Preffered Bias Method

[Geezer]

  Hoffman Amplifiers
    > Tube Amp Building - Tweaks - Repairs
        > Preffered Bias Method      
 
<< Prev Topic | Next Topic >>  
Author  Comment  
James Louie Lewis
Junior tube assistant
Posts: 1
(2/18/04 11:35 pm)
 Preffered Bias Method
--------------------------------------------------------------------------------
 My name is James and I'm new to the forum and to amps, as far as working on them goes. Have worked on and built guitars for years. My question is this which method of biasing
do most of you, the readers of the forum prefer? The method
I'm going to start with is going to be the Cathode resistor method because it seems the safest but I am interested in the
Output Transformer Shunt Method which I read about in the Gerald Weber book from Train Wreck and the Duncan amp page. My test amp is my 1980 50 watt Marshall JMP MKII
2204 Lead head. It currently has Groove Tube EL34's and previously it had 6550's which is what it came with. I just want to check the bias to get use to doing it. The Duncan
method for cathode resistor method I pretty much understand
and feel comfortable with. I the reason I would like to try the transformer shunt method is just to be able to perform another method. I do realize that when you put your meter
in series to read amperage you better have your head on
and be carefull. I have a Fluke 87 III Multimeter

I would like some assistance as to where or do I understand properly where to hook my probes on my particular amp
for the Shunt method. According to Trainwreck I would connect the Positive + lead to the Center Tap of the Output
Transformer and the negative - lead would be connected
to Pin#3 of any of the output tubes as long as they are
EL34, 6550, 6L6 those types not EL84's. They say by doing this you do not have to unsolder any leads, but on my amp the Center Tap is hooked to the H.T. fuse along with the Choke at the same point. If I hook up my meter in the fashion described will it be a problem on my amp. You can refer to this method on page 179 of the Gerald Weber book.

I do have another few questions about some things as they refer to bias on this amp. With the Tubes out it reads 413 V
at the number #3 (Plate) pin of both output tubes. With the tubes in it reads 375 V across the #3 pin on both tubes. I
realize that the tubes are drawing current when they are in
which would make sense for the drop in voltage but the 413 V
kind of seems odd to me but since this is my first amp that I
will be working on I don't have the experience to know. The
amp was biased by a good friend of mine many years ago when we put the EL34's in, and the amp works fine and has no apparent problems. This and my Peavey Ultra Plus are all I have to work on for now. I used to think Peavey's were lousey but the Ultra is really a good sounding amp.

Sorry for such a winded question but it's time to get my feet wet but don't want to do anything foolish if I don't have to.

James
 
Tiny Daddy
I will work on all amps
Posts: 493
(2/19/04 3:10 pm)
 Re: Preffered Bias Method
--------------------------------------------------------------------------------
 A preferred method is to install a 1-ohm resistor in the cathode lead of each power tube. Measure the voltage drop across the resistor, for example 40mV = 40mA. The 1-ohm resistors may be left in the amp permanently.
This is far safer than connecting a meter to the B+.
 
bluetonene
Junior tube assistant
Posts: 25
(2/19/04 3:13 pm)
 Re: Preffered Bias Method
--------------------------------------------------------------------------------
 I agree with Tiny Daddy just remember to use resistors with a 1% tolerance as the readings you get will be affected by the actual resistance.
Tom

Edited by: bluetonene at: 2/19/04 3:14 pm
 
James Louie Lewis
Posts: 2
(2/19/04 4:44 pm)
 Preffered Bias Method
--------------------------------------------------------------------------------
 Thanks for taking the time to answer. I fugured that that was going to be the safest method just was interested in the Shunt Method. I have some 1 ohm resistors coming from Doug and should be here today.
 
es347
Junior tube assistant
Posts: 16
(2/19/04 6:49 pm)
 Re: Preffered Bias Method
--------------------------------------------------------------------------------
 I have done the tranformer shunt method like you decscribed in my 5f6-a. I also got it from Webers book, but I only did it until I got the Parts from Hoffman to make my own Bias checker. I like that much better.
 
 
 << Prev Topic | Next Topic >>

 

 Topic Commands
 Click to receive email notification of replies
  
jump to:  Tube Amp Building - Tweaks - Repairs (New)New Product Suggestions (New)Praises - Complaints - Suggestions (New)Buy - Sell- Trade (New)Archives (New)    
 

- Hoffman Amplifiers - Tube Amp Building - Tweaks - Repairs - Hoffman Amplifiers -



Powered By ezboard® Ver. 7.32
Copyright ©1999-2005 ezboard, Inc.