Muck'n About with Ohms' Law

  • Thread starter Thread starter rayc
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rayc

rayc

retroreprobate
Hi Folks,
I bought a tube amp attenuator - the Tube Cube. I had to opt for either the 80w max or 160w max version & as it was for a Marshall 100w bass head I opted for the latter but didn't fully read the specs on that version.
The 80w madel has resist/impedance selection from 4 up & the bigger fella from 8 up.
Naturally, the speaker I wanted to work with is a 4 ohm beast (great big single 200w Etone bass beast).
Can I add an inline resistor or something between the attenuator & speaker to match i4 ohms?
How much diff/damage will running into a 4 with a unit made for an 8 cause?
I can select resist with a dial at the back of the amp - will that help?
I remain,
Yours in ignorance.
(I have to admit to guessing my way through my final high school science exams - passed but guessed nonetheless - so you can easily GUESS my science/physics/accument).
 
Hi Ray; If that is a variable L-pad type attenuator an impedence mismatch could really f'k things up. Try hooking your speaker up to the out on the attenuator, and check the resistance at the input throughout the volume range. It will probably vary. Variable L-pads on high power amps scare me. I experimented with one once, and I much preferred my homemade fixed type.
 
i'ld want to see a schemo before making any real comment but generally a tube amp is real picky about output impedences... especially lower... or extrememly high (like none!!!)
 
I'm not sure where to even start looking for an inline resistor for that kind of wattage- :eek:
 
Anfontan said:
I'm not sure where to even start looking for an inline resistor for that kind of wattage- :eek:
Click to expand...

You just stack 10W or 20W resistors in parallel until you get what you need.
 
RadioShack has high wattage Non-reactive (non-inductive)resistors for about $3 ea get a bunch and hook them up in a "series/parallel arrangement. Google "series-parallel" and you should find directions and diagrams.
I believe that you will be able to build whatever you need and for far less money than you would spend on a "ready made" unit. Added plus!!!!!you will end up with a greater understanding of resistors in the process.



chazba
 
Hi Folks,
Some interesting directions thus far. I've dug up the info that came with the unit - no schematics I'm afraid.
There's a selector on top for 8, 12, 16 & 20 ohms.
Also a 12 point chicken knob to adjust "volume" from OFF at 1 to FULL ON at 12.
So do what, where?
Here's a snap:
 

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"So do what, where?"

You could try a 4 ohm 50w resistor in series with the attenuator. Set the unit, and your amp at 8 ohms. You can check the resistance your amp will be seeing at the input of the tube cube while it is hooked to the speaker. Ideal would be about 6.5 ohms.

It's hard to tell from the photo, but it looks similar to a wall mount L-pad for a central audio system that has been mounted in a box.
 
JCH said:
"So do what, where?"

You could try a 4 ohm 50w resistor in series with the attenuator. Set the unit, and your amp at 8 ohms. You can check the resistance your amp will be seeing at the input of the tube cube while it is hooked to the speaker. Ideal would be about 6.5 ohms.
Click to expand...
Agreed about the 4-ohm, 50W resistor in series. This resistor will be soaking up half the output of the amp, so you get some sound reduction right there. Also, since your amp will be running into 8 rather than 4 ohms, it will be delivering about 1/2 of the wattage it would be delivering into 4 ohms.

So you could see a significant sound reduction just by adding a series resistor.

The series resistor may effect the sound of the driver.
 
rayc said:
Hi Folks,
I bought a tube amp attenuator - the Tube Cube. I had to opt for either the 80w max or 160w max version & as it was for a Marshall 100w bass head I opted for the latter but didn't fully read the specs on that version.
The 80w madel has resist/impedance selection from 4 up & the bigger fella from 8 up.
Naturally, the speaker I wanted to work with is a 4 ohm beast (great big single 200w Etone bass beast).
Can I add an inline resistor or something between the attenuator & speaker to match i4 ohms?
How much diff/damage will running into a 4 with a unit made for an 8 cause?
I can select resist with a dial at the back of the amp - will that help?
I remain,
Yours in ignorance.
(I have to admit to guessing my way through my final high school science exams - passed but guessed nonetheless - so you can easily GUESS my science/physics/accument).
Click to expand...

Generally, a 4 ohm load will not hurt an amp that's set for 8 ohms. you don't want to go any lower, though.
 
I enlisted the help of the manufacturer (silly me didn't do that 1st off...why? Silly I guess).
From him I've discovered that I select the 8 ohm setting on my amp and connect it to the cube.
The speaker can be 4ohm, 2 ohm, 8 ohm or 16 ohm, and the cube doesn't care as the important connection is from amp to cube, not from cube to
speaker.
In the 8 Ohm setting, The cube will keep 8 ohms on the amp, regardless of the speaker.
The input and output are isolated and the amp is protected regardless of the load you connect up to the cube.
This design is not an L-Pad, but slightly different in this regard.
So now I can go and play about & try for the cranked tube/valve sound with out the volume.
I'll post a review of the unit when I've explored it.
Thanks for your help everyone. I learnt about ohms law on a more concrete level.
 
Glad you got your answer. :)

The Cube must use a transformer for impedance isolation, I'm guessing.
 
Thanks for the comments from the Heart of Gold.
I'd say you're right but then again I don't know so I'll accept your definition. Give Trillian a g'day from me.
 
Hi,
I started a new thread with my initial impressions of the tube cube.
Thanks all.
 
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