Tube amp issues

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shred_head

shred_head

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Hello,

I got a Randall Power Tube 1000 II (the head),
and of course I got it used

First off, there were some issues with the Main volume pot, that you could have it all the way off, then bring up the volume up to about 2 or maybe 3, (everything fine) anything past that, and it was just dead air.

So i took off the pot, (which had a 151 capacitor connected to it) and had an electronics expert check it,
he said it looked perfectly fine,
he cleaned it up with some contact cleaner and sent me on my way

so I cleaned off all the old solder
tinned the pot, and tinned the wires (after recutting them and stripping them)

and tried it out, only for the pot to do it again,
then after about 5 minutes of leaving it at 2 or 3,
I could smell as if some plastic was melting/burning, after about 15 seconds of that, the amp powered off, and now won't power on

Could it be possible that it is just the tubes that need replacing?
would have the tubes being a bit faulty would have caused the main volume pot issue?
How can you tell if a tube needs to be replaced by physical appearance?
could something else be causing it not to power up now?

If this isn't really the right forum, or site, please direct me to where I may find help

thanks
 
Last edited:
shred_head said:
Hello,

I got a Randall Power Tube 1000 II (the head),
and of course I got it used

First off, there were some issues with the Main volume pot, that you could have it all the way off, then bring up the volume up to about 2 or maybe 3, (everything fine) anything past that, and it was just dead air.

So i took off the pot, (which had a 151 capacitor connected to it) and had an electronics expert check it,
he said it looked perfectly fine,
he cleaned it up with some contact cleaner and sent me on my way

so I cleaned off all the old solder
tinned the pot, and tinned the wires (after recutting them and stripping them)

and tried it out, only for the pot to do it again,
then after about 5 minutes of leaving it at 2 or 3,
I could smell as if some plastic was melting/burning, after about 15 seconds of that, the amp powered off, and now won't power on

Could it be possible that it is just the tubes that need replacing?
would have the tubes being a bit faulty would have caused the main volume pot issue?
How can you tell if a tube needs to be replaced by physical appearance?
could something else be causing it not to power up now?

If this isn't really the right forum, or site, please direct me to where I may find help

thanks
Click to expand...

Two things, and neither are particularly good news. First, I can't think of how a bad tube can take out a pot on the faceplate. Second, a bad smell followed by the amp shutting down is usually a very bad sign, and something that you'll want to get checked out by a reputable amp tech rather than doing any more putzing with yourself. You've very likely got some significant damage to one or more components of the amp, and trying anything else with it that involves trying to power it up has a high probability of making things worse (read: more expensive).

Stop messing around with the amp and take it to someone who knows what they are doing and has the tools of the trade. Of course, that's free advice and worth every penny.
 
Many of the electronic components in amplifiers like yours - resistors, capacitors, diodes, rectifiers, and even volume pots - work because of an internal reaction requiring smoke as a catalyst. If you let the smoke out, they stop working, as most of us have noted through our past experiences.

I think that bad smell you noted when the amp stopped working was smoke that leaked out of one or more of the components. As ggunn noted above, you really need to take the amp to an electronics repair person so he can put the smoke back in properly. If you keep fooling around with it and let the smoke out of more stuff, it can get very expensive.
 
crazydoc said:
Many of the electronic components in amplifiers like yours - resistors, capacitors, diodes, rectifiers, and even volume pots - work because of an internal reaction requiring smoke as a catalyst. If you let the smoke out, they stop working, as most of us have noted through our past experiences.

I think that bad smell you noted when the amp stopped working was smoke that leaked out of one or more of the components. As ggunn noted above, you really need to take the amp to an electronics repair person so he can put the smoke back in properly. If you keep fooling around with it and let the smoke out of more stuff, it can get very expensive.
Click to expand...


wow. smoke? that is 100% bullshit.
 
crazydoc said:
Many of the electronic components in amplifiers like yours - resistors, capacitors, diodes, rectifiers, and even volume pots - work because of an internal reaction requiring smoke as a catalyst. If you let the smoke out, they stop working, as most of us have noted through our past experiences.

I think that bad smell you noted when the amp stopped working was smoke that leaked out of one or more of the components. As ggunn noted above, you really need to take the amp to an electronics repair person so he can put the smoke back in properly. If you keep fooling around with it and let the smoke out of more stuff, it can get very expensive.
Click to expand...


What if I got a smoke machine, wouldn't I just need it, and maybe a small funnel to put the smoke back in?
 
shred_head said:
What if I got a smoke machine, wouldn't I just need it, and maybe a small funnel to put the smoke back in?
Click to expand...

Nope. Wrong smoke. Smoke machines are solid state.
 
so aside from taking it in to a tech....what is the difference between the pre-amp tubes and the other main tubes?

can you not match up certain ones? or is it ok provided that they are compatible with the amp in their own right?
 
shred_head said:
What if I got a smoke machine, wouldn't I just need it, and maybe a small funnel to put the smoke back in?
Click to expand...
No, you need a 5ml syringe with a #30 needle, and you have to know just where to stick it.
 
shred_head said:
so aside from taking it in to a tech....what is the difference between the pre-amp tubes and the other main tubes?

can you not match up certain ones? or is it ok provided that they are compatible with the amp in their own right?
Click to expand...
The point here is:

1) Something was wrong with something (most likely not a tube) that caused the original malfunction.

