Absolute Begginer in Search of Wisdom

[chris thoen]

Hello all,

I'm Chris and I am looking for some help. Of course I am totally green with just enough theory to probably electrocute myself. I have been looking for folks who are interested in talking about amplifiers and the art of creating them and have found few willing to part with their wisdom.

I met a guy here in Portland at the local Guitar Center (I imagine there being a hiss in the crowd here) who gave me the url to this community and told me to lay it out for you all. I was looking for a pair of 6V6GT tubes for a Laney amp (Laney Pro Tube AOR series combo 30watt maybe? with a 10"speaker circa 1980ish) that I aquired about 7 years ago. The amp has some problems and it makes me sad so I want to help it.

I can't find the schematic for this particular amp anywhere. I have emailed Laney a couple times with no result. I have found some schematics but I believe that this amp was made slightly before the closest schematic I could find which is a Laney PT30 Series II from 11/5/89. I can't really read the schematic other than to pick out the pretty symbols.

I have pictures of the amp and questions galore, but I wanted to say first off that I respect people who do this in whatever capacity and I humbly ask for forgiveness in advance for my stupidity and taking up your precious time. I have searched for threads for begginers and for some basic information about how to get started to no avail.

Would anyone be willing to outline for me where to start and the first 10 or so things I should DEFINATELY know before even considering messing up this wonderful little amp? I have purchased a set of Mesa Boogie 6V6GTA tubes, and <i>"The Complete Guide to Guitar and Amp Maintenance: A Practical Manual for Every Guitar Player"</i> by Ritchie Flieger. I have a soldering iron and the barest undertsanding of the principles of electricity.

Thanks for reading this and feel free to PM me or whatever. This is literally my second time on the board so I am still trying to figure out the basics of using the forum.

[Structo]

Welcome Chris,

You're in Portland, eh?

I'm just to the South.

Anyway, what you need to do is read about safely working on a tube amp.

Most tube amps have anywhere from 300 to 500 volts DC inside.
Of course that is more than enough to stop your heart if you are unlucky enough to get that across your chest ( from hand to hand).

That's why one of the safety methods is to stick one hand in a pocket when probing around inside an amp to prevent the deadly current from going across your heart.

You will need a decent DVM. Most here recommend Fluke meters as they are the tops in that category.
I have a Meterman 37XR that is similar in quality.

Then you need to read up on how to safely discharge the power supply filter capacitors. Many times these can hold a lethal charge even when the amp is unplugged.
If you have read that the way to do this is to short them to ground with a screwdriver, that is not only dangerous to you but damaging to the caps.

Here is some info on that, be aware that some people will give bad advice that could be dangerous.
http://studentweb.eku.edu/justin_holton/caps.html

I would recommend that you get some books about tube amps if you are the type of guy that can learn from reading. Some can't but I think it is pretty much a requirement in any technical field.

The book you have is a good one.
There are others such as:
The Guitar Amp Handbook: Understanding Tube Amplifiers and Getting Great Sounds by Dave Hunter

Tube Guitar Amplifier Essentials by Gerald Weber

The Tube Amp Book by Aspen Pittman

Then you have the internet, where you will find an infinite amount of information about them.

http://www.aikenamps.com/

http://www.geofex.com/

http://www.tone-lizard.com/Table_Of_Contents.htm

http://www.pmillett.com/tecnical_books_online.htm

http://www.freewebs.co.uk/valvewizard/index.html

http://www.guitaramplifierblueprinting.com/index2.html

[Structo]

What I should have told you and I kind of glossed over it is, that since you don't seem to have any electronics background that you need to learn basic electronic first in order to understand tube theory.

You need to start at the beginning.
Like what is voltage, what is current, the DC and AC theory.
Ohms law plays a very important part in all of this and the math that goes along with Ohms law.

Then you need to familiarize yourself with electronic components and how they affect a circuit.
Like what is a resistor and what does it do?
What are the properties of series and parallel resistive circuits.

What is a capacitor and what affect does it have in a circuit.

The same for inductors (transformers and chokes).

Then of course tubes, what they do and how they do it?

You might see if you can pick up a used book or two about electronic theory at a college book store or other technical book source.

