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Tube amp knowledge for dummies.


willsellout

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So I was making a private exchange with Kindness because as some of you know I have a vintage Ampeg V4B coming my way within the next couple weeks. This was an exchange where at the end I was honestly more confused that when I started; which is saying something because I know very little about tubes.. Instead of going back and forth I figured I'd post a thread because there might be other people that want to learn.

 

My question is about biasing and tubes.

 

So this is what I thought biasing was:

Basically setting the amount of current running through the tubes based on the type of tubes and their tolerances.

 

I thought most tube amps had to be biased when they were re-tubed if not more often. I know some tubes had a fixed bias (400+) but apparently there is more to it.

 

I thought that tubes are matched in pairs.

 

 

So, feel free to post your own tube questions as well. If you can explain this to me as well I'd be really grateful.

 

Also any advice on general upkeep that I can do myself would be nice.

 

 

Thanks!

 

 

Dan

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I've played mainly tube rigs since I started playing bass in 1977. I've replaced a couple preamp valves, but have never had to replace the entire set.

 

Having said that, I've got a few points to share FWIW...

 

Use the stand-by setting (if your amp has one) or start-up.

 

Never, ever, power up a tube amp unless it's connected to a cabinet.

 

Keep your chassis clean. Tubes run hot, and all the dust, grease, crap & whatnot that builds up acts as insulation.

 

My two cents. :wave:

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Find an excellent tech whom you trust... I, and others in my band, use Jim at Amphead in PDX... Very honest, and stands behind his work. He's a small shop, so sometimes lead times can be a smidge long, but he is always upfront with that info...

 

My bandleader / harp player has two original '59 Bassman amps and will not let anyone else touch them...

 

-robert

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Biasing sets the idle current through a tube. It can off by a pretty wide margin and cause no real damage but the sound will be different. You can rebias when you replace tubes or not. I don't unless the tone is off and then the bias would be way off. On your V-4B, the bias is fixed at -60v so all we do is verify the bias supply is working. It 's acceptable for it be off a few volts. Some people will modify an Ampeg for adjustable bias but it's not necessary. I would never do it.

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Biasing sets the idle current through a tube. It can off by a pretty wide margin and cause no real damage but the sound will be different. You can rebias when you replace tubes or not. I don't unless the tone is off and then the bias would be way off. On your V-4B, the bias is fixed at -60v so all we do is verify the bias supply is working. It 's acceptable for it be off a few volts. Some people will modify an Ampeg for adjustable bias but it's not necessary. I would never do it.

 

 

The one I'm getting is adjustable with test points for each tube I believe.

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Tube amp knowledge for dummies = plug in and play. If something sounds wrong, it is and you should take it in for repairs. Solid state amp knowledge for dummies = plug in and play. If something sounds wrong it is and you should take it in for repairs.

 

The only other practical advice I'd give is on a solid state amp don't go below the minimum rated impedance and on a tube amp try to stay right at or around the rated impedance and don't use an amp with an output transformer without a load connected.

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What do you use to clean it? One of those air sprayers?

 

 

Just make sure the unit is unplugged before you go blowing off anything in the cabinet. Notice I said "unplugged". There are still a couple of live components in any electronic device if it is just turned off. About pairing of tubes. The common addage is to replace them in pairs, as the tubes are amplifing one half of the signal each in the amp path. Back to the cleaning tips, if you are going to have the case apart anyway to blow it out, get some potentiometer lube from Radio Shack or some other supplier and spray out your pots while you are at it.

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Back to the cleaning tips, if you are going to have the case apart anyway to blow it out, get some potentiometer lube from Radio Shack or some other supplier and spray out your pots while you are at it.

 

 

Don't do this unless it's necessary. If it's not done correctly you'll make a quiet pot noisy.

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Could you tell me why? Does it just have something to do with the optimal amount of current running through it?

 

 

Short answer: A transformer is used to match low impedance speakers with high impedance tube output stages. The matching of the impedances improves the efficiency of the output section. Having multiple secondaries, you can optimize transfer of power into various cabinet loads.

 

More involved answer: In a solid state amp, the output section is typically connected directly to the speakers. The output is a constant voltage, so the lower the speaker impedance, the greater the current flow/power. (Ohm's Law: V=IR: Voltage = Current x Resistance) A solid state amp is typically designed for an optimal output load where a lower impedance speaker will draw more current than the amp can comfortably supply (causing overheating/meltdown) and a higher impedance speaker will not draw as much current as the amp can supply (causing a reduction in power).

 

By contrast, a tube amp's output section is typically connected to the speakers through an output transformer. The output is a constant current, not a constant voltage as in solid state amps. This is key, but first a brief interlude regarding transformers:

 

A transformer works by interleaving coils. The first coil is the "primary" the second is the "secondary." There can be third, fourth, fifth coils, but they are generally referred to as multiple secondaries, rather than tertiaries, etc. The voltage between the primary and secondary(ies) is transformed based on the ratio of turns between the two coils. If they have an equal number of turns on both sides, there is no transformation. However, if the secondary has twice as many turns as the primary, the voltage on the secondary will be twice as high.

