Pulling out tubes from a 2:90 to reduce the power

[margoulin]

Hello,

First, let me introduce myself. My name is Marc Fascia and I am a french guitarist living in Lyon. I have been using some MesaBoogie gear from more than 10 years , along with a Marshall cabinet (Mesa cabs are so expensive here !) and some Ibanez guitars.

I red several posts where people mention the possibility to pull out some amp's tubes for reducing the power and I would like to test this on my 2:90 which is a great great amp but too powerful for my personal day-to-day usage.

I have the 'old' 2:90 with the blue led. It's tubed right now with 8 Sovtek 6L6.

The info I found is that I can remove the 4 outer tubes without harm. Is that right ?

Do you guys know some other option to reduce the amp power so I could use it at home without blowing my ears and the neighbours ones up ?

Thank you !

-Marc

 

[metalgarth]

My understanding is that pulling tubes out of any tube amp to decrease its wattage is not really recommended. I'd talk to someone at Mesa and ask if this is a good idea. I'd bet $1.00 that they say no.

 

[margoulin]

I just phoned the MesaBoogie customer service and they told me that pulling out tubes was not possible because of the way SimulClass works. My english is not perfect (and my electronics skill is lot worse) so maybe I got the explanation slightly wrong ...

The operator who spoke to me advised me to buy an attenuator like a THD HotPlate or similar.

So metalgarth, I owe you 1$ ! )

thank you for your answer.

 

[trippy]

yes, the 2:90 is a monster.
There is another way to reduce power beside the THD hotplate and it calls Yellow Jackets http://www.thdelectronics.com/products/yellow_jacket.htm its from THD as well.
Its cheper then the hot plate.
I wish boogie have made the 2:50 like they did the 50/50 - with a low power switch allowing it to drop from 50/50 to 15/15 (which is by the way still VERY powerful).
I use a 50/50, mainly because of that reason, but still it is very strong, running at 15/15 on rehearsals and it is very strong..

 

[v6spider]

why don't you try running in half drive? I have been running in half drive for years with my 2:90. Here's some the benefits:

• 
  PA Distortion at lower volumes
  More controlled feedback
  More Compressed tone for leads
  Better Feel
  Don't Wake up the neighbors

The only thing I have not liked is the lowered bass response which I compesate for by boosting low end in my TC G-force. However it is not an issue when running 2 4/12's in stereo... in fact it is heavenly and soooo sweet with two 4/12 1960a Marshall tops running in 4 ohms. 8)

 

[disassembled]

Half drive does not give you power amp distortion but it just distorts the preamp tubes more. The only way to get PA distortion is to turn up, attenuate, or pull tubes.

You can run the 290 in class A just like you can a Mark IV head (in essence the 290 is a Mark IV times 2). You can make the change by adding some switches or you can just pull tubes (the 4 inner tubes). The 4 outer tubes are class A which can also be used as EL34's.

Also, you can put either side in "standy by" by removing the 1amp fuses near the tubes. I pulled the fuse closest to the outside to put Channel B in standby and i pulled the 2 inner tubes (Class A/B) to run channel A in in class A. Once I hit the electronics market I will pick up a bracket and install a few switches for Simul/Class A and Triode/Pentode. In the meantime i am not wastefully burning all 8 tubes.

Use the same speaker out as normal or else you could redplate the tubes.

The tubes are arranged like this:

A = Channel A
B = Channel B

A1 = Class A
B1= Class A

B1 B A A1
B1 B A A1



Greg

 

[v6spider]

Straight from the Simul-Class 2 Ninety Manual. BTW the 2:90 is a power amp.

1/2 DRIVE
Like Deep, this LED indicates the 1/2 Drive mode has been selected. Again - connect the
TriAxis switch jack to the 1/2 Drive jack on the rear left of the 290. Program it to be activated
under the desired preset. When 1/2 drive is activated the 2:90's overall output is reduced by
roughly half and you will hear the volume drop correspondingly. Notice that the top end gets
a little brighter in a higher frequency and just starts to break up. When listening to 1/2 Drive
pushing a clean sound these break-up brights may seem excessive. Switching to a high gain
sound however, tells the real story right away! The top end gets softer and the harmonies line
up and start to dance. The whole amp gets spongier and feels easier to play. When using 1/2
Drive for giant crunch chords it provides the haze that's hard to beat, especially when used in
conjunction with the Deep feature. Half Drive was originally conceived as a way to get more
gain from the power section to combine with high gain sounds in the preamp. This works
beautifully. There is, however, another great way to put this feature to work that is easily
overlooked. Dial up a clean sound from the pre-amp, then increase the front end gain as high
as possible without pushing it into full blown distortion ... you know, just to the threshold of
distortion. Trigger the 112 Drive to unleash a truly cool and bluesy medium gain lead sound.
Darken or brighten to taste either at the tone controls or with the 2:90 presence controls. With
an "analog" (non-programmable) pre-amp this becomes a useful "extra mode". With TriAxis
many such possibilities await. As we mentioned earlier, engaging the 1/2 Drive causes a
volume decrease. This is a possible problem only to non-TriAxis owners. Since the 2:90 was,
from its inception, the TriAxis counterpart in a "system" to other pre-amp owners, we now
apologize for any inconvenience and hope you make a B-line to your nearest MESA/Boogie
Pro Center straight away to hear the TriAxis. To all you TriAxis owners, you will soon find this
"could be fault" a truly great feature! You've now doubled all your great programs at lower
volumes, great for choruses where the vocal needs to stand out but you want more gain or
"intensity". Or you might program the TriAxis Master higher and whip the 2:90 in 1/2 Drive
harder for that extra grind when moshing. Whatever your individual application, 1/2 Drive will
enhance the 2:90's already toneful nature greatly.

