Anyone opened a Tom Scholz Power Soak before? - diyAudio
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Old 13th January 2013, 11:32 AM   #1
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Default Anyone opened a Tom Scholz Power Soak before?

Hey guys, I was lucky enough to score a Tom Scholz R&D Power Soak (PSII) on the 'bay, was hard enough to find one of these in the US but somehow I managed it in Europe.. (Go me! )

Anyway, the Power Soak is basically the mother of all designs for tube amp attenuators, it's basically just selectable high wattage wirewound resistors. I have used a Weber MASS before, it had a voice coil as the dummy load and hence was supposed to load the amp more "gracefully" than a purely resistive load, but to me it didn't quite do the trick, so I'm eager to try this unit out, as it's supposed to be quite legendary!

Still waiting on the delivery of the unit, but I'm curious whether anyone has opened one up before? The Power Soak is meant for amps which put out 100-120W RMS. Since my amp is 150W, I suspect replacing the resistors with higher wattage ones would increase the power handling capability of the unit??? Also, I've read stuff about "non-inductive" wire wound resistors, but it doesn't make sense to me since, for a tube amp, having a resistor bank load which pose an almost purely resistive load would be worse than resistors which has some inductance because of the construction method, i.e. for wirewound, which pose a reactive load ? Am I thinking wrong?

TL;DR - got a Tom Scholz Power Soak PSII, wondering if anyone has opened it up or modded it before
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Old 13th January 2013, 02:23 PM   #2
kevinkr is offline kevinkr  United States
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Moved to Instruments & Amplifiers per forum rules, a power soak is generally used with a musical instrument amp as opposed to a hifi amplifier. Please read the subheaders in Tubes / Valves before posting there. Thank you.
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Old 13th January 2013, 02:34 PM   #3
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Thanks kevin, sorry about that! D'oh!
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Old 13th January 2013, 03:35 PM   #4
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Yes, you're thinking wrong.

From the amp's point of view, a purely resistive load is more benign than an inductive load. The problem is, a resistive load changes the frequency response and dynamics of a tube amp, as far as guitarists are concerned. That's why the Weber uses speaker voice coils - to match the inductance of actual speakers.

The Power Soak is indeed legendary, it's the source of all the "Dude, you gonna blow your amp up!" legends.
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Old 13th January 2013, 09:42 PM   #5
Tesla88 is offline Tesla88  Italy
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What is the purpose ? When i play at lives i don't attenuate my amp , 100W into 4X12 Celestion V30 and I smile At home , i just use a speaker emulator between line out of amp and audio monitors ..

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.
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Old 13th January 2013, 10:31 PM   #6
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If you are thinking of modding it, or beefing it up then be prepared to spend a lot of time and money. Those units are filled with resistors with oddball values. What kind of amp are you using that has 150 watt output?
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Old 13th January 2013, 10:59 PM   #7
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Originally Posted by Tesla88 View Post
What is the purpose ? When i play at lives i don't attenuate my amp , 100W into 4X12 Celestion V30 and I smile At home , i just use a speaker emulator between line out of amp and audio monitors ..

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.
Well I don't play too many live shows, mostly I play inside my apartment so it's difficult to get that saturated, slightly compressed tone from the amp, which you can get only by cranking up the master volume... They're not really amp killers, their purpose IS basically to absorb part of the energy put out by the OT, so it's obvious tubes will get hotter and will last less longer and all of that, but basically if you were to run the amp at same output level, with the volume it SHOULD have been (i.e. without the attenuator), the tubes last the same time. Soooo...
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Old 13th January 2013, 11:00 PM   #8
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Originally Posted by Dr-Joned View Post
If you are thinking of modding it, or beefing it up then be prepared to spend a lot of time and money. Those units are filled with resistors with oddball values. What kind of amp are you using that has 150 watt output?
I'm using an ENGL Invader 150. I'm definite that they use pretty standard values, even if not; once I get to know the how the impedance switching works, I can pretty much replace the resistors with similar values as long as I match with the impedance switch.
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Old 13th January 2013, 11:19 PM   #9
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windmarp
the invader 150 appears to be a classic push pull output stage rather then "soaking" the output try removing the four "outside" output tubes leaving the "middle" pair your neighbors will be happier and you too.
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Old 14th January 2013, 01:16 AM   #10
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Originally Posted by turk 182 View Post
windmarp
the invader 150 appears to be a classic push pull output stage rather then "soaking" the output try removing the four "outside" output tubes leaving the "middle" pair your neighbors will be happier and you too.
Well taking tubes out is not an option, primarily because there's not much of a difference in the levels at 50W and 150W, maybe about 5-6 dB and that's with 4 out of 6 power tubes removed.
Moreover, removing a pair or 2 pairs of output tubes will not yield half power scaling so matching the impedance of the OT and the speakers will be a problem!
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