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Post subject: Original Blues Deluxe Bias Problem
Posted: Tue Dec 14, 2010 2:11 am
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I have a '93 Blues Deluxe and I am going to change the tubes in it. I have been playing it more often now than I did before and I would like to get the best possible sound that I can from it. Here's my dillema:

This amplifier has no biasing pot. I am trying to find out if there is any way I can adjust the bias for the tubes without the pot. I know it probably isn't possible, but I want to get the most I can out of this amp until I decide to upgrade. I will keep the 6L6s and the 12AX7s, so I will not need to actually rebias the amp. I just want to be able to monitor and adjust the amperage going into each power tube.

What is the optimal mA for each tube? I have read everything from 60mA to 30mA. I know this question will probably be for naught since there is no pot for the adjustment.

The amp has factory GTs in it and to my knowledge these tubes have never been changed. I have had the amp awhile, but did not play it as much as I do now and I can tell the tubes are degrading. What good replacement tubes would you suggest besides the GTs? I have heard good things about EuroTubes, but I am hesitant to buy anything that I haven't seen first for the sake of receiving tubes that do not match the bias settings I need since I probably cannot adjust it.

Any help is appreciated and I am an electrician by trade, so technical advice is welcomed as well.


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Posted: Tue Dec 14, 2010 3:29 am
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Schematic said Blues Deluxe' 6L6's run at 427 volts DC so an average bias is 42 milliamp, Hot is 40 ma and cold is 35 ma.

For adjusting bias negative voltage, you have to change R87, a 27 K ohms resistor. Sometime also R84 a 3.3 K ohms.

Smaller R27 = less neg volt : tubes run hotter.

If you are able to solder, a adjustable bias pot modif is possible


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Posted: Tue Dec 14, 2010 3:36 am
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http://www.fender.com/community/forums/ ... ce7820e6b4


Go to look at this post


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Post subject:
Posted: Tue Dec 14, 2010 6:09 am
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stratele52 wrote:
Schematic said Blues Deluxe' 6L6's run at 427 volts DC so an average bias is 42 milliamp, Hot is 40 ma and cold is 35 ma.

For adjusting bias negative voltage, you have to change R87, a 27 K ohms resistor. Sometime also R84 a 3.3 K ohms.

Smaller R27 = less neg volt : tubes run hotter.

If you are able to solder, a adjustable bias pot modif is possible



Do you know about R87 and R84 from experience? I compared the BD schematic with the Hot Rod Deville schematic and it looks like R87 may be it. The circuitry is pretty close with the exception of a few extra diodes and resistors, but I need to be sure before I start messing around with the components. If it is an easy pop and swap, I could take out the resistor and solder in a same rate pot. I doubt it is this easy though. Will the fender support give me advice on their circuitry for these kind of mods? I appreciate the help....


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Posted: Tue Dec 14, 2010 7:28 am
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Yes ,I have lot of experience about repair amp and modification. But I'm not sure what is your amp\ schematics.

I have to see to be sure , but schematic I foud is for Blues deluxe.


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Posted: Tue Dec 14, 2010 8:02 am
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This is the bias circuit from your amp in my opinion. Do somes check if right.
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Posted: Tue Dec 14, 2010 8:05 am
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Tis is you can do if you want an adjustable bias pot . It's from Blues Deville.
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Posted: Tue Dec 14, 2010 9:02 am
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Hi MrGreenGenes,

You are right, the first BDRIs have no bias adjust. stratele52 gives some good advice for bias setting, and for a mod to the bias circuit. This should be the correct schematic for your amp, the later versions use a different schematic.

http://support.fender.com/schematics/guitar_amplifiers/Blues_Deluxe_Schematic.pdf

It isn't easy, you have to remove all knobs and nuts from the pots, and remove the nuts from the jacks, to be able to pull the circuit board to have access to the back for soldering. If you do want to add a pot, I would change R87 to around 15K ohm (plus or minus a little) and put a 25K ohm pot in series with it. You could just replace R87 with a pot and it would work fine, but it is a good idea to keep a resistor in series with the pot, so as not to accidentally have too low a voltage which could ruin the output tubes and cause other damage. If you add a pot and resistor, start with the pot in the middle position (about 12K ohms), with the output tubes removed, turn on the amp and check the voltage at C- and adjust to get the voltage on the schematic, -46.7 VDC. Then, turn off the amp, install the tubes, turn on the amp, and adjust from there, monitoring the Plate current and set to the specs for the tubes that you use. stratele52 may have some better advice for the value of resistor and pot to use. :)

