Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: BlackCrowe604 on April 17, 2025, 04:16:32 am
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Hi Everyone,
I got a new OPT for a vintage hiwatt that had a blown musicman OPT installed. The old non-original OPT had leads for 4 & 8ohm only.
I have recapped the amp, and am now installing a Mojo "partidge style" remake.
I just wanted to check-in regarding the connector used for the impedance switch. I haven't seen one of these before so bear with me for a moment.
It appears that the 4 & 8ohm selector lugs were/are jumpered together.
It also appears that the 16ohm and adjacent lug were jumpered together, and the adjacent (unused) lug has the NFB connected to the 16ohm tap.
So my question is:
Am I correct if I remove the jumpers and just attach the 4/8/16 OPT taps? 16ohm would have had the NFB connected, I would just wire it to the same lug as the 16ohm and not use the extra lug.
The reason I ask is there appears to be the hiwatt style "black paint" over these lugs with the jumpers, so I am not sure if they are original or not. My feeling is that someone added the paint, and added the jumpers to prevent a no-load condition when using a different fender OPT originally.
Photos and schematic attached.
OPT part no is mojo785: wiring is here: https://www.mojotone.com/Hiwatt-50-Watt-Output-Transformer?srsltid=AfmBOoqevbfatmwk28gontYeD0vPxsYNEwTLY0TnSvqUuDHd5Mjjo33y
amp is DR504, schem is here:
After looking at the "archive" photos below, looks like the selector tags are in fact jumpered together, and original OPT does not have seperate taps for each impedance?
https://hiwatt.org/pix.php?p=DR505_78_chassis
https://hiwatt.org/pix.php?p=DR504_topR
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Why not doing wiring like a DR103 ?
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I can, I am just wondering why the jumpers connecting the lugs. If there was a reason.
Amp is 504, not 103.
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I can, I am just wondering why the jumpers connecting the lugs. If there was a reason.
The OT only had 4 and 8 ohm taps. But the switch has four positions. The lugs were jumpered together so there would always be a tap connected to the speaker jack. IOW, never gonna be a switch position that selects an open circuit to the speaker jack. It's all about protecting the OT from an open circuit.
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I can, I am just wondering why the jumpers connecting the lugs. If there was a reason.
Amp is 504, not 103.
Does't mater it is the same secondary 16-8-4 ohms as yours.
Switch wiring is the same.
Or there is something I don't understand ?
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I can, I am just wondering why the jumpers connecting the lugs. If there was a reason.
The OT only had 4 and 8 ohm taps. But the switch has four positions. The lugs were jumpered together so there would always be a tap connected to the speaker jack. IOW, never gonna be a switch position that selects an open circuit to the speaker jack. It's all about protecting the OT from an open circuit.
Yes, this is what I was wondering. Makes sense. I thought maybe the jumper put them in paralell or something to get the 8ohm tap down to 4, but that doesn't make sense.
Looking at the rest of the photos on Mark HUSS's page, I see only two taps like you said.
Thanks!
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I can, I am just wondering why the jumpers connecting the lugs. If there was a reason.
Amp is 504, not 103.
Does't mater it is the same secondary 16-8-4 ohms as yours.
Switch wiring is the same.
Or there is something I don't understand ?
You're right, except that the stock amp I guess only had two taps as mentioned by SEL49, explaining why jumpers are present. I was curious about the original config.
I will delete the jumpers and wire as per 104 schem.
Thanks both of you!