opinions on attenuators.

[danwelsh]

Hi fellas. As most of you are aware I built a jtm45 amp. It's insanely loud and I was thinking on building an attenuator with a 100 watt 8 ohm lpad. Anyone use a diy attenuator for a 50 watt amp? And what's your opinions on them.

[Leevibe]

Not sure if an L pad would work on a 50w or not. I'm betting that resistor is going to get hot though!

[danwelsh]

Not sure if an L pad would work on a 50w or not. I'm betting that resistor is going to get hot though!

I was gonna wire up 2 fans inside to keep the heat down......one sucking across and the other blowing the same way to dissipate the heat

[danwelsh]

If I use a 100w stereo lpad and wire the 2 lpads in series will that give me 200w?

[RobA]

I have a little attenuator box that'll work on up to a 15W amp. When I have my 5W Marshall cranked into it, it get's fairly warm, but it's completely enclosed and just has the main resistor screwed to the aluminum enclosure.

Mines a bit different from normal attenuator usage because I don't run it out to a cab. I attenuated the signal down to about line level and then buffer it and run it in to my computer for digital processing. Given that, I tried L-pads, pi-pads, T-pads, and bridged T-pads. I settled on the bridged T because I thought it did the least to the frequency response.

The bridged T would be more difficult to set up as a variable attenuator, but when I was playing with the whole idea, I tried an idea of using a fixed bridged T followed by a variable L pad. That seemed to work pretty well to me.

One of things to note is that you have to attenuate way down for the volume drop to really be effective. The Marshall amp has a built in L-pad attenuator that's supposed to drop the output to something like a 0.1W. That's still too loud to play in the house at a conversational level and get the power tubes into saturation.

[RobA]

If I use a 100w stereo lpad and wire the 2 lpads in series will that give me 200w?

You can wire resistors in parallel to distribute the current, but then you need to recalculate the resistors you need to get the attenuation you want.

If you were to wire the L-pads one after the other, the first one would drop all the voltage and take all power and the second would see very little.

[Morgan]

There is a one sheet write up for an air brake out there. It's a proven design and lots of people use them on trainwreck amps. Its a thing that you want to use the correct, time tested parts for (ohmite rheostat, etc) and I think the parts total is around $100 these days.

It'll work though, and it should last forever.

[Guitarmageddon]

edit^ yep, just like Morgan says!
I built a Trainreck Express, volume goes from off to max by about 3, loudest 45 watts I've ever heard and I own a '74 50watt JMP, also loud.  I built an Airbrake (search over at Ampgarage) there are two versions, one for "bedroom" playing that uses a rheostat, most think it sucks tho, don't bother, just build the 8 step one, still lowers the volume significantly for gigs or recording- best thing I ever did.
Cost about a hundred bucks because you need two big ass wire wound resistors. But a pretty simple build, and way less expensive than the commercial options. Works great, at the max atten there's a very slight tonal change, but not really an issue. Other great thing about it is it attenuates the hiss- an Express is high gain, so this really keeps it quiet. I never use it without it now.
Enabled me to get a bunch of great new tones from my old (non master vol)JMP too.
Airbrake doesn't have a line out, prolly the only issue, but I've never really heard one that was concincing anyay.  I'm a very happy guy with mine.  OK, cross post!

[alanp]

How do you find the tone of the Express in general?

[gordo]

And is it true that TW's don't like pedals?

[jtn191]

Also look into VVR voltage scaling. Arguably more effective than attenuators

[davent]

Also look into VVR voltage scaling. Arguably more effective than attenuators

And so much easier/gentler life extending of your expensive output tubes. London Power (Thunder Bay ON) has a number of options for power scaling the amp.

http://www.londonpower.com/power-scaling-faq
http://www.londonpower.com/selecting-power-scaling-kit

In no way associated with London Power but have used one of their scaling circuits (scratch built) in three different cathode biased amps, no issues what-so-ever.

[cooder]

Also look into VVR voltage scaling. Arguably more effective than attenuators

And so much easier/gentler life extending of your expensive output tubes. London Power (Thunder Bay ON) has a number of options for power scaling the amp.

http://www.londonpower.com/power-scaling-faq
http://www.londonpower.com/selecting-power-scaling-kit

In no way associated with London Power but have used one of their scaling circuits (scratch built) in three different cathode biased amps, no issues what-so-ever.

That has also swayed me towards incorporating VVRs in my amps so far... I just haven't had a chance to use an airbrake or other attenuator, just wondering if you guys can give your opinions on sound and other pros and cons related to taht if you have had a chance to trial either.

Another thing that makes me concerned about attenuators is that they are between Ot and speakers, so if they fail for some reason the OT might get sucked down the drain as well...?!?
I had VVR Mosfets failing on me (because I got a batch of fakes of ebay... ) which didn't cause the PT to be damaged or so, it didn't even blow the fuse and the amp kept on going, just on full voltage rather than regulated down. That was an annoying but otherwise harmless affair...

[Morgan]

I absolutely love power scaling on lower power amps, especially cathode biased amps like 5E3's and 18 waters. They are the cat's pajamas.

But as the circuits get more complicated, so does the scaling.

I've got a 5E7 tweed bandmaster that I incorporated the fixed bias VVR on (quite basically two VVR circuits connected to a dual ganged pot that scales the B+ and bias voltages in tandem). I haven't got it to work yet. First try, the bias voltage wouldn't scale. I pulled it out to find that the bias MOSFET and associated diode had blown. Reinstalled it with new components and immediately blew the B+ MOSFET. I pulled it out again and went over everything with a fine tooth comb. I'm going to try it one more time.

Separately, I've got an amp that it is basically a fender standalone reverb circuit I front if a tweed bassman preamp and phase inverter, with an early 50's era cathode biased 5881 power section (tweed bassman transformers). It's the buggiest amp I've ever built by a mile. Bugs aside, it hates the VVR circuit. I'm scaling the whole amp, which makes the reverb go to mush and oscillate when the voltage gets too low. And taking the reverb out of the circuit, it just doesn't sound right when it is scaled. It sounds like a bad master volume - gets real thin, real quick. I'm this case, I should probably just scale the power section, which is tricky in my experience. You end up with a hot signal coming out of the PI that gets way out if balance with the power section as you scale it down. The trick there is to use a PPIMV to attenuate the PI signal as you scale the power tubes. But then you end up with two controls you have to fiddle with every time you want to back off (infuriating on a dark stage while the sound guy is waiting on your ass).

Anyway, sometimes having an airbrake around is nice in that they are pretty bullet proof and you can move them from amp to amp. And sometimes with larger bottle, fixed bias amps like a JTM 45, it works a bit better to leave the voltages as is and just dump some power on the way to the speakers as needed.

I'm sure scaling can be great in something like that; it just might take a bit of doing!

[Morgan]

Oh, I also have a simple switchable L Pad attenuator that switches between 6 and 12 dB of attenuation. I basically built it for 5E3's. It sounds really good at 6dB, but sounds like crap at 12dB. I find that it's actually a great attenuator for blackface fender circuits - it seems like a lot of times, attenuating 6dB us all you need with those circuits (unless you're the guy playing a twin reverb in a small bar ).

It's pretty money with a deluxe reverb for the bars I typically play. Turn everything to 6 (I like to get 'em cooking), reverb to taste, attenuate 6dB, mic it and point it at my head, BAM - Morgan's in la-la land for the next 4 hours and no one is complaining about the loud guitar player.