Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jellyfish on December 15, 2021, 02:37:31 pm
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Hi everyone! This is my first post so apologies if anything looks wonky. I'm doing a small mod to my 1964 Silvertone 1482. During the mod I thought of a question that I'm having difficulty answering on my own: Is it acceptable to wire the chassis to the negative terminal of the speaker? I'm not necessarily going to do this... and maybe this is a dumb question. But in theory, wiring the ground lug of the ground tap of the output transformer (which is securely fastened to the chassis) directly to the speaker is not necessary as long as the speaker is wired to ground, correct? In other words, if I were to add a speaker jack to the chassis and another to the speaker with a speaker cable connecting the two, the ground tap of the output transformer would no longer need to be directly wired to the negative terminal of the speaker... right?
I tried it out already: I desoldered the wire connecting the ground of the output transformer to the negative terminal of the speaker. The ground of the output transformer is still screwed tightly to the chassis. I then took an alligator lead and hooked one end on the negative of the speaker and the other on to the chassis. Sound came through as normal. I'm just wondering if one technically shouldn't do this/if there are any possible consequences of wiring it up this way. Thank you!
Ryan
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Totally legal.
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Heyo. As Sluckey says, yes.
Your question raises the subject of good grounding practices, of course. Sometimes you get lucky and can ground things in odd places with few problems, which manufacturers did willy-nilly back in the day. So if you don't get any ground hum issues, no worries. But if you do, consider a star ground system (although for a Silvertone amp, you might have to reconfigure the entire grounding system to do that - so, not worth bothering with in this instance perhaps, but it may be encountered on some other build or mod).
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Figure it yourself.
1) The speaker does not need to be grounded to work. Any more than a flashlight or pocket radio.
2) User safety demands the secondary be grounded. What if the primary B+ insulation breaks down, and someone is touching both speaker and a lightswitch or radiator? Grounding the secondary ensures no shock (altho maybe fire).
3) Many negative feedback schemes expect the speaker side to be referenced to a common point, "ground" for slang. (Your amp has no NFB from the speaker.)
99% of tube table radios had grounded speaker for shock-safety; it was a check-point in UL tests. Sometimes built-in: one side of OT or speaker not even brought out except through a metal mounting foot.
The vast majority of tube Hi-Fi had grounded speaker, for safety and also for NFB.
Some "Professional" tube PA amplifiers had ungrounded secondaries. The system designer was expected to design the safety and signal grounding to suit requirement, and document a strapping scheme for the installer. Speakers on top of poles at a sport stadium might not be a safety hazard, but are subject to lighting strikes, which changes the grounding goal.
You are not doing anything clever/goofy. Ground one side. Yes, on paper going OT to ground and ground to speaker is all the same thing. In practice, especially on a rusty chassis found behind the surf-shop, you want to ask if you can trust those connections, both for safety and for not crapping-out on tour.
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Perfect! Thank you so much everyone!
If anyone's curious, the (probably very common) mod I'm doing to the 1482 is adding an output impedance selector switch to choose between 4Ω and 8Ω. I am not drilling any new holes into the chassis in case I want to sell it later, so I'm going to place the switch on the speaker connector. There's plenty of space between the positive and negative lugs to drill a hole for the switch (I don't feel bad about drilling a hole in that). Will swap out the footswitch (mono) jack for a stereo jack but will cut one of the tips to keep the handy lug (now isolated from any connection of male 1/4" plug) for connection of the two 330Ks to the tremolo strength pot. This new jack will be the speaker out, where the hot lug takes the switch's output (either 8Ω or 4Ω tap from O.T.). Small gap between chassis and cabinet provides sufficient space for the 18 AWG wire to go from switch (on the speaker) to output jack (in the guts of the amp). The stock Fisher speaker will have its own cable soldered to its lugs and will sit cozy in the cabinet if another speaker is going to be used (such as my Bogner 4x12 8Ω cab).
Once again, thanks for the kind replies!
Ryan