Weber Mass Lite Attenuator / Peavey Classic 100 application

[Chuck H]

tbonuss, I just looked over the schematic (thank you for posting it). You may have already determined this, but if you do decide to pull four tubes you'd be pulling one "row" rather than simply two from each row. Which could also do, but you'd have to know exactly WHICH two from each row since either row uses two tubes from each side of the push/pull circuit. So the simplest method would be to just pull a row.

[Chuck H]

I wouldn't say that it's not an attenuator. Take a look inside.

[ATTACH=CONFIG]40122[/ATTACH][ATTACH=CONFIG]40123[/ATTACH]

Yep, it's an attenuator. It uses the speaker motor as the load, but an attenuator is an attenuator and they all require circuitry to make the actual speaker and the attenuator load variable (like a rheostat and a couple of resistors) and also often include tone shaping circuitry (capacitors). Which some of the Weber attenuators do also.

[tbonuss]

IMO you could put some signal in, with a generator, or any audio source and dial in about 1v peak to peak or .7Vac RMS if that's what your meter reads. Then, crank your amp up all the way briefly while your meter gets a reading ON THE SPEAKER. You may want to use your speaker and wear some headphones so as to not risk blowing the attenuator. If you do it fast hopefully, hopefully, you won't create enough heat to hurt anything. This will give you some idea of the real RMS wattage on 11. At least that's what I'd do... and ideally into a suitably rated speaker cabinet. At least it will get you an idea of it. Most pickups are a few hundred millivolts, I THINK, and this varies obviously a lot from pickup to pickup and depending on strings and strength of strumming and pickup height etc... This is all ballpark.

All this said, I agree with others in that I tend to go with twice the rated power for speaker loads... especially being I like overdrive.

Hahaha. Yes I would do that with speakers at a MINIMUM. I wasn't thinking about it like that for some reason. I guess I was initially thinking it was rated for 100W amps, not that it was rated at 100W dissipation.

I'll give that a shot with the input. I was hoping for a reason to get a good o-scope though

[tbonuss]

As a side note: can you post a couple MASS pictures?

Never saw them in detail or closeup and guess nobody in Argentina owns one.
Thanks.

Are Bob P's googled pix what you wanted; I'm happy to get closeups of whatever you are trying to see.

[J M Fahey]

Are Bob P's googled pix what you wanted; I'm happy to get closeups of whatever you are trying to see.

Thanks, what I saw was enough.

No magnet visible but I guess there´s some NEO slug inside or under the polepìece, definitely no ceramic on sight (might have an alnico slug though).

As of MASS itself, which is the coneless speaker, I stand by my earlier statement that *it* is not an attenuator, to be so it should be either in series with the driven speaker or in parallel with it, absorbing whatever power is needed; I see two *rheostats* doing the real attenuation and MASS as a silent speaker/reactive load .

In a way, similar to valvestate amplifiers, they do *contain* a tube but are not tube amps, the actual work is done by SS parts, here I see the attenuation done by (variable) resistors.

Hope the conceptual difference is clear.
If not, no big deal, anybody sees it as he believes

[Justin Thomas]

I just call anything that reduces an amp from "Fire-Spewing Implement Of Sonic Annihilation" to "Care-Bear-Crunch-Box" an attenuator. I don't really care how they do it. So maybe "Castrator" is a better way of putting it - that is a blanket umbrella term that us techno-incapable peons can use to cover anything from a MASS to a BFR.

Anyway, I'm saying all this as a joke...

Justin

[tbonuss]

Thanks, what I saw was enough.

No magnet visible but I guess there´s some NEO slug inside or under the polepìece, definitely no ceramic on sight (might have an alnico slug though).

As of MASS itself, which is the coneless speaker, I stand by my earlier statement that *it* is not an attenuator, to be so it should be either in series with the driven speaker or in parallel with it, absorbing whatever power is needed; I see two *rheostats* doing the real attenuation and MASS as a silent speaker/reactive load .

In a way, similar to valvestate amplifiers, they do *contain* a tube but are not tube amps, the actual work is done by SS parts, here I see the attenuation done by (variable) resistors.

Hope the conceptual difference is clear.
If not, no big deal, anybody sees it as he believes

Yes. Very clear. I thought of it the same way, that the rheostats are the attenuators. In support of this notion, the two rheostats are labeled 50 watts and the thing is rated to dissipate 100 watts. I really like your analogy with the tubes. They can really effect the tone (I currently have three valvetronix and a zoom g9 that has two tubes) but they aren't tube amps.

***LATER: Correction. I got curious and I popped the top, this one has two 100 Watt rheostats. I really should know better than to assume...I must have been looking at a picture of a different version or somehow thought the 50 ohm stamp was the wattage.

[tbonuss]

tbonuss, I just looked over the schematic (thank you for posting it). You may have already determined this, but if you do decide to pull four tubes you'd be pulling one "row" rather than simply two from each row. Which could also do, but you'd have to know exactly WHICH two from each row since either row uses two tubes from each side of the push/pull circuit. So the simplest method would be to just pull a row.

