Biasing Marshall Super Lead 100

[funkgang49]

I just had a friend drop off a '74 Marshall Super Lead 100 MKII to have all the multi-cap cans replaced (current ones are original-6 total!) and to replace the 4 power tubes with a new pair(2) of power tubes. He wants to run it with just 2 power tubes - the amp being just too damn loud at its full 100watts.
I will be installing the new tubes in the #2 & #3 power tube socket positions.

My questions:
1. Will installing the pair of new tubes in the #2 & #3 positions be correct for running the amp at half power?

2. Will leaving out the full quad of tubes alter the way that I will need to bias the amp?

I am new to setting bias on fixed bias amps. All my experience has been with cathode bias amps so far.
I have done research on how to set bias by measuring plate voltage and will be using a bias probe plugged into my DMM to read milliamps and set bias according to tube requirements and standard calculations.

[TUBEDUDE]

You still bias the tubes for the desired dissipation. Look online for Jims bias.tables. You'll see the chart for EL34 tubes, under the measured plate voltage, select the desired dissipation, and adjust the bias control for the listed current.
Try playing the amp to see which bias setting sounds best.

[funkgang49]

That's what I thought. I just wanted to make sure - new territory for me.

Thanks!

[Phil_S]

Remember, half power doesn't equal half volume. Assuming it is now a 50W amp it will still be (too) frikin' loud. To get 100W down to half volume, you will need to get to about 10W, which isn't possible on this amp.

[old tube]

With 2 EL34's you should change the output impedance. So if you have a 16ohm cab, set the amp at 8ohms (I hope I've got that the right way round!)

[funkgang49]

Remember, half power doesn't equal half volume. Assuming it is now a 50W amp it will still be (too) frikin' loud. To get 100W down to half volume, you will need to get to about 10W, which isn't possible on this amp.

Yep, got that - was going on the idea that cutting the wattage in half would equate to a 3db drop in volume which in this case, 100 screamin' obnoxious watts, should help.

I will also set bias so plate dissipation is closer to 75% for a slightly earlier breakup.

With 2 EL34's you should change the output impedance. So if you have a 16ohm cab, set the amp at 8ohms (I hope I've got that the right way round!)

Wasn't even thinking about the effect on impedance - thanks for the heads up!

[Stevem]

Have your friend open up his wallet and get a Attenuator , period!
Trust me, other than getting a different 30 to 40 watt amp this is the best way to go!!!!

[funkgang49]

Ok, so I have my bias table set:
New filter caps in place, new power tubes (2XJJEL34's-max 25watt) & preamp tubes in place, bias probe connected, etc.

My plate voltage is 472v.
25watt max. / 472v = .052 or 52mA
.052 X 70% = .0364

Problem is that the bias adjust trimmer when set to max. is only giving me 25mA which works out to only 50%!

Which resistor do I need to change in order to achieve a higher mA range?

[img720]http://s25.postimg.org/qgfxh7onj/Marsha ... _point.jpg[/img]

[Phil_S]

It looks like there is a voltage divider with 15K on the supply side and 33K + 25K pot on the ground side. Verify this, as I can't see the traces on the board. Try changing the 33K.

If that doesn't do what you need, try increasing the dropping resistor before the diode, not shown in your picture.

[Roe]

watch out for redplating. the power supply won't sag much with 2 el34s and mismatched load

[Stevem]

Yup!
What I also do is install trash (used up) output tubes , but clip off pins 3 and 4 to keep the heater voltage where it should be and in turn the intended gain levels of the amp.
Then a dot of silicone sealer at two spots on the base will hold them in nice!

[pdf64]

Yes, to best retain the intended tone and response, an attenuator etc seems a much better idea.
An SL with 2 tubes removed has a super stiff power supply, pushing up the (full load) operating points, stressing the 2 tubes more.
And likely puts out >60 watt, saving barely 2dB.

There's a lot of stuff on Myles Rose' site that seems a bit iffy, but he puts things well on the 3rd to last paragraph here http://www.guitaramplifierblueprinting. ... ences.html