2) Either the original problem got worse when the smoke was released, or the original problem caused one or more secondary problems.

3) The problem(s) is almost certainly not a tube - more likely a capacitor, resistor, or, heaven forbid ($$$$) a transformer.

Somebody who knows how to troubleshoot and repair tube equipment will need to look at it - otherwise it will stay dead. You're not going to fix it by switching tubes around, and could possibly make the problem worse (read: more expensive to fix.)
 
shred_head said:
so aside from taking it in to a tech....what is the difference between the pre-amp tubes and the other main tubes?

can you not match up certain ones? or is it ok provided that they are compatible with the amp in their own right?
Click to expand...

No offense meant, but questions like this is the reason I advised you to quit messing with the amp and take it to a pro. At your level of experience there is nothing you can do to make it any better and plenty you can do to make it worse. That's not even to mention the fact that within your amp there are circuit points which carry lethal voltages, even after the amp is powered down and disconnected from power.

But we all have to start somewhere, eh?

Usually, the preamp tubes are small and the power tubes are large. The preamp tubes shuttle the signal around to the EQ sections, reverb, tremelo, etc, at line level, which is to say at about 1 volt AC. The power tubes receive this signal and boost it up to where it will drive speakers, something in the tens of volts and over a hundred volts in big amps.

All the tubes have a DC high voltage supply line that will usually be between 300 and 500 volts; these voltages are stored on large capacitors which can retain a charge for days or longer after the amp is powered down, and touching one of these nodes when you are grounded can result in painful and potentially lethal electrical shock. There are many points in the circuit which are connected to these capacitors, and touching any of them will zap you. One should never attempt to work on an amplifier without first draining the capacitors, and if you don't know how to do that, you should not even open it up.

I don't want to discourage you from learning about your amp, but leaping before you look can get you killed.
 
ya i'm taking it to a tech no doubt....just was curious on the workings of it,

as well about compatibility.....any suggestions when getting new tubes and replacing old ones, as for what power tubes work with what pre-amps....or anything goes?
provided it is a compatible one with the amp?

any recommendations for tubes?
 
shred_head said:
ya i'm taking it to a tech no doubt....just was curious on the workings of it,

as well about compatibility.....any suggestions when getting new tubes and replacing old ones, as for what power tubes work with what pre-amps....or anything goes?
provided it is a compatible one with the amp?

any recommendations for tubes?
Click to expand...

That's a larger question than you might imagine. Your amp is designed with specific tubes in mind, though some substitutions will work. Others will not, and swapping out tubes without a working knowledge of what tubes are interchangeable and which are not, especially power tubes, even though they will physically fit, can severely damage the amp.

There are gray areas as well, where for example, a substitute power tube will work, but the bias voltage to the tube will have to be changed. Some preamp tubes with identical pinouts are close enough in gain to interchange, but how they will sound in your amp may be terrible.

Where you are at the moment, I would advise you to stick with the designed tube numbers for that amp and restrict yourself to trying out different manufacturers' offerings within those parameters.
 
ggunn said:
Where you are at the moment, I would advise you to stick with the designed tube numbers for that amp and restrict yourself to trying out different manufacturers' offerings within those parameters.
Click to expand...

So would you say if it's a 6l6 power tube, then just try 6l6 of different brands....

same as for the preamp tubes but of their size/value?


and say if I were to replace any or all of the power and pre-amp tubes (have replaced) with their appropriate number....then they will always need to bias the amp again, correct?
 
shred_head said:
So would you say if it's a 6l6 power tube, then just try 6l6 of different brands....

same as for the preamp tubes but of their size/value?
Click to expand...

Yes and yes. As you learn more about this stuff, you'll maybe try a 12AX7 in a socket designed for a 12AT7, or something like that, but wait until you understand why you'd want to do that; don't just start trying things at random. Your chances of making something better with random choices are pretty small.

and say if I were to replace any or all of the power and pre-amp tubes (have replaced) with their appropriate number....then they will always need to bias the amp again, correct?
Click to expand...

If your amp has adjustable bias, then it's a good idea to re-bias when you replace power tubes. Some amps are fixed bias, so you can't change it.

Biasing is only for power tubes; preamp tubes don't have biasing. Biasing is different from DC power; they all have that, and the voltage is fixed by the design of the amp.
 
not too sure it that would be it, but on the back of my amp there is a reset switch.....would that be the auto bias option?
 
shred_head said:
not too sure it that would be it, but on the back of my amp there is a reset switch.....would that be the auto bias option?
Click to expand...
No. Maybe a circuit breaker or polarity switch (if amps still have those.)

No major problem's to report except the bias adjustment, there is an adjustment pot on the circut board for this, the chassis has to be removed to get to it and you have to be around some very high voltage's 500v +, I strongly recomend checking or haveing the bias checked, but if your not sure how to do it DON"T!!!! THE VOLTAGE'S ARE LEATHAL!!!!
Click to expand...

from:
http://reviews.harmony-central.com/reviews/Guitar+Amp/product/Randall/Pro-Tube+1000+II+Head/10/1

There are some anecdotal tube substitution suggestions there also.
 
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