Getting Started Help Required

[hdave]

Hi,
I am a player and Electronic Technician interested in starting up servicing, mods, etc.  I have tried to find someone locally who might let me "observe" for a few days and learn a bit about typical service issues, required equipment, parts that I should have on hand etc.  So far no luck, if you are somewhere close to the Barrie (Central Ontario Canada area) and can help I would appreciate hearing from you.

Any other advice on setting up from those of you farther away would also be appreciated!
Thanks,
hdave  :D

[Rich]

Wouldn't hurt to read the reference section; http://www.el34world.com/schematics.htm#Mods , http://www.el34world.com/schematics.htm.

Lots of mod info and reference how tos.

[tubesornothing]

Since you are an electronic tech, I am sure you have lots of the equipment, but here is a list anyways:

Required:
- Meter to 1000V (analog is just fine and sometimes better than a DMM)
- soldering iron
- chopstick
- alligator clips
- books on amp repair (weber comes to mind)
- good solder sucker
- lightbulb current limiter or variac
- circuit breaker jumper (see doug's library)

Optional, but useful:
- dummy load
- low distortion signal generator
- old, cheap dual channel scope (but still good condition)
- capacitor and resistor decade boxes (these are especially good if someone wants to modify and amp)
- Doug has a circuit for a signal injector



As for parts:
- since you are close to thetubestore, You might not want to stock tubes, but here is a popular list:
   - 12ax7, 12at7
   - matched 6L6GC, 6V6, EL34 and EL84

As for the small parts, you might be better to order as needed, then get a few extras as you go along.  Since you are close to so many electronic outlets in Barrie, I am sure you can get next day service pretty inexpensively.  Doug has fast mailing to Canada.

- 1/2W MF resistors (carbon comp are good too) - various sizes
- 500V pico mica caps - various sizes
- 500V poly film caps - various sizes
- 500V electrolytic caps

Don't bother to stock transformers, speakers or other big stuff - it just doesn't blow all that often.

Make sure you brush up on your high voltage circuit precautions.

ToN
 

[hdave]

Hey guys, thanks for the suggestions espcially the list of stuff ToN.
I have not used my electronics for a number of years and hope it's like riding a bike to some degree.  I've read a couple of the books including the Aspen Pitman.  I'm told I should buy TUT1 by Kevin O'Connor.  Also Doug may have a video series he did?
Probably dumb, but I'll be damned if I can remember the best way to bleed the caps.  Can they all be done at once, or do you do them one at a time.
I seem to recall using an alliator clip with a higer value resistor (value?) and touching this to ground till the cap is discharged.
I guess one of those little plug in gizmos to read current value when biasing would also likely be a good tool.
I could look for a scope and signal generator after I get going I suppose.  I have been told about the tubestore but that is a good tip.
What kind of rates do techs typically charge?
You sound like you know Barrie ToN, are you here in Ontario?

[jhadhar65]

Here's a good place to start:

http://www.hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&thispage=parts13.htm&ORDER_ID=!ORDERID!

I'd say a good DVD these days is almost as good as working with someone.  I guess it is anyway.  I've never actually bought one, but they didn't have DVD when I started, just huge video discs.  That was before I even had my first VCR, but it doesn't matter since nothing "tube amp instructional" was available then anyway.  I also never really had anyone to work with, either, but if I had to do it over again now, I'd certainly reach for a good starter DVD.

[hdave]

Thanks,
I was contemplating the DVD and books already but I just put through an order.

[tubesornothing]

TUT1 is an OK book (although a little disorganized) - but all books have there good points.  For me I just end up collecting a lot of them.  There are books that are more about designing and building your own amps versus repair.  Look at the table of contents of books and it should give you an idea.

There was a recent thread on bleeding caps.  There is the right way and then my way, so you'd be better off finding the thread.

Check out Doug's library for building your own bias checker - yes that is an absolute must tool. Also read up on the 456 ways of biasing amps just so you know the lay of the land (check out aiken amps tech info).

As for rates, ask at various the music stores what local techs charge.  Out here we see from $45-60/hr CDN.

I am on the west coast, and the tubestore is the only Canadian place that stocks a lot of guitar tubes, so I deal with them often.

If you can afford to be picky, avoid PCB amps.  They are a *&%$#* pain.

[hdave]

Thanks again ToN,
Appreciate your assistance.  Not a lot of other "weigh in's"going on here.  We Canucks are sticking together I guess.
It's all a little intimidating even with an electronics background.  Obviously there are a lot of do's and don'ts as well as a lot of tricks learned with experience, just like with most things.
I was hoping to get up and running fast as my main business is really, really slo right now but I guess it's going to take a bit more time to try to absorb some more materials first.
I did not see a real clear answer on the bleeding caps thread but I'll look again, maybe I missed something or there is a clearer post there now.  I do believe there was one saying to bleed the whole circuit off pin 1?, but what if there is an open somewhere?  I don't think things are going to bleed then.
Thanks again!

[tubesornothing]

A super easy way to get started is just by re-tubing and biasing amps.  $40-$50 per amp + tubes.  That way you can start getting some money to fund the book collection.  Also when an amp comes in for biasing, compare it to a schematic, see how it is put together.  Start to become familiar with common circuits.  Search or google on the amp to see what other people are doing to the amp or what troubles they are running into.  A good one to look at is the fender blues junior.  Nice little amp, but does not have an adjustable bias, so must be adjusted by getting the right rating of tube.

