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FDP Forum / Amp Mods, Repairs, and Projects / Variacs, what are they for as applied to Guitar amps?
(This message was last edited by amphead4 at 12:40 PM, Sep 9th, 2006)
FDP Forum / Amp Mods, Repairs, and Projects / Variacs, what are they for as applied to Guitar amps?
Next 20 Messages
Jeano
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USASep 9th, 2006 08:39 AM Edit Profile Print Topic Search
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I'm going to start working a project to rebuild a Valve Junior Head, replacing transistors, caps and stuff.....Purely a newbie in this area. Am currently reviewing safety recommendations and gathering information and parts/equipment. To start up a rebuilt amp do I need a variac? If so what type, voltage amps and size? Thanks Jean
jazzguitar
Contributing Member
Delicious tone !
Looking for the perfect jobSep 9th, 2006 09:04 AM Edit Profile Print Topic Search
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Forget about that. They are heavy, dangerous things best kept to where they belong to: electronic labs.
amphead4
Contributing Member
Cincinnati, USASep 9th, 2006 09:04 AM Edit Profile Print Topic Search
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A variac can be used to soft start so electrolytic caps can form slowly but is more useful with amps that have solid state rectifiers.
It can also be used to run the amp at a very low voltage to help find shorts in the power supply.
If you have a very old amp designed to run on say 110 vac and the voltage at your house pretty high, say 125, you can also dial in a lower voltage to baby the amp. This generally is not necessary if the caps are up to snuff.
You can do a whole lot of amp repair without a variac. A cheap but effective tool for soft starting/current limiting is a lamp with a 100 watt bulb in series with power outlet that supplies the amp under test. You can build a rig for this for $10 by using a 4x4 box, power cord, duplex outlet (with the jumper tab broken off) and combo outlet light switch for the other duplex side. This is cheap compared to paying retail for a variac and if gives you a couple more outlets for you bench too.
I have a variac and but use the light bult current limiter more often. The first time I plug in an unknown amp, it gets connected to the current limiter just in case it has a bad short in it. (Also check the fuse for proper value too.) This tool has been discussed here a lot - search for more info. It's also covered in one of Weber's book with details on why you add the switch.
BTW, you don't run the amp with the current limiter very long if the tubes are in because it annoys the tubes to be run on a low voltage.
willie
Contributing Member
Walton County, GA.
Guitar and amp servicing since 1962Sep 9th, 2006 09:23 AM Edit Profile Print Topic Search
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What amphead4 said...we use one on the bench that sets the line voltage standard and also reads the current drawn by the amp or other device plugged into it.
willie
musicmenders.com
Leftee
Contributing Member
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NC
Hatching the Mother of all SuggestionsSep 9th, 2006 09:30 AM Edit Profile Print Topic Search
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I've got one and rarely use it.
Jeano
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USASep 9th, 2006 09:45 AM Edit Profile Print Topic Search
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Amphead, is there a simple diagram, or can you explain how to build the light bulb soft start tool. I know how to wire a light bulb to a switch, but my mind is fuddled on what you are describing. I'm sure I have all the stuff I need. Jean
Jeano
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USASep 9th, 2006 10:08 AM Edit Profile Print Topic Search
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Sorry I did a search and cant find anything on Current limiter/light bulb limiter or anything like that. I get an idea of what is necessary material wise to build the limiter but my mind is messed up on the exact wiring.
Billm
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New Jersey, USA
Hey! What exit?Sep 9th, 2006 12:04 PM Edit Profile Print Topic Search
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> replacing transistors, caps and stuff
Um, no transistors in a Valve Junior. Don't worry so much about Variacs and light bulbs. Instead, get a good book on basic tube amp electronics like the Dave Funk Tube Amp Workbook, read the Jack Darr service book. As long as you can tell plus from minus on a filter capacitor, you won't have a problem.
Were you planning on the DC filament mod? The capacitor polarity is important there, but the directions are pretty explicit.
Leftee
Contributing Member
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NC
Hatching the Mother of all SuggestionsSep 9th, 2006 12:10 PM Edit Profile Print Topic Search
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VJ head has DC fils already. Those amps are dead quite all the way up.
amphead4
Contributing Member
Cincinnati, USASep 9th, 2006 12:37 PM Edit Profile Print Topic Search
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Search phrase: light bulb current limiter
I will describe. Make a sketch based on this. If it doesn't make sense, get help from someone who can do house wiring.
Install all this in a metal 4x4 electrical box with a cover for two duplex outlets. There will be three outlets and one light switch in the box. A power cord comes out of one of the knockouts, through a romex clamp, so you can plug it in. The whole rig is like an extention cord with a metal box on the end, with three outlet and one switch.
________
[ O1 SW ]
[ O2 O3 ]=========={=
--------
How it works:
The lamp plugs into Outlet1. The amp under test plugs into O2. With the switch off, the lamp is in series with the amp and limits the current. If the lamp is bright, the amp is shorted but the fuse doesn't blow because the amp limits the current. If the lamp is very dim, the amp is drawing very little current and can be plugged in normally. Turn the switch on and O2 becomes a normal outlet with no current limiting. O1 quits working because both sides are hot and there is no neutral.
Outlet3 is never limited and can always be used normally.
=======================================
Materials:
14 gauge grounded power cord.
4x4 metal electrical box.
