Dummy load for guitar amp head

[Dr Gris]

About to do some mods, service and recap on my Fender amps.
One of them is a Bassman 135. I've found 10 ohm 50W resistors in a local shop.
Do you think it's safe to use two in parallel as a dummy load, just for checking that
everything is ok? I'm not gonna blast the input with a sine wave. Only for setting bias
and check other voltages. Output from amp is 4 ohm.

Any tips on a heatsink solution, prefably not a bucket of oil/water....

Best
Magnus

 

[mjrippe]

Buy four of them and wire them in series/parallel (ie, two series pairs, wired in parallel).  This will give you 5 ohms with plenty of power handling.

*EDIT* That will give you *10* ohms with plenty of power handling, but higher ohms is OK ;-)

 

[CJ]

you can also use a speaker cabinet placed face down with blankets over it while using a little cotton in the ears, inductive load may or may not change bias setting,

 

[Dr Gris]

Thanks for the tips guys. Haven't started this yet.
What about those big resistors you can find on eBay, 4 ohm 300W for instance??

Best
//M

 

[mutterd]

Hmm... I'm not sure if an inductive load would change biasing...

But a speaker whistling 1k doesn't do much for my troubleshooting skills - I have played that game and i dont like it...

I have a 35R 50W and a 10R 50W in parallel for 7.78R - it will get hot if you crank 30V across it for long enough - but as you wouldn't need to do anything like that under normal circumstances - you should be fine with the 50W...

 

[Dr Gris]

Like this one:
http://www.ebay.com/itm/300W-4-OHM-Audio-Power-AMP-Test-Dummy-Load-Non-inductive-Amplifier-300-Watts-/201062177646?pt=Vintage_Electronics_R2&hash=item2ed03d5b6e

Cheers
//M

 

[PRR]

If you are just checking BIAS.... you want NO signal. Use a speaker. If there's anything over a murmur, that's signal current on top of bias current and screwing your bias reading.

I would however have a full-power dummy for smoke-testing. After cap-job you might have BIG hum (oops, fergot a wire) which would stress an unloaded OT. Other changes can cause motorboating. Even just poking around can cause squeals (g-amps can be quite fussy about wire placement).

Smaller tube amps can be low-loaded. Cathode bias can even be shorted. The Bman 135 is in the high-strung class where shorts are bad. However I would not fear a 3.3 Ohm load, which (with 10r 50W parts) gives a 150W rating.

Obviously you can't get "power ratings" with a 3.3/4 mis-match. But I've only known a few amp dudes equiped and inclined to actually test the power output. If it will push near nominal voltage (23V in 4r or 15V-18V in 3.3 Ohms), a Bman 135 will be Loud Enough on-stage; if not, it's sick, and shouldn't be working until it is healed.

Don't diss the water bucket. I did days of testing on 180W amps with a 44W resistor in a coffee can.

 

[Dr Gris]

Thanks guys,

This is all new to me, I'm no amp tech but know enough to be dangerous inside my
three different Bassmans (Bassman 50 Silverface, Bassman 135, Bassman Tweed Combo) 

The reason I would like to do it without a speaker is the cabinets are big and my little workbench
is in my small home studio/mixing room at home in my apartment. The cabinets reside with my
other big stuff in my rehearse studio.

Best
Magnus

 

[PRR]

> the cabinets are big

Then use a little speaker. Biasing should be "no signal". On the monster stage-amps, you may have 0.1 Watts or less of stray hiss and hum, no problem. A small hi-fi speaker will manage that. When the small speaker makes big sounds, shut-down-NOW and investigate.

Still in all, a Power load is a good thing to have and a handful of 10 Ohm parts is close enough for rock-n-roll.

 

[outoftune]

i built a box that is switchable between 2 / 4 / 8 / 16 ohms using these:

http://www.ebay.com/itm/4-Ohm-4R-100W-Watt-Power-Metal-Shell-Case-Wirewound-Resistor-/250911322824?pt=LH_DefaultDomain_0&hash=item3a6b7af2c8

i built it into a metal hammond project box (box acts as a heatsink)

it has a 1/4" send for the speakers and also a 1/4" thru that i hook to my scope.

have used it many times for troubleshooting tube amps.

don't flip the switch while the unit is on!!!

 

[t9crepair]

You could buy one of these:http://www.parts-express.com/4-ohm-100w-non-inductive-dummy-load-resistor--019-015
Heatsink is built in. I use several of these in different ohmages for my bench.