Rebiasing Original Fender Blues Deluxe

[TheGrandWazoo]

I have an original '93 Fender Blues Deluxe. I am interested in changing the tubes as I have been playing the amp much more often in the last few months and they are starting to degrade pretty rapidly. There is no rebias pot on this amp, so I was wondering if any of you have any idea on how to bias the new power tubes. I will probably get "matched" tubes, but they are not actually matched 100% and I want them to be within .10mA if possible.

I understand that if I replace tubes with something different, i.e. 6L6s with 300Bs or 6146s, I will need to rebias the amplifier, but I am not changing the wattage output or gain in this amp as I will use 6L6s for sure. I would just like to be able to view the amperage for each tube and to be able to adjust that amperage accordingly. I am just worried that if I simply swap out the tubes, with the age and wear on the amp, there will be a bias difference that isn't healthy for the amp or tubes.

I have read that the amp is rated for 60mA/60mV @ 1 ohm per tube. Is this correct? I would think that 60mA would be too high. I was thinking more like 30mA per tube for a combined 60mA. I will settle for simply changing out the power and preamp tubes with comparable tubes and not worrying about the rebias IF I can be certain this will not damage the amp. It is old and the guy who had it before myself never changed out the tubes, so this amp isn't in tip top shape.

So basically, do any of you know a way to adjust the amperage for the tubes w/o the pot? I would guess this isn't possible, but you never know. I would appreciate any information you could give.... thanks

[52 Bill]

I'll take a shot here. I guess that you are concerned about changing the tubes in the amp and causing some sort of damage to the amp or the new tubes unless you rebias the amp.

While your concern is good, you should not get overly worried about this, as the circuit has been designed to work within the tolerances of the standard manufacturing processes and nearly any 6L6/5881 tube that you install will work just fine.

If you really want to rebias the amp, there is a resistor on the board that sets the bias. While not adjustable, you can change the value of this resistor or add an adjustable resistor to the circuit that will make a bias adjustment possible.

When the amp was manufactured, the assembler reached over to a large bin of output tubes and installed a "matched" pair of output tubes and the amp was tested and then shipped out to a dealer. It has worked ever since.

If you were to take the amp to a repair shop or a Fender dealer, they would take a new set of tubes and install them, turn on the amp and listen for any problems. If everything worked ok, they would then return the amp to you.

Just get yourself a good set of tubes, install them and play the amp. Then if there is a problem, face it and fix it.

[TheGrandWazoo]

Thanks Bill. I posted something similar to this on the Fender community forums and got some good reponses. I looked at the schematic and compared it to a fender amp that has the biasing pot. It ain't rocket science to figure it out and the job isn't too tough, but I will take your advice and just not worry about the bias unless the new tubes cause an issue. These amps are pretty durable, but I see durable electrical equipment break everyday so I am always weary about things like this. Again, thanks for the advice.

[Billy R aka DynaFreak]

Wally you are correct to be somewhat concerned about setting the bias level for your amp when installing new tubes, especially since current production 6L6's vary widely from brand to brand and even from one set of tubes to another of the same brand. Although it is unlikely that you would damage your amp by not checking the bias, certainly the tone of the amp and/or the longevity of the tubes could suffer. The 60mv spec across the one ohm cathode resistor for the bias test point on the Hotrod Deluxe is for both tubes, so if the amp is at spec then you are getting an average of 30ma per tube.

I can't vouch for other Fender techs/shops but if I am installing new power tubes in a Blues Deluxe or other amp without adjustable bias I will: 1. install a set of power tubes that I have checked for matching. 2. Check for proper current at the test point or with a bias probe. 3. If is pretty close then we are good to go. 4. If not, I will install another pair with a different bias spec. Some times this means going with a different brand of tubes. In some cases I will install a bias pot at the customers request, or change the bias set resistors. This is most often the case when the customer has installed new tubes (to save money) then sends it to me to set the bias.

[TheGrandWazoo]

I went ahead and ordered two JJ 6L6GCs that are specifically for Blues Deluxe Originals and the Reissues. The kit also comes with 3 ECC83Ss for the preamp. Some guys at the Fender Forums recommended them, so I will try them out. I went ahead and bought a bias probe as well to check and MAKE sure that these tubes are matched. Hopefully I will get lucky, but if not I will have to install the pot and another resistor in series to protect against low votages. I will just have to figure out an easy way to install the resistor in series being there will be not spot for it. I could use jumpers, but I would rather have everything soldered hard to the card. I may have to get creative, but I will probably end up with jumpers. It will just not look too pretty. If anyone has any advice on installing the resistor, it is more than welcome.

I am an electrician by trade, I just have no experience with amp mods. Technical advice is appreciated as well.

[MWJB]

Replace R87 with a top adjust, multiturn 50K 0.75W Cermet pot, wired as a variable resistor. Mount it in place of R87 & secure to the board with silicon cement, wire it to the old resistor legs using 20 guage solid core wire, be quick & light to avoid the old leges dropping out the back of the board, or pull the board and solder the hook up wire straight in...& whilst you are in there replace the 2 sand block 470ohm 5W resistors in the switching supply (R85 & R86) with resistors that are elevated away from the board, if this has not been done already.

[TheGrandWazoo]

Replace R87 with a top adjust, multiturn 50K 0.75W Cermet pot, wired as a variable resistor. Mount it in place of R87 & secure to the board with silicon cement, wire it to the old resistor legs using 20 guage solid core wire, be quick & light to avoid the old leges dropping out the back of the board, or pull the board and solder the hook up wire straight in...& whilst you are in there replace the 2 sand block 470ohm 5W resistors in the switching supply (R85 & R86) with resistors that are elevated away from the board, if this has not been done already.

That would all be fine, but I want to install another resistor in series; something around 25k ohms to protect the tubes in case of voltage issues. That causes me to jerry rig something up either on the board itself soldering a small cermet pot in series with a resistor or putting it all on a small board mounted to the chassis with jumpers to the board. There is also a kit available with the full size fender adjustable pot along with instructions on mounting to the chassis. I will decide what I want to do after the tubes get here.

[MWJB]

"That would all be fine, but I want to install another resistor in series" Not "would be" but IS fine. You're making life hard for yourself, cermet pots like the one I describe are multiturn (takes 10-20 full turns to go from one end to the other), so that it takes more than a slip with a screwdriver to burn up your tubes, the cermet pots don't adjust themselves.

[TheGrandWazoo]

"That would all be fine, but I want to install another resistor in series" Not "would be" but IS fine. You're making life hard for yourself, cermet pots like the one I describe are multiturn (takes 10-20 full turns to go from one end to the other), so that it takes more than a slip with a screwdriver to burn up your tubes, the cermet pots don't adjust themselves.

I just don't think the pot itself will provide enough protection is there are voltage or current issues. Also, if the pot opens there will be nothing to curve the amps going to the tubes. At least there isn't according to the schematic. Looking at other Fender schematics (ones of amps with bias adjustment installed) there is always a resistor in series for this purpose. You also called for a .75W pot while everything I have read recommended a .25W. It is difficult to install the resistor in series properly because of space issues. You could install it on the back of the card, but I haven't been able to draw up anyting that would work as well as I want it to. I was thinking of soldering in a 500K pot in parallel with the 27k resistor as that would bring the total resistance to around 25k. I have found a 500k multi-turn (14) cermet that is rated for .25 W online. I will still have to draw it out and do the math to make sure this set-up won't screw up the impedence or anything.

[MWJB]

"I was thinking of soldering in a 500K pot in parallel with the 27k resistor as that would bring the total resistance to around 25k. I have found a 500k multi-turn (14) cermet that is rated for .25 W online. I will still have to draw it out and do the math to make sure this set-up won't screw up the impedence or anything." What the hell for? Impedance? It's a simple voltage divider. Parallelling resistors/pot will only allow you to go hotter on the plate current, what if you fit tubes that need less plate current? Why use 0.25W when 0.75W will fit happily (I know it fits because I've done it) & is readily available, not much bigger & overrated W. If the wiper of the bias pot goes (never seen this happen) your amp will stop working, either because current will rise uncontrollably, or disappear altogether...either scenario results in an amp that doesn't work, I don't know how you will find a fixed resistor that will work for every brand of 6L6, one brand might read 6mA in circuit another 36mA. If you're concerned that the pot represents a weakness, stick with a fixed resistor & rebias for new tubes or buy new tubes from someone who can match plate current to your current pair. When Fender eventually decided to fit bias pots to Blues Deluxes/Devilles/Bassman RIs they revised the layout so that the parts would fit...this is where we came in, you have a pre-bias pot amp & little space to fit a pot...easiest thing to do is sell the amp and buy one with a bias pot.

[TheGrandWazoo]

"I was thinking of soldering in a 500K pot in parallel with the 27k resistor as that would bring the total resistance to around 25k. I have found a 500k multi-turn (14) cermet that is rated for .25 W online. I will still have to draw it out and do the math to make sure this set-up won't screw up the impedence or anything." What the hell for? Impedance? It's a simple voltage divider. Parallelling resistors/pot will only allow you to go hotter on the plate current, what if you fit tubes that need less plate current? Why use 0.25W when 0.75W will fit happily (I know it fits because I've done it) & is readily available, not much bigger & overrated W. If the wiper of the bias pot goes (never seen this happen) your amp will stop working, either because current will rise uncontrollably, or disappear altogether...either scenario results in an amp that doesn't work, I don't know how you will find a fixed resistor that will work for every brand of 6L6, one brand might read 6mA in circuit another 36mA. If you're concerned that the pot represents a weakness, stick with a fixed resistor & rebias for new tubes or buy new tubes from someone who can match plate current to your current pair. When Fender eventually decided to fit bias pots to Blues Deluxes/Devilles/Bassman RIs they revised the layout so that the parts would fit...this is where we came in, you have a pre-bias pot amp & little space to fit a pot...easiest thing to do is sell the amp and buy one with a bias pot.

I appreciate the advice. I got new tubes and put them in the other day. Everything sounds great, but I will still check the bias when I have time. The probe I bought doesn't have the 1 ohm resistor so it is read in amps only. My meter doesn't have an amps setting, so I will buy a cheap ammeter. If these tubes are "matched" like advertised, I will not even worry about doing the mod any time soon. I will likely upgrade to a better amp in the future anyhow. Again, thanks for the info.