Marshall 1959SLPX Power Tube Removal

[saltydogg]

Currently I run the amp through a Marshall Powerbrake to ease the volume. I have read on another forum that by removing 2 of the 4 power tubes this will tame the volume. Would someone mind explaining to me which of the power tubes to remove? Will the amp need to be re-biased? Is there a substantial difference in volume with the 2 power tubes removed? Clearly, I've never done this before, but it sounds like a reasonable idea. Has anyone here done this before with better results than a attenuator? Thanks in advance to anyone who can shed some light on this issue.

[WaltC]

You can remove either the inside pair or outside pair of the power tubes (I'm assuming the SLPX is a 100 watt amp and has 4 power tubes), but the amp should be re-biased if you do this and you'll probably want to plug the speaker into a different output jack (if your speaker is 8 ohms, plug it into the 4 ohm output jack, etc..

All that said, you'll not really drop the overall volume all that much and it won't come close to the drop you get by using a PowerBrake (which I don't much like BTW, but that's just me, I'm partial to the TrainWreck designed Airbrake or the THD hotplate or Weber MASS).

[saltydogg]

Thanks for your input Walt. Yes, the amp is 100W with 4 tubes. Is there a link or advice for re-biasing or should I leave it to a professional? The amp runs into the cabinet at the 16 OHM setting (speakers). Why the hard feelings toward the Powerbrake?

[winkofaneye]

This is a common thing for people to want to do,but you must remember to lower the impedance on your amp.I'm not sure whether you would have to rebias.I wouldnt really advise doing this,as the amp was designed to run with four tubes,and you may be putting strain,even small,on the output transformer,and for the small amount of difference in volume,its not worth it.The Marshall Powerbrake is an attenuator that colours the tone considerably v unattenuated.You should always leave re-biasing to a pro.Amplifiers can kill when switched off and unplugged.

[Rosewood]

My problem with this sort of thing is the power and output transformers are so big for a hundred watt amp that with 2 tubes pulled it's not that much quieter. Attenuator maybe?

[FortinAmps]

Originally Posted by WaltC

You can remove either the inside pair or outside pair of the power tubes (I'm assuming the SLPX is a 100 watt amp and has 4 power tubes), but the amp should be re-biased if you do this and you'll probably want to plug the speaker into a different output jack (if your speaker is 8 ohms, plug it into the 4 ohm output jack, etc..

All that said, you'll not really drop the overall volume all that much and it won't come close to the drop you get by using a PowerBrake (which I don't much like BTW, but that's just me, I'm partial to the TrainWreck designed Airbrake or the THD hotplate or Weber MASS).

+1 to that. It will give you a slightly different feel to the amp. Give it a try, it's pretty straight forward.

[saltydogg]

I appreciate all the suggestions. If I was to look into buying a Train Wreck by Airbrake where would be a good place to look- preferably on-line? I know the larger chains carry the THD Hotplate. Any other quality un-attenuators available? Thanks.

[winkofaneye]

The Ultimate Attenuator has a great reputation,see other threads on here.I get on ok with my airbrake,and I bought it here in the emporium.They come up now and again.The good thing with the airbrake over the hotplate is its not ohms specific,and will work with 4,8 and 16 ohm amps.The hotplate is ohms specific,so for the SLP you would have to get a 16ohm hotplate if you are using a 16 ohm cab with the amp set at 16 ohms.I can use the airbrake with my twin reverb and marshalls.

[WaltC]

Here's some info that may help:

http://www.duncanamps.com/technical/lvbias.html

Info from Marshall:
"
Here's the Marshall Service Bulletin on biasing the JCM2000 series amps: 

Procedure: 
1. Make sure the amp is connected to a load with the proper impedance selected. 
2. Power up amp on Standby, and let the circuit stabilize for a couple of minutes. 
3. Locate the male three pin molex connector (CON2) with the two mini pots (PR1 and PR2) (located on each side of the molex connector) on both ends found at the bottom of tube bay. 
4. Connect DMM (set to read mV) with alligator leads, reference common lead to center pin (pin2) on molex connector CON2 and positive lead to pin1 on CON2. 
5. Take amp off Standby with no signal, adjust mini-pot (PR1) closest to pin that the positive lead from your DMM is connected to and set it to mV voltage that is listed below. 
6. Repeat steps 4 & 5 for pin3 mV and until pin1 & pin3 mV readings are the same. 

Bias Settings: 
DSL50 45 mV TSL 601&602 80 mV 
DSL100 90 mV DSL 401 1.375V 
TSL100/122 90mV DSL201 .675V 

Here's a quick bias lesson: 
What you want to do is measure the plate voltage (B+) from pin3 of one of the power tubes. It should measure between 450-500V. Now the max plate dissapation (Pa Max) for EL34s is about 25W. To calculate the correct bias, you want to: 
(Pa Max/B+)(.7) = 38-45mA depending on the exact plate voltage of your amp. Check and adjust via the molex connector and pots from above.
"

Equipment to help you bias your amp:

http://www.compu-bias.com/
https://taweber.powweb.com/biasrite/br_page.htm
http://www.alessandro-products.com/accessories.html

[saltydogg]

Originally Posted by winkofaneye

The Ultimate Attenuator has a great reputation,see other threads on here.I get on ok with my airbrake,and I bought it here in the emporium.They come up now and again.The good thing with the airbrake over the hotplate is its not ohms specific,and will work with 4,8 and 16 ohm amps.The hotplate is ohms specific,so for the SLP you would have to get a 16ohm hotplate if you are using a 16 ohm cab with the amp set at 16 ohms.I can use the airbrake with my twin reverb and marshalls.

Have you used the Marshall Powerbrake? Generally, I don't brake the volume that much- because my gear is in my detached garage. However, to get the tubes a little hotter by way of turning the volume up it will shake my garage's foundation. I'll never use a "bedroom level" volume. I'll look into the Ultimate Attenuator. Some of these are more transparent than the Powerbrake I guess? Thanks...