Cover fitting box that will accept two duplex outlet.
duplex outlet, 15A (duplex is like a normal outlet configuration with two outlets, one above the other).
combo switch/outlet, 15A, duplex configuration.
3/8" romex clamp.
14 gauga solid wire, THHN or similar. The stuff that comes out of romex is good.
maybe a wire nut or two.
=====================================
Description of wiring:
Outlet3 is always hot. The White wire from the cord goes to O3 white and O2 white.
The black wire from the cord goes to O3 black, one side of the switch and O1 black.
A wire connects O1 white and O2 black.
The other side of the switch connects to O2 black.
This is important! The duplex outlet that makes up O1 and O2 will have two jumper tabs. One connects O1 and O2 blacks. The other connects O1 and O2 whites. Both of these tabs must be broken so that the two outlets can be wired independently.
==========================================
Here's a list of the connections. Make a sketch!
O1 black connects to the black power cord wire AND one side of the switch AND O3 black.
O1 white connects to O2 black AND the other side of the switch
O2 black connects to O1 white AND one side of the switch
O2 white connects to O3 white AND the white power cord wire.
O3 black connects to the black power cord wire AND one side of the switch and O1 black.
O3 white connects to the white power cord wire AND O2 white.
=========================================
Yes, I realize this is confusing without a sketch but I'm too lame to make one. Draw it out and make sure you understand that O3 is powered all the time, O2 is powered only when the switch is on and that O1 and O2 are in series when the switch is off.
If you have the ability to sketch it electronically, email it to me and I can review it before you build.
Dale
TeleThom
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Chicago, ILSep 9th, 2006 12:40 PM Edit Profile Print Topic Search
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As long as you have a decent DMM (Digital Multi-Meter) and know how to use it, with a good working knowledge of Ohm's Law in its variety of forms, you'll have most of the diagnostic tools you'll need.
For all tube amp projects, learn and know how to discharge filter capacitors (THE HI-VOLTAGE STUFF). There's tons of info here and elsewhere on the net to help you.
TeleThom
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Chicago, ILSep 9th, 2006 03:16 PM Edit Profile Print Topic Search
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Great post on a current limiter, Dale.
amphead4
Contributing Member
Cincinnati, USASep 9th, 2006 03:25 PM Edit Profile Print Topic Search
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It's long anyway. If people don't understand, I'll tweak the wording until it's better.
willie
Contributing Member
Walton County, GA.
Guitar and amp servicing since 1962Sep 9th, 2006 05:57 PM Edit Profile Print Topic Search
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Point of order here...there are tools and equipment that a home user or casual experimenter might need, and those that professional technician would use. A variable voltage transformer, isolation transformer that monitors current and voltage is highly beneficial if not essential to a pro. We see so many different types of gear...not just guitar amps mind you...and there are many situations where such gear is a tremendous help in our work. That said, we seldom form caps anymore, though we do upon occasion depending on the job at hand and/or the instructions from a customer.
When you toss amps for a living, you need all the help you can get. Thats reason enough unto itself. ;)
willie
musicmenders.com
Leftee
Contributing Member
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NC
Hatching the Mother of all SuggestionsSep 9th, 2006 06:16 PM Edit Profile Print Topic Search
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I'll bring up a new build on my variac. That's about it.
Forming modern production caps is unnecessary.
Jeano
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USASep 10th, 2006 11:05 AM Edit Profile Print Topic Search
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Amphead 4 (Dale): Believe it or not I do house wiring......Just couldn't get it together in my head. What a wonderful, concise and comprehensive lesson....it is clear as a bell to me now. I just have one more question... Can I use (would there be any benefit to using) a dimmer switch? You know, like wired to a light switch in a house? Your the greatest Man!
Jean
Jeano
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USASep 10th, 2006 11:14 AM Edit Profile Print Topic Search
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One more thing, the project is on a Valve Junior Combo that I converted to a head unit. I also have a Valve Junior Combo new in the box to use also. I have bought two Valve Junior upgrade kits from S2 Amplification and am using their upgrade kits, I think they cost me 29 bucks apiece. I am not going to start till I get all the questions out of the way to include (as stressed on this forum) Safety first of all, then I will take it one step at a time. I hope that this will open the door to a new hobby for me. Jean
amphead4
Contributing Member
Cincinnati, USASep 10th, 2006 12:45 PM Edit Profile Print Topic Search
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Don't use a dimmer on an amp!
Billm
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New Jersey, USA
Hey! What exit?Sep 10th, 2006 12:56 PM Edit Profile Print Topic Search
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Expanding...
Dimmers put out pulsating DC, not AC. That's a sure way to fry the output transformer. There are a few expensive dimmers (Leviton, etc.) that put out variable AC for fluorescents and for transformer-powered halogen lights, but they can't handle enough current to run most amps.
Not to be a nag, but if you've got the National Electrical Code under your belt, you can certainly learn enough basic electronics to mod amps and know what you're doing instead of replacing parts by rote. Get a couple of books. And get this whole feed-the-amp-lower-voltages thing out of your head.
Jeano
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USASep 10th, 2006 05:34 PM Edit Profile Print Topic Search
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Thanks guys,
Bill, I will get the books. Jack Darr and Dave Funk, I'll see if I can get them on the internet somewhere. I will get the lower voltages stuff out of my head. Is the National Electrical Code understandable??? It sounds really intimidating. I will study the books before I open an amp. I just hope I can understand the stuff without any formal electrical training.
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