 

[disassembled]

I am going by the schematic....... which doesn't mislead like the manual. All the manual states is that the volume is reduced by half and you get a more distorted tone. Nowhere does it state that the power tubes are distorting or that the puts out less wattage. This is because the amp is still running 180 watts but the preamp tubes are distorting inside the 290.


Greg

 

[v6spider]

I am going by the schematic....... which doesn't mislead like the manual. All the manual states is that the volume is reduced by half and you get a more distorted tone. Nowhere does it state that the power tubes are distorting or that the puts out less wattage. This is because the amp is still running 180 watts but the preamp tubes are distorting inside the 290.


Greg

I agree the manual does not refer to the actual mechanics of how half drive works. But the explaination of the effect on tone and how the 2:90 reacts with half-drive engaged is indeed accurate. It would be interesting to know what Mesa has to say about how half-drive actually works. I would think that it occured in the driver section of the 2:90 and yes those are pre-amp tubes.

 

[disassembled]

If you call and talk to any old customer service rep they will probably just regurgitate the manual or agree with whatever you tell them. If you read off stuff from the schematic they will probably have no clue what you're talking about unless you speak to someone that knows the amp.


Referencing the schematic:

When you engage half drive 2 LDR's are also engaged (1 per channel) changing the resistance going to pins 3 + 8. All that the LDR's do is put a 470 ohm (R333) resistor in parallel with a 168 ohm (R334) resistor. This happens with both phase inverters (V2+3) so half drive ONLY affects the phase inverter tubes which are 12AX7's(i commonly refer to them as preamp tubes out of habit). On pins 3+8 the voltage is lowered from 25volts to 14 volts (this is also on the schematic).1/2 drive does nothing to the power tubes themselves. The schematic is on tubefreak .com if you have any questions. The lowered voltage gives you lower volume and earlier breakup.


I am not trying to be argumentive but just trying to squash any misinformation from being spread.

Greg

 

[v6spider]

Ahhh that's what I thought.... but it doesn't lower voltage going to the power tubes in any fashion? Interesting.... I appreciate the detail of your response. Sounds like you have studied the schematic thoroughly.

I agree about not passing around misinformation. Yes, it is not easy to reach real techs at Mesa. I actually got lucky one time when I needed help and was referred to one. One of the capacitors in the bias shorted out and a repair tech was able to instruct me how to test and verify the problem over the phone. Not too often does that occur!

 

[v6spider]

I did a comparison between pulling the four center tubes out of the 2:90 and running in fulltime class 'A' and half-drive with all the tubes in.

I still prefer half-drive to pulling the tubes and running in fulltime class 'A'.
It sounded to brittle to me in that Class 'A' only mode. Half-drive with all the tubes installed sounds fuller and less harsh with more low mids to me. So I won't be doing the mod for the class 'A' switch anytime soon...

I suggest others do the comparison for themselves. I'd be interested to hear some different opinions on this.

 

[chris.goth]

So would it be possible to remove 4 tubes AND run it in half-drive mode?
It would be a much more manageable volume.

Otherwise, are there any stereo power attenuators around that are as good as the THD Hot Plate (without costing 1000$).

 

[v6spider]

Don't see why not... half drive just lowers the cathode voltage to the inverters. Don't see why it would be an issue, but get a second opinon. 'Disassembled' has studied the schematic well. It might be good to ask him as well, as I am not 100% sure, but it definately seems feasable.

 

[Henz]

I mostly agree with disassembled. No biggy but resistor 334 is a 15k ohm. And based on schematic, half drive does increase voltages going to power tubes. 428v going into the two .1 coupling cap Orange drops from 311v via the C node power supply. Don't know if it really matters though.