Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: slash1986 on February 13, 2015, 04:32:45 am
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Hello everyone , I am writing to you for advice. :w2:
I would like to build an attenuator to put between the out of my amp and the cone , which is a Celestion Vintage 30 .
The amplifier is a 20w head , with 2 EL84 in the power amp .I wish I could turn the volume of the Master without breaking the windows of my room .
On the net I saw a kind of potentiometers called L -Pad , which should do what I need .
However,it seems, that a large reduction in volume , has a negative influence on the tone , particularly at high frequencies .
Can you tell me a circuit that uses this L -Pad , but also has a kind of compensation on the high frequencies ?
If my amp is 20w which L -Pad should I use : 50W or 100W ?
I hope you can help me ,
I thank you in advance .
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Hi Slash I don't know about the L Pads but I play a Princeton Reverb head all the time and lightly use a Weber Mini Mass attenuator that works quite well to soak up some power out and get great overdriven sound when I want to.
Attenuator Info (http://www.tedweber.com/atten.htm)
The other attenuator people use a lot is that THD Hotplate.
Hotplate (http://www.thdelectronics.com/product_page_hotplate.html)
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Have you considered Dana Hall's VVR?
http://hallamplification.com/main.html?src=%2F#2,2 (http://hallamplification.com/main.html?src=%2F#2,2)
http://el34world.com/Forum/index.php?topic=6899.0 (http://el34world.com/Forum/index.php?topic=6899.0)
OR have you tried a LarMar PPIMV (post phase invertor master volume)
with respect, Tubenit
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On the net I saw a kind of potentiometers called L -Pad , which should do what I need .
However,it seems, that a large reduction in volume , has a negative influence on the tone , particularly at high frequencies .
Can you tell me a circuit that uses this L -Pad , but also has a kind of compensation on the high frequencies ?
If my amp is 20w which L -Pad should I use : 50W or 100W ?
This will work fine for what you want: http://www.parts-express.com/speaker-l-pad-attenuator-50w-mono-3-8-shaft-8-ohm--260-252 (http://www.parts-express.com/speaker-l-pad-attenuator-50w-mono-3-8-shaft-8-ohm--260-252)
If necessary you can always place a cap & a switch across the input/output lugs to help high frequency make-up but I've heard positive results with these without them. Up to you & your personal preference. For the price I'd try it before anything more expensive and to be sure you even want attenuation or not?
If you get this, please report back on your findings-
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Of this type will go well? http://www.musikding.de/Attenuator-50W-8-Ohm (http://www.musikding.de/Attenuator-50W-8-Ohm) which is the capacitor do you recommend ?
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Here some info about the LPad
http://www.colomar.com/Shavano/lpad.html (http://www.colomar.com/Shavano/lpad.html)
(http://www.colomar.com/Shavano/lpad3.gif)
K
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If I wanted to add together the power reducer a DI BOX would be possible ? I saw attenuators with built- DI .
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Seems like the L-pad I built will work for you. Search for "L-pad" in this forum section.
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I'm with Tubenit. If your amp is cathode-biased I would look into VVR instead of an attenuator. Easy to implement especially if you buy a kit.
There are fixed bias versions too but they are a little more complicated and if you are not using a transformer on your bias tracking circuits your results may not be all that great. London Power does excellent fixed-bias Power Scaling kits but they are rather pricey.
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This is the Schematic..
where should I put it??
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If your amp is cathode-biased
It looks to be fixed bias since the cathodes are grounded. So you're back to L-pad at the speaker?
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If the solution of " Tubenit" is not applicable to my amp,the only way I have is an L -pad right ?.
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There are also VVR for fixed bias amps, not only for cathode bias amps
only the price is higher
http://www.hallamplification.com/main.html?src=%2F#2,2 (http://www.hallamplification.com/main.html?src=%2F#2,2)
http://www.londonpower.com/power-scaling (http://www.londonpower.com/power-scaling)
K
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However, it seems that a large reduction in volume has a negative influence on the tone , particularly at high frequencies .
That's part of the cost of business when you employ heavy attenuation. I have a Weber MASS and three different THD Hot Plates (for different impedance) and they all dull the high end at higher attenuation levels. They all have high end boost circuits (the THD also has a low end boost) but I find that the on board high end boost on these - which I am guessing is essentially a capacitor - is a bit on the harsh side. The trick is to find a balance of amp volume and attenuation that minimizes this effect.
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That's part of the cost of business when you employ heavy attenuation.
+1 (especially on standard resistance-based units). But this is a sensitive/touchy subject even with the best normal volume control or something like a standard PPIMV (or anything w/ resistance and/or voltage divider), just plain lowering volume in this way will affect high end w/out a treble bleed cap or network. Same principle for guitar for that matter. (this also supports Alerich's comment below about seeming "harsh"). This is why a bit of resistance in the treble bleed path can help with this also but nothing is "perfect" in dealing with resistance. This also supports a bit of the VVR side. However when voltage controlling, this is going to have it's impact on performance as well.
but I find that the on board high end boost on these - which I am guessing is essentially a capacitor - is a bit on the harsh side.
It IS a capacitor however I think I've seen it be a high value, like 25uF and non-polarized IRRC?
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Slash, there's other ways of manipulating that circuit to lower output power such as triode mode and/or increasing screen resistors but you'd be better off making a single EL84 amp instead or even a less powered tube depending on what kind of wattage you're trying to get down to of course. Best not to take a higher powered amp to only neuter it.
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Slash,
If you want to really see an exhaustive debate over attenuators, take a look at this. If you don't scratch your eyes out by the end, you may find what you are looking for. It was Jojo's fault, by the way....
Jim
http://el34world.com/Forum/index.php?topic=17793.0 (http://el34world.com/Forum/index.php?topic=17793.0)
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It was Jojo's fault, by the way....
Yeah, Jojo's fault. :BangHead: :cussing:
Brad :laugh:
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wait just a second...bringing up NiChrome toaster wire sucking those precious dynamic wattages away was all Jeembob's idea and and and .... no it was all SG's fault!
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:l2:
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And Jojo said this in that thread, which is quite insightful concerning attenuators;
I think they work & function the best adjusting your amp/set-up total power within reason while going easy on how much attenuation you use. This retains most of the sound and feel as it would be under "normal" conditions without one. Nobody should just turn all knobs to e11even and crank down the attenuator. No attenuator will overcome that, plus it's damn hard on your amp and running it needlessy high & and just begging for something to give way. An attenuator needs time to get used how you control it along with amp settings and style to get the most out of them and they compress things, which is why I feel they should be used like you use salt on your food. Just a little goes a long way and it's very easy to use too much and ruin an otherwise great meal/amp tone!
And this is precisely why I said I judiciously use my Weber MiniMass. Experimentation will reveal the sweet spot and the sound you are looking for.
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I recently stumbled across a lengthy discussion at Freestompboxes.org about Weber Mass Attenuators.
http://freestompboxes.org/viewtopic.php?f=2&t=2287&hilit=mass+attenuator (http://freestompboxes.org/viewtopic.php?f=2&t=2287&hilit=mass+attenuator)