Yes, thank you. I understand that the actual topography is side by side, not across as the schematic drawing. Thank you for your consideration in helping me avoid a potential oversight.

[diydidi]

Not a fan of Attenuators. They are bad for most valve amps IMHO. Hard on valves and OP transformers.
Rather try pulling two tubes as stated or get a softer amp...

[Chuck H]

Not a fan of Attenuators. They are bad for most valve amps IMHO. Hard on valves and OP transformers.

What? Nope. I'm going to qualify your statement for you while simultaneously discrediting it! Watch... Nothing up my sleeve.

When an attenuator is in regular use there is a MUCH greater likelihood of an accidental open load due to user error. Too much plugging and unplugging WRT the most important thing for the amp. The load! And if overdrive is your thing, well, an attenuator makes it possible without a dirt box! But the added wear on the tubes at that point has nothing to do with the attenuator. The wear on the amp would be equal or greater if the stock speaker load were used. Also, the risk of an open speaker cable doubles because there are two in use instead of one. It also can't be denied that an attenuator is an extra device in the speaker load chain so that can also increase the probability of a failure because there's one more thing that can fail. But these are the only reasons a properly designed attenuator, used properly would put an amp at any risk. It has nothing to do with the load offered by the attenuator. Tubes and OPT's are actually better off running into a flatter load of the correct impedance. All things being equal, and the correct attenuator used correctly, the attenuator is a better load for the amp than the speaker.

[bob p]

I'm somewhere in the middle.

A resistive load is a standard stress test. If an amp fails into a resistive load then there's a problem with the amp.

Sure, you can wear out parts if you abuse them. In that respect an attenuator should be viewed as providing normal wear and tear at an accelerated rate. That shouldn't surprise anyone, so if an attenuator is used plan accordingly. Build transformer protection into your amps and plan regular tube changes.

[tbonuss]

I'm somewhere in the middle.

A resistive load is a standard stress test. If an amp fails into a resistive load then there's a problem with the amp.

Sure, you can wear out parts if you abuse them. In that respect an attenuator should be viewed as providing normal wear and tear at an accelerated rate. That shouldn't surprise anyone, so if an attenuator is used plan accordingly. Build transformer protection into your amps and plan regular tube changes.

I also have a Tom Scholz Power Soak (120W). FWIW, what you guys are saying mirrors what Tom says: It won't hurt your amp. Its like driving a car fast all the time, it'll just wear out faster.

I've now got one attenuator on each of the outputs (please don't feel like I'm ignoring the input here--my schedule just hasn't allowed me to physically move the amp to a low traffic area where I can pull the tubes, measure the voltages etc.). This seems like the best idea to me, because no matter what happens it'll always have at least one as a load. Its got parallel outputs, I've got the amp set at 4 ohms and the Rockman set at 8, and the Weber set at...well whatever it does. Its convenient because the matching cab is a stereo 4*12 Peavey Sheffield rated at 150 watts per side.

So...I will put a half power switch idling half the tubes, but I'm out for the next couple of weeks. When I do that, is there a recommended thread on what I should build for o/p xfmr protection? I'd like to build that if possible while I've got it moved and open.

As always, thanks!

Doc

[bob p]

The OT protection topic comes up quite often, so you'll find several good threads when you search. Things to think about would include:
1. power resistor across the speaker terminals.
2. zoebel
3. flyback diodes
4. MOV

While you're thinking about protection, some PT protection measures to consider would include:
1. MOV on the AC inputs
2. fusing each of the secondary circuits
3. backup SS rectifier diodes for tube rectifiers

I can't think of anything else at the moment...

[Chuck H]

It's been my experience that a shorting jack is the best and easiest safeguard. I use an attenuator and do some bench testing with amps out of their cabinets so it HAS come up that I have failed to have a correct load on an amp accidentally. I've never lost an OT that was shorted. Another example would be a poster here some years ago asking questions about how he was operating his stock Bassman head. He was using it as a preamp by plugging a cable from the "ext" speaker jack to another amps input. With NOTHING plugged into the "speaker" jack!!! Apparently, due to the low impedance there was enough signal bleeding onto the ext jack to be useful in driving another amp. And he had been doing this for years with no failures.

[tbonuss]

I'm back and had a few minutes to mess with the amp. I used a "true" RMS clamp meter into a Scholz Power Soak and couldn't coax (viz, banged as hard as I could on and Epi Dot Studio with stock pickups straight into it) more than 3.4 amps of it on the clean channel with EVERYTHING dimed. 3.4 * 3.4 * 8 =~ 93 Watts

Thats more than I expected. Nothing got hot.

Not a ton of progress as I just got back to this, but I thought I'd post something initially in case I'm way off base here. Oh, and BTW, this started out as a Weber Mass thread, and I'll get back to that, its just that I'm far less worried about damaging the resistor bank in the Power Soak than I am with the speaker thingy in the Weber. Plus, it makes the calculations much easier. I think...