Help! I tried to recap my amp!

[docz]

Ok, here's the story. I dug up an old Teisco 10 amp from the basement. It was my father's old amp from his glory days, and I thought I'd put it to use. When I tried to power it up, it worked fine for a while, then it started to get a lot of static and bad sounds. Some wise men on the net told me to change tubes and caps. So I change three caps two 33uF 450V and one 10uF 350V, when I powered it up everything looked allright, but no sound. And no glow in the 12AX7 tubes.

So then I took this as an opportunity to learn more about how all this works, and I decided that repairing/restoring this old amp would be a good place to start learning. So a couple of weeks later after a whole lot of googling, ebaying, and asking questions here I am. A bit wiser, but with a lot more questions!

So far I've located one broken resistor, 80k between the ground and center pin on the Reverb potentiometer. One broken cap 33uF 10V between the center of one 12AX7 and ground.

I started a thread over on the My Les Paul forums, and I've gotten a lot of advice, but they refered me here to you guys.

Here are some pictures, and schematic:
[img747]http://farm5.static.flickr.com/4038/419 ... 9933_b.jpg[/img]
[img486]http://farm3.static.flickr.com/2726/408 ... f597_b.jpg[/img][img768]http://farm3.static.flickr.com/2587/422 ... 64b5_b.jpg[/img]

So far I have a lot of questions:

1. Where do I start trouble shooting?
2. Should I replace the remaining electrolytic caps?
3. Can I change 10V and 16V caps with 25V caps?
4. Is it possible for me to add a "GAIN" knob to get a crunc sound out of it?
5. Where can I find a general explanation of each of the elements that make up this kind of tube amplifier, that describes the purpose of all the components?

Merry Christmas

Doc-Z

[M Fowler]

The jack with the two resistors is the high input.

Solder the resistor back there seems to be enough lead left to do that. Resolder the broken cap unless it needs to be replaced and that one looks to be 600v cap.

Yes you can replace the other noted caps with 25v caps but get the amp working before going further.

110v Primary and secondary is 260v on that power transformer and has 6.3v filament supply. Are you handy with a DMM? Also you know to always have a speaker connected when powering a tube amp up right?

Mark

[M Fowler]

Yes those are turret strips that have a lug going to ground bolted to the chassis.

The blue and white twisted wires come from the 6.3 or 3.15-3.15AC voltage off the power transformer to light up the filament heaters for the tubes. The pilot has its own supply per your chassis gut shot.

Since its hard to tell from pictures and this is a point to point circuit and no spaghetti wire covering was used over the capacitor and resistor leads check to see that they are not shorting out against the chassis, lugs or other unwanted bare wire areas. Naturally some join together at the same terminals but make sure they are not shorting out.

Your tubes more than likely are bad do you have replacements?

[tribi9]

I'd start off by....



resoldering that 33 uF broken cap first to see if I get any sound.

I'd also remove that .022 cap from the switch.

check that all my grounds are good

Last but not least that I'd check that the speaker is plugged in.

If still no sound I'd go around with a chopstick to check for any loose connections, cold solders etc. Also do a visual see if any components look burned or damaged.

You can swap the 10V and 16V for a 25 V. As long as you're over you should be good.

You can swap the coupling caps but if they don't sound bad I wouldn't.

(Sorry I myself just started learning so if I'm wrong maybe somebody can chime in and correct me

Oh and don't forget to drain the caps before you start poking around. Caps can store lethal charges even when the amp is unplugged. Safety comes first.

Edit: It took me a bit to type this but I see there are some replies now. So yeah just do as they say. They know what they're talking about.

[docz]

Thank you for your replies.

All tubes were replaced with new ones. I also tried swapping back to the old ones, but still the same result. The broken resistor is a bit of a mystery - I can't seem to find it anywhere in the schematic, so I'm not quite sure where it goes. The same goes for the big caps, the schematic says 40/10/10 but the ones in the amp are 33/33/10 why is that? And what the heck are the resistors marked with "L"'s like "1/2L100k" or "1L1k" and what does the "1/2" mean? half resistors?
Why do I need to remove the .022uF cap from the switch?

