18.5 VAC on speaker terminal?

[Perkinsman]

With power on, I measured 18.5 vac on the + terminal & 12mv on -. Amp has a bad hum. I’ve replaced the main filter caps but still have hum and sounds like speaker crackles. I’m thinking possibly a bad ground but don’t know enough about whether this voltage on the speaker is excessive & if so, would that be due to a bad OP?

[Enzo]

Does the speaker move one direction and stay there? Set the meter for DC volts, do you get some or do you get zero DC?

18v on the speaker? Unless the music is playing loud, there shouldn't eb aany voltage on the speaker.

Also, measure speaker voltage across the speaker, not from each terminal to ground.

[J M Fahey]

With power on, I measured 18.5 vac on the + terminal & 12mv on -. Amp has a bad hum. I’ve replaced the main filter caps but still have hum and sounds like speaker crackles. I’m thinking possibly a bad ground but don’t know enough about whether this voltage on the speaker is excessive & if so, would that be due to a bad OP?

For Godīs sake, WHICH amp?
Schematic - supply voltages - etc.

[Perkinsman]

I was asking a general question, doesn’t really matter what amp it is, does it?

[Helmholtz]

With power on, I measured 18.5 vac on the + terminal & 12mv on -. Amp has a bad hum. I’ve replaced the main filter caps but still have hum and sounds like speaker crackles. I’m thinking possibly a bad ground but don’t know enough about whether this voltage on the speaker is excessive & if so, would that be due to a bad OP?

18.5V AC or DC at output without input signal is certainly "excessive". Means over 40W heating power to an 8R speaker.
And amp type/schematic always matters if we want to talk about troubleshooting.

What is an "OP"?

[The Dude]

......What is an "OP"?

^^^^ Yes, what is an OP?

......amp type/schematic always matters if we want to talk about troubleshooting.......

That, too. We don't even know if it's a tube or SS amp.

[xtian]

Also, measure speaker voltage across the speaker, not from each terminal to ground.

This.

[Perkinsman]

It was a ‘65 Ampeg Jet I’ve been working on. I thought the OT (not OP) was faulty but I was missing a cathode cap. It caused the amp to distort/low volume. Once installed, it’s all better. Thanks.

[J M Fahey]

Yesterday, 07:47 PM



I was asking a general question, doesn’t really matter what amp it is, does it?

It is not a "general question" by any means, since you are trying to troubleshoot and repair a specific amplifier.
TONS of different possibilities depending on amp type, technology, etc.
As mentioned above, we didnīt even know whether it was tube or solid state !!!!!

It was a ‘65 Ampeg Jet I’ve been working on. I thought the OT (not OP) was faulty but I was missing a cathode cap. It caused the amp to distort/low volume. Once installed, it’s all better. Thanks.

But you did not complain about *that*

And it does not cause or explain 18.5VAC at the output.

We donīt even know its frequency.
It might have been 50/60Hz, some squealing Audio frequency, inaudible HF oscillation where you only hear the exagerated supply ripple caused by overconsumption, etc.
All different, all different possible causes, all different troubleshooting and repair suggestions.

Amp brand/type/model DOES indeed matter!!!!

[Helmholtz]

We discussed/troubleshooted this amp here: https://music-electronics-forum.com/...und#post917349

I think this is what happened :

The cathode cap for PI triodes was missing, strongly increasing the NFB signal even introducing some positive feedback.
It is likely that this caused heavy HF oscillation (and also increased power supply ripple).
So the large AC at the output would have been ultrasonic HF.

[vintagekiki]

I was asking a general question, doesn’t really matter what amp it is, does it?

General question means a general answer, what kind is question, such is the answer.
The amplifier has a malfunction, intervention is required in an authorized service.

[drewl]

That would be VERY LOUD!

I had one of them Ampeg copper panel Jet amps that had a nasty high frequency oscillation, couldn't hear it, but my output ac meter jumped way up and scope showed the oscillation.

One of the shielded wires had a bad ground.