So I learned something about the Ultimate Attenuator

[alivegy]

I've posted in a few threads about the UA and mentioned that I was having some issues with a crossover type distortion sound as soon as I started to push my amp a bit. I have a 65 bassman that wants a 4ohm load so obviously there is some concern with pushing it through a nonreactive 30ohm load. I replaced all of my tubes as it just needed to be done but the problem persisted. So I contacted Mark through email, explained the problem that I was having and sent him a rough sound clip.

He thought that it might be caused by what he called the plexi circuit in the UA. I thought that it was coming from my amp, but he seemed pretty sure. My UA came without the "plexi-switch" option but he gave me the option of installing it myself if I didn't want to send it back to him. I can pretend to use a soldering iron so he sent me some instructions on how to install it to bypass the plexi business. After a few minutes and a couple "Hey Honey, will you hold this for a second"'s I had the switch installed.

Flipped it on, turned the plexi circuit off with my new switch and the fuzz is gone! It worked. I finally have the deep, rich clean that I was shooting for with out the sizzle. I love the overdrive of my amp so much I just use a compressor to push the front end of the amp and get a killer tone. No TS9 required althought they play nicely too. Turn the amp up to 6 and I have a great smooth distortion with that beautiful fat midrange that works magic with my Fralin Steel Pole 42. I'm in love with my amp all over again and I'm sure my neighbors will be happier too since the sound is virtually the same at any volume minus the speaker interaction of a pushed amp.

Everyone talks about how well the UA works with the amp dimed, but it works beautifully with a chimey fender tone as well. Just make sure you get the Plexi-switch. You can get a nice clean tone with the amp turned down on it's own, but the sound can be thin and you don't get that midrange push that adds a bit of fatness to the notes to keep them from sounding too brittle and that natural tube compression. So people start buying an army of tubescreamers and are disappointed that the overdrive doesn't sound natural. Now I can overdrive the amp it self and it just roars.

Now all of that said I will admit I'm not entirely comfortable running the bassman at such a high load, my amp doesn't have a lot of ventilation around it and it can get very hot if I play for long enough. So i'm looking into getting one of the weber Zmatchers to take some of the load off so to speak. I'll report back on how that works when I get it. Will probably be a couple of weeks.

 

[mesanatic]

Interesting. So the 'Plexi' circuit is on by default and you need the Plexi option to turn it off. Is that correct.
I'll make sure I order mine with that option then.
Thanks for the info.

 

[alivegy]

Yes. The plexi switch turns the plexi option off.

 

[sinner]

Thanks for the info. I had some questions about this and other features/options. Found this:

Using the Standard UA with a 4 EL-34 or 4 6550 amp ( Marshall Super etc.)
If you want to use the Standard UA with a 4 EL-34 or 4 6550 amp, you must purchase and use the 100v ac tap. This will lower the voltage enough to make the amount of heat safe for the unit. If you don’t want to use the 100v tap, you can pull out a push-pull pair of output tubes (either the outer two or inner two). Remember to halve the impedance setting on the amp (compared to the cabinet) if you don’t use the attenuator.
Example: 8 ohm amp setting into a 16 ohm cabinet. If you don’t want to do this either, you must use the Super UA. Thermal damage caused by running a 4 EL-34 or 4 6550 amp straight into a standard UA without the above stated precautions is not covered under warranty.

A word about impedance and load.
With the UA you don't need to worry about different impedances, mismatches or even plugging in your speakers at all. In other words, you can use any amp with any impedance, 4, 8 or 16 (we even have a 2 ohm option). You can run a 4 ohm amp into a 16 ohm cabinet or any combination you want. You can also use an A\B box to switch cabinets as well making the UA a true impedance matching device."
A word about Scaling and Watts.
Scaling is the UA's ability to go from zero db's to the matched full volume of your un-attenuated amp. For example, The UA is rated for 100 watts and the average Marshall Super Lead amplifier with 4 EL-34 power tubes can produce 150 watts before clipping. That is a 30% deficit between the UA And the Super Lead. In other words, the UA with the volume maxed will be less loud by about 30% or approximately 2db's.
You might say, "I bought the UA to turn the volume down, so what's the difference?" This is a good argument and we have lots of UA's on tour with players using Marshall Supers. If you want full scaling for amps rated up to 300 watts, I would recommend the SuperUltimate Attenuator.

