Question for the amp techs...

[boobtube21]

As some of you know I've been slowly blackfacing my SF Vibrolux.

The next (and probably final) step is replacing one of the power supply resistors. SF value for this resistor is 4.7k, 1w. I would be changing it to the BF value of 10k, 1w. As I understand it this should lower the plate voltage from 200 to 170.

My question is, is this what makes the silverface version 40 watts instead of the blackface 35? Or is that more a result of the different ouptut tranny?

Thanks guys, sorry if this is a noob-ish question...

[Kap'n]

I suspect the differences are from higher voltages, but the specs are subject to interpretation. What frequency? What THD? etc.

That resistor is one of the least likely to produce a change in sound. I wouldn't sweat it.

[boobtube21]

Well, the reason I'm curious is I need to buy an attenuator in a hurry, like yesterday, and the only one I can find local for sale is a 50 watter. As you know Kap'n Weber recommends buying one rated for 2x the rated wattage of your amp.

I'm thinking if I could dumb down the wattage at least a little, I would have less of a chance of frying the thing until I could get my hands on a 100w version. I don't plan on cranking up past 6 or so anyway, so I doubt it would be putting out much more than 40w as is. Just better to safe than sorry.

[Kap'n]

Weber mainly suggests an overrated attenuator because there's always the folks who use them to crank their full stack to 13 with five fuzzboxes in front of it to play grindcore in their basement.

With more normal usage - just enough to bring the amp's sweet spot down to a more manageable level, you shouldn't have much of an issue.

[boobtube21]

Gotcha. Right on, thanks for the help Kap'n!

[yankeerob]

Just to add my two-penneth - as you seem committed to this amp I'm not going to suggest that what you really need for less power is a lower power amp... I've just never found an attenuator that doesn't change the sound and response of an amp... quite drastically in some instances... but so be it - I don't know everything and certainly haven't tried them all so I'm just gonna say is good luck with it...

Wattage ratings? If the attenuator is truly capable of handing 50W you shouldn't have any trouble using it with the Vibrolux - despite what you might read - I've never come across an amp with the published plate voltage of a VL running 6L6's that produced more than about 30 honest clean watts anyway so you should be fine provided that attenuator is rated honestly...

I'm also guessing that you're referring to the first power resistor after the choke return hence your reference to a plate voltage of 200 to 170 - I take it you're referring to the PI plate voltages... changing the 4K7 to 10K is likely to have very little if any effect on the clean output power of your paticular amp - you're only changing the potential slightly on either side of said resistor and the effects, with regard to how they would affect output power, would virtually cancel each other out... my guess is that you might wind up with a very slight increase in pre-amp distortion (as you'll need to drive the PI harder to get the same out of it) so the clean output power of the amp might drop but only slightly - I've never seen a drop of as much as 5W by doing such... a different OT is - as you've guessed - more likely to result in a change of output power....

So - if you like the sound of the amp with the BF mod - that's what's to be considered as I very much doubt you'll notice any change in power... again good luck!!

[barentsman]

As to attenuators: I have a Weber miniMass, 50W max. When I had a HRDlx (40W), I drove the HRDlx on full throttle at rehearsals, also on gigs, once 3x 45 mins with breaks in between. It got a bit warm, but worked fine. Still do.
So I suppose as long as your amp has lesser wattage than the rating of the attenuator, and impedances are ok, theres no frying going on.

[boobtube21]

Thanks for the input guys.

Reason I'm doing this is, I have a gig with a band on Friday in which I play pedal steel and regular guitar. The pedal steel needs to be mostly clean--around 5 on the volume--and the guitar needs to be just over the edge of breakup--also around 5 on the volume.

Reason why the steel guitar is clean at the same volume setting as overdriven guitar, is I'm using a volume pedal with the steel that's cutting the input sig down considerably. Then once I plug in the guitar, it's way too loud. 2 1/2 is a more reasonable volume setting for this venue, but you know what a tube amp plays like set at 2 1/2, makes you want to turn it up!

We're practicing today, I'll practice toggling between normal and bypass mode when switching instruments, and see how it goes.

FWIW I too have been flogging this thing (Weber 50w Mini Mass), amp volume on 10, att. volume on 1 for the past couple days. I can't help it, it's been a while since I had overdrive at low volume. I have a couple overdrive pedals, but I can't stand any of them. It does have the considerably rolled off highs, but that's to be expected at max attenuation...and it STILL sounds better than an OD pedal to my ears. I imagine with the way I'll be using it it'll just tame the icepick freq's a little.