Because in order to make sense of a tube amp, you really do need to understand what the different bits and pieces do inside one.

Unfortunately, they really don't teach tube theory anymore and when I was in college even back in 1976 they spent maybe two or three classes on tube theory because everybody knew those little three legged devils were going to replace and do a much better job of it than vacuum tubes....
Little did they realize that guitar players and a lot of studios still use tube equipment because a analog tube simply sounds better to the human ear than a rack of solid state gear.

I will seem daunting at first and depends on how fast you are able to absorb the knowledge.

A good thing to do after you have learned a bit is to build a tube amp kit.
Like a Fender Champ or other similar simple amp.
Having a hands on experience is always better than a book. It will force you to understand how an amp works and what the different components do inside an amp.

Weber, Allen, GDS and a few others have amp kits that have instructions for assembly.
Most of the time with kits you are just given a schematic and layout and you are on your own to figure out how to build it.

So if you are mechanically inclined and are a do it yourselfer type person that would be a great project to learn on.

Like I said earlier though, be sure to educate yourself about the dangers of high voltage and how to work around it safely.

[Phil_S]

One of the first things you need to do is learn how to solder properly. Do this by making a couple of guitar cables. These are fairly easy to do and mistakes won't be costly. Also, search youtube for soldering demonstrations. Some are better than others. Here's one:
http://www.youtube.com/watch?v=I_NU2ruz ... re=related

See if you can buy Tom Mitchell's book and video on How To Service Your Own Tube Amp. This one of the book only:
http://www.youtube.com/watch?v=I_NU2ruz ... re=related
Don't order from Amazon, as they don't actually have it and it will take 2 months to deliver it. I know this from experience. Order from a source that has it in stock. This is a very good beginner's book. It's not too technical, but hits the important points very nicely.

[chris thoen]

Thanks guys. I'll look into the links you have provided.

I took some engineering classes about 8 years ago and learned some of the basics of electronic circuits but it would be a good idea to refresh the memory of that.

Do you think it would be better to do an amp kit before I mess with the little combo I have or does it really matter? I want to learn and I also want to play and this Laney is all I've got right now.

It may not be easy for me to fix because I see that there is a burn mark on the green board (PCB) under a ceramic-looking something...resistor maybe. It says 1000oms and 5W8534 on one side and OHMITE and 4623 on the other side and is whitish. It doesn't look burned itself and looks different than any other component so I wonder if it was replaced when I took it in to the shop a few years ago. Is it probable that any of the components before it might not be doing their jobs to cause the original problem?

I'll look into the safety stuff first. Thanks again for the advice.

[Structo]

That sounds like a power resistor and yes, they can get quite warm.



Here is a schematic for a Laney AOR 30:


http://www.schematicheaven.com/newamps/laney_aor30.pdf

Is that the right one?

Looks like R67 1K 5W.

It's a screen resistor and yes they can burn up if one or both power tubes have shorted.

Look on the schematic above the power tubes for R67.

[chris thoen]

It may be the right resistor and I don't think it's the right schematic. If I understand correctly, that is the schematic for the 30 watt head. What I have is the 30 watt combo. The closest thing I can find to what I have is the schematic for the PT30 Series II. It has 2 6v6GT power tubes and 4 12AX7 preamp tubes. I am having a problem understanding the schematic for the PT30 because it looks like there are 4 power tubes and 5 preamp tubes.

Here is the schematic for the PT30:

http://www.schematicheaven.com/newamps/ ... 0_PT30.pdf

Here are the pictures I was talking about before.

[gahult]

Chris,
That is the schematic I had for you. Glad you found it.
Gary

[gahult]

Chris,

The schematic is OK as far as the tube compliment.

V1b is 1 tube a dual triode 1 side used ECC83... AKA....12AX7
V2a/V2b is 1 tube a dual triode 2 sides used ECC83... AKA....12AX7
V3a/V3b is 1 tube a dual triode 2 sides used ECC83... AKA....12AX7
V4a/V4b is 1 tube a dual triode 2 sides used ECC83... AKA....12AX7
V5 is 1 final 6V6GT
V6 is the other 6V6GT

hope this helps.

Gary

[Firestorm]

Hi Chris.

Welcome and all that.

That amp is going ruin you as an amp tweaker/repairer. I've been doing this for years and I still hate to see a PCB in an amp I'm not familiar with -- you have to trace the circuit, figure out what's connected to what, then start guessing what's wrong.

If you want to learn, get a blackface or silverface Fender (something simple like a Princeton or a Deluxe). Those amps are laid out so it's pretty clear what's going on. Maybe if you read all the stuff already suggested, you could just download a Fender schematic or layout and get a sense of how it's all supposed to work. Then, you could tackle the Laney.

[M Fowler]

Chris,

Pardon my ignorance but what did you say was wrong with the amp in your opinion? It doesn't work at all or makes loud hissing noise? Are there internal fuses that blew?

Mark

[chris thoen]

The amp powers up fine. It plays but has some pretty raspy sound when driven hard. It's fairly complex but suffice it to say that the distortion from the preamp is really nasty above half or so. Under certain conditions the amp goes quiet. Playing at maybe half master volume with both preamp 1 and 2 levels over half, the sound fades out. Sometimes it comes back briefly but if I remember correctly it goes away completly eventually. I can't determine how long until it occurs because there are too many factors involved but I just played it at about 2 on master volume with the preamps in multiple different positions for about 15 minutes and the amp didn't fade out. The distortion was raspy so I believe that the speaker needs to be reconed, the power tubes may need to be replaced and/or biased or there may be some foul play in the components before r67 on the PT30 schematic. This is my opinion and not much of one. I am imagining that the fade out is the power tubes but what do I know?

[Structo]

That resistor is likely a replacement.

Your pictures are not clear enough on the components to tell what the other resistors look like.
But if the rest are the square cement type and this is the lone cylinder type, I would conclude that it has been replaced.


Look at the schematic I referred to.
It has the two preamp volume pots.

It has 4 preamp tubes and two power tubes.

V5 and V6 are the power tubes, V4 is the Phase Inverter.

V1 through V3 are the preamp tubes.

They are only using one side of V1 in the first stage.

I think you are not understanding that on a schematic they show both halves of the dual triode preamp tubes.
That is, V1a and V1b, are one tube.

The first thing to try on any tube amp is to swap in known good tubes.
Then if you zero in on one tube as the trouble maker swap sockets with the tube to see if the problem follows the tube or stays at the socket.

[chris thoen]

I think I get it now, thanks man. I have some new 6v6GTA tubes from Mesa Boogie. I have been told they are Sovtech tubes and they will work in place of the original 6v6GT tubes that are in there now. I am happy to replace the old tubes but I am wary of firing them up without biasing them. I am not ready to be able to adjust the bais. Istill have yet to get a meter or a scope.

Does anyone use a computer scope like one that connects to a PC with a USB cable? Is there a particular brand of scope I should look for? I don't beleive that I need an expensive one, is it okay to get any old scope? Do most of you use a scope or a bias plug?

[Structo]

That amp would be very simple to add a couple 1 ohm 1% resistors from the cathodes to ground.
Then you would be able to measure the bias current in mv since the 1 ohm resistor converts the current to voltage.
You would put your red probe on pin 8 and the black probe clipped to chassis ground.

You don't need a scope for this, and besides, then you would have to learn how to use a scope.

You just need a good DVM to set bias with this method.

Right now it probably has pins 1 & 8 jumpered and then a short lead right to the ground lug nearest pins 1 & 8 on the power tubes.

You would remove the jumper from the pins to ground and solder one end of a 1 watt, 1 ohm 1% resistor to pins 1 & 8 and the other end of the resistor to the ground lug.
Do this to both power tubes.

Like I said, just clip the red probe to the pin 1 & 8 junction and the black probe to chassis ground and set the meter to milli volts. You are reading the current across the 1 ohm resistor.
Using Ohms Law, we know that R x I = E so we know the R value is "1" so any current is converted to voltage.

Be sure speaker is plugged in, turn all volume pots down and tone controls at 12 O'clock. No guitar plugged in either.

This will set your idle bias current.

Adjust for 18 - 20 mv on each power tube.

I attached a layout the way my amp is.
My amp has the test lead jacks but you don't really need them.

EDIT:

I modified the schematic to show you where the bias resistors go.