It is currently Tue Mar 17, 2020 12:34 am

All times are UTC - 7 hours



Post new topic Reply to topic  [ 3 posts ] 
Author Message
Post subject: Biasing a Twin Reverb Reissue tube amp
Posted: Mon Feb 04, 2013 2:11 pm
Offline
Hobbyist
Hobbyist

Joined: Wed Dec 05, 2012 2:54 pm
Posts: 12
Can someone point me out on how to bias a Twin Reverb Reissue Amp? My tech biased the power tubes to -50mv according to TP26 shown in the schematics. The amp sounds okay but the tubes seem to get very hot even though they dont show signs of red.

I have a Radioschack digital multimeter and would like to bias it correctly if suggested.

The method he used was placing the black probe on one of the chassis screw and the other one on pin 8 of one the power tubes or just accross the resistor laying in each power tubes. Is this accurate enough?


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: Biasing a Twin Reverb Reissue tube amp
Posted: Mon Feb 04, 2013 2:35 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Fri Feb 05, 2010 4:31 am
Posts: 14049
Location: Province de Québec, Canada
jalexquijano wrote:

The method he used was placing the black probe on one of the chassis screw and the other one on pin 8 of one the power tubes or just accross the resistor laying in each power tubes. Is this accurate enough?


You probably miss something or your tech is not qualified.

Chassis screw is ground and power tube pin 8 are connected to .....ground . Not voltage reading is avalaible.

TP26 ; - 50 V on schematic . This is probably ( not sure ) Fender adjustment with the original tubes. This do not mean your tubes have the proper and best bias .

And if you replace power tubes , this -50 V need a bias reading ON the power tubes itself thru a 1 ohms resistor or a use of bias probe.

Different tubes brand / year could need another negative bias voltage other than the -50 V.

Question ; you don't trust your "tech " ? You could be right , on guitar bussiness, not many tech know electronics.


Top
Profile
Post subject: Re: Biasing a Twin Reverb Reissue tube amp
Posted: Mon Feb 04, 2013 5:08 pm
Offline
Rock Star
Rock Star

Joined: Mon Apr 05, 2010 2:37 pm
Posts: 8708
Location: Natural Bridge, Virginia
+1 stratele52.

Measuring the bias supply voltage just tells you the that bias power supply is set to spec. It tells you nothing about the actual currents flowing through the tubes in an idle condition.

In order to properly set the power tube bias, you need to measure both the plate current flowing through the tube as well as the plate voltage present at the tube for the measured current You then multiply those two values together and compare the result to the rated maximum output for that tube. In the case of a 6L6GC, 30 watts is the maximum rated output for a single tube. In an amp like the TRRI, you adjust the plate current so that the product of plate current and plate voltage is between 50% and 70% of the max power.

However, it is not easy to measure the plate current. The best and safest way is to measure both cathode current and plate voltage using a bias probe and a DVM. Since the TRRI has four power tubes, a four socket bias probe should be used although you can get away with a two socket (or even a single socket) since the bias adjustment is universal for all four tubes. The advantage of four sockets is that you can easily see how closely matched the four tubes are. A good two socket probe (the one I use) is this:

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

Weber sells a four socket probe that includes the meter:

https://taweber.powweb.com/biasrite/br_page.htm

Using cathode current is close enough for government work and actually gives you a little safety margin to avoid biasing the tubes too hot.

There are many You Tube videos available that show the proper way to bias amps in general and for specific amps. Although specific to the Hot Rod amps, this video is a good one and the basic principles are the same:

http://www.eurotubes.com/eurotubes-Hotr ... lts-mA.htm

Finally, you might want to read these to learn more about amp biasing:

http://www.aikenamps.com/WhatIsBiasing.htm

http://www.aikenamps.com/Why70percent.html

Keep in mind that while biasing an amp, the amp is powered on and you are exposed to high voltages which can injure or kill. This is a good read also:

http://www.aikenamps.com/SafetyTips.html

Good luck, have fun, and be careful if you decide to do this yourself. With the proper tools and procedures as outlined, biasing an amp is very safe.

_________________
Bill

Image


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 3 posts ] 

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: