Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mike_J on April 28, 2015, 08:01:15 am
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I am trying to repair a friends amp. He has terrible output jacks in the amp so I want to replace them. The amp has separate 8 ohm and 16 ohm jacks instead of an impedance switch.
If I use output jacks with a tip, shunt and ground lug do I need to connect the shunt to the ground so the tap that is not in use is grounded?
Thanks
Mike
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No, you don't want to do that. you only want one "wire" from the secondary grounded, no taps. unused taps shouldn't connect to anything.
I imagine the jacks you are replacing are non-switched, or without the shunt. I'd replace with the same style jacks. If all you have is a switched jack, then leave the shunt unconnected.
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No, you don't want to do that. you only want one "wire" from the secondary grounded, no taps. unused taps shouldn't connect to anything.
I imagine the jacks you are replacing are non-switched, or without the shunt. I'd replace with the same style jacks. If all you have is a switched jack, then leave the shunt unconnected.
Terminalgs
My goal is to replace these low quality jacks and to isolate the grounds on the jacks and secondary and ground them in the same place the B+ and screen caps are grounded.
You are correct that the jacks I am replacing are the non-switched type. I was considering using long bushing Switchcraft type 11 jacks but don't have any on hand. The long bushing jacks are better when using the isolation washers Doug sells when using a thick chassis which this amp has. I have a number of Cliff mono jacks on hand. They have shunts on both the tip and the ground but are isolated by design. I would prefer to use them if possible.
Please confirm that I would tie the 8 ohm secondary to the tip of one of the jacks and the 16 ohm secondary to the tip of the second jack. I would then tie the grounds of both jacks together and run it to the ground for the OT secondary which has been isolated from the chassis. At no point do I want to make a connection to any of the shunt connections.
Thanks
Mike
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Yes, that would work fine.
FWIW, there is no advantage to tying the OT's secondary to the same ground as the power tube cathodes, or the reservoir cap's ground. it doesn't hurt anything, but it doesn't really provide any benefit. If you have NFB, a slightly better place to ground the speaker output is the ground under the cathode of the stage were NFB is applied (usually the PI of LTPIs, or the gain stage just prior to the PI on cathodynes...).
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> ground for the OT secondary which has been isolated from the chassis.
If the amp has NFB around the power section, you usually MUST ground one end of the secondary to complete the NFB loop.
Truly "floating" speaker outputs are rare special-cases. Why would you want that?
(There is even a potential safety risk: if the OT insulation breaks-down primary-secondary but not to core, the secondary could be pulled to + 400V DC. The amp will play fine even after it kills its musician and technicians. Grounding one end assures it can't have dangerous voltage on it.)
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> ground for the OT secondary which has been isolated from the chassis.
If the amp has NFB around the power section, you usually MUST ground one end of the secondary to complete the NFB loop.
Truly "floating" speaker outputs are rare special-cases. Why would you want that?
(There is even a potential safety risk: if the OT insulation breaks-down primary-secondary but not to core, the secondary could be pulled to + 400V DC. The amp will play fine even after it kills its musician and technicians. Grounding one end assures it can't have dangerous voltage on it.)
PRR
This is an AC30 clone that a friend of mine purchased second hand for his recording studio. It wasn't working when he bought it but I got it to work with the exception that it has too much hum to be used to record with. This amp was constructed with grounding points all over the place. I made a Hoffman AC30 board, used 10 gauge buss wire (because I had some lying around) and plan to use grounding techniques that have worked for me in the past.
The AC30 does not appear to have negaitive feedback that I can identify. Certainly nothing going back to the speakers as most amps have.
I probably should not have used the word floating if I did. What I meant was I was isolating the jacks and OT input and grounding it in a different area that I had luck with in making a very quiet amp. The reason that amp was quiet may have nothing to do with the way I grounded the OT jacks and OT ground but I like to repeat the steps that have given me good results in past builds. Anyhow they will be well grounded.
Thanks
Mike
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My goal is to replace these low quality jacks and to isolate the grounds on the jacks and secondary and ground them in the same place the B+ and screen caps are grounded.
If this amp has a negative feedback loop (NFB), then the OT gets grounded to the phase inverter (PI) filter cap. Because the NFB puts the PI inside the power amp loop.
Please confirm that I would tie the 8 ohm secondary to the tip of one of the jacks and the 16 ohm secondary to the tip of the second jack. I would then tie the grounds of both jacks together and run it to the ground for the OT secondary which has been isolated from the chassis. At no point do I want to make a connection to any of the shunt connections.
Yes, yes and yes.
Read this on grounding;
http://www.valvewizard.co.uk/Grounding.html (http://www.valvewizard.co.uk/Grounding.html)
And here's a link to Doug's grounding schem;
http://el34world.com/charts/grounds.htm (http://el34world.com/charts/grounds.htm)
Brad :icon_biggrin: