Torres bias kit problems

[strat100]

Just installed a torres bias kit to my 90,s fender blues deluxe and now i seem to have a problem with it,no matter what resistor i use on the pot i get the same
reading which is 40ma,i cant adjust it downwards but it will adjust
upwards.The resistors i used were,18k,22k, 27k,33k an 47k.I connected lead from bias pot to R87,after removing the resistor,as i far as i can see i have connected everything as shown,
At the same time i changed the screen reistors to 1K5,as i want to try 6v6 at a later date.

any hints?

[MWJB]

Can't access the Fender website at the mo' and don't have a schematic to hand, but I do remember from past experience that the Blues Deluxe doesn't generate particularly high negative voltages. Even if you went up to 100K at R87 (after a point, increasing the value here won't make any difference) and reduced the value of R84 (also lowers current) you might still find that some brands of tubes still run too hot (SED/Phillips WGC?).

What brand are you using and what negative voltage do you have at pin 5 of the power tubes?

[strat100]

Thank you for you reply MWJB,

I am using Groove tubes,so i am assuming they are the original ones,the negative voltage at pin 5 is -43volt on both tubes.

[Amp Kat]

Are you sure you have enough - voltage to start with ? What is the negative voltage without the pot connected ? You may have to install a voltage doubler which is very easy to do if your negative voltage isn't high enough.
Who told you that you needed 1k5 screen resistors for 6V6's, because you don't.

[MWJB]

-43v seems a bit low, you should be able to rebias happily with the Reflector Russian Groove Tubes. You should be able to get -50v+. Assuming your bias pot is in the R87 portion of the circuit, I would reduce the value of R84 anyway.

What is your voltage at either side of R84, what resistance to ground are you getting from the lower voltage side of R84 to ground?

Amp Kat - Isn't the circuit a voltage doubler already?

[Enzo]

No, it is a simple half wave. CR19 rectifying 38VAC. Should be about -53 on C37, and about -47 across R87. -43 is a hair low, but not unreasonable. If you want colder, you can increase R87, but -53 is all you can get there without going to a doubler or a HV tap. Making R87 100k gets you up to -52 for example.

[strat100]

The negative voltage from the topside of R87 is -52volts.The voltage either side of R84 is 52.3 and 58.6.As a complete newby i am unsure about the readings for of R84 to ground i get 2 different readings,5.64 and 17.4 and probably me taking readings wrong.

http://studentweb.eku.edu/justin_holton/bias.html#tubes
is where i got the information about the screen resistors.

Many thanks for your help.

[strat100]

The bias pot connects to the topside of R87 via wire,i clipped the resistor out and connected the wire to the main board and not the tag as instructed,as i was replacing the dreaded R85 and R86,so i soldered it into place.

[MWJB]

Your bias pot and whatever fixed resistor you have at/replacing R87 are your load for the bias voltage divider (when the pot is set to coldest setting). R84 is the dropping resistor. We have -58.6v coming into R84.

With -58.6 coming in to work out what the divider puts out:

v in * (load/load+dropper) = v out, or

-58 * (3.3K/17.4K+3.3K) = v out

-58 * 0.84 = v out

v out = -48v...less a few v when the tubes are installed, near enough to your -43v.

You want the load (bias pot & R87) to be a substantial value at full cold setting (max neg voltage at 6L6 pin 5), as Enzo suggests 100K, will maximise your sweep. You should be able to get around -56v. This should be enough to bias up most Reflector/Sovtek/EH 6L6 or 5881, maybe even JJ too.

[strat100]

Connected a 100k to bias pot and bias is at 28.4 at cold and 30.4 at hot,is this ok now or should it idealy be a bit hotter for the tubes?Negative voltage was brought up to -46volts.

Thanks for all you help i realy appreciate it.

[MWJB]

28-30mA is fine for the tubes you have in there now.

When you're feeling brave (or want to change the tubes for another type/brand) rewire the fixed resistor & pot for say 22K and a 50KL pot. You probably won't be able to run EH/JJ6V6 (as per your earlier intent) with your current set up (too cold). Sounds like you currently have a 10K pot which won't allow much variation from hot to cold with that large value load resistor.

[strat100]

Changed the 10k pot to a 50k pot and added a 18k resistor,and can now bias between 20 and 40mv,added 2 jj 6v6 and biased to 22mv,and now have the tone i have been looking for.
Torres have certainly lived up to there reputation on here,never again will i buy anything from them.