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

[Steve Conner]

Another suggestion to add to the mix (pardon the pun ) Pull two of the power tubes and set the impedance selector to 8 ohms for a 16 ohm load. This will cut the power in half, and it might sound better than an attenuator. Just a thought, never tried it since I've only owned amps of 50W or less.

However, maybe the bottom line is that speakers just sound their best right before they catch fire.

[firehouserecording]

I am FireHouse Recording, , baby! Lol

I've wanted to pull two tubes for a while. I read a lot of warnings against it, though. I do heed the warnings when they seem reasonable. Anyone have an opinion on running two tubes?

So that's attenuator vs. dummy load vs stacked cabs vs two tubes. I ordered an attenuator, but I will probably end up recording my old way. I'll just buy a fire extinguisher.

Thanks,

Paul

[soundguruman]

Another suggestion to add to the mix (pardon the pun ) Pull two of the power tubes and set the impedance selector to 8 ohms for a 16 ohm load. This will cut the power in half, and it might sound better than an attenuator. Just a thought, never tried it since I've only owned amps of 50W or less.

However, maybe the bottom line is that speakers just sound their best right before they catch fire.

Anybody who has tried this knows:
There is almost no volume difference between 50 and 100 watts.
Yes it cuts the power in half, but the actual volume drops (according to human ear) only a small amount.

In hi-fi repair shop, we conclude:
It takes 8-10 times the power, to double the volume, according to the human ear. Especially with typical low efficiency speakers.

[soundguruman]

I've wanted to pull two tubes for a while. I read a lot of warnings against it, though. I do heed the warnings when they seem reasonable. Anyone have an opinion on running two tubes?

So that's attenuator vs. dummy load vs stacked cabs vs two tubes. I ordered an attenuator, but I will probably end up recording my old way. I'll just buy a fire extinguisher.

Thanks,

Paul

It can run on two tubes, no problem. But you can't compare the result to an attenuator. The attenuator rules.
But, as you pointed out, 25 watt low efficiency speakers will also saturate and lower the overall volume.
(those 25W speakers are essentially hi fi speakers, w/ little tiny voice coils)

[soundguruman]

What a truckload of bull manure.
Nobody never ever does or did that.
Ridiculous suggestion.
If you have 2 cabinets and one head, you just plug *both* and play all night long without trouble.
Which by the way doubles sound pressure, punch, thump, oomph, you name it.

This is standard procedure, for Marshall amps, used all over the world.
The speakers used 25-30 Watts are well known to overheat after 40 minutes.
But what do you know about playing thru a Marshall wide open? Not much I can see.

[Alex R]

pulling a tube from each side and running it into 16 ohms with the selector on 8 ohms will work just fine, standard practice used all over the world to coin a phrase lol. You will be able to use your 4 x greenback cab and it won't burn up the speakers. The OT won't saturate as much but you may find you get more dynamic range and prefer it - give it a try, it's quite safe.

[firehouserecording]

I've wanted to pull two tubes for a while. I read a lot of warnings against it, though. I do heed the warnings when they seem reasonable. Anyone have an opinion on running two tubes?

So that's attenuator vs. dummy load vs stacked cabs vs two tubes. I ordered an attenuator, but I will probably end up recording my old way. I'll just buy a fire extinguisher.

Thanks,

Paul

[soundguruman]

pulling a tube from each side and running it into 16 ohms with the selector on 8 ohms will work just fine, standard practice used all over the world to coin a phrase lol. You will be able to use your 4 x greenback cab and it won't burn up the speakers. The OT won't saturate as much but you may find you get more dynamic range and prefer it - give it a try, it's quite safe.

To make it sound "really" good, turn down the bias, until the output tubes turn just "slightly" red. Just a hair, that is.
And the tubes will last about 20 hours.
But man, what a sound, I mean killer.
Yes, there are people who do this on purpose. With a $425 dollar set of Bugle Boy output tubes.
HEY, you want to record killer guitar sound? That's how.
DON'T FORGET: you have to use an attenuator that MATCHES the output impedance of the amp.
It MUST match the impedance of the speakers.
You CANNOT plug "any" attenuator into "any" amp. You will burn your sockets, or melt your output transformer.
YOU have been WARNED.

If you are a wimp, who does not play the Mashall WIDE OPEN, then ignore all this. Stick to your distortion pedal, and pretend...
For the Marshall does not sound LIKE a Marshall, until the output tubes are tortured, and screaming for mercy.

[firehouserecording]

I agree with everything you write. I did buy a 16 ohm attenuator that does match. Thanks for the warning. I totally want to try your tube idea. Since I run the bias at about 70 to 80 percent of the plate voltage equation (plate voltage divided into 25), any idea how far down to bias before they get slightly red?

Thanks Paul

Do your Marshalls come with big read warning stickers that warn not to turn them past four? I'm just saying...

[soundguruman]

I agree with everything you write. I did buy a 16 ohm attenuator that does match. Thanks for the warning. I totally want to try your tube idea. Since I run the bias at about 70 to 80 percent of the plate voltage equation (plate voltage divided into 25), any idea how far down to bias before they get slightly red?

Thanks Paul

Do your Marshalls come with big read warning stickers that warn not to turn them past four? I'm just saying...

Oh, you are so right to agree, hahahahahhaahhh!
But 'this one goes to 11, that's one more isn't it? well isn't it??
"any idea how far down to bias before they get slightly red?"
aahhahahahahahahahhh!
try in a dark room, and I mean slightly. You will see the plate turning red, just a bit around the edges. You don't want the whole plate red, just on the edge boarders..."bit" is a bitty bit.

And I forgot, mic the outer edge of the speaker, not the middle.

You see, the whole thing is sensitivity.
That's what the factory engineers say. I have talked with them...
1/2 mv input for slight clipping on output. (knobs more or less max) If you are not getting that, something is still wrong, for example the bias is too cold.
Not gain, not distortion, not loudness.
When you crank the bias hot, it's increasing the sensitivity. Then drive it till the output tubes beg for mercy. And that is when it comes alive, finally sounds like a Marshall.
Maybe it's no wonder people use the variac to drop the line voltage.
The original was made for 110VAC or 115,117, now we run it on 120 or higher.
If you have a power transformer that is 'just like' original, maybe you are overdoing it, without a variac to drop the line voltage.

But as I said don't let the heaters fall below 6 volts with the variac.

And the last thing is make sure you have 1K screen grid resistors on the output tubes. (EL34)
The earliest plexi did not have screen grid resistors. Make sure what is on your schematic....
For modern EL34, the screen resistors have to be installed. For the old vintage EL34, does not need it.
But don't try to run modern without resistors, cause the fuse will blow. And that is why old Plexi has reputation for blowing, more or less.