Anyone opened a Tom Scholz Power Soak before? - Page 2 - diyAudio
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Old 14th January 2013, 10:05 AM   #11
Tesla88 is offline Tesla88  Italy
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IMHO don't expect good result by using power attenuator at home at bedroom levels , the problem is speaker related , at low power - bedroom levels you give power about 1 to 5W to speaker , most of the compression and lovely sound at high levels is due to the power compression made by the speaker and also when you send low power to speaker you get a thin fizzy sound because your not moving the cone properly and you get a flat response , when you send power to the speaker the sound is more mid focused and frequencies over 5kHz are deep cutted ...so power attenuator can't do this!
What kind / genere of music do you play ? If you want i'll post some results made by using a speaker simulator ...
Be carefull with power soak , sometimes amps are not happy with that kind of load...
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Old 16th January 2013, 08:57 AM   #12
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It is also nice to make all hiss and hum disappear.

I've had many (maybe 4 or 5) of the Sholz Power Soaks, as well as a Power Plate, and several Marshall Power Brakes. Some of the eBay power soaks arrived with blown resistors, but repair by replacing the resistors was easy, and all the values were available and affordable in small quantities. The resistor values do increase somewhat when they get hot. On the power soak it is nice that you can just measure the resistance and know approx what the load on the amp is, but make sure you review the internal circuit of the Soak because I can't remember the details of how it switches resistors in series and parallel as you change the settings. If it uses fewer of its resistors in the louder settings where it doesn't burn off much power, it still might be a lot of power per resistor...so check it out 'cause I can't remember the details. The Marshall Power Brake is really kind of neat, I vaguely recall it uses a multi-tap transformer (not a choke???) and some damper resistors.

If you use a partial power switch to turn off some tubes, that might reduce the power enough to be safer with an attenuator, but be a little careful 'cause the output transformer primary becomes a rather low impedance load to the remaining tubes so you might want to use a different impedance speaker tap on the secondary and you might want to be careful what kind of load the attenuator presents with an already less-than-ideal match.

Sorry I'm not much help and can't remember more, but I had to sell all that stuff years ago in order to keep my house.

You can also look into various high-voltage regulators and sag resistors etc.

Be careful, have fun.
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Old 19th January 2013, 03:22 AM   #13
JMFahey is offline JMFahey  Argentina
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Quote:
try removing the four "outside" output tubes leaving the "middle" pair
to begin with, and then use your attenuator as-is.
Do not butcher it.
Using "higher wattage" resistors (supposing you can find them same size) is missing the point; those resistors will *still* be inside the same chassis
And *where* will all that extra heat go?

Do NOT try to "match output impedance to 2 tubes instead of 6", just use it as "officially labelled".
You are trying to *lose* power after all.
If you are the fearsome type, split the difference and pull just 2 , one on each side.
Everything else still applies.
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Old 19th January 2013, 10:28 AM   #14
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Quote:
Originally Posted by Tesla88 View Post
Some of those attenuators are amp's killers. Not a Legend , i serviced an old JCM whith open output transformer due to use with a power reducer.
As long as a 'power reducer' maintains the correct impedance, then it won't (can't) do any damage - so either the 'reducer' wasn't correctly designed, or it wasn't used correctly.
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Old 22nd January 2013, 04:55 PM   #15
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Quote:
Originally Posted by JMFahey View Post
to begin with, and then use your attenuator as-is.
Do not butcher it.
Using "higher wattage" resistors (supposing you can find them same size) is missing the point; those resistors will *still* be inside the same chassis
And *where* will all that extra heat go?

Do NOT try to "match output impedance to 2 tubes instead of 6", just use it as "officially labelled".
You are trying to *lose* power after all.
If you are the fearsome type, split the difference and pull just 2 , one on each side.
Everything else still applies.
Interesting point but one of us has it wrong. Here's my reasoning:

What I recommend: If you reduce the number of tubes in parallel, they make less current, and a 'corrected' transformer load would be a higher impedance that demands less current. Since most of the load is transformed, an attempt to make an optimal match would have the speaker load also be a higher impedance than the given labelled transformer tap. That would also make the speakers quieter. You imply that 'correcting' the tap would be an adjustment to make it louder, which is not true, that's adjsuting the wrong way. When I pull tubes, I make the load higher impedance than the tap label, which makes it easier on the amp (requires less current) and a bit quieter (which also agrees with the objective).

