• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Dummy load for tube amp

chrisng

Member
2015-07-30 5:55 am
I want to build a amplifier switcher for my friend to compare his tube amps ( switching two amplifiers with one pair of speaker ).
Is a 25 ohm dummy loading resistor safe for ANY tube amp? or I need a higher/lower value resistor?
Your advise would be most appreciated.
 

rayma

Member
2011-04-29 8:37 pm
Is a 25 ohm dummy loading resistor safe for ANY tube amp? or I need a higher/lower value resistor?

There's always a quirky amp around, but I'd use a non-inductive resistor equal to the impedance tap
that is being used (with enough power rating), like 8 ohms on the 8 ohm tap, etc.

Also have a higher value (like 100 ohms) permanently connected directly across the output of all
of the amp channels to avoid open circuits.
 
It doesn't really need to be non-inductive - a speaker is not non-inductive!

It should be not too different from the intended load impedance, especially if you are driving to full output. It is there to load the OPT so when clipping takes place with the valve switched off by grid drive and the OPT self-resonates the stored energy has somewhere safe to go.

Don't switch with signal present.
 

chrisng

Member
2015-07-30 5:55 am
Thank you, DF
I have some 25ohm 100w resistor on hand, will parallel it to 8.3ohm 300w dummy load.
 

Attachments

  • 25ohm100w.jpg
    25ohm100w.jpg
    15.8 KB · Views: 529

rayma

Member
2011-04-29 8:37 pm
Thanks for your input, rayma
What is the reason to use non-inductive resistor and why the resistor to be matched
with the amp output impedance?

The non-inductive type is similar in price to others, and is potentially useful as an amp test load.
Some tube amps could be damaged if operated with a load impedance much different from the rated.
The ones you have will be fine, though. Bear in mind that the resistor's rated power only applies
if they are mounted on a heat sink.
 
Last edited:

Tarzan

Member
2004-05-23 6:54 pm
Genk
Non iductive power esistors? Use these fixed on a heatsink and at half the mentioned power.
MP9100-1.00-1% | Caddock MP9100 Series TO-247 Radial Power Film Resistor 1Ω 1% 100W -20 → +80ppm/C | Caddock
AP101 2R F 300PPM | Arcol AP101 Series TO-247 Axial High Power Resistor 2Ω 1% 100W 300ppm/C | Arcol
These are available in values ranging from 1 Ohm to ...
Max power 100W but read the spec sheet and apply no more then 50W.
I use them on my dummy load (up to 800Watts) on computer processor heatsinks with a temp controlled ventilator.
 

Koonw

Member
2013-04-09 9:37 pm
200W 4ohm Aluminium Shell Braking Resistor Resistance Dummy Load for Audio | eBay

Tolerance +-5%, 40W without heat sink. I mount a pair back to back and the speaker plug is connected directly to wire lead, for very quick manual switching between the speaker and dummy. If your amp has gNFB, it's probably ok without a load, because gNFB has to clamp down the output when unload, but if zero feedback must have a load.
 

chrisng

Member
2015-07-30 5:55 am
Sorry I didn't make it clear, the 25 ohm resistor is " kind of " permanent.
When the selector at "off" position, 25ohm resistor parallel with the 12 ohm or 5 ohm resistor to form a load close to 8 or 4 ohm load on the amplifier terminals.
When the selector at the position between "off" and "A", relay 1 and 2 connect amplifier to speaker while the 25 ohm resistor still in the circuit.
When the selector turn to "A" position, relay 3 will turn on and disconnect the 25 ohm resistor from the circuit.
Relay 1 and 2 are 40/60A SPDT automotive relays, four needed per channel. I also added a MY4Z Omron relay parallel to the contacts.
 

Attachments

  • control.jpg
    control.jpg
    137.4 KB · Views: 215
I use an 8 ohm 10 watt resistor in a glass of water. Works great.

Over time electrolysis may weaken the leads where they enter the resistor causing sudden and unexpected failure, and just as a general rule I try to keep water and electricity apart.

I've used heavy mineral oil, the kind you can buy at a pharmacy (in the US anyway) in exactly the same way. Just make sure it doesn't get too hot. This is also a great trick for running resistors WAY over their rated voltage, since surface arcing is the usual mode of failure. Same trick with variable capacitors, except the capacitance changes of course. (And yes, I was a HV spark throwing nut for several years, there's nothing quite like a Marx generator to convince your kid that you are a mad genius, and that they should stay away from your work area...)