Available options for your playing style​

Plexi / hi-gain
True bypass switch customized for amp type. Standard Plexi option is optimized for the presence \ crunch response of a late '60's Marshall Lead amp. When used with cleaner amps, will act as a presence boost. Much preferred over post-amp tone controls. Two settings of bypass and engaged offers very useful tone and crunch options. Bypassed can be considered creamier in tone.
Variable line out
Consists of 1\4" line level out jack and volume knob. Located on back of UA. Useful for adding post-amp mono or stereo effects with the addition of amp\cab(s). Great for the "Van Halen left-right wet, center dry setup". Also useful for DI applications with or without a cabinet emulator (not offered).
100 volt AC tap
3 prong female AC tap for plugging in your amp to drop the voltage to around 100v. Useful to slightly drop plate voltage on amp. Tonal effects are minimal. Feel may have slightly more give. Useful for older British amps that may run current production tubes a little hot. Easier on vintage transformers. Also useful for extended coffee fueled jams with extreme attenuation for easier (cooler) amp operation. Not to be used as a courtesy outlet.
Wattage doubler
Switch located on back that enables UA to drive a second cabinet with total dynamic control of the second cabinet, from off to equal loudness of first cabinet (louder if more powerful or more total speaker surface is employed, i.e.; more or larger speakers). No sonic artifacts are discernible in the second cabinet and no effect whatsoever is detected in the original or first cabinet. When switch is disengaged, unit operates as a standard UA.

 

[soldano16]

Thanks for the info. I had some questions about this and other features/options. Found this:

No mention of the dual volume feature which is super cool. If you're a guitarist who doesn't play with volumes all the way up, you can use the UA's footswitchable "volumes" to give you different levels of compression/overdrive at the same volume.

 

[JLee]

Did you happen to try lifting the ground? I was also getting a great amount of hum along with fizzy distortion and loss of headroom. Mark suggested lifting the ground to alleviate the hum, which he thought was from a ground loop, but also suggested installing the Plexi Switch. On a whim I thought I'd also call Yu Kong Ho, since the UA is his design. He thought the ground loop was the culprit to all of the issues. I used a 3 prong to 2 prong adapter to lift the ground and it solved all of the issues! I have a Ebtech Hum Xterminator on order though, since I'm not too comfortable using the amp without a ground.

 

[alivegy]

Hmm, I'm not currently having any ground hum issues that I'm aware of. I'll have to give it a shot to see if it makes any difference. Installing the plexi-switch made the UA much more useable. With the plexi-circuit on the UA is really only good for high gain stuff.

 

[iim7v7im7]

Hi,

I also typically keep my Plexi-Switch engaged. I am using the UA with a Allen Accomplice with the output load set at 8 ohms. I have it set up with 6L6s. I am not using the UA for bedroom volumes. I have my amp set somewhere between 5 - 7 in volume which with 6L6s is the edge of clean and distortion. I have the attenuation control somewhere between 9 to 10 o'clock, which is the equivalent output volume when the amp is set to about 2 in volume 9which still is fairly loud and moves some air!). I find the plexi switch adds some high frequency content, that these old ears like. I also recommend people ask for this if they order a unit.

Bob

 

[iim7v7im7]

Cochese,

My understanding was the opposite of yours (butm I could be wrong). When I ordered mine with the bedroom switch option, I originally did not ask for the plexi options. They recommended that I add it. Since it gooses the highs a bit, I think it is useful if attenuating down to very low levels. This why I believe that they recommended itin combination with the bedroom option. Simplest way is to e-mail UA or Ho to find out for certain.

My $.02

Bob

 

[alivegy]

Maybe I misunderstand but I thought that the unit has the Plexi circuit engaged at all times unless you order it with the switch. This enables you to turn it OFF. That circuit adds highs much like the bright switch on the Hotplate. I've found that the UA will make the amp sound dark without the switch engaged at lower attenuation levels. However if you turn your amp way up and attenuate it down to really soft levels it creates fizz (much like the Hotplate) and then needs to be turned off.

This is correct. The switch allows you to turn it off, otherwise it is always on. Get the switch.

