PLEASE CONFIRM MY POS/NEG TERMINALS

[zozoe]

Hi all,, This amp is a Webster 81 15 2x6V6 mono P.A. amp, and since it has those old 4 pin speaket sockets, I'm going to run wires from the old sockets to the new 1/4" jack, and according to the same 'B' version schematic, the red cables are hot, while the black is ground,,,,please say yes,,,,? Beneath the black tranny, is the Impedance selector, & under that are the unlabeled +/-- posts.

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Thank you all & stay safe~~

[xtian]

I'd say that's a good guess. But a simple check with your meter will confirm the black wires are directly connected to chassis.

[zozoe]

How does that work, Xtian? What should it read, zero?

[Stevem]

Yes, the end/ beginning / negative of the secondary winding of the output transformer always goes down to chassis ground, so it will read zero resistance when tested with a meter!

If you have a good enough digital voltmeter with short leads the the secondary ( speaker taps) should resistance wise read as such.

4 ohm tap = .5 ohm

8 ohm tap =.7 ohm

16 ohm tap = 1 ohm

Note that the amp will always sound its best when you are using the 16 ohm tap with a 16 ohm speaker since then you are using the full secondary winding , not just 66% of it, or worse yet 33 some odd percent of it by using the 4 ohm tap.

[zozoe]

Thanks, Stevem~~
I always wondered about it using more iron via the 16ohm tap was better,,, I've been told both yes & no, but I'm on your side!! So, exactly how does one connect a 3 prong to an older amp? Please be as specific as possible.
Kenny

[Phil_S]

Several of us have pointed out the dangers of not wiring the mains power cord correctly. I was not anxious to reply because I have real concerns about whether you can do this safely. I'll say it again. If in doubt, take this amp to someone who knows how to do this. If you do this on your own, you and anyone else who might come in contact with the chassis may be at risk of electrocution (death.) It is a serious matter.

In the end, I concluded you will probably do it anyway, so providing information isn't likely to hurt and may actually help. Here is a graphic illustration of how to wire a mains cord in the USA. The lamp can be omitted. If you don't understand this picture, then you really should not do the job.