Help! I tried to recap my amp!

[docz]

No offense taken! I am quite aware that all this is totally new to me - and that I need to practice a lot! I am only used to soldering circuit boards, this is much easier - at least to me. And thank you for your little tutorial. I find it a bit cumbersome to do this, wish I had one extra hand to hold the iron. Just a quick question, how do I do it if the terminal doesn't have a hole in it? Do I just wrap it around? Or should I use a shoe or something?

Phil, I was thinking of just hooking up the 6.3V wires, and disconnect the high voltage wires.

The OT says "4 - 5 ohm" on one side and "8 ohm" on the other. If it needs replacing, maybe I can replace it with something that can handle both 8 and 4 ohm speakers?

Cornelius I live in Tønsberg. Cool to see other Norwegians here as well!
It is both the 12AX7's that doesn't light up, I believe one is used as a preamp and the other is used for the two effects. Before I started all this the amp worked, but after a couple of minutes it started getting noisy, a lot of static or scratchy sounds. Reverb, Tone and Tremolo all worked like they should. After I got some advice on another forum I changed all the tubes and the three big electrolytic caps. After that I thought it was ok, but there was no sound coming out of it. Not even hiss, none of the potentiometers had any effect on sound. I noticed that the rectifier and power tube lighted up, but the two 12AX7's didn't.


DocZ

[docz]

Will this OT work?

http://store.triodestore.com/tfchxfwi48oh.html

DocZ

[Cornelius]

Not every tube have a clear visible glow; some can be real hard to see if they're glowing. If the preamp tubes gets lukewarm after a couple of minutes with power on, they do glow. Have you tried the old preamp tubes again after the 'operation'?

As long as you can measure 6,3VAC between pin 4-5 and 9 they really should glow. oxidized/dirty tube sockets could of course lead to no connection to some pins, and lots of noise and crackle. Faulty filaments (the wire that glows) in the tubes are rare. (i think...)

Extra hand?
But you DO have an extra arm; your mouth! The wire/component in your left hand; soldering iron in your right, and soldering tin in your mouth.

I just put a guitar cable in the speaker output instead of the speaker on a SE amp i got here, and measured 2,4ohm on the secondary winding on the OT, with no powertube in the socket. (and no power to the amp while measuring... )
And Yes, if you really suspect the OT, the one in your link should be ok.

You say that you doesn't even hear a hiss now. Since you have swapped the el.caps, i think there is a good chance that you now have a bad soldering on one or more of those caps. Put your iron on the soldering points and hold it there until you can see that all the tin melts, and then dab a bit more on. (but do not overdo it. )
As long as the power and output tube glows and there's no sound through the speaker, i suspect that the output tube does not get any high voltage.

NOTE:
Put a 10k resistor over the + and - on each of the big caps and hold it there for 5-10sec. before sticking your nose in there.... there's no fun getting those shocks...

If you're really stuck, you're welcome to visit if you are in these parts of the neighbourhood.

EDIT:
I took a closer look on the post where you're measuring the output tranny, and that setting you're using are not suited to measure ohms i think (i might be wrong though...); it is for measuring diodes/transistors, and it can be used to measure connectivity in wires. Use the 'ohm' setting on your meter at the lowest setting.

[martin manning]

Just to help myself not get confused, I made a color coded version of the schematic to help me out when tracing each section in the real circuit. Could you guys check it to see if I got it right? I also wrote some questions on it, it was easier to write them on the schematic - rather than trying to explain it in writing.

So here it is:

[img747]http://farm3.static.flickr.com/2664/423 ... 3f09_o.jpg[/img]

DocZ

You've got it pretty well figured out. Answers to questions:

The input with the 200K series resistor is the low input, and yes the 1K cathode resistor sets the bias of that stage.

Yes, you could add a master volume, but it will go between the coupling cap and the 500K resistor to ground.

We'd call the pre-effect amplification stage a "driver" over here.

The tremolo oscillator is sourcing its current through the cathode resistor of the second gain stage. This will modulate the voltage on the cathode of the gain stage and therefore the current through it at the trem frequency.

MPM

[Phil_S]

Will this OT work?
http://store.triodestore.com/tfchxfwi48oh.html
DocZ

Yes, it's a good choice. Price is good. I think shipping will be reasonable. Duty to get it into Norway...unknown, but how much could it be if based on price? Don't be too quick to replace it. You may not be measuring correctly. What you say it measures says it is blown. See about getting the tubes to glow first. This is probably the first thing to focus on.

I think Cornelius was joking, but don't put solder in your mouth. It's poison He's also correct about doing no mods until you have it fixed.

There is no problem disconnecting the high voltage secondary. Plug in all the tubes and see if they light up. Cornelius pointed out, you can't always see the filaments easily. Turn off the lights. Touch the tube to see if it's warm, don't get burnt.

[Structo]

If the terminal doesn't have a hole in it, you still want to tin the wire, then wrap it around the terminal tightly, then solder.

Remember to tightly twist the wire before you tin it, that way you don't have any stray strands hanging out that can short circuit something.

Just take a look at the amp builds some of these guys here do.
You will see a common theme in the neatness and quality soldering in them.
Lead dress ( the way the wires lay in the amp) is very important to keeping noise down in the amp.

