It is currently Mon Mar 16, 2020 6:47 pm

All times are UTC - 7 hours



Post new topic Reply to topic  [ 39 posts ]  Go to page Previous  1, 2, 3
Go to page Previous  1, 2, 3
Author Message
Post subject: Re: Honest Tube amp repair guys ?
Posted: Thu Dec 13, 2012 7:42 am
Offline
Roadie
Roadie

Joined: Thu Feb 21, 2008 12:52 pm
Posts: 292
I am far from an amp guru like some of you on here. I work on them because I dig it and learn as I go. I have refurbed a dozen or so old Fender amps and have 6 of my own. When I do work on other peoples amps I usually take booze :lol: as a payment and of course cash for the parts. I love great tone and thin I can coax it out of the old Fenders. Its a great platform already so make it healthy and go from there. I have even woked on amps from owners I never met before. Sent them a PM and mentioned I would like to help them. It has always worked out. No tpatting myself on the back at all just pointing out that there are honest guys out there willing to help out.
BTW....much that I have learned were on sited like this from guys like you all willing to part with experience and information. I am just trying to keep that train moving so to speak


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: Honest Tube amp repair guys ?
Posted: Thu Dec 13, 2012 8:24 am
Offline
Professional Musician
Professional Musician
User avatar

Joined: Sun Aug 05, 2012 9:50 am
Posts: 1399
I'll ask a beginners question since my SCX2 the first tube amp I own and am sober enough to care about. Type A amps do not need biasing after tube replacement and type A/B amps do?

I do have a deluxe reverb that an ex-wife is holding hostage. The agreement is she can't sell it. She paid for it to be repaired and I will give her that money if she needs to get money for it. I do not wish such a situation upon her. This spring I'll get the serial number. I think it is a '65

_________________
I have some Fender gear.
(Telecaster, Jaguar, Villager 12 String. Mustang l, Bronco, SCX2, Rumble 350, HRDIII, Rumble 2x8 Neo Cab)
http://www.reverbnation.com/modwiz


Top
Profile
Post subject: Re: Honest Tube amp repair guys ?
Posted: Thu Dec 13, 2012 9:13 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
If the amp is cathode-biased (which 99% of Class "A" guitar amps are) --- you do not need to worry about biasing. The output tube will somewhat self-level, just like all the 12AX7 and 12AT7 front end tubes.


Top
Profile
Post subject: Re: Honest Tube amp repair guys ?
Posted: Thu Dec 13, 2012 10:03 pm
Offline
Professional Musician
Professional Musician
User avatar

Joined: Sun Aug 05, 2012 9:50 am
Posts: 1399
BMW2002Ti wrote:
If the amp is cathode-biased (which 99% of Class "A" guitar amps are) --- you do not need to worry about biasing. The output tube will somewhat self-level, just like all the 12AX7 and 12AT7 front end tubes.


A little more, if I may. The SCX2 is an AB amp that is solid state diode rectified. Will biasing be required when swapping out tubes. It is (1) 12ax7 and (2) 6v6's. Thanks for helping a tube noob.

_________________
I have some Fender gear.
(Telecaster, Jaguar, Villager 12 String. Mustang l, Bronco, SCX2, Rumble 350, HRDIII, Rumble 2x8 Neo Cab)
http://www.reverbnation.com/modwiz


Top
Profile
Post subject: Re: Honest Tube amp repair guys ?
Posted: Thu Dec 13, 2012 10:52 pm
Offline
Rock Star
Rock Star

Joined: Mon Apr 05, 2010 2:37 pm
Posts: 8708
Location: Natural Bridge, Virginia
modwiz wrote:
BMW2002Ti wrote:
If the amp is cathode-biased (which 99% of Class "A" guitar amps are) --- you do not need to worry about biasing. The output tube will somewhat self-level, just like all the 12AX7 and 12AT7 front end tubes.


A little more, if I may. The SCX2 is an AB amp that is solid state diode rectified. Will biasing be required when swapping out tubes. It is (1) 12ax7 and (2) 6v6's. Thanks for helping a tube noob.


Yes.

Assuming the power supply and power output stage of the SCX2 is the same as the SCXD, there will be a bias adjust pot and a test point with a 1 ohm resistor that gives the sum of both power tube's cathode current. You must also measure the plate voltage as you adjust the bias voltage to determine the idle plate dissipation and calculate the % of maximum plate dissipation. Shoot for 50% to 70% max plate dissipation.

http://support.fender.com/schematics/gu ... ematic.pdf

_________________
Bill

Image


Top
Profile
Post subject: Re: Honest Tube amp repair guys ?
Posted: Thu Dec 13, 2012 11:31 pm
Offline
Professional Musician
Professional Musician
User avatar

Joined: Sun Aug 05, 2012 9:50 am
Posts: 1399
I appreciate the answer Bluesky, and I downloaded the pdf. gave it a good look, got on my knees with a flashlight and peered inside. I guess being happy with how it sounds will have to do until I have a mentor. What you informed me of would have to translate into something a little more coherent for me. I am a medical massage therapist and detail and how things work is not a challenge for me. Electronics is another language and techno alphabet, so to speak. I guess certain measuring tools/equipment is part of the equation?

