It is currently Tue Mar 17, 2020 5:12 am

All times are UTC - 7 hours



Post new topic Reply to topic  [ 6 posts ] 
Author Message
Post subject: Bias Measurements For A Fender Blues Deluxe Reissue
Posted: Thu Apr 21, 2011 10:57 am
Offline
Rock Star
Rock Star

Joined: Mon Apr 05, 2010 2:37 pm
Posts: 8708
Location: Natural Bridge, Virginia
As I was getting ready for bed last night, I started thinking about the method I use for measuring and setting bias in my Fender Blues Deluxe Reissue (Strange things sometimes pop into my brain at 2:00 am. :lol: ).

I use the following bias probe to measure cathode current and plate voltage in each power tube:

http://www.amp-head.com/product_info.ph ... c9587ff6ba

It uses a 1 ohm, 1% tolerance resistor in each socket to measure cathode current in each power tube individually. I then plug the measured numbers into the Weber bias calculator to come up with approximate plate current and percentage of maximum power dissipation:

http://www.webervst.com/tubes1/calcbias.htm

Anyway, I started thinking (Always dangerous.) about the fact that each bias probe socket includes a 1 ohm resistor in each cathode circuit and that the BDRI also includes a 1 ohm, 5% tolerance resistor that is set up to measure the cathode current of both tubes combined.

So the question is this: Would the cathode current measurement be more accurate if the Fender resistor were to be removed from the circuit, or would the difference be neglibible?

Looking at the schematics, the original Blues Deluxe just has the two power tube cathodes tied directly to ground:

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

The Blues Deluxe Reissue has the two power tube cathodes tied to ground through the 1 ohm resistor and a diode (I guess for protection from any reverse current?). See page 11:

http://www.blueguitar.org/new/schem/fen ... eissue.pdf

The resistor and diode are easily accessable without removal of the circuit board and could be removed and replaced by jumper wires with no problem. See page 14 of the above link.

Does this look like a worthwhile mod? Anyone see any problems in this or is my engineer mind just overthinking things? :lol:

_________________
Bill

Image


Last edited by bluesky636 on Thu Apr 21, 2011 5:56 pm, edited 1 time in total.

Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: Bias Measurements For A Fender Blues Deluxe Reissue
Posted: Thu Apr 21, 2011 12:49 pm
Offline
Professional Musician
Professional Musician

Joined: Sun Dec 19, 2010 12:00 pm
Posts: 1389
1 Ohm * 0.032^2 = 0,001W compare to 15W to 20W idle power , my point of view negletible. 8)

_________________
Tissan

*'52 Telecaster RI, '57 Stratocaster RI, Gibson ES175 RI, Washburn Hawk 1980, Epiphone Casino MIK 2000, 57' Deluxe Knotty Alder, BDRI ...*
https://soundcloud.com/pierrou
http://www.youtube.com/user/Jazzenfamille


Top
Profile
Post subject: Re: Bias Measurements For A Fender Blues Deluxe Reissue
Posted: Thu Apr 21, 2011 4:16 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Tue Jun 29, 2010 1:20 pm
Posts: 9640
Location: Indiana
I don't see a problem, and your idle current setting will be the same, whether with one or two resistors since they will be in series. The voltage dropped across one resistor is only about 70 mV, which is negligible considering that the plate voltage is well over 400 volts. Maybe you could use a switch across the amp's 1 ohm resistor temporarily, to check the difference before soldering a jumper in place (there won't be much difference). I think that the diode across the 1 ohm resistor is just to protect the resistor, so if you remove the resistor, then the diode will be unnecessary. Of course if you just jumper the resistor, the diode will be jumpered too, and you wouldn't have to remove either.

When I was a tech in the service industry, we used to joke about the engineers being our enemies because they liked to make things too complicated, and we liked things to be as simple as possible. You never hear the term "over-teching" something. :lol:

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


Top
Profile
Post subject: Re: Bias Measurements For A Fender Blues Deluxe Reissue
Posted: Thu Apr 21, 2011 4:26 pm
Offline
Rock Star
Rock Star

Joined: Mon Apr 05, 2010 2:37 pm
Posts: 8708
Location: Natural Bridge, Virginia
shimmilou wrote:
When I was a tech in the service industry, we used to joke about the engineers being our enemies because they liked to make things too complicated, and we liked things to be as simple as possible. You never hear the term "over-teching" something. :lol:


Guilty as charged. Maybe I just need to have my doctor adjust my night time meds so I don't think about things like this as much. :lol:

_________________
Bill

Image


Top
Profile
Post subject: Re: Bias Measurements For A Fender Blues Deluxe Reissue
Posted: Thu Apr 21, 2011 5:02 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Tue Jun 29, 2010 1:20 pm
Posts: 9640
Location: Indiana
Don't change a thing friend! You keep the rest of us in line by thinking of the things that we wouldn't.

Do you and Tis-san agree that the diode across the 1 ohm resistor is just there to protect the resistor from high reverse current? It is a big diode, and I can see no other purpose for it. There are fly-back diodes for each output tube already, is the one across the resistor even necessary, or is it just an extra measure of protection for the resistor? (you know, over-engineered :lol: ) :?:

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


Top
Profile
Post subject: Re: Bias Measurements For A Fender Blues Deluxe Reissue
Posted: Thu Apr 21, 2011 5:57 pm
Offline
Rock Star
Rock Star

Joined: Mon Apr 05, 2010 2:37 pm
Posts: 8708
Location: Natural Bridge, Virginia
GAAAA!!!!! I just realized I posted the wrong link for the Reissue. I have fixed it now.

_________________
Bill

Image


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 6 posts ] 

All times are UTC - 7 hours

Fender Play Winter Sale 2020

Who is online

Users browsing this forum: Google [Bot], stratele52 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: