making sure I understand OT secondary ground

[dmartn149]

I'm using the transformers from this radiorca.pdf to build an amp similar to a Silvertone 1482 silvertone1482.pdf
As you can see the output trans secondary leads on the radio are separated from ground. My plan is to attach either one of the OT sec. leads to ground near the preamp and also make sure the speaker jack has a good ground. Is that correct?
Thanks

[R.G.]

Correct or not is hard to say. In an amp with no feedback from the speaker output, there is no particular reason to ground the speaker leads for the circuit's operation. You can ground one of them, but if you decide to do it, be sure that you do it properly. Speaker jacks in this kind of amp (i.e. no feedback from the speaker secondary side) can be grounded at will, as long as you don't try to use the chassis for one of the return wires back to the OT. Running speaker currents through the chassis is an open invitation to oscillation. The transformer secondary wires should run to the speaker jack, which can have its bushing grounded. This forces the current that went out of the OT to the speaker to come back into the OT on return without passing through the chassis.

[dmartn149]

Thanks R.G. I have a ground bus for the preamp I was thinking I would wire the OT and the speaker jack to that. I also think that I might want to add a neg. feedback loop. Not sure yet.

[loudthud]

An output transformer can have significant capacitance between one end of the primary and the secondary. If you were to play a loud cord, hold the cord touching the strings on the guitar, and touch the shell of an ungrounded speaker cable, it will shock $#&*% out of you. Ground one side of that speaker.

[pdf64]

Also OTs, especially vintages ones that are being tortured in a guitar amp, have the potential to short primary to secondary. A solid chassis ground reference for the OT secondary will protect the user from possibly fatal HT voltage should that happen.

[Chuck H]

I decided to sticky this thread for it's safety information.

[dmartn149]

Thanks everyone. The radio that these transformers came from had the OT mounted on the speaker with the B+ coming and going on a three prong plug

[J M Fahey]

Thanks everyone. The radio that these transformers came from had the OT mounted on the speaker with the B+ coming and going on a three prong plug

Not that surprised.

IF it had a field coil, +B goes through it.

And if permanent mgnet, speakers sometimes had an extra winding called "anti hum", in that case a few turns only.

[sparkies]

I'm using the transformers from this radio[ATTACH]54965[/ATTACH] to build an amp similar to a Silvertone 1482 [ATTACH=CONFIG]54966[/ATTACH]
As you can see the output trans secondary leads on the radio are separated from ground. My plan is to attach either one of the OT sec. leads to ground near the preamp and also make sure the speaker jack has a good ground. Is that correct?
Thanks

When I establish the speaker output jack. I ground one side of the transformer to the grounding point in the power supply. Because the function is to pop a fuse if the transformer fails. Of course if you use the output in the cathode circuit you wouldn't need to ground it at all. Which i always wondered why the standard way is using the plate circuit. after all, the gain is 1 or less in that stage. But back to what you are doing....

The silvertone schematic is ok, with the exception of the .05 capacitor tied from the ac line to ground. That should not exist in your new build.

The grounding scheme, I do a little bit different than most because I build mine so that in the case of using a load box to record with it doesn't ground loop.

The green wire from the AC cord gets directly bolted to the chassis and measures less than 1 ohm from the ac plug to any metal place on the chassis.

Then the DC power supply is grounded to the chassis using this circuit:

[sparkies]

If you are building a separate head amp, I would put a 100-400 ohm resistor across my combo speak-on connector in case it was accidentally turned on without a speaker. This will assure that some load is applied on the transformer at all times, and becomes insignificant when the speaker is plugged in.