Great thread with lots of really good info that has now made its way in to the FAQ's stickied thread. 
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Learning amp repair
- Thread starter JimmyM
- Start date
Don't get all stuck up on us now you're famous Jimmy!
The book by Jack Darr is called the Electric Guitar Amplifier Handbook. First published in 1965 by Sams. My latest copy is from the 1971 printing. It has schematics for a lot of US made amplifiers both tube and SS including your B15's. The SS is out if date but the same servicing principles are still full applicable today.
If you PM me your mailing address and (trying to do a little good in these trying times) make a $10 donation to a charity or food bank of your choice, I'll mail you my copy of the book at my cost.
Paul
Wow Paul, that's really nice of you and I thank you for the offer, but I don't want to take your book if it's your only copy.
There's nothing like a bit of hands on training / mentoring / apprenticeship, if you can get it going. One reason is that working with someone will let you learn how to use some of the tools such as an oscilloscope, before you have to invest in those things for yourself.
One thing to know is that design and repair are different beasts. I am a good designer, but an experienced repairman can run circles around me, in terms of coming up with the correct diagnosis on the first try.
One thing to know is that design and repair are different beasts. I am a good designer, but an experienced repairman can run circles around me, in terms of coming up with the correct diagnosis on the first try.
Really? Why would that be? I'd think if you could design them, troubleshooting them would be a breeze.
Design and troubleshooting are completely different arts.
Getting good at repair requires a mind that is focused on pattern recognition. The repair world celebrates speed and high percentages getting it fixed the first time. Most equipment fails in very predictable ways, which a tech gets better at predicting with heavy experience.
Design takes a different mind, one that is great at balancing trade-offs and then weighing the trade offs against the idealized end result.
Both are higher order skills.
Getting good at repair requires a mind that is focused on pattern recognition. The repair world celebrates speed and high percentages getting it fixed the first time. Most equipment fails in very predictable ways, which a tech gets better at predicting with heavy experience.
Design takes a different mind, one that is great at balancing trade-offs and then weighing the trade offs against the idealized end result.
Both are higher order skills.
Here's a couple of links that you might want to check out:
http://www.amazon.com/How-Service-Your-Own-Tube/dp/0962817007
Gerald Weber DVD plus others:
[Invalid or Expired Link Removed]
Weber does amp repair seminars.
[Invalid or Expired Link Removed]
http://www.amazon.com/How-Service-Your-Own-Tube/dp/0962817007
Gerald Weber DVD plus others:
[Invalid or Expired Link Removed]
Weber does amp repair seminars.
[Invalid or Expired Link Removed]
As an Amazon Associate, TalkBass may receive commissions from qualifying purchases made via links on this site.
Bruce Egnater was doing regular seminars, but you would have to fly to his shop. He was doing like weekend amp seminars where you end up building your own amp with him guiding. I would love to do such a thing myself, but it will likely never happen.
have a fender bassman 1200 pro head amp, will turn on, no output,, opened it up and see a blown resister at r230 but is burn beyond recognizing the colors on it, as far as I can tel they are red,black,green gold but would like to know if anyone knows for sure. cant seem to find the schimatic for this. can anyone help?
R230 on the two schematics I have found show a 10ohm, 1/4 watt resistor. On the schem, sector B7/B6.5 towards the bottom with the grounds.
Schem download page
Other schem
4 Bar code colors for a 10ohm resistor would be BROWN/BLACK/BLACK and then GOLD for 5% and Silver for 10% tolerances.
If your colors are correct on the resistor that you pulled, then it would be a 2M resistor and incorrect according to the schems above.
Schem download page
Other schem
4 Bar code colors for a 10ohm resistor would be BROWN/BLACK/BLACK and then GOLD for 5% and Silver for 10% tolerances.
If your colors are correct on the resistor that you pulled, then it would be a 2M resistor and incorrect according to the schems above.
There's a cat local to me & you Jimmy. He makes these 3way leslie cabs. Often, he parts out old hammond organs, and sells the all tube power amps for next to nothing. Search "hammonator" on craigslist. You'll find him. It would be a great amp to experiment on, I would think, and very low cost. Pretty sure he's at the monthly swap meet in Mt. Dora too.
Mr. Foxen
Commercial User
Chances are it is something else that has made the resistor die by pulling too much current through it, see what it is supplying and check there.
