Bassman bias balance pot dilemma

[Baxtercat]

[duplicate]

[loicdulux]

A trimmer like part number PT15D-5K on this page is what I am suggesting: http://www.mouser.com/catalog/catalogusd/646/873.pdf

There are actually 4 pins, so you'd have to add a couple of eyelets to the bias/rectifier board. There are two wiper pins, one on each side instead of just one at the top as shown in my drawing.

I finally got all the parts (just discovered a part shop next door... will avoid heavy cost in ordering online for next times..)

Just a stupid question because I'm a real newbee who needs to learn and read a lot... In your schematic when you use resistor, would you use at that place 1/4watt or 2 watt resistors? (I bought both).

Thanks!

[martin manning]

Use the 2W. That way you are sure to be ok.

[loicdulux]

It makes a lot of sense indeed!

I'll post pictures of the mod this weekend, hopefully. Thank you a lot Martin!

[loicdulux]

Hi Martin,

Here we go!

I simplified the power entry: I removed the ground switch and its capacitor, removed the plug as it is wired to a transformateur 110->220, and installed a plate for the future master volume.

You can see the Piher I bought, 5K. Now, I'm not 100% sure how should I connect the Piher according to your schematic. Beside, how should I stick it: glue it? Or should I do some holes near the diodes/cap like it was a PCB?

(and once done, what is the procedure to bias the tubes: should I wire a 1ohm resistor to the grid to do the measures?)

Thanks for your help so far!

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[martin manning]

You are in Luxembourg? I was just watching the finish of the Tour of California, where Andy Schleck looks to be steadily improving. I'm looking forward to this summer's TDF!

Anyway, that trimmer looks like it will do the job, and is the same pattern as I drew in my sketch. Can you get a picture of the rectifier and bias supply board in your amp? If it is like the original Fender layout, I think it would be best to add another eyelet or two to accept the three legs of the trimmer, which you would then solder in place. If your electronic parts store doesn't have eyelets, perhaps you can find some at a craft supply store. Plain brass or copper will work, tin plated is best.

I think the 1 ohm resistors are a good idea. They will go from the cathodes to ground, so the voltage (in mV) at the cathodes will be numerically equal to the current in mA. Can you get a good picture looking straight down on the power tube sockets?

[loicdulux]

I'm french, living in Germany, just at the border with Luxembourg where I work. Yes, Andy is quite famous here as you can imagine in such a small country! I hope he will be able to do the TDF, last year was a pretty sad story...

I get the pictures right now! Yes the bias is what you think I'm sure. i think the eyelet is the best option: I need to document myself how to put these without removing all the diodes and so on.

Still confused what the procedure is when there is two pot for bias (the balance + the new one I will put there)


EDIT:


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Again, thank you so much for your time!!!

[martin manning]

It would be nice if the spacing of the trimmer's legs matches the spacing of the existing eyelets with the jumper wire. Get some de-soldering braid to draw the solder out of the existing eyelets. I think I would place a heat sink (an alligator clip, say) on the diode lead so it doesn't get damaged by the heat. You could also just put a new 1N4007 diode in, as they are very cheap.

I see the cathode grounds are soldered directly to the chassis. If you are going to add 1 ohm resistors, I would un-solder the existing ground wires from the tube sockets (Use de-soldering braid) and leave the other end attached to the chassis. You can then connect one end of the 1 ohm resistors to the socket and the other end to the old ground wire.

To set the bias I would set the existing balance pot in the center and adjust the new trimmer to get the bias current level approximately where you want it by measuring the voltage across the 1 ohm resistors. Then you can put your meter across the two cathodes (non-ground side of the 1 ohm resistors) and adjust the balance pot to get zero volts. Then check the voltage across the 1 ohm resistors once again and tweak the level if necessary.

[loicdulux]

Yes, looks like the trim will go in existing eyelet! Thanks for the advise, I have alligator clips.

The thing is I've got only cheap metaloxyde resistor, should this do the job for bias and larmar volume or should I buy carbone stuff?

[martin manning]

I think the metal oxide will be fine.

You could probably get away with just soldering the two trimmer legs into the existing eyelets and wrapping and soldering the lead of the resistor going to the balance pot around the third leg. Bend the third leg out so the trimmer can lay flat on the board.

[loicdulux]

Exactly what I started to do as the two legs to diode and cap fit in the board

Ok let's go for the metal then.

[loicdulux]

Hi Martin,

I think I'm done with the Bias mod! But I prefer to check with you before switching the beast on!

Here are the pictures below. So I finally placed
- Resistor Metaloxide 2 Watts / 470 Ohms
- Resistor Metaloxide 1 Watt / 1 kOhms
- Trimmer pot Piher 5K

Does the layout look correct to you? Should I turn completely counterclockwise or clockwise the trimm pot before switching on the first time?

I hope the resistors are fine (wattage, composition).

For the bias, I ordered with the parts that kit with the 1 ohm resistor, I think for more convenient I should order a second one.. It will avoid any additional solder!

Next step: wire the PPIMV pot. I'll move to the other post for that!

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[martin manning]

To test your bias mod remove the power tubes (to be extra safe) and turn on the main power. You can leave the standby off. Measure the voltage on the outer lugs of the bias balance pot with the trimmer at both extremes. You should get a range of negative voltage centered around -45V or so. Set it to the most negative voltage when you power it up with the MV installed. That should be at maximum clockwise rotation

Looks like you have a bias probe kit there. I was suggesting putting 1 ohm resistors on the sockets inside the chassis so you could use your multimeter to check the bias.