The speaker is plugged in, it is soldered to the output transformer.

The coupling caps seem to all be electrolytic, I was under the impression that all electrolytic caps over 10 years should be replaced, but is that not the case?

Doc-Z

[rfgordon]

I rebuilt a Teisco Checkmate 25 for a client a few months ago. Though they are considered budget amps from days of yore, they can be built into really nice tone machines.

Check to make sure you've got good filament voltage on all sockets. If not, that's why your tubes don't operate. Be sure the filter caps are grounded. Check B+ voltage on all plate connections, along with cathode voltages.

Teisco used paper in oil signal caps, and it's likely that all of them in your amp leak significant DC, so they probably all should go. If the caps in the Tremolo circuit leak, the trem won't work.

This amp, as built, only has two gain stages before the power tube, so it's kinda like a Champ with Tremolo and the world's wonkiest reverb tank.

[Phil_S]

Some wise men on the net told me to change tubes...

Others seem to be giving you lots of good advice. I didn't see anyone address the tubes. Tubes don't go bad in storage. The get used up with many hours of use. It is quite possible some, maybe all the old tubes are still OK. Don't be so quick to discard them. In some cases, old used tubes are better than new tubes you can buy today. Not always, but they don't make 'em like they used to, so hang onto them until you are sure they should be discarded.

Maybe you'll get into this later. First, though, you need a set of known good tubes to make sure the repairs you are making fix the amp.

Good luck.

Phil

[docz]

Check B+ voltage on all plate connections, along with cathode voltages.

Teisco used paper in oil signal caps, and it's likely that all of them in your amp leak significant DC, so they probably all should go. If the caps in the Tremolo circuit leak, the trem won't work.

This amp, as built, only has two gain stages before the power tube, so it's kinda like a Champ with Tremolo and the world's wonkiest reverb tank.

This is probably a string of silly questions, but what is B+? and what is B-? What is B? And how do I meassure the voltage on B+?

Ok, so all non-electrolytic caps should go as well... time to ebay, do I replace them with other type like ceramics? Or will I need PIO's? Is there a way to test them? Or should I just rip 'em out of there in cold blood?

Is it possible to modify so that the two gain stages uses separate volume controls? Or swap that honky reverb for an extra gain stage? Would love to get this thing to crunch at a tolerable volume, when I tested it out before I started working on it, I discovered it was loud - in fact - ridiculosly loud! My behringer B212A active PA 12" cabinet could infact not compete with it, without hurting the speaker.

Well thanks again for your help guys. I'll get to the checking, as soon as I get a new 33uF 10V cap to replace the broken one, and I know how to check what I am supposed to check

Cheers!
Doc-Z

[Structo]

NO!
Don't replace all the caps.
Not all the caps in there are electrolytic.
Usually the only place you find them is in the power supply and on the cathodes of the preamp tubes if they are bypassed.
Then they are called bypass caps.
If the amp is cathode biased on the power tubes, then there will most likely be an electrolytic cap there as well.

You do want to replace all the electrolytic caps.
These are pretty easy to identify, sine they usually have a + and - on them.

The L designation is for Linear, pertaining to a pot's taper.
There are audio taper and linear taper.
Audio taper pots were designed because of the way the human ear hears volume, so it works better for volume controls and such.

You should try googling a lot of your questions as there is always an answer to your questions online.
I would suggest that you get some books on tube amps.
There is a thread somewhere here that lists a lot of books to buy.

Point to point amps such as yours can be a nightmare for the beginner to work on.
Many times the amp is modified at the factory because they found a better way to do something.
Therefore often times they are not true to the schematic.

Before you do much more on that amp, take lots of closeup pictures so if you forget where something goes you will have something to go by.
Use the Macro setting on your camera to get blur free closeups.

B voltage or B+ voltage is the high voltage for the tubes plates.
The letter B is leftover from when they used to power amps with batteries.

Many times the coupling caps are a film type cap and a lot of the old ones are prized for their tone, so don't go ripping any out until you are sure it is too leaky to work well.

Regarding electrolytic and high voltage caps.
You can always exceed the voltage rating in the amp.
Never go below what is specified for the amp.
Frequently old amps have a cap with a different capacitance (uf) than is currently available today.
So you want to get as close to the original value but not below what is called for.
But you don't want to exceed the value by too much either.