This is something I've seen mentioned from time to time. Can anyone elaborate on this? It TERRIFIES me!
I just bought a used tube amp last night, then took it to practice and that's when it's issues came to light. I'd like to learn how to repair it myself - but I don't want to kill myself doing so!
It's not something I'm going to jump into in the coming days - but hopefully in the coming months. First I'm going to get some soldering experience by building some kits (drum machine, etc).
Is there a specific part of the amp that would kill me, like, say, the transformers? If I stay away from that part, should I be (mostly) safe?
Given the issues that cropped up I was thinking of re-wiring the amp, and possibly replacing the potentiometers (those are the tone/volume knobs, right? clueless!) and inputs/outputs. I was thinking I'd replace each wire one at a time so that I don't get lost - is this an acceptable way to re-wire an amp?
I'm going to explore the links posted in this thread and buy at least one book...and possibly take a class on building electronic instruments...but I just wanted to throw a few questions out there to see if I could get some quick answers to some of my more immediate questions. Thanks in advance!
Mr. Foxen
Commercial User
The bit that kills you is a weak heart. voltage in itself isn't a big deal, its current that does the damage, high voltage just makes it go through you more readily, ever got zapped when you touch your car, that is really high voltage getting you, not enough current to do much though. The capacitors are the bit that gets you if you are not cautious, they store charge when the amp is unplugged, so you aren't safe just because its off at the wall. If you are healthy and not in a place you can fall off of, discharging a cap into your finger is likely more educational than fatal. I've had it a few times in amps where people have got creative with their DIY wiring, I made a discharge tool, to drain the caps safely, but the same degree of caution applies to using it, just means you can be sloppy without it mattering later.
The power supply (transformer to rectifier to filters) are the dangerous area. The power supply caps are large metal, paper or in old cases wax coated cylinders. If a tube amp, voltages present can be from 200-750 volts and energy is "stored" in these like short term high energy (current capable) "batteries". Most designs include "bleeder" resistors (that also act as a filter) to discharge this stored energy safely but you can NEVER take for granted they are there or working properly.
The transformers are dangerous when the amp is connected to the mains or powered on.
Rewiring an amp is in most cases not needed unless burned or insulation has failed. Wire dress is very important and if not fallowed exactly or incorrect original dress not corrected noise will be injected into gain stages.
Yes "pots" are the controls behind the knobs.
Power supply caps can be drained safely if you know the proper techniques and fallow safety precautions.
The transformers are dangerous when the amp is connected to the mains or powered on.
Rewiring an amp is in most cases not needed unless burned or insulation has failed. Wire dress is very important and if not fallowed exactly or incorrect original dress not corrected noise will be injected into gain stages.
Yes "pots" are the controls behind the knobs.
Power supply caps can be drained safely if you know the proper techniques and fallow safety precautions.
Search up bleeding or draining capacitors. Simple wire rig with a resistor in it, to drain the caps slowly to ground instead of all in one ahot.
And yes, never count on bleeder resistors, always assume they are charged no matter how long it's been sitting and always keep one hand in uour pocket while poking around in there, at least until you've verified it's "dead" at least twice.
And yes, never count on bleeder resistors, always assume they are charged no matter how long it's been sitting and always keep one hand in uour pocket while poking around in there, at least until you've verified it's "dead" at least twice.
Thanks for the info, guys! I'm "this" close to signing up for a class on building electronic instruments such as theremins, mono-synths, fuzz pedals, etc. I figure the instructor should be able to show me how to bleed my caps safely.
I'm both a little bummed and a little excited that my "new" amp has some issues. Of course I want to be playing it now (and I am, but not without frustration), but it should be fun and rewarding to learn how to get it ship shape.
I'm both a little bummed and a little excited that my "new" amp has some issues. Of course I want to be playing it now (and I am, but not without frustration), but it should be fun and rewarding to learn how to get it ship shape.
Those will all be low voltage gadgets.
You can get a low voltage power supply, or battery, and never get near high voltage or line. High voltage, deadly, you get zapped, but line voltage, worst, you get electrocuted with constantly flowing current. Some DIY forums will not even discuss line power supplies.
I doubt you'll find a school that lets you get above low voltage in the beginner classes.
Still fun. And maybe you'll find you can build a good fuzzbox/distortion/compressor that gives you all the tone you were ever looking for.
