps, but maybe see what windings it's connected to. Schematic will definitely help you.
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thedudear
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Re: 1972 OR80 PT Wiring "SCR"

Post by thedudear » Fri Apr 23, 2021 1:06 am

Hi Jondog, thanks for the reply!

I have repeatedly referenced the schematic, the 72 circuit schematic makes no reference to this connection. The DC negative side of the bridge is connected to the ground, not the transformer (as it was previously wired).

However, the 74' schematic does show a ground connection to what appears to be the core of the transformer, which is why I'm thinking it might be what we're seeing here. Let me know you're thoughts!

Thanks!

Nick

Jondog
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Re: 1972 OR80 PT Wiring "SCR"

Post by Jondog » Sat Apr 24, 2021 10:50 am

If it’s an original transformer and schematic I’d follow that and check all your connections. Should be a ground somewhere.
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bclaire
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Re: 1972 OR80 PT Wiring "SCR"

Post by bclaire » Sat Apr 24, 2021 11:52 am

At this point, I'd seriously consider having an amp tech look at it.

Where are you located?

thedudear
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Re: 1972 OR80 PT Wiring "SCR"

Post by thedudear » Sat Apr 24, 2021 11:15 pm

I wound up (Har Har) connecting that point to ground as it was when I got it. That, and grounded the positive side of the bias supply (which also is not grounded according to the schematic) and the amp is powered up and working great. Biased the KT77s to 42mA and this thing shouts!! I'll have some videos and pictures up soon. Plate voltage is pretty high at 560 volts, I'll probably end up swapping the mains primary PT connections to the 115-240v winding to bring the B+ down a bit.

Cheers,
Nick

thedudear
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Re: 1972 OR80 - Restored on Turret Board - My First Build

Post by thedudear » Wed Apr 28, 2021 3:37 am

Here are some photos of the final build. This forum seems sort of quiet at times, I hope to get some feedback! This is my first "build". I've done repairs on other amplifiers here and there, but have had plenty of experience with projects of the high voltage sort in my past. What a joy it is to actually play one of your projects!

I wanted the preamp tube sockets to be accessible, as well as have the heater wires come straight upward off the tube sockets, which is why I pushed the turret boards off over the OT. This has *mostly* been okay, except for the inductor in the HF Drive circuit at the very bottom of the board. Some oscillation or feedback occurs as treble and HF Drive is dimed, and the voltage across that inductor increases during the event. I assume this is due to the inductor coupling with the OT, and there is some work to be done here, perhaps in the form of a shield to go over this portion of the turret board.

I ended up reconnecting the incoming neutral to the 115v Primary PT connection and placing the voltage selector at 240v. This lowers the secondary voltage to a more acceptable range. I still have 475 volts at the screens, however. I biased the amplifier to 34/35.5 mA (slightly mismatched tubes) with E34L's. This thing is loud!

I have a new set of knobs from Orange to install, however, they seem to be a slightly smaller spindle size than the pots currently installed have. I will drill them out slightly to fit, and then install them.

I ran out of the red and black stuff, so the preamp heater wires had to be done with white wire. Rest assured they are wired in phase.

The marshall cab it's hooked into is an 87' 2551A with its original G12T75's.

Hope to hear some feedback/comments/concerns. All criticism welcome, I'm still learning about amplifiers.


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Last edited by thedudear on Thu Apr 29, 2021 5:36 am, edited 1 time in total.

bclaire
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Re: 1972 OR80 - Restored on Turret Board - My First Build

Post by bclaire » Wed Apr 28, 2021 2:49 pm

Looks great!

Jondog
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Re: 1972 OR80 - Restored on Turret Board - My First Build

Post by Jondog » Fri Apr 30, 2021 12:57 am

Looks good. My only real criticism would be to try and get those big filter caps secured a little better. The small width turret board isn’t ideal for longer cals and bending the wire underneath like that and having them raised makes them susceptible to movement and weakening on the solder joint. May never be an issue, especially if the amp isn’t being trucked and thrown around, but for me it’s not ideal. You could ty-wrap them down or try some silicone or hot glue to secure them.
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thedudear
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Re: 1972 OR80 - Restored on Turret Board - My First Build

Post by thedudear » Sun May 02, 2021 5:33 pm

Jondog wrote:
Fri Apr 30, 2021 12:57 am
Looks good. My only real criticism would be to try and get those big filter caps secured a little better. The small width turret board isn’t ideal for longer cals and bending the wire underneath like that and having them raised makes them susceptible to movement and weakening on the solder joint. May never be an issue, especially if the amp isn’t being trucked and thrown around, but for me it’s not ideal. You could ty-wrap them down or try some silicone or hot glue to secure them.
That's a good insight, over time the leads might fatigue. You're correct that it won't be lugged around. The leads do "wrap" around underneath the board and back up through an adjacent post. I will figure something out to secure them next time I break into the amp. For now, I'm enjoying it quite nicely.

Nick

Jondog
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Re: 1972 OR80 - Restored on Turret Board - My First Build

Post by Jondog » Thu May 13, 2021 1:55 pm

thedudear wrote:
Sun May 02, 2021 5:33 pm
Jondog wrote:
Fri Apr 30, 2021 12:57 am
Looks good. My only real criticism would be to try and get those big filter caps secured a little better. The small width turret board isn’t ideal for longer cals and bending the wire underneath like that and having them raised makes them susceptible to movement and weakening on the solder joint. May never be an issue, especially if the amp isn’t being trucked and thrown around, but for me it’s not ideal. You could ty-wrap them down or try some silicone or hot glue to secure them.
That's a good insight, over time the leads might fatigue. You're correct that it won't be lugged around. The leads do "wrap" around underneath the board and back up through an adjacent post. I will figure something out to secure them next time I break into the amp. For now, I'm enjoying it quite nicely.

Nick
You could use that empty tube socket hole and wire in a can capacitor if you ever felt the need.
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