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How do you change tubes in an amplifier?

M

MichelD

May 19, 2014
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#1
I had a tech replace my power tubes and one preamp tube in my Ampeg V4B and it still makes a frying bacon sound in the speakers. Rather than take it by I figure I should just put a new preamp tube in myself to replace the last old tube in there.

How do you do that? Dumb question perhaps but I have never ever tinkered with the insides of any electronic equipment.
 
#3
Amp off... and let it cool down ;) Tubes get hot! Be careful putting them back in. Make sure the larger gap between pins lines up with the corresponding pattern on the socket. If you try to force the tube in when it's not aligned right, you'll bend a pin and if you put the tube in the wrong way around it may cause some serious damage.

I'm not familiar with the V4B, but if it's built like my SVT it has some more preamp tubes hidden where you have to partially disassemble the amp to change them. Not a place you want to go unless you know what you're doing, there are lethal voltages in there (that can linger after the amp is turned off).

Did the tech tell you the tubes were causing the frying bacon sound, and that replacing them would fix it?
 
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#7
MichelD said:
I had a tech replace my power tubes and one preamp tube in my Ampeg V4B and it still makes a frying bacon sound in the speakers. Rather than take it by I figure I should just put a new preamp tube in myself to replace the last old tube in there.

How do you do that? Dumb question perhaps but I have never ever tinkered with the insides of any electronic equipment.
Click to expand...
Did the tech not test the amp after he replaced the tubes???
 
#9
A sound like that is quite likely caused by a plate load resistor in the the preamp. I would take it back and get it looked at before replacing another tube. Preamp tubes last a long time, and if they are original they may well be better than new ones available today.
 
#12
Ronnie Robinson said:
Im not understanding something....if this was at a tech why was this issue not identified? I'd be speaking to the tech about it.
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This is why I asked about the other items in the signal chain to be ruled out.

The Tech gave it an ok because it is ok at his shop.
Then OP plugs back in with the wonky whatever again at home and the problem is back.
May not be the head at all.
 
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#14
If the first step of changing tubes DOES NOT, include a tube tester, (or other test equipment) you are shooting in the dark. If money's no object then I suppose it could be for your enjoyment but you are grabbing at straws.
And sometimes tubes fail because of an associated component has failed.

Imagine if you repaired your car that way.
 
#15
Jim C said:
If the first step of changing tubes DOES NOT, include a tube tester, (or other test equipment) you are shooting in the dark. If money's no object then I suppose it could be for your enjoyment but you are grabbing at straws.
And sometimes tubes fail because of an associated component has failed.

Imagine if you repaired your car that way.
Click to expand...


I think this is correct but perhaps a bit overstated.

Although tubes can last 10,000 hours, they can also fail after only a few hours. Also if they are not seated properly they won't work properly.

I think it's a good idea to reseat the old tubes to see if it fixes the problem. If not, I think its a good idea to roll a new tube of the correct type into the amp. There is small chance you could damage your spare tube, but I think its pretty low.

Small signal tubes are generally self biasing. For the most part, you just pull them out and install a fresh tube and see if it fixes the problem.

Some amps have cathode biased output tubes, which is also a form of self-bias. However most bass amps use something called fixed bias for the output tubes. You need to know what your are doing to replace the output tubes in a fixed bias amp, or you may wind up destroying a brand new set of tubes or causing serious damage to the amp.
 
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