What watt rating does a fender bias pot need to be ?

[catnine]

 I got one of the trim pots from hoffman but can't find it anywhere. It was the 50k linear. I also got a small linear 50k pot that if I recall was rated at 1/2 or maybe 1 watt . I can't recall the brand I just know I must have checked it out before ordering it, it may be a peavy wire wound 50k pot and I cut the shaft down and made a new screw driver slot in it , I did this so I could adjust the bias without removing the chassis each time. It was a smaller outside diam than a alpha or CTS pot. 

[sluckey]

Depends on the circuit. Got a schematic?

[catnine]

Depends on the circuit. Got a schematic?

 It is exactly the same as a fender 6G2 princeton without the trem . It does have a bit more plate voltage 410 VDC on the plates of the JJ 6V6's due to the Allen amps PT which is a replacement for a champ or princeton. For both the bias board resisters I used 3 watt because than was what i had on hand . I built this amp 4 years ago and at the time I was very careful to use the best parts I could get . I have not played with the bias for some time , I do know the neg voltage was - 35 volts and I can adjust it below or above that . I can use and have used JJ 6L6's with the allen amps PT and OT I have in it and a JJ GZ-34 rect tube and used the same bias pot with that set up but now I have it running JJ 6V6's and a 5Y3 rect . I see fender CTS 10k bias pots and they do not give the specs on watts now Alpha pots say 1/2 watt 500 volt for linear and 1/4 watt 250 volt for audio pots and they are what I use for both my amps vol and tone controls .

[sluckey]

It is exactly the same as a fender 6G2 princeton without the trem

Are you sure? I don't see a bias pot on the 6G2. Can you post a pic?

[tubeswell]

1/4W trimpots will work fine for bias adjustment in fixed-bias circuits, because all you are doing is setting the grid voltage, which is no-to-fairly-low-current anyhow.

For a tweaking bias adjustment in a cathode-biased amp, then you are more than likely going to be using a low-resistance (100R - 500R?) pot in series with the cathode, which will see quite a bit more heat dissipation, and what you decide to use should be at least about 2 x the wattage [2 x (tube current x voltage across the pot)].

[stratele52]

I use 1/4 watt cernet multitour pot. Or standard 1/2 watt pot are enough

[sluckey]

You can easily calculate the power dissipated in any resistor. Just measure the voltage across the resistor, square that number, then divide by the value of resistance. This gives the actual power dissipated in the resistor. Double that number for a safety factor and choose the power rating accordingly.

For example, you measure 35V across a 50K resistor...

P = E²/R = 35²/50000 = 24.5mW

Double that for safety factor...  24.5 x 2 = 49mW

In the above example, a 1/4W resistor would be more than sufficient. In fact, 1/4W is sufficient for most typical bias circuits, but without knowing the numbers, how will you ever really know?

[catnine]

 It is a 6G2 princeton circuit . No the factory 6G2 does not have an adustable bias pot. I built this amp just like an 6G2 circuit without the trem . I did this because I used a fender music master bass amp chassis for the build and it had only two inputs and a vol and tone pot so that's why I used the 6G2 without the trem . So the front end is exactly like the 6G2 and the power section is a 5E3 but I added the fixed adj bias . Also the 6G2 stock PT does not have a bias tap the Allen amps TP25 in this amp does so I used the 50 Volt tap and had to use a higher range resister . I would have to pull the chassis and check but I think I used a 1k range and a  a 27k to ground , the 50k bias pot is in seriers between  the 220k ohm resisters junction then the 27k ohm resister to set the bias . Still I would need to pull the chassis to be certain what values the resisters are all I do recall is with the pot set to zero ohms I had -35 VDC on the 6V6 grid and to set the bias to 18mA per each JJ 6V6 I barely add a bit of a turn on the 50kL pot perhaps a 1/4 turn . The only way I can check for certain is measure the grid voltage with the 6V6's pulled or pull the chassis and get the readings and resister values , don't know why I never wrote them down , I have photo's of the bias board yet the colors are hard to tell in the photo. This is the circuit of the amp attached

[sluckey]

I'm not sure there's a question here. Maybe you just need to pull the chassis.

[catnine]

I can't recall where I came up with the value of the range resister or the bias resister. The PT I used could run either a pair of 6V6's with a 5Y3 or a pair of 6L6's with a GZ-34 so I had to reach a happy medium so I could bias either set of tubes . The Allen amps TP-25 will run either quite well. It has a 50  VAC bias tap and I do recall palying around with the range resister and bias resister values so I could bias either set of tubes. This is to say what ever I did a lot of thought went into it . This is the primary reason why I used a bias pot . So far after three years I have not had one issue with the bias board I built.

[PRR]

There is at most 70V across the pot.

Possibly more like 35V.

It appears the pot is 50K?

Voltage squared divided by resistance.

70V^2/50,000 = 0.1W  -- 1/8W pot will do.

35V^2/50,000 = 0.0245W  -- any pot will do

[catnine]

 I have to pull the chassis just to see what I used for the range resister and biss resister.  I knwo when I built a few other 6G2's without trem the PT had no bias tap and the 100k was easy because it came off the pin 4 of the 5Y3 and was higher voltage and I know I used a 50k pot and a 10 k resister in series with the 50k pot .

 With this build it has a 50 VAC bias tap so I know I did not use a 100k range resister , I know I used a 50k pot .

 can it be possible to use the same bias / all values the same to bias a pair of 6V6's with a 5Y3 or a pair of 6l6's with a GZ-34?

