HV on speaker

[Two Tone]

Hello everybody,
I have been working on a ī55 Gibson GA 20 amp,
putting in new caps,NOS tubes and an old Jensen speaker.
The amp sounds very good and is as loud as any 14w amp
Iīve played.The OTīs prim. measures 19k,which is a bit high for
two 6V6 tubes, B+ is 390v.
The OT is mounted on the speaker frame,there is no chassis ground
connection to the speaker.
Problem is there is HV on the speaker(390v!)
Whatīs going on here?

[52 Bill]

Problem is there is HV on the speaker(390v!)
Whatīs going on here?

Sounds like you have a field coil speaker in your amp.

The high voltage runs through a coil in the speaker frame to create the magnetic field for the speaker motor to operate. This coil is sometimes also used to replace the choke in the power supply.

If there was 390 volts on the voice coil, it would burn out in no time at all.

[Two Tone]

No, itīs a Jensen P12N from 1961....

[Don Moose]

I don't think I've seen a tube amp that doesn't have B+ on the OT.
The lack of chassis connection to the speaker itself isn't that suprising - the OT secondary and speaker coil are a complete circuit.

Add some heatshrink around the leads to provide some extra insulation if you're uncomfortable.

Hope this helps!

[Two Tone]

maybe I wasnīt clear enough,
thereīs HV on the OTīs sec.

[Don Moose]

Oooh. HV relative to what, though? If you have a high DC voltage _across_ the voice coil, it should be jammed tight all the way in or out and not work at all.

If the voltage is relative to chassis GND, but (nearly) the same on + and -, the secondary and voice coil are just floating on the OT's leakage. You _could_ run a chassis gnd wire down to the area and add a 1M resistor from the secondary's + and - (both) to that gnd wire.

... or did I fail again to understand? (quite likely)

Hope this helps!

[Enzo]

Are you saying there is B+ voltage on the frame of the speaker? (With respect to the chassis?) Or only on the VC?

Either thing points to a bad OT. The B+ flows through the primary winding of that OT. Normally the wires are insulated from anything else, just as they would be on a chassis mounted OT. If there was a primary to secondary short, then the transformer could still work, but now B+ would be on the VC. SInce the speaker and transformer are not grounded, it can get away with it. If the primary shorted to frame, then the transformer could still work, but now B+ would be on the OT frame and since they are bolted together, the speaker frame as well.

That make sense?

[jrfrond]

This is a wild one! But there MIGHT be some sense to be made here.

If you are reading from chassis ground, check it to both sides of the secondary. If you have the SAME +390V on both sides of the voice coil, it will not burn out. Think of dynamic mics with +48 phantom power on both sides of the coil. There is no problem because the potential is the same on both sides.

The next step would be to disconnect the transformer primary and secondary and check it to see if you have a DC leakage path from one winding to the other. If this is an interleaved transformer, it is a definite possibility.

[Steve Conner]

Sounds like the OT primary is shorted to the secondary, all right. Try measuring continuity between the B+ node and speaker terminals with your DMM. If you get a reading less than infinity, start to worry.

Try grounding the speaker minus terminal, and if things start to smoke and blow out, it's time for a new OT. The speaker minus should be grounded for safety anyway, to prevent a tech getting bitten in this very situation. (Nobody expects B+ on the speaker terminals.)

If nothing bad happens, maybe the OT is just a little leaky, if it was stored in a damp basement or something. Digital meters can read voltage with only a minuscule current.

You might like to try measuring the leakage current with a milliamp range, once the above grounding test has verified that it's not a dead short that would blow your meter out, then make a judgement call. 1 microamp would be fine. 1 milliamp would suggest a trash can in your OT's future. 1 amp means the trashcan for your OT, your DMM and your rectifier tube :-O

[Enzo]

Try grounding the speaker minus terminal, and if things start to smoke and blow out, it's time for a new OT.

Steve, please tell me you didn't just hand out that advice. B+ don't belong there for sure, but why burn anything more up? Maybe a 10k resistor if you want to test things.

[Two Tone]

Thank you all ,
Iīll check the amp on thursday,
I believe there is HV on both poles
of the speaker.
I will report in a few days.

[jrfrond]

.....Try grounding the speaker minus terminal, and if things start to smoke and blow out, it's time for a new OT. The speaker minus should be grounded for safety anyway, to prevent a tech getting bitten in this very situation. (Nobody expects B+ on the speaker terminals.)

If nothing bad happens.....

And I thought cowboys were strictly an American thing!!!

From now on, I will refer to you with your cowboy nickname: SPARKY!!!

[Jag]

LOL Maybe it's Friday in Scotland, sure sounds like Friday advise to me

[Two Tone]

I measured for continuity between B+ and Speaker terminals,
there was about 500Meg.
Both speaker poles carried the same voltage,
thatīs the reason the speaker wasnīt damaged.
However I didnīt feel comfortable with this setup
and replaced the OT with a tweed Deluxe type(8,2k prim)
Tonewise thereīs a bit more crunch,
probably due to the lower prim. Impedance(the old OT had 19k prim)
Great forum,thanks a lot!

[Steve Conner]

Yes, it was Friday afternoon

I figured that the best thing to do with a fault like this is try to goad it into an outright failure. You don't want the thing continuing to work in an unsafe condition, but you don't want to throw away a working OT either. And DVMs can be *really* sensitive to tiny levels of leakage.

So I figured that if it can withstand grounding the speaker, it's probably OK, and if it can't, it had to be trashed anyway.

Actually I should have suggested grounding the speaker basket too, since the OT core was bolted to it.

Of course replacing the OT was the proper, sensible solution But I still suggest grounding the speaker negative terminal and basket to chassis, anyway.