ToddW Posted June 13, 2008 Report Share Posted June 13, 2008 Nice to know you didn't get zapped (yet ) Anyway, gain, drive, volume . . . are usually the same thing in an amp. It sounds like they're adding a pot somewhere in the amp, basically a variable voltage divider so you can adjust how much of the signal passes to the next stage, and how much goes to ground. That'll let you increase or decrease the "Drive". Odd that'd be an add on. Sure it isn't a Sag control? Anyway, and more importantly, that amp has a power attenuator, why would you need to add another one? Todd Quote Link to comment Share on other sites More sharing options...
djhollowman Posted June 13, 2008 Author Report Share Posted June 13, 2008 Nice to know you didn't get zapped (yet ) Anyway, gain, drive, volume . . . are usually the same thing in an amp. It sounds like they're adding a pot somewhere in the amp, basically a variable voltage divider so you can adjust how much of the signal passes to the next stage, and how much goes to ground. That'll let you increase or decrease the "Drive". Odd that'd be an add on. Sure it isn't a Sag control? Anyway, and more importantly, that amp has a power attenuator, why would you need to add another one? Todd haha yeah thanks! The amp already has a thing called a Powerator - quote "A power attenuator located AFTER the power stage so that the sound of an all tube amp can be obtained at low volumes This is not equivelant to the preamp gain/master volume type circuits which starve the power tubes. This POWERATOR allows the power amp tubes and transformer to run "full out" but have the output volume from Speaker Out attenuated to the desired volume." And yet, as you rightly say Todd, here I am about to fit something which would appear to do exactly the same! I don't fully understand that. Is the fact that the existing attenuator is AFTER the power stage that's significant?? Is this new one going BEFORE it??? What I know for certain is that when you have the amp up and running through the owners 2x12 Marshall cab it is just FAR too loud for "bedroom practice" levels! This is the owner's grumble - he wants to be able to use this amp at low volume levels but getting the tone of the valves being driven. (Which is what I thought this Powerator did!) I think I've found a way to let you all read the instruction I got on how to do the job. I'll try to post it next. DJ Quote Link to comment Share on other sites More sharing options...
djhollowman Posted June 13, 2008 Author Report Share Posted June 13, 2008 Right then! Got the .pdf converted - sorry about the watermarks! 4 pages: comments welcomed! Thanks! DJ Quote Link to comment Share on other sites More sharing options...
ToddW Posted June 13, 2008 Report Share Posted June 13, 2008 (edited) Hi DJ Putting the power attenuator after the power stage is exactly what you want to do if you want to be able to overdrive the power tubes at a lower volume. You can overdrive the pre-amp tubes and simply turn down the master volume for preamp distortion. Think about it like this, which is sort of schematically. Coming in from the left you have your guitar signal going to a control grid on a tube. It's a TINY voltage swing. From the tube top to it's bottom is a very high voltage and inside the tube, the tiny voltage swing on the grid causes a big swing in the current that flows from the tube top to it's bottom. (it's really bottom to top, but ignore that). If you put a resistor in that high voltage current, then as the current flow changes, that voltage also changes We tap that voltage swing, and with our drive/gain/volume control, send a portion of it to the right and into the next tubes control grid. Now it's a bigger voltage swing than you got from your guitar. Could be 40x bigger with a pre-amp tube, could be 1000x bigger with a power tube pentode. So your pre-amp takes the signal voltage from tiny to small, possibly using several gain stages, and then you use that voltage to control the current running through your power tubes. At this point, you don't take your voltage swing, you take that current and put it thru one side of a transformer. On the other side you get a MUCH lower voltage, but higher current signal which goes through your coil, makes a magnetic field, and moves your speakers. Whoo hoo, sound. Because of how the tubes work, they amplify different frequencies different amounts, add some cool harmonics, and . . . if you hit the control grid with too big a swing, they clip or . . . . distort . . . and we like that sound if it's done well. Here's the kicker. It's fairly easy to get the little preamp tubes to clip, or you can just run your signal through several stages to add harmonics, but waste a lot of the gain so you don't clip. But the harmonics and tone is different than what you get pushing the big power tubes into distortion. That's what you get running an amp full out on a stage. I describe it as less buzzy, but that may be a lame description. Anyway, the post power section attenuator is taking a portion of the signal that should go to the speakers and turning it into heat. It might even be using some of the power output to run a lightbulb If your friend wants to push the power section into distortion, he needs to hit the control grids with a big voltage swing. With a lot of voltage across those power tubes, that makes a big current change, and without the attenuator, it'd be super loud. With power scaling, you drop the voltage, ie. starve the tubes, and surprisingly, it still sounds great with certain tubes. But the reality is, it's hard to get big amp tone at bedroom volumes. I built a 1watt amp that comes close, and a single ended amp with power scaling that does it better. I've never tried a power attenuator. I don't like the idea of intentionally wasting power, but it should let your friend drop the volume. Realize, however, that even if the attenuator wastes 99% of the 50 watt output, if you run it into an efficient cab, that 1/2 watt is going to be VERY LOUD in a bedroom. I can't crank my daughter's 1 watt amp and carry on a conversation even running it through an inefficient 10" Weber driver. Put it through a Marshal 2x12 and you wouldn't want to be in the room. Long post, hope it helps a touch. Todd Edited June 13, 2008 by ToddW Quote Link to comment Share on other sites More sharing options...
ward Posted June 16, 2008 Report Share Posted June 16, 2008 This mod you're trying to do, it looks like it's just a pot and two caps. So I'm guessing what it does is simply attenuate the preamp volume somewhere. I don't think this gives a more desirable effect as the power attenuator. The power tubes will be less driven. Preamp overdrive will be there, powertube distortion not. I agree with Todd, bedroomlevel in a 95dB efficient speakercab is about 1/4Watt, not 50!!! Is there no schematic with the mod ??? Quote Link to comment Share on other sites More sharing options...
djhollowman Posted June 20, 2008 Author Report Share Posted June 20, 2008 Hey all! Update - job done, total success! I didn't get killed, tha amp still works, the "drive control" does exactly what it's supposed to (in volume terms anyway!), and the owner is delighted! Phew! The owner got a friend of his, who does electrical testing, to test the amp was fully discharged first. The actual fitting of the parts was a piece of cake. And it really does work! I don't know enough about what that amp sounds like running with the tubes at full tilt, but you really can turn it down to any volume level without making it sound all thin. Anyway, owner is happy, and that is the main thing! Thank you all for your help and comments with this one! DJ Quote Link to comment Share on other sites More sharing options...
Prostheta Posted June 20, 2008 Report Share Posted June 20, 2008 Excellent - good work on not deading yourself. Quote Link to comment Share on other sites More sharing options...
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