Then as amps come in for other repairs, do what we all do here: search the archives & books for a similar problem  - if you cannot find it, post a note. 

For bleeding caps you can use a 10k resistor with an alligator clip.  Touch the resistor to B1, B2, etc.  (or to the positive side of the electrolytics).  Double check with a meter.

[hdave]

ToN,
Thanks aagain.  Some good thoughts!  My last post may have sounded like I missed the posts by jhadhar and Rich but that is not the case.  Thanks also to them, much appreciated !  Just felt like ToN and I were going back and forth a fair bit here which of course is also a positive thing!
Always nice to know there are others out there who want to try to lend a helping hand!
Happy Holidays to all by the way!
hdave

[2twang2]

And when you're discharging those caps (or doing anything inside an amp with power on) please remember to keep the other hand behind your back.

[jjasilli]

I ditto the above suggestions.  But seeing as how you're not a novice, get a traditional Champ kit and build it.  Or, buy an old Fender Silver face Champ and rebuild it.  New Power Supply; maybe gain mods.  Possibly buy the Torres Champ mods; it's fast, clear and easy.  Anyone could follow those instructions, and then you won't need them anymore.  If you run into trouble it will be easy to get help on this among other Forums. 

The Champ is real easy to work on; there's darn near nothing in there, and it sounds great.  Once you have done that, guitar amps & schematics will make complete sense to you.  That's just about all you need to know, except for modern hi-gain amps with relays, bells and whistles.

[hdave]

ToN had suggested that I get started by doing re-tube and Biasing on amps.  A great suggestion, but I know that not all amps need to be bised, or at least are not set up for this operation.  So I don't look foolish taking in amps for this process that I shouldn't, is there a list somewhere available that tells me what amps are the ones I should be taking in?

I just got the Weber books and DVD delivered that I ordered here.  They look pretty good so far and I have already started to fill in some gaps and grey areas in my knowledge.

Thanks to Doug for the quick fill and delivery of my order!
 :)
Dave

[tubesornothing]

I have not seen a list, you just need to look at the schematics.

In general, if an amp has a cathode biased power stage it will not need biasing.  You can tell cathode biased by looking at the schematic - the cathodes of the power tubes will be connected to ground via a resistor and bypass cap.   

If an amp has fixed bias power stage it will require biasing.  You can tell a fixed bias amp by looking at the cathode and seeing it is grounded.  Then look at the power supply to the grids and you will see the grids are supplied with a negative voltage.  Now here is the problem:  not all fixed bias supplies have an adjustment pot for the bias current. (Blues Junior is a good example).  BUT you can still bias them by hand selecting a tube that gets the current close to the 70% mark.

Take a look at a bunch of schematics and it will all start to come clear.

[hdave]

Sounds like one needs to have  a few tubes on hand.  Thanks for the info ToN.  Sounds like experience is the guide again like most other things in life..

[tubesornothing]

You are so close to the tube store, I bet you can get them overnight.  But it is only required for those few amps that are fixed bias without an adjustment - that may not be a lot...

[Greasehorse]

For discharging filter caps, you can take a DVM type probe end, open it up, and solder a 10k 1/2 or 1 watt resistor to the tip and solder a length of wire (10" or so) to the other resistor end. Then solder and an alligator clip to the loose wire end. Clip the alligator clip onto the chassis/ground and tough the probe to the + side of the cap lead and hold it a few seconds on each cap. Then, check them with a meter to make sure they are discharged...use one hand on tube amps because the shock from one hand to the other goes through the heart and a real hot one will burn a path between the places you touch...through the heart also.

[PRR]

> So I don't look foolish taking in amps for this process that I shouldn't

What's the problem?

I have been invoiced for "Alignment" on cars which had no adjustment. One time they argued that "check" and "set" alignment were the same price (it has to be set-up on the same rack), and I paid. Another time I knew (I watched) that it had never been on the rack, and they admitted a "mistake" (ha!).

The average car owner would never know.

The average amp owner would never know.

As in that first case, even if there is no adjustment, it is occasionally useful to "check". The car was very old/tired and unknown history. It could be bent or wobbly. Although there was no routine adjustment, if it were way-off, I'd know that some non-routine repairs might be needed. New rubber biscuits, frame-bending, whatever had gone wrong. Stage amps have a rough life; it is not "foolish" to check bias any time it comes to your bench.

Check the bias. Unless you suspect it "was in a crash", you don't need to know what the design bias IS, you check for "reasonable". Voltage times current must not exceed the rated dissipation. And it probably should not be less than half of rating either. Two sides of push-pull should not be grossly different.

If one tube of a pair seems "off", look for a bias adjustment. No adjustment? Swap the tubes. Does the problem follow the tube, or stay with the socket? If it stays with the tube, try your spare tube.

A Bias-Rite or similar is essential to this service. Otherwise you must pull the chassis to probe.

If you are very lazy, use an IR thermometer to shoot the tubes. 250F to 400F and similar across a pair is likely acceptable.

On most amps, the set-up to "check" is bigger than the labor to "set". Unless you bill strictly by the minute, you charge a flat-fee for a general service, which may take 2 minutes or 20 minutes. You charge an average price because that is easier than strict timing. So you pick your "bias service" price to cover a lot of checks and a few adjusts.

You might have an "SE bias" price for true Champs and Juniors. There's only one big tube, no adjustment, and the only likely failures are catastrophic. You can put your trained hand next to an SE 6V6 and know if it is OK. And if it makes several-Watt sound and does not glow in the dark, it is good to go.