Here is a slide show of the history of guitar amp attenuation
and highlights some of the similarities of the overall design, UA excepted.
When you click the link, it will download a Powerpoint document. When you open it either press the F5 key to play or click slideshow in the toolbar and click play. To stop, click esc
Here is a slide show of the history of guitar amp attenuation
There has been a lot of lively discussion on TGP and the Vintage amp Forum about the safety of mismatched loads and in particular, of the 30 ohm load in the UA. Here is a response I made to a customer that I will reprint here.
There are two serious considerations for load devices like the UA or other attenuatos. They are
1.Flyback voltage from the load
2. High heat and high voltage from the amp
Both are dangerous to your amp. The higher the impedance of the load and the more voltage an amp produces, the more flyback voltage can become a concern.
The lower the impedance of the load, the more heat and voltage is produced. It’s kind of a balancing act.
We have been asked why we don’t match the impedance of the load more closely to an OT’s winding spec?
There are two reasons. One is tone. The 30 ohm load sounds better. The other is safety. At 30 ohms, the amp doesn’t work as hard or produce as much voltage and heat which are your amp and tube’s enemy. In principle and practice, the 30 ohm load is safe for the amp and doesn’t introduce unacceptable risk for the amp and tubes at any impedance in theory and practice until about 2 ohms. That’s pushing it a bit with the mismatch but generally amps with 2 ohm taps are lower wattage and lower voltage which helps. If you are nervous about it, plug your amp into the UA’s 100v tap. It drops the voltage about 10%, not low enough for cathode stripping of the tube to be a concern, but low enough to make the amp and tubes run safely when the amp is cranked for long periods of time. Is it as safe as running straight into a cabinet? Yes, maybe more so at the same playing volume and intensity (while using the 100v tap). Why then, do techs and amp makers not recommend using attenuators (except their own brand?). I suspect that anyone that warrants a product or repair would prefer not dealing with issues like current tube production failure rates that can cause damage to an amp. Who can blame them. Playing hard can reveal problems with tubes and amp circuits and playing through an attenuator encourages playing hard. That’s my take on it. If your amp runs at an unusually high voltage or you have a shaky output tube, I don’t recommend using an attenuator. Otherwise, play the amp. That’s what it’s there for.
Mark Gregg, Magus Innovations.
Here is a response to some posts on the Vintage Amp Forum by the forum administrator and owner of Plexi Palace and Mojave amp. Victor Mason is considered to be an expert in his field. You can find the complete thread here:
by plexi on Wed Feb 11, 2009 6:56 am
With respect to the Ultimate Attenuator there is a difference. The first palce this difference exists is the load resitor. The Ultimate Attnuator, also known as the “UA” uses a 30 ohm load resistor. I do not know exactly why that value was chosen, but I do know this. It’s safer. A lot safer. Your amps power output is a mathematical equation. If you match the load to the ohmage tap on the transformer, say 8 ohms to the 8 ohm tap on the amp, your amp will provide maximum power efficiency. That means it works hard and will pass the maximum current. That also means heat. Heat in the Power transformer and the output transformer. That is because its passing loads of current. Dangerous for an old amplifier. This is not an ideal scenario for a vintage Marshall. The older original transformers were simply not designed for this kind of use or better yet “Abuse”. With the Ultimate Attenuator, your amps current is reduced leaps and bounds.
For instance, if your amps set for a 8 ohm load, the UA is only going to see a maximum of around 25 watts made by your 100 watt amp. That is because the load is approximately 4 times larger. The result is the power amp is full throttle but the power output is low. That is good. It saves not only your output transformer but your power transformer. This is probably a good answer to the Mercury Magnetics concern now that the amp is not actually making any where near its full power even though the power amp is full tilt. The power transfered from the amp to the laod simply cant be any higher then the equation allows for.
Now what about the tone. The load is so far mismatched, how can the sound be correct? The UA has a built in low impedance power amplifier. The term low impedance means strong high frequency response and with their well crafted internal audio coupling, they have managed to retain and preserve the sound quality while keeping the amp at a low volume. The reduction of actual power is safer and the audio quality is preserved and enhanced via the audio coupling circuitry as well as the built in low impedance power amplifier, which does exactly what you need for low volume high frequency preservation.
Because of these two basic facts, the Ultimate Attenuator is definitely the safest product you can use and it delivers that low volume sound with the least impact which results in a very accurate quality of sound reproduction. For what its worth, this is just my opinion and my explanation was provided here to try and help some of you understand the way these things work.
Over the years, I have always told my customers that it was safer to use both a Hot Plate and a Variac together. The reason was simple, you can knock down the volume with the attenuator but you need to lower the power that is fed to the amp which results in how much power is made by the amp to protect it from catastrophe. I also recommended that the customer not use his best tubes since a variac will lower the heater voltage and thus reduce the temperature of the cathodes and that causes the preverbal Cathode Stripping, which simply put, ruins your tubes very quickly.