 

[mightysteve]

"A word about impedance and load.
With the UA you don't need to worry about different impedances, mismatches or even plugging in your speakers at all. In other words, you can use any amp with any impedance, 4, 8 or 16 (we even have a 2 ohm option). You can run a 4 ohm amp into a 16 ohm cabinet or any combination you want. You can also use an A\B box to switch cabinets as well making the UA a true impedance matching device.""

I think this is highly misleading. The UA presents a 30 ohm resistive load to any amp you plug it into (though I believe the 2 ohm option changes it to around a 15 ohm load). Perhaps the fact that it's not a reactive load (hence the load is not frequency-dependent) makes it safer, but I'm not sure I believe that. Certainly, most people would not plug a 4 ohm amp output into a 16 ohm speaker cabinet (a 16 ohm reactive load).

 

[cg]

Are the units marked differently for the 100w and 200w versions? If so, where?

 

[drbob1]

I'm not convinced about the UA but the continuous noise about loads brings up an interesting question: A speaker load more than 2X the output impedance of the amp is pernicious in part because when the speaker moves back to its resting position, it generates a voltage in the secondary winding of the transformer (which is why Shure can use a speaker as a mic for bass drums). This is not true of a purely resistive load. And since the collapsing field around the OT isn't being goosed, I can't actually see what would result in a larger flyback voltage than you'd get using a lower resistive load (in fact, doesn't discharging a coil thru a resistor slow down the discharge, resulting in a decreased flyback peak voltage?). I'm NOT an engineer, so any illumination on this would be appreciated!

 

[ozspawn]

I talked to Ho yesterday about the Plexi-switch and he said it will make the amp sound brighter (more fizz?). I was hoping he was going to say it eliminated the fizz because my UA (the 2 Ohm model) is unusable with my Super Reverb. It sounds like frying bacon, and the signal seems to cut out and cut in intermittently. Ho suggested I might have a ground-loop problem, and I should disconnect the ground in the UA. I did so with no change in the problem. I'm going to take the UA and the Super Reverb to Ho's shop next week to see if he can fix it. Ho is a very nice guy and a pleasure to work with.

 

[JLee]

alivegy,
Can you post the pic of the mod? Mark tried forwarding your photo to me, but forgot to attach the photo in the email.

 

[sinner]

Other options to consider: 100 volt tap

Anyone getting that option?

I've read it's good for vintage amps that were actually made (60s, 70s) when voltage was lower and now in the US voltage has crept up to more than 100v. It can keep components cool.

Good idea for new reissue amps as well?

Do I understand correctly that you plug your amp into this 100 volt tap on back of the UA and then your amp gets a steady and accurate 100 volts? Is this anywhere near the "brown" sound rating?

 

[muddy]

question; what about just getting a variac? what are good ones to buy (brand, rated for how many amps, etc) for lowering 120v to 100v?


ml

 

[alivegy]

AbbeSauniere, I'll post that pic tonight when I get home from work. Sorry didn't see your post till this afternoon.

ozspawn, if you have a UA with no plexi-switch the plexi circuit is active all of the time. I don't know if this is causing your sizzle, but it was mine. The switch bypasses the circuit which does make the sound darker, but you can turn the treble on the amp up to compensate. I think the plexi circuit is there to add some sparkle to the high gain sound of a marshall voiced amp. If you use the the UA to get more of a bluesy clean overdrive the plexi circuit is really going to work against you.

 

[jackaroo]

Mudslide-

Mine has the 100v tap- it is indeed in the browning territory. 90v is the lowest most say you should go without a separate transformer for the heaters.

A cranked amp with a tube rectifier and on the 100v tap is very squishy. It can be a little mushy for some I bet. I do like it both ways with my JTM 45 but I'm leaning towards more clean headroom and snappier response of the wall over the 100v tap now. But mine is a dead nuts repro I'm told- and that means it's pretty loose and woofy to begin with. A tighter amp like a 1987Xplexi would benefit from it more I bet.


Here's some pix of mine with

100 watter
dual level
100v tap
Plexi switch
effects loop (2 levels too)

I've since installed an off/on switch for the fan as it's a bit loud for recording at quiet levels- for stage work- no problem.


BTW- It works.

 

[sinner]

Thanks, Jack, for the info and pics.

"Dual Volume" that looks promising--come with foot switch? I do see "FSW" on back of unit, must be for a foot switch.