 Man I tell ya my memory has faded and I have photo's in color of the bias board but cannot tell what the actual color bands are . It is either a 10k ohm range resister and a 27 k ohm bias with the 50k pot . All I recall is building this bias board to work with either set of tubes and how I arrived at the values is beyond me and now it's driving me nuts. What I do remember is at first this amp was a cathode biased amp with the 6L6's and GZ-34 rect. and in order to use the 6V6's and 5Y3 I had to change the cathode resister valuse. I had this 71 music master bass amp chassis in a hand built by me tall cab that I had 2 twelve 8 ohm speakers in it in parallel to have 4 ohm load and built this tall cab for a 73 SF champs and called it the champ tower which is the same width and depth at thwe chassis end as a MM bass amp cab but twice as tall to have the 2 twelves and the MM bass amp chassis is the exact same size as a SF champ so then I went and installed the mm chassis in the same cab and I wanted power so I used the aleen amps PT that can run either set of tubes and went with the 8L6's and GZ-34 and added a small fan because the PT did get warm , not hot but warmer than with 6V6's . The allen amps OT  a TO25 can run 6L6's with a 2 or 4 ohm load of 6V6's with a 4 or 8 ohm load so I have two output jacks and used one or the other to still have a 4 ohm load.   So somewhere along the line I took out the cathode bias and made an adjustable fixed bias that somehow I found a range resister and bias resister that would bias either set of tubes . Now I have this chassis in the original MM bass amp cab with 6V6's and one 12" 8 ohm speaker simply because 2 twelves with the 6L6's was far to much amp for home use. even 2 twelves with 6V6's was far to loud . Not that what I did is really all that important , what ever I did works fine . Heck I can't even recall what year it was that I built this amp perhaps late 2004 or sometime in 2005 . Last time I had the chassis out was over 2 years ago when I added a switch to have the 6G2 NFB loop whcih takes out the 25 uf cap on the second gain stage and connects to the stock 56k 6G2 NFB resister but this was really a waste. I thought that I could use a 12 ax7 in V1 and make it into a 6G2 and perhaps an 6G2 does breakup early but I always use a 12 ay7 in V1 because it has a lot of headroom and bottom end .

 Well that's the end of my rant.

[sluckey]

According to the pic of your bias board, the cap is installed backwards. The resistors are 1K and 10K.

[catnine]

According to the pic of your bias board, the cap is installed backwards. The resistors are 1K and 10K.

 Yes I know the cap looks backward . What I did was instead of building the bias borad in the traditional manner where the neg end of the cap would be jumpered under the board directly across from the diode  which is also the junction at the diode that ties to the junction of the two 220k ohm resisters to the 6V6 grids . What I did since of lack of space and to make the layout of wires cleaner is I ran the wire from the junction of the two 220k resisters to the diode as normally done then from that same eyelet on the bias board I turned the cap around and ran the jumper under the board to the neg side of the cap . The eyelet on the neg side of the cap has the jumper off the diode and the wiper (yellow wire )of the 50k pot . Then the bias resister in series with the pot has it's own eyelet at each end . one end goes to the leg of the pot ( green wire) and the other end of the bias resister jumpers across to the pos side of the cap and from there both share the same white ground wire . On the far left is the red/blue 50 volt bias tap off the PT. You see on the far lower right of the board there are three eyelets layed out like a triangle , this was because at first I was going to use the trimmer 50kl pot hoffman sells , instead I wanted access under the chassis so the pot is between the PT and bias board just below the gray fuse holder. I could have built the board the tradition way and used another ground but I wanted all the leads as short as possible and where the leads to the pot were not over or under the board. It's crowded in that area , I mounted the board on a nylon sleeve so it stands above the eyelet board with a machine screw tapped into the fiber board . This is the area where the 10watt cathode resister and bypass cap used to be. Right between the bias board and first filter cap are the OT's seconday leads . Also there are other leads under the bias board so I insulated the underside of the bias board with a section of black fiber board so as not to create a short .

 This amp has been through a lot of changes . If I were to do it over I would build my own eyelet board a bit shorter and included the bias board as part of it , as it is it was sort of build a 5E3 then design and adapt from there. I used the fiber 5E3 eyelet board weber sells and it just fit and this chassis is the exact same size as a SF fender Champ .

 Sorry for the long story , it would have been easier to draw a diag of the bias board . I just don't want anyone to think I slapped this thing together without any thought involved.

[sluckey]

Guess you don't need to pull the chassis afterall.

[catnine]

Guess you don't need to pull the chassis afterall.

 How on earth could you see the colors of the resister bands Sluckey? I kept looking but could not tell for the life of me. But 1k and 10K do sound about right . I just don't know why I needed 1k for a range resister to knock down 50VAC bias tap but then with a 10k bias resister and a 50k pot it all combined sets the neg voltage.

[catnine]

I pulled the chassis just to make sure of what I did . On the bias board I have 50.8 VDC on the PT bias tap then there is a 10k range resister then the diode . Right after the diode there is - 35.4 VDC . Then I have a 50kl pot in series with a 26K bias resister to ground along with the pos side of the cap. I have -35.4 VAC at the junction of the two 220k ohm resisters and measured at the grid on the 6V6 after the 1.5k grid resister I have 34.4 VDC.

 I basically built it just like shown in the hoffman library of info just different resister values mainly because mine has the 50 VAC bias tap from the PT.

 One other difference was I connected the lead from the junction of the two 220k resisters looking at the back of the pot to the left leg of the pot and jumpered that to the ( center) wiper leg of the pot then the right leg of the pot connects to the 26k bias resister which along with the pos side of the cap goes to ground. As far as the pot goes it seems to be one AES sells for a peavy and is 50kl wire wound with a long metal shaft no splines and a slot for a screw driver.

 I recall this was a way of doing it in case the wiper failed and if it did you would have 26k + 50k to ground which if I recall would lower the mA to  the 6V6's ie draw less current . Perhaps over kill but this was one way I was told to do this.