If your using an attenuator at present, I strongly advice you incorporate a Varaic to dial back the power level the amp can make. Reducing the power by 30 volts is enough to prevent catastrophe. The core temperature of the transformers can get very high internally and that will melt insolation between wire and cause shorts. The attenuator that is set way down for good volume control and even if it sounds good to you will be absorbing full power. Full power means full throttle on the current flowing through the transformers and full throttle on the internal core temperatures, several hundred degrees F. That can damage your amplifiers original transformers. Newer transformers like Mercury have developed improved insulation and with a quality improvement in over all design materials, they can withstand the increased core temperature much better but it’s still a risk not really worth taking if you can simply avoid it.
The UA offers a higher load to start with and that makes it safer. You can mismatch a passive attenuator buy using a 16 ohm Hot Plate or similar attenuator on your amplifiers 8 ohm output tap, thus reducing the power in half, but the mismatch and the fact that your using a passive system will further erode tone quality if not dealt with some how. Again, the UA has adapted audio coupling and incorporates the low impedance power amp which resolves the tone issue.
I’m in no way claiming the UA is perfect but it does the best job to my ears and technically speaking there is a safety factor which is worth considering especially if you want to keep those old transformers from blowing or over heating.
I was informed that there is a protection circuit to prevent a major issue if you plug up your amp to the UA in reverse. That is to say you plug the output of the amplifier into the speaker output of the UA. That would result in a problem if there were no protection for that. Thanks to Tone-Freak, I have made the correction to my previous comments that you could have a problem with the UA because of its built in power amp. It’s not an issue and that is very cool to know.
Sorry to admit it but I am guilty of this mistake. Thank God it was a passive Hot Plate. I did plug the amp into the speaker connection and the speaker into the amplifier input of the Hot Plate. There is no damage and no ill effect. The unit simply does not work as an attenuator. If you reverse the wires your back in business and all is well. one of the safer factors of a passive system.
For the money and trouble of buying both a passive Attenuator and a Variac and the fact that you will have to lug around an additional 13 pounds of iron using the variac and think about your tubes dying off, It makes a lot of sense to stick with the UA. The unit provides safety, good tone and you can use your best Mullard power tubes. The UA does have a 100 volt power outlet on the rear of the unit to allow you to run your amp 20 volts down for a good measure of safety especially if your using an old JMI that runs on 100VAC or 105 VAC. That goes for old Marshalls as well. We did many old Coffin logo, Block logo and Script logo Marshalls. They run exactly right at 240VAC but if you set them for US voltage, typically 105-115VAC on the back of the amps selector, they are over voltage at 120VAC. 100VAC is ideal for the older amps and will keep those internal voltage low enough not to blow up your original capacitors. Just another good reason to have the UA. When you guys use your older Marshalls or Vox amps or any British amp that was originally built for UK use, be sure to check this out or have someone check it out for you. You can see if your amps running hot buy measuring the heater voltage on pins 4 and 9 of the pre amp tubes. The ideal voltage for the heaters is 6.3VAC. If your looking at 8 VAC, you’ve got a problem and your running the amp with a very high internal voltage most likely. This is where a Varaic can help. It will lower the power down until the amps running at 6.3 Volts AC on the heaters. Now you can see where your amps AC input needs to be. If it sits at 100VAC, then that is the input you can safely use. If the running voltage works out to say 115 VAC input, your probably ok to just go ahead and run 120VAC since the difference is not outrageous and your not jeopardizing the amp. This is off topic but I mention it because the heaters reflect the internal voltages. That is to say if the heaters are way high, then your going to most likely have very high internal voltages inside your amp and then your in danger of over voltage on the old capacitors and putting tremendous stress on older tubes and power output will increase as well. Al these things are tied together.
That is my 2 cents.
The UA has a protect circuit that must be switched on by passing a signal through the unit. This is simply accomplished by turning on the amp and turning it and the guitar up enough to open the circuit. The circuit was included to protect parts in the UA vulnerable to the voltages it would see if plugged in backward. This saves time and money on costly repairs and in no way affects tone. Once on, you need not open the circuit again until the unit is unplugged from the AC main, and plugged back in again (to the power).
RECOMMENDED POWER UP SEQUENCE
1. Make cable connections with the “speaker” jack going to the cabinet and the “amp” jack going to the speaker out on your amp.
2. Set the UA to off. Turn on the amp and listen for blowby or signal to make sure the proper connections are made and that the cables are working properly.
3. Set the amp volume halfway or more to your intended volume setting.
4. Set the UA volume all the way to the left (no sound) and turn the switch “up” to
4. Turn guitar volume all the way up and spank a chord. The protect will now be off
and you won’t hear a sound. If you want to hear the protect defeated, turn up the
UA slightly when you spank the chord.
5. Turn up the UA to your desired volume.
If you experience hum or squealing from your amp, lift the ground on the UA using a 3 to 2 prong adaptor.
Hello and welcome to my Blog. My name is Mark Gregg and this is Stupid Amp Tricks or sit, stay, good boy! These should be loose, fast and devoid of spell check. Basically it’s about how to get the most out of your amps and UA and the experience I’ve gained blowing up my own amps and attenuators. I’ll be naming names and I have a lot of ideas I want to get across and this seems like a great way to archive them for your perusal. I’ll try and give them pertinent names for ease of search but no promises. This is just the introduction and I have a feeling I’ll be spending some time figuring out how to upload this. Stay tuned!