You don't necessarily have to have nice 90 degree bends in the wires, that is more of a aesthetic thing just for beauty rather than function.

You have a great opportunity to learn on the amp you are repairing.

JUST BE CAREFUL!!!!

Lethal voltages and all that.
Do you know how to safely discharge the filter capacitors?

You do know they can retain a charge of voltage even when the amp is off and unplugged right?

[docz]

I spent a lot of time reading about how to safely discharge the caps, some say just short the + lead to the chassis with a screwdriver. My brother, he is an engineer, told me best way was to use a resistor, if I didn't have that I could use some metal that wasn't magnetic, just make sure to short it long enough for the caps to drain. And yes I know they can recharge, that is what they are designed to do isn't it?

I always measure them for voltage before doing anything inside the amp. But so far I have not gotten them to charge anything.. But after skimming through that NASA "thesis" on soldering I now know I have to re-solder everything I did. I'll get to that tonight.

When it comes to import duties, everything that is USD 34$ or less is duty free. If it is over it is added sales tax of 25% and a customs fee of about USD 60$.

About the tremolo, what I didn't understand (well I didn't understand how it worked either - so thanks for explaining that ) was where the signal was fed back into the chain. On the Reverb section - at least to me - it looks like the signal is fed back through the 100k resistor over the B1M pot. But on the tremolo the line is just cut off, and there is a dashed weird line shape between the switch and the pot.

I like that you guys think I have a cool project. I myself really enjoy working on it. I learn a lot every day, and I started thinking about new projects after this one - let's just say I'm hooked on electronics! Even though I don't understand all the complicated theory and physics, I am quite confident that when I devote enough time and get enough experience, I will get the hang of it.

I'll report back when hook up the heaters to check for glow.
I'll also go into some re-soldering to make everything connect properly and look prettier.

DocZ

[martin manning]

...But on the tremolo the line is just cut off, and there is a dashed weird line shape between the switch and the pot.

The dashed line is common notation for a mechanical connection between two or more components. In this case, the trem switch is mechanically activated by turning the pot shaft. Dual pots and multi-pole switches will be shown similarly.

MPM

[Cornelius]

Will this OT work?
http://store.triodestore.com/tfchxfwi48oh.html
DocZ

I think Cornelius was joking, but don't put solder in your mouth. It's poison He's also correct about doing no mods until you have it fixed.

If you're worried about lead poisoning, it hasn't been present in standard soldering tin here in Norway for years; now it's about 95% tin, 4% silver and 1% copper. (RoHS)

So no, i wasn't joking.
That said; if soldering are your job, chewing soldering tin all day would probably not be healthy, even without the lead.

But i'm not worried for that very small amount of time i use my mouth as a third hand.

[docz]

Ok guys just finished re-doing all my soldering.

Is it better than the first time?

[img:500:375]http://farm5.static.flickr.com/4021/423 ... 73393a.jpg[/img]
[img:375:500]http://farm5.static.flickr.com/4035/423 ... 305c3b.jpg[/img]

I also tried powering this thing on with just the heater wires connected, but still no glow.
[img768]http://farm5.static.flickr.com/4053/423 ... e0f7_b.jpg[/img]

So I thought I'd try out one of the old tubes, just to see.
[img768]http://farm5.static.flickr.com/4068/423 ... c26a_b.jpg[/img]
it lit up like a christmas tree!

I guess that russian guy screwed me on the replacement tubes I ordered...... Maybe they are the wrong kind? I don't see "12AX7" printed anywhere on them, I only see "OTK1" and "12-something-81", did I kill my amp by using the wrong kind of tube? The ad said "12AX7" arrg... just my luck...

Ok, guess I'll order a fresh set of those tubes, but where do I go from here? Can I keep on fixing using the old tubes? What will be the next step?

There is also a faint humming sound comming from either the rectifier or power tube - or both. Not from the speaker since it is unconnected right now, it comes from the tube. Is that normal?

DocZ

[docz]

I contacted the seller, he now tells me:

"Please Note! These tubes are not direct replacements for 12AX7, ECC83, etc. Filament pins schould be rewired for 6.3V
Pin 9 is the shield between two triodes."

Well.. that was not in the ad when I bought them.

How do I do this? Do I move the pin 9 to one of the two that are connected?

DocZ

[malt]

Hi, if you want tubes in Norway you can try demostenes he will help you http://www.demostenes.no/

[ampgeek]

Warning DocZ!! The amp shouldn't be operated without a load (speaker or artificial) if you have the power tube installed. It will seriously jeopardize your output transformer and power tube among other things.

Hang in there!

Dave O.

[Aurora]

Tubes in Norway.....
also look up Motron - Brynjar Kvanlid in the Yellow Pages...( he doesn't have a web page)
in addition to selling tubes and Hammond transformers, he's also a highly qualified tube tech.......

Best regards from way up north...

[docz]

Warning DocZ!! The amp shouldn't be operated without a load (speaker or artificial) if you have the power tube installed. It will seriously jeopardize your output transformer and power tube among other things.

Hang in there!

Dave O.

Thank you for the warning, I had disconnected everything from the power transformer, except for the filament heaters. So basically I was just using all the tubes as light bulbs. I was told this was ok.

DocZ