#1 in working on a tube amp is to be aware of lethal voltages inside.
You must know what you are doing or you can kill yourself.
Learn how to safely discharge the filter caps.
Remember, the amp doesn't have to be plugged in to have a lethal shock stored in the caps.
Many amps have power supplies that will store a charge in the caps for a long time.
So learn how to get rid of that voltage before sticking your hands inside the amp.

[docz]

Thank you for your reply. The L's are definately not potentiometers, the potentiometers are clearly marked with arrows in the schematic as you can see. How can I check if the PIO caps are leaky?

And why should I remove that .022uF 1000WV cap between the switch and the 110V terminal on the transformer? What is it's purpose?

Google is my friend, but the problem is that I often have no clue whatsoever to search for. But be sure that along side writing on this forum, I am eagerly googling, wikipediaing, reading and studying all this stuff. But it is a very complex thing to understand, at least for me as a total newbie.

Doc-Z

[MBD115]

The schematic looks simple. The gut shot looks terrible . This is not an easy amp to work on. The way those circuits are laid out (not!!!) is going to make it pretty difficult to trace.

Look a your schematic and the gut shot and try to trace out the circuits. This alone is going to teach you much. You will then spot the mods. Once found they can be figured out what they do.

Do a complete voltage check. Write down what you find and where. Check the voltages on each of the tube sockets and write that down. This is very important.

By looking at schematics and pics of other amps you can learn too. You learn what a typical circuit looks like and how its usually wired. You can then apply that knowledge to your circuits which will help you to understand them.

Be very careful. Tube amps contain very high voltages and are dangerous to work on. Learn where the high voltage is and how long it stays on the circuits after the amp is shut down. Shut it down with the standby switch still on should help kill the volts, but even then Check for high voltage before sticking your hands back down in there. EveryTime!!!!!! this is very important.

And good luck (by the look of that gut shot - you're going to need it)

[badtweed]

Clipped...

Teisco used paper in oil signal caps, and it's likely that all of them in your amp leak significant DC, so they probably all should go. If the caps in the Tremolo circuit leak, the trem won't work.

clipped...

Quoted for agreement. I've rebuilt/repaired a number of 60's and 70's guitar tube amps from Japan such as Univox and Teisco and others that used the paper in oil caps and most needed to be replaced due to dc leakage. Total pita but necessary.

[Richie]

Quoted for agreement. I've rebuilt/repaired a number of 60's and 70's guitar tube amps from Japan such as Univox and Teisco and others that used the paper in oil caps and most needed to be replaced due to dc leakage. Total pita but necessary.

i'd agree with that also,and mostly the ones using those gray caps.

[ampgeek]

Looks like a cool project! I love re-habbing old amps.

A quick heads up on the schematic: It indicates that B+ node 2 (power tube g2-screen grid) is connected directly to ground. I can't believe that is correct...but...hoping another set of eyes can check and confirm.

Also, I haven't had as much bad luck with old coupling caps (PIO or other) as others have. I suggest saving replacement of those (as needed) for the end of the re-hab process to avoid confusion and frustration.

Good luck!
Dave O.

[docz]

Thanks for all the input guys!

More questions!

The broken cap went from the center of the 12AX7 (the metal pin in the middle) and then a small wire went from the middle to pin 3 on the 12AX7 tube(Cathode 2 I believe?). I can't find anything that that middle pin might connect to when I look at the tube, is this just used as a terminal for the cap, resistor and pin 3?

What does that resistor and cap actually do? Is that some kind of bypass?

Can I test the amp without that cap in there?

The broken resistor seem to have been connected to the center terminal of the reverb pot and ground, is that to get a custom range on that pot? The pot is 1M and the resistor is 80KOhm.

I'm not sure if pin 9 of the EL84 is connected to ground, it seems to be connected to a number of resistors, caps and then to the 12AX7 tubes.

Do you guys think it would be possible to power up a 4x12 or 4x10 cabinet with this thing?

What are some similar amps this thing might be "modeled" after?


Cheers

DocZ