What you recommend: If you left the speakers on the original tap and unplugged 4 of 6 output tubes, that would be like having your transformer tap wrong by one and a half taps, driving a load that's too low an impedance for your amp, trying to make the speakers louder. That might bring on your breakup mode in the output tube earlier, but it will make your transformer hot. You might overheat your remaining tubes or even (desipte the reduced power) you might hurt your output transformer trying to demand too much current with a load of such mismatched low impedance.

I just think of it this way: if you pull tubes to make less current, I make the load require less current and hope everything will be OK.
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Old 22nd January 2013, 07:46 PM   #16
Enzo is offline Enzo  United States
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If you pull two tubes out of four, the impedance on the primary side doubles. You are removing parallel impedances, right? That means the relative impedance on the secondary side also doubles. (The transformer has no impedance of its own, it only has ratios of primary to secondary) So what used to be the 8 ohm tap now acts as a 16 ohm tap. And the former 4 ohm tap now acts as an 8 ohm tap. SO if you had an 8 ohm cab, with half the tubes gone, you'd need to plug it into the old 4 ohm tap to have the 8 ohm impedance match the tubes.

So if you pull tubes and leave the speaker in the same tap, it will now be in too HIGH an impedance tap.

The point of a transformer is that it trades voltage for current. But it also matches impedances between the high impedance of the tubes and the low impedance of the speaker.


Mismatching impedances wouldn't hurt the transfomer anyway, what it does is make the tubes operate on the wrong parts of their curves.

Last edited by Enzo; 22nd January 2013 at 07:48 PM.
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Old 22nd January 2013, 07:49 PM   #17
JMFahey is offline JMFahey  Argentina
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Let's split it step by step:

Quote:
If you reduce the number of tubes in parallel, they make less current,
True.
Tubes are "current limited".
Less tubes, less current available.

Quote:
and a 'corrected' transformer load would be a higher impedance that demands less current.
True again.
Switching taps will adapt the load to the new reduced current capability and offer a better match.
Problem is, if we pull 2 out of 6, we are losing only 1/3 power, which is un-noticeable.
By the way, that's what everybody notices, so theory matches practice. Good.
So the wide problem becomes that the "technically correct" solution, (on what I agree) , does not meet the user desire.

Now, if you do *not* match impedances, you lose even more.

How much more?
Power follows a square law , depending either on I^2 or V^2, so when NOT matching, result is not linearly dependent on the available current reduction but on its square.
Since ear response is logarithmic and power reduction (when not trying to match) is exponential, they tend to correlate better.

Quote:
Since most of the load is transformed, an attempt to make an optimal match would have the speaker load also be a higher impedance than the given labelled transformer tap.
True.

Quote:
That would also make the speakers quieter.
No. Better matched is louder, unmatched is quieter.
See above.

Quote:
You imply that 'correcting' the tap would be an adjustment to make it louder, which is not true, that's adjsuting the wrong way.
See above. Corrected will be louder than not corrected.

Quote:
When I pull tubes, I make the load higher impedance than the tap label,
Please rephrase it. There's a word missing.
You might have meant either :
When I pull tubes, if I make the load higher impedance than the tap label, .......
or
When I pull tubes, I need to make the load higher impedance than the tap label, .......

Quote:
which makes it easier on the amp (requires less current)
True.
But provides less attenuation than not correcting.

Quote:
and a bit quieter (which also agrees with the objective).
Exactly, "a bit".
Usually not enough, as perceived by most.

Quote:
What you recommend: If you left the speakers on the original tap and unplugged 4 of 6 output tubes, that would be like having your transformer tap wrong by one and a half taps, driving a load that's too low an impedance for your amp, trying to make the speakers louder.
True, that's the idea.