[boobtube21]

Hey, I forgot about this thread.

For anyone who cares, the Mini Mass works great. The vol control is a little jumpy with some weird spots around 1, probably from the previous owner overheating the rheostat. I myself have gotten it pretty hot at home a couple times, cranking the amp up to 9 or 10 and lowering the Mass vol to bedroom levels. Probably not a very smart idea...

Either way it works and sounds great live. Also great for recording: To my ears--crazy as it sounds--it sounds near identical to the amp just running direct to the speakers, only it sits a little nicer in the mix. Hmmm....

Overall very happy with it.

[steve25]

Even Fenders own BF era schematic call for different output tube plate voltages with the same PT, 440 to 465 volts in the 6L6 amps seems to be all just a difference in the way they happend to be setting the bias, and that voltage difference will be seen all the way down the to the first preamp power supply node.

[boobtube21]

Hmmm. I guess maybe that could account for some amps having a little of their own character. Especially in a SF amp where the PT voltages could also have fluctuated, but the bias was non-adjustable.

At any rate I did replace that resistor with the 10k a month or so back. I don't get enough chances to crank this amp up...so I can't really remember exactly what it sounded like before the swap...but I do love the wsay it sounds now.

Just played a show on Wed night, same venue, no attenuator, at about 2 1/2 on the vol. Sounded killer for the bluesy/country songs we were playing.

Then a guy from another band borrowed it and had it up to about 5, wow it sounded f'n awesome. Actually he and his band were awesome, but I'd like to think my amp had something to do with it!

[ziess]

I do love the wsay it sounds now.

That's all that matters. Isn't it great having an awesome amp?

[boobtube21]

Yes it is!

[Gris]

Just saw tis thread. I'll be the contrarian. To my ears small diffs in voltages translate into appreciable sonic diffs. i actuallythink voltages are a big part ofthe magic of tube amps... that and tubes

[boobtube21]

You know I can say that it sounds different with pedal steel. Not as full and bloomy and saggy as it once did. Funny though when I plug in a regular guitar, it seems to have that quality in spades. Just not as much for steel like it did before.

This may have more to do with the PI coupling cap though (changed from .01 to .001). That swap definitely lowered the amount of ~200-500hz freq's in the sound. Again I can't remember exactly when i really started noticing which difference in the other sounds, but the PI cap for sure yielded that change.

Still make me happy when I think about how good that other guitarist sounded through it with his band. It's nice to hear your own gear from the audience perspective and know it sounds that good.

[DanD]

I read in a guitar mag interview of Neil Young's tech that Neil could hear if the voltage was low at sound check.

The tech stated Neil would mention that the voltage was low while he was playing and the tech would look down at his meters and lo and behold, the voltage was low.

He said Neil could hear the difference between 117 and 120 volts.

I think Gris is on to something here.

[boobtube21]

Ahh I don't really believe that. Maybe...and maybe Eric Johnson really can hear the difference between batteries. But I doubt it.

Makes a good story though!

[Gris]

i can hear it. but you cant hear it then it doesnt matter (for you)

[boobtube21]

Really? A difference of 3 volts? Wait are we talking line voltage or plate voltage here?

[chuckocaster]

i met neil's tech (larry cragg), and trust me as loosey goosey and cavemanesque you think neil is, he's not. every time larry has tried to change something neil can hear it and pick it out. he had some great stories he shared with me. larry's a cool dude, and i'm a huge neil fan so i wasn't complaining.

[boobtube21]

No I believe it Chuck. Neil for some reason always struck me as a musician who knew what they needed to hear and how to do it.

I actually read an interview with a guy that engineered a lot of his solo stuff over the years, his name escapes me. Great interview though.

In the interview he says when they were tracking "Hear Of Gold" Neil told the drummer he was playing too busy, to make it simpler. So the drummer sat on his left hand for the entire song.

[chuckocaster]

i didn't mean YOU in particular colin, i meant the public. he gets a bad rap as a fumbler. his genius to me is him embracing who he is as a musician and doing what he does best, which is to play like neil young.

[boobtube21]

Yeah I know some people that don't get Neil. You can't explain it to them, if they don't get it, they don't get it.

[chuckocaster]

exactly. there are 3 arguments you'll never win: music, religion, and politics. so i stay out of them.

[boobtube21]

Good call Chuck