What are the TP numbered things?

_________________
I have some Fender gear.
(Telecaster, Jaguar, Villager 12 String. Mustang l, Bronco, SCX2, Rumble 350, HRDIII, Rumble 2x8 Neo Cab)
http://www.reverbnation.com/modwiz


Top
Profile
Post subject: Re: Honest Tube amp repair guys ?
Posted: Fri Dec 14, 2012 2:58 am
Offline
Professional Musician
Professional Musician

Joined: Fri Jun 26, 2009 6:53 pm
Posts: 2252
Location: Harvard IL
Hey Mod, as far as a mentor goes, you couldn't do any better than Bill and Beemer!!! Did I forget Louis(Sratele52) and Shimmy. These guys are all over this stuff !!! Art

_________________
None of Us are free, if One of Us is chained !


Top
Profile
Post subject: Re: Honest Tube amp repair guys ?
Posted: Fri Dec 14, 2012 4:49 am
Offline
Professional Musician
Professional Musician
User avatar

Joined: Sun Aug 05, 2012 9:50 am
Posts: 1399
I fully acknowledge the names you mention. I am such a beginner it might be difficult for them to help me. I barely speak the language. I can solder though. :D

_________________
I have some Fender gear.
(Telecaster, Jaguar, Villager 12 String. Mustang l, Bronco, SCX2, Rumble 350, HRDIII, Rumble 2x8 Neo Cab)
http://www.reverbnation.com/modwiz


Top
Profile
Post subject: Re: Honest Tube amp repair guys ?
Posted: Fri Dec 14, 2012 12:55 pm
Offline
Rock Star
Rock Star

Joined: Mon Apr 05, 2010 2:37 pm
Posts: 8708
Location: Natural Bridge, Virginia
modwiz wrote:
I appreciate the answer Bluesky, and I downloaded the pdf. gave it a good look, got on my knees with a flashlight and peered inside. I guess being happy with how it sounds will have to do until I have a mentor. What you informed me of would have to translate into something a little more coherent for me. I am a medical massage therapist and detail and how things work is not a challenge for me. Electronics is another language and techno alphabet, so to speak. I guess certain measuring tools/equipment is part of the equation?

What are the TP numbered things?


First you need to get the correct scematic for your amp. I only posted that one on the assumption that there would be minimal if any changes to those parts of the amp. You can contact Fender at

consumerrelations@fender.com

and ask for a copy the SCX2 schematic.

Biasing an amp requires working on a live amp. One thing to keep in mind when working on a tube amp is that touching the wrong thing can give you quite a shock and can even kill you. If the thought of that makes you squemish, it would be best to have a good amp tech do the job for you.

If you want to do it yourself, there are a few tools that you will need. First is a digital voltmeter (DVM). You can pick one up in most well equipped hardware stores like Ace, Lowes, Home Depot, etc. A decent one will run around $30 to $50. Avoid the real cheapos. The entire bias job can be done with just a DVM, but the best and safest way to do it is to use a bias probe:

http://www.amp-head.com/product_info.ph ... ucts_id=70

The sockets of the probe plug into the power tube sockets and then the power tubes plug into the bias probe sockets. The leads of the probe then plug into the DVM. This allows you to measure both the cathode current and plate voltage which are necessary to calculate the bias settings.

Once you have things connected up, you can measure each tube to see where things are set. Cathode current is measured across a 1 ohm precision resistor in the probe. By Ohm's law, 1 volt measured across a 1 ohm resistor equals 1 amp. You won't be measuring values that high, it is just an example. By flipping the switch on the bias probe, you will also be able to measure the plate voltage which will be in the range of 350 to 375 VDC. These are the values that can kill you.

Once you measure the cathode current and plate voltage, you multiply those values together to come up with the idle plate power dissipation. Note that you actually need to use plate current, but cathode current is close enough for government work. Most modern 6V6GT tubes are nominally 14 watt tubes. You want to see an idle plate power of 50% to 70% of that values, or 7 to about 10 watts. If the current bias setting doesn't give you a value within that range, adjust the bias pot with a small screwdriver and remeasure both cathode curent and plate voltage and multiply them together. Adjust the bias pot until you get it within that range and the amp sounds good to you. Biasing the tubes colder than 50% will result in a harsh sounding amp. Biasing the amp hotter than 70% can burn out the tubes quickly.

That's the basic procedure. If I missed something, I'm sure one of the others will fill in the hole. It is actually easier to to than it is to write about it. :wink:

PS:

The "TP" stands for test point and is a place on the amp where meaurements can be made. Using a bias probe eliminates having to access the test points.

_________________
Bill

Image


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 39 posts ]  Go to page Previous  1, 2, 3
Go to page Previous  1, 2, 3

All times are UTC - 7 hours

Fender Play Winter Sale 2020

Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: