• TGP is giving away a Strat, Tele, and Jazzmaster. Click Here for full details.
    Click Here to upgrade your account and enter today!

Yet another question on attenuators

Paul86

Member
Messages
1,504
Once again I'll ask TGP.
A friend just gave me his old Marshall Powerbrake. It's fully functional and it's OK for what I need - to knock out a few db's when I go out to play, and it gives me a passable tone for practice.
Now, the question. It has a 8 - 16 ohms switch. Will it hurt my amp if I set the output of the amp to 8 and the switch on the PB to 16 ohms?
I remember reading somewhere here that it's actually better for the amp to present it with higher ohms.
 

wyatt

Member
Messages
4,169
Once again I'll ask TGP.
A friend just gave me his old Marshall Powerbrake. It's fully functional and it's OK for what I need - to knock out a few db's when I go out to play, and it gives me a passable tone for practice.
Now, the question. It has a 8 - 16 ohms switch. Will it hurt my amp if I set the output of the amp to 8 and the switch on the PB to 16 ohms?
I remember reading somewhere here that it's actually better for the amp to present it with higher ohms.
You don't mention what amp it is but...no, it's not. You either got bad advice or remember/read wrong.

Set it to 8 ohms.
 

Paul86

Member
Messages
1,504
Here, I found it.
Said Mr Hogy
"An attenuator with a load setting and a line out. You can also use a load resistor.
I don't want to get into a discussion about attenuators and how safe they are to use. Religious debate is frowned upon on TGP. Instead I'll just tell you what works. For an amp with a 16 Ohm output, use a 25 Ohm resistor. No, that is not a mismatch. Impedance is not the same as DC resistance, a 16 Ohm speaker is only 16 Ohms at a certain frequency. 25 Ohm is a very good approximation of how an amp "sees" a 16 Ohm speaker. It is very easy on the amp/tubes and most importantly, it sounds great.
If you're a DIYer, by yourself a 225 Watt, 25 Ohm Ohmite wire wound resistor. This will handle all your 100W amps comfortably. Put it into a nice enclosure or mount it somehow. It will get warm/hot. Make a line out with an 82k resistor and a 5k linear pot. Done."
And Hogy was quite emphatic too
"I do know that a 25 Ohm resistive load works and is safe. I trust that based on years of experience. Not going to gamble my old amps with something unknown at this point."
 
Last edited:

pdf64

Member
Messages
7,057
For a valid response, you need to clarify what technology does the amp in question use, eg solid state, tube.
In regard of tube amps with beam tetrode / pentode configured outputs, the tubes have 2 key elements with dissipation limits, the plate and the screen grid.
As the load impedance is reduced below the intended nominal value, the plate dissipation will tend to increase above the design intention (and screen grid dissipation decrease).
As the load impedance is increased above the intended nominal value, the screen grid dissipation will tend to increase above the design intention (and plate dissipation decrease).
Very generally, the screen grids tend to be run closer to their limiting value; when overdriven (and attenuators may encourage that), it's often exceeded, so it can be beneficial not to use higher than intended load values.
I'm not sure if Hogy had considered this aspect.
Speaker impedance varies massively with frequency and at bass resonance can rise to >10 times the nominal, so it's not super critical.
Note that Hogy advises a 25 ohm load on a 16 ohm output; not 32 ohms, 50 or whatever.
 
Last edited:

Paul86

Member
Messages
1,504
Thanks for replying everyone.
It's a tube amp a friend put together from a kit.
It's kind of a clone of an old Marshall JTM 45.
 

pdf64

Member
Messages
7,057
With a JTM45 loaded with it's nominal rated impedance, you're generally starting off with a highish OT primary, usually either 6k6 for RS Deluxe types or 8k for Drake 784-103 type.
So if the secondary is loaded with a much higher than nominal impedance, the primary impedance will tend to go rather high, maybe up to 16k p-p with a Drake type OT and a 16 ohm load on the 8 ohm secondary output.
I'm sure some guys can wail away for hours a night for decades with such a set-up without any trouble, but I seem to attract trouble and strongly suspect something bad would happen if I tried it, so can't recommend it.
 

Paul86

Member
Messages
1,504
He had some parts laying around, and I think he bought the transformers and other small parts online.
 




Trending Topics

Top