 

In tube amps, the output tubes are run at very high voltage, with relatively little current and have a very high output resistance (typically in the thousands of ohms). On the other side of the transformer the speakers are run at a lower voltage, with higher current into much lower resistance (8 ohms/4 ohms). As I said above, the transformer will try to maintain a constant current. Therefore, when you connect a lower impedance cabinet and the current stays the same, the voltages on both sides of the transformer decrease. (V=IR on both sides) A voltage decrease at the output tubes lowers the power output and is generally safe, though there can be issues (it can stress the emission material and current capability of the tubes and can create heat issues in one or more of the transformers). On the the other hand, when you connect a higher impedance cabinet, the voltage at the tubes increases, which can be a dangerous condition. Depending on how far from overload the plate voltage is in the output section, causing it to increase can be okay or deadly to the tubes/amp.

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It's not supposed to. No V-4/V-2 ever had that.

 

 

This one was modified quite a few years ago.

 

Now back to knowledge, I understand that the caps can hold lots of voltage. I reading about general maintenence and the article I read said that if you are going to clean it, that you let it sit unplugged for a couple days to let the electricity drain. It also said that if you wanted to be extra safe to run an insulated wire with alligator clips from the positive on the cap and to touch it to the chassis to disperse anything leftover.

 

For the question; do the caps drain voltage? I thought it was just stored in there.

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Now back to knowledge, I understand that the caps can hold lots of voltage. I reading about general maintenence and the article I read said that if you are going to clean it, that you let it sit unplugged for a couple days to let the electricity drain. It also said that if you wanted to be extra safe to run an insulated wire with alligator clips from the positive on the cap and to touch it to the chassis to disperse anything leftover.


For the question; do the caps drain voltage? I thought it was just stored in there.

 

 

Caps operate by holding a charge. Some circuits automatically discharge the caps and others do not. Some circuits are supposed to, but you might have something fouled up in the amp preventing it from doing so. I think it is always best to assume a cap is live until you can verify otherwise. You can look in the safety thread I wrote for more info.

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Well I put a lot of effort into the links in my signature, especially the one on how the JTM45 operates.

 

I understand that and appreciate it. I actually read it and none of the questions I have asked were apparent to me in that thread. I probably could have just posted this is that thread though.

 

 

Caps operate by holding a charge. Some circuits automatically discharge the caps and others do not. Some circuits are supposed to, but you might have something fouled up in the amp preventing it from doing so. I think it is always best to assume a cap is live until you can verify otherwise. You can look in the safety thread I wrote for more info.

 

OK, so much like a gun, treat it as if it were loaded. Not that I'm going to do anything with the amp anyway.:D

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I understand that and appreciate it. I actually read it and none of the questions I have asked were apparent to me in that thread. I probably could have just posted this is that thread though.

 

No, it's cool to have this separate thread. You keep changing the game though. :D

 

Tube amps dummies = plug in and play. If something sounds wrong, it is and you should take it in for repairs. Also, try to stay right at or around the rated impedance and don't use an amp with an output transformer without a load connected.

 

How does a tube amp work = http://acapella.harmony-central.com/forums/showthread.php?t=2245311

 

Deeper exploration of the circuits = Navy Electricity and Electronics Training Series and the writings of

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No, it's cool to have this separate thread. You keep changing the game though.
:D

Tube amps dummies = plug in and play. If something sounds wrong, it is and you should take it in for repairs. Also, try to stay right at or around the rated impedance and don't use an amp with an output transformer without a load connected.


How does a tube amp work =
http://acapella.harmony-central.com/forums/showthread.php?t=2245311

 

I'm not changing anything sucka!

The thread title was a bit simplistic though for what I was looking for. I was basically looking for info without getting too technical. Yes/no answers or quick explanations, which I received. So thanks to everyone...I'd like to learn a bit more but honestly its a bit hard for me to grasp.

 

 

 

Dan

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In tube amps, the output tubes are run at very high voltage, with relatively little current and have a very high output resistance (typically in the thousands of ohms). On the other side of the transformer the speakers are run at a lower voltage, with higher current into much lower resistance (8 ohms/4 ohms). As I said above, the transformer will try to maintain a constant current. Therefore, when you connect a lower impedance cabinet and the current stays the same, the voltages on both sides of the transformer decrease. (V=IR on both sides) A voltage decrease at the output tubes lowers the power output and is generally safe, though there can be issues (it can stress the emission material and current capability of the tubes and can create heat issues in one or more of the transformers). On the the other hand, when you connect a higher impedance cabinet, the voltage at the tubes increases, which can be a dangerous condition. Depending on how far from overload the plate voltage is in the output section, causing it to increase can be okay or deadly to the tubes/amp.

 

 

This is not correct.

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Perhaps you misunderstood your other sources but what you wrote was completely wrong. I wouldn't know where to begin.

 

 

How about you post your explanation instead of correcting Kindess' post? That might be easier and I'd love to read it honestly.

 

 

Dan

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Yeah...saying that something Kindness posted is completely wrong without providing any kind of explanation is really kind of useless, and contrary to the educational nature of this thread.

 

So long as we're trying to go for basic knowledge though, without the electrical specifics/formulas, suffice it to say that matching the output resistance between the amp and cab is important. Failure to do so can cause components to overheat, degrade in performance, or in the worst case, fry themselves.

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