Quote:
That might bring on your breakup mode in the output tube earlier,
True.

Quote:
but it will make your transformer hot.
No. In fact it will heat less because of the reduced current.

Quote:
You might overheat your remaining tubes
True. It's a price to be paid for having bought a too loud for the job amp in the first place.
Unless tubes are redplating (which they shouldn't), tubes will wear somewhat faster, yes.

It's part of the Rock World mentality (gettink killer overdrive) vs. making amps for, say, Churches, Home Hi Fi or Supermarkets PAs (clean sound with lowest maintenance possible).

Quote:
or even (desipte the reduced power) you might hurt your output transformer trying to demand too much current with a load of such mismatched low impedance.
No, the transformer will actually work cooler.
The current limited device which will work more is the tube.

Quote:
I just think of it this way: if you pull tubes to make less current, I make the load require less current and hope everything will be OK.
It depends on what OK means
If it means: "get the max. power out of available current": yes.
If the idea is "bandmates hate me, I'm too loud": no.

By the way, I didn't invent this, look at what Mesa Boogie Mark I and II did: they simply opened the cathode connection of 2 6L6 and called it a day.

PS; try it yourself: pull 4 tubes and try it both ways, matched and unmatched.
Let your ears be the judge as to perceived attenuation

Last edited by JMFahey; 22nd January 2013 at 07:59 PM. Reason: Typos and formatting.
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Old 24th January 2013, 02:45 AM   #18
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Yes, unmatched is quieter. Too low a load impedance demands more current, not less, and ultimately the amp can't deliver enough current to make the voltage track, tubes running outside their optimum range. Really hard on the amp.

Too high a load impedance and the power output section just loafs with extra current capability that can't do useful work within its voltage capabilities.

Re-matched (adjusted for the tubes you pulled) the tubes will sound similar to when the stock setup is cranked, but quieter. Isn't that the objective?

So why would you risk your amp, especially with a power soak when you might not realize how hard you're making the remaining tubes work? That's probably part of how power soaks get a reputation for damaging amps.

I said that when I pull tubes I increase the impedance of the load. That's exactly what I mean when the output transformer has only one tap; I either throw a switch that puts a resistor in series with the speakers (often the switch was on some kind of power soak) or I plug in another speaker cabinet in series (admittedly not getting speaker breakup).

Admittedly, NOT changing the tap or the load when you pull tubes may make the remaining output tubes break up or sound nice, but that's a different sound; re-matching for having pulled will sound more like the stock setup cranked. Isn't that the objective?

It's not a big deal with most amps, and you're not alone in saying it's safe to mismatch but my Sound City has 6 output tubes, my Super Twin has 6 output tubes, Peavey Classic 100 has 8 output tubes, and I think the original question was about an amp with 6 output tubes. Pullng them down to just 2 tubes may be quite a change, and desppite what you might do, is it really safe enough to make a broad recommendation? I sure don't know. All that said, I'm really still not very experienced with my own tube amps, and when I roadied I had a mandate to be extra cautious when responsible for other peoples' gear. So I haven't done much experimentation, but I have seen amps really suffer long long sets under hot lights. They're supposed to be overbuilt to survive abuse, but I try to avoid abuse anyway.

Enzo, we're saying the same thing I'm just using the wrong words.

And I do appreciate all the advice from the more experienced.

The only point I was making was that if the power soak is small for a powerful amp, it's usually safe to set the power soak to be a rather high impedance load for the tap, and pull some tubes. It might well sound better to set the soak to be a lower impedance load on the amp but it's not as safe when pulling tubes.
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Old 30th January 2013, 09:19 PM   #19
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That's the unit i remember a buddy of mine had bunch big power resistors inside and they would melt etc his got so hot I swear you could light a cigarette off of it lol
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Old 30th January 2013, 09:22 PM   #20
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as far as the sound hmmm I remember it cut **** load of highs on a 50watter which kinda helped but really that sucker was notorious for those resistors going south and blowing up output transformers.
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