How do I check output voltage on my Marshall Super Lead 100 vintage 1971 head?

[firehouserecording]

I blew three speakers. The speaker tech thinks there might be DC current harming their voice coils. He told me to check the output voltage of the amp to make sure there is no DC voltage. I have the head apart. I have my meter ready. I have no clue where to put the probes. Help, please.

Thanks,

Paul

[g1]

Sounds like the speaker tech thinks you have a solid state amp. Did you tell him it was a tube amp? The output transformer in a tube amp will prevent any DC from getting to the speakers.
Solid state amps can put DC on the speakers and cause them to blow, so this is probably what he is thinking.
What was going on when the speakers blew? Playing at high volume? Connected to a different amp?

[firehouserecording]

And yes, he knew it was a tube amp.

[R.G.]

He may be trying to see if there's an internal short in the output transformer. In either case, you clip your meter probes onto the speaker leads or speaker jack. Set it to DC first, just to be sure there's no DC to speak of on the output, then switch to AC. Do this with the speaker hooked up to the amp if it's a tube amp.

[g1]

Bit of a catch22 here. As RG mentioned, you have to hook your speaker up because it is a tube amp. So you run the risk of damage to your speaker (if there is a short in the transformer putting DC on the speaker).
Also, if there is DC enough to damage speakers you will probably hear hum (more than normal) and should see the cone move in or out and stay there.

[firehouserecording]

With One speaker hooked up at max volume, I got readings ranging from negative 10 millivolts to positive 2 milivolts with spikes up to 20 millivolts. Any thoughts on these readings. The meter was very jumpy. I heard no hum. I didn't notice the speaker moving.

Thanks guys,

Paul

[firehouserecording]

It was a tricky test, because I am alone, so I had to put the guitar next to the speaker to produce ear splitting feedback, then rush through the readings. Without the volume, I wasn't getting readings. At that volume, I am guessing 20 millivolts is acceptable.

[Steve Conner]

Tube amps can't put DC on a speaker. Well, technically they could, but it would take two simultaneous faults (output transformer secondary blows open and then primary shorts to it) and even then the power supply could probably deliver less than 1 amp before a fuse blew.

I guess the speaker tech just doesn't know this. The speakers more probably blew from being cranked too loud.

[Jazz P Bass]

A few questions that come to mind.
What speakers were they, anyhow?
Manufacturer?
Model?
Wattage rating.
What cabinet configuration (how many speakers. Series or parallel wiring)?
What tap do you use on the back of the amp (4, 8 or 16 ohm)?

[firehouserecording]

25 watt celestion greenbacks. In a 4x12. I think they were wired in a series. They are 16 ohm. One black and one red wire come from the jack to the speaker. I use the 16 ohm tap.

Thanks,

Paul

[Jazz P Bass]

If the speakers are 16 ohms each, then a 2 in series & 2 in parallel configuration will come out to a 16 ohm cabinet.
The overall rating of the cabinet will be 100 watts.
Max.
If one speaker goes out in the series loop, then both of them will be off.
The parallel speakers will now be at 8 ohms, 50 watts.
I would think all 4 speakers in parallel, which would be 4ohms, would be a safer bet.
At any rate, you need to find out the exact configuration of the cabinet.
And as Steve has mentioned, if you are playing at full volume, you may want to consider a speaker that can handle more wattage.
Speaker Impedance

[firehouserecording]

It's a 16 ohm cab with 16 ohm speakers. The first two blew, and all four were silent. I rewired to run the two speakers that still had continuity around them. Then the first one blew of the two blew, but they were both still on.

[J M Fahey]

You blew them with power.
No DC involved.
4 greenbacks are not enough for a Super Lead 100, simple as that.
Even Marshall warns about it.
You need 2 similar cabinets.

[firehouserecording]

Yes, Juan, I am convinced you are correct. I've been running it this way for 14 years. Before yesterday, I had blown a total of four speakers. It's bad enough that I didn't know these fundamentals - I can't believe that the "experts" that I took my problem to didn't explain the obvious to me. Thanks to everybody for their replies. You seem like a good group. I am going to buy an attenuator.

Paul

[olddawg]

You blew them with power.
No DC involved.
4 greenbacks are not enough for a Super Lead 100, simple as that.
Even Marshall warns about it.
You need 2 similar cabinets.

Or an attenuators or a dummy load. Faced with a similar problem once I added a ceramic resistor to each speaker and wired it for the appropriate load.