• 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.

Discharging a capacitor - should I??

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cederholm

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Joined 2011
#1
Hi all,

I'm new here and new to messing around with a tube amp and I'd love some advice. I own a Create Vintage Club 30 guitar amp that cuts in and out once warned up. I changed the power tubes over a year ago thinking that was the issue, but it wasn't. In my frustration I stuffed the thing on a shelf (unplugged).

No one to give up, I've recently decided to look into it again and I've found multiple posting on the web about this amp and this problem. Many say to check the solder joints as they're know to come loose and ack up.

I feel very comfortable resoldering any issues, but capacitors scare the heck out of me. Should I attempt to discharge them and have a look around or just take the amp to a pro?

Thanks for any and all advice.
Carl
 
#3
Most amps have bleeder resistors built in so after a few minutes the voltage should be almost nothing. If your amp has been sitting on the shelf its most certainly discharged by now.

Lastly you can always measure the voltage with a meter to be sure.
 
#4
Cederholm,
First, make sure unit is unplugged and disconnected from everything else. If you have a volt meter, or can borrow one if necessary, I recommend checking for voltage on caps, BUT BE CAREFUL! Set voltage range to highest range (if it's not an auto rage meter), connect one lead (usually negative) to chassis and/or ground, put one hand (left or right) in your pocket, and use other hand to probe cap terminals. By following this proceedure, you won't electrocute yourself. If caps are dis-charged, you wil have no or low voltage readings, then you can poke around and solder connections.
Good luck,

Mike
 
#5
astouffer said:
Most amps have bleeder resistors built in so after a few minutes the voltage should be almost nothing. If your amp has been sitting on the shelf its most certainly discharged by now.

Lastly you can always measure the voltage with a meter to be sure.
Click to expand...

FIRSTLY measure with a meter to be sure. I have an old Pioneer SX-838 that holds about 80V for _weeks_ - and that's transistor!!!
 
#6
Michael Bean said:
put one hand (left or right) in your pocket, and use other hand to probe cap terminals.
Click to expand...

Probably doesn't need clarification, but here goes:

DO NOT use your other hand to probe the cap terminals. Use this free hand (i.e., the one not in your pocket) to carefully hold the voltmeter probe, which you will touch to the cap terminals looking for residual charge.

Why place a hand in a pocket? To hopefully avoid an "arm to arm" discharge path should you touch a charged lead--the arm-to-arm path would cross the chest and heart, which is very unhealthy to do!
 
#7
cederholm said:
I feel very comfortable resoldering any issues, but capacitors scare the heck out of me. Should I attempt to discharge them and have a look around or just take the amp to a pro?
Click to expand...

Before doing anything, it's essential to use your DMM to see if there's any residual voltage across the filter capacitors. Most hollow state equipment can retain enough of a charge to give nasty shocks. For solid state, residual charge can poof sensitive transistors and ICs, such as any MOSFET, or anything based on CMOS.

It's a good idea to prepare a soft discharger from a 500R / 10W resistor fitted with alligator clips. Clip that across any hot capacitors to bleed down the voltage before shorting across the terminals. It's also a good idea to keep that short in place while working since some types like electrolytics can "recharge" themselves if left open circuited (though this is more of a problem with solid state and its static charge sensitivity). Just remember to remove that shorting link before powering up. :redhot:
 
#10
Want to drain your capacitors without those messy resistors and test clips? Crank your amp playing something loud and hit the off switch. You will hear the sound fad off pretty quick. That is your capacitors discharging. The heaters will be glowing long enough to use up the power in the capacitors. Much easier with amps that have a standby switch but in the end the same thing happens. Empty capacitors. I am assuming you will head the others and check the big ones though.
 
#12
Can vouch for music NOT discharging the caps if the heaters are switched off at the same time as HV - with 250V only half is eaten before tubes stop conducting.
If there is a standby switch that kills HV first and leaves the heaters on then yes, caps are down to 2 volts by the time last crackling sound comes from the speaker.
 
#14
dovla said:
Can vouch for music NOT discharging the caps if the heaters are switched off at the same time as HV - with 250V only half is eaten before tubes stop conducting.
If there is a standby switch that kills HV first and leaves the heaters on then yes, caps are down to 2 volts by the time last crackling sound comes from the speaker.
Click to expand...
Actually the standby switch would not work for this amp as it is past the filter capacitor. Yes a resistor would be a better method but as long as you check with a meter afterward and find minimal voltage you should be good to go. This amp has a bleeder resistor so the power supply will not be charged forever.

Two volts and still crackling? Not sure if I would concern myself with 10.
 
#18
after you make sure you have it discharged, you could put a large-ish value resistor over it and it will drain it constantly for you, two 150k 1/2W minimum resistors in series to make 300k across that will draw about a mA across the cap, ensuring it gets drawn down to zero. just might take a minute or two. that C32 looks like a good spot to put it across, rest of the capacitors in the circuit looks like would drain with it. still, check with probe first after doing a 'test' poweron

C34/C35 should probably be drained too
 
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