The easy way would be to replace your tubes with the same GTs that you have now, and the bias will likely be OK as is. That's the whole idea of the colored labels for the GTs. These are likely the output tubes that you have now (link below), just select the same color label that you have now. Blue are the "1-3 GT rating" for early breakup, white are the "4-7 GT rating" for mid range, and red are the "8-10 GT rating" for clean headroom.

http://www.guitarcenter.com/Groove-Tubes-Gold-Series-GT-6L6-R-Matched-Power-Tubes-105811988-i1125717.gc

Edit:
I sometimes forget to remind everyone that the capacitors in the amp need to be discharged before taking anything apart. :!:

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Posted: Tue Dec 14, 2010 10:53 am
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+1 shimmilou


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Posted: Tue Dec 14, 2010 1:31 pm
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+1&+1 Strat&Shimmi

Voltage at C point C is given mainly by: V(c)=V(C37)*R87/(R84+R87), playing on one (or both) resistor(s) you can adjust your current by dichotomy allowing other 6L6GC matched power valve brands. For sure a pot mod is an option to be done by a tech.

(For Stratele: I've checked on my BDRI schematic, the bias have R76 (1.5K), R82 (25K, pot) and R77 (100K), I think on BD original V(c37) is -100V and on BDRI V(c43) is -53V that makes the big difference betwenn R84 and R76)

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Post subject: Re: Original Blues Deluxe Bias Problem
Posted: Tue Dec 14, 2010 2:08 pm
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MrGreenGenes wrote:
I have a '93 Blues Deluxe and I am going to change the tubes in it. I have been playing it more often now than I did before and I would like to get the best possible sound that I can from it. Here's my dillema:

This amplifier has no biasing pot. I am trying to find out if there is any way I can adjust the bias for the tubes without the pot. I know it probably isn't possible, but I want to get the most I can out of this amp until I decide to upgrade. I will keep the 6L6s and the 12AX7s, so I will not need to actually rebias the amp. I just want to be able to monitor and adjust the amperage going into each power tube.

What is the optimal mA for each tube? I have read everything from 60mA to 30mA. I know this question will probably be for naught since there is no pot for the adjustment.

The amp has factory GTs in it and to my knowledge these tubes have never been changed. I have had the amp awhile, but did not play it as much as I do now and I can tell the tubes are degrading. What good replacement tubes would you suggest besides the GTs? I have heard good things about EuroTubes, but I am hesitant to buy anything that I haven't seen first for the sake of receiving tubes that do not match the bias settings I need since I probably cannot adjust it.

Any help is appreciated and I am an electrician by trade, so technical advice is welcomed as well.


If you know what you are doing inside of a tube amp (they can kill you, ya know, even when they are unplugged), you can add a bias pot as suggested. Here is a link to a kit to do just that:

http://www.torresengineering.com/fenbluesdevn.html

The stock Fender bias is very cold, somewhere around 30 mA per tube (when measured at the Fender bias test point, this equates to about 60 mV). I have my JJ 6L6GC tubes biased at about 37 mA per tube.

If you are not comfortable working on the insides of a tube amp, just replace your output tubes with the same type as Shimmilou suggests. No sense taking chances with your life. :wink:

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Posted: Tue Dec 14, 2010 2:10 pm
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Thanks for the advice guys, I really appreciate it. I was hoping there was a much easier way to go about all of this, but the whole modification is not too big of a deal. I have already had to do some soldering due to cold joints and a few loose components. I will probably just replace R87 with a pot and put a resistor in series, like shimmilou said.

I am an electrician/electronics tech, so the mod will not be a problem for me to DIY. So, back to tubes.... do you all recommend just staying with the GTs?


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Post subject:
Posted: Tue Dec 14, 2010 2:18 pm
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MrGreenGenes wrote:
So, back to tubes.... do you all recommend just staying with the GTs?


My tube choice in my BDRI:

Fender Blues Deluxe-Deville 6L6GC
Price: $66.25
Description: A hotter matched pair of the JJ 6L6GC's for the reissue or original fixed bias 90's Blues Deluxe or Deville for a big warm tone along with three JJ ECC83S's with one balanced for the phase inverter in V3. These will reduce the harsh mids, the brittle highs and make the drive channel much more usable.

https://ssl.eurotubes.com/cart/index.ph ... ory_id=122

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Posted: Tue Dec 14, 2010 3:37 pm
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MrGreenGenes wrote:
...So, back to tubes.... do you all recommend just staying with the GTs?


I do! :D

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Post subject:
Posted: Tue Dec 14, 2010 4:06 pm
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shimmilou wrote:
MrGreenGenes wrote:
...So, back to tubes.... do you all recommend just staying with the GTs?


I do! :D


GTs stink!












J/K shimmilou. :lol:

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