• 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.

Tube Power Supply Question

Hi Folks,

About 15 years ago my father built me a stereo LA-2A audio compressor that runs off an external tube power supply. The unit has been in storage up until recently when my father passed away. I'd like to get the unit working again. I realise that it's not a good idea to plug it in and turn it on, but I'm unsure as to how to proceed. I thought that the first step would be to make sure the power supply is working. I have two questions about this. Firstly, is it advisable to replace the electrolytics in the power supply due to their age? Secondly, does a tube power supply require a load or can it be run without one? If it needs a load, what sort would it need?

I have medium-level knowledge of electronics - I've built a few tube guitar amps although in the past I've had my Dad around to keep an eye on what I was doing.

Cheers,

Chris
 

rayma

Member
2011-04-29 8:37 pm
Firstly, is it advisable to replace the electrolytics in the power supply due to their age?
Secondly, does a tube power supply require a load or can it be run without one?

The 15 years isn't all that long, and the capacitors will likely be ok once reformed.
If you have a Variac, hook everything up and gradually bring it to 100% (120 VAC)
over an hour or so. Monitor the Variac voltage to make sure it doesn't go any higher.
It may not be a good idea to run the power supply without a load.
http://www.jblproservice.com/pdf/Vintage JBL-UREI Electronics/UREI-LA-2A manual.pdf
 
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rayma

Member
2011-04-29 8:37 pm
I don't have a variac - any other options?

You could use an external power transformer wired to output half your usual line voltage,
and run it off that for an hour. The supply has iN4007s, so the filter caps would still partially reform.

Otherwise, you can connect a standard 100W incandescent bulb in series with the power input,
to limit the current draw when power is applied.
 
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DAK808

Member
2012-05-27 7:34 pm
Thanks, I think I'll put together the dim bulb tester (as you suggest) as I've got some other old gear that I'd like to get working.

I agree that if you have other gear to work on then a variac would be a more than worthwhile investment. I find that i use mine as much as my multimeters. The ability to do testing at low voltages is a very big plus when doing measurements and testing not to mention the capacitor reforming thing. Check out epay and you might get one for fairly cheap. cheers.