Peavey VK100 Valveking Bias Mod

[krhnyc]

About 6 months ago, a thread was posted about installing a bias pot in a valveking. I had a couple questions and was hoping you could help;

(1) with whatever pot you are using, you would solder the high side to the trace where one leg (of the old resistor) would go and solder the wiper + low side to the other?

(2) you would solder the "safety" resistor to the low side+wiper?

(3) On my valveking, the resistor in question is 39K, what would be good values for the pot and resistor?


Thanks in advance or your time.

Best Regards,

> KRHNYC

[Enzo]

When you solder the wiper to one end, you turn the pot into a variable resistor - which is what you want here.

In the basic sense, you would replace the 39k R205 with this variable resistor of similar value, so yes, one end would solder to where one end of the resistor was, and the other end to where the other end of the resistor was.

But that allows us to turn the bias to zero, and we don't want that to happen. You can tell me you would never do that, but trust me, it will eventually. So we add a resistor. That way, even if the pot is turned to zero, there will still be the resistor.

SO in the simple example above, we pick one end of the pot, and add an extra resistor between it and the circuit board. Now we still have a variable resistor, but now it only goes down to whatever that resistor we added was.

So: since we started with 39k - lets call it 40k - we could replace it with a 40k pot. That is an odd value, so we would really use a 50k pot. But for safety reasons, we would prefer to use somehting like a 15k resistor in series with a 25k pot. That gives us the 40k we started with, but allows us to turn it down as far as 15k. (pot at zero)

Not only does this prevent us from turning the bias to zero, but we now can use a lot more of the pot travel range than before. That makes fine adjustment easier. Instead of just using the upper third of a 50k pot, we can use most of a 25k pot.

Now we just replaced a resistor with a pot+resistor. It doesn;t matter which one is on top and which closer to ground. Together they form a resistor that varies from 15k to 40k - it has no polarity.

[krhnyc]

Thanks, that is what I suspected, but wanted some confirmation before I wrecked something.

Judging from what I'm reading, the end result is something that should allow more voltage to pass. Is that the primary goal? Are these amps known for running 'cold'?

Wouldn't it be easier to strap the resistor across the wiper and one side of the pot? It would serve the same purpose, yes?

Finally, the older thread mentioned cermet (trim) pots, any thoughts here? A multiturn would give much finer adjustment, to idiot proof it. Lastly, I now that bias is measured in mV, but is there any wattage to be concerned about? 1/4W resistor? Higher?

Thanks again.

[tedmich]

Enzo has dial up, I believe, so I will kibitz,

the typical commercial amp is usually biased cold to get the best tube life, and the resistor in question is part of a voltage divider (two resistors to ground with a tap from their middle, nice calc here:Voltage Divider Calculator) to let more of the voltage pass. Its really a matter of current, which is why many designs include a 1ohm/3w resistor so voltage readings across directly relate to current (mV->mA). Fancier set ups even allow for separate tube biasing, to unbalance "matched tubes" or match unmatched ones; some say unequal bias sounds good too...

with -17v and ~40mA on my EL84 PP amp...wattage is of little concern and this is generally the case I think.

I think adding a resistor from wiper to leg on a pot changes the taper; its parallel but the safety resistor needs to be in series...I could be wrong..

cermet trimmers are good, multiturn is more accurate but single turns let you see ballpark where you are, the little screw on a 15-20 turn unit is mum and adjustment needs to be followed with a meter in my experience.

[CharlieP]

I am also about to do this to my VK112. I have some 25k fender bias pots and I have a 50k pot on order from Weber. I have some 6.8 k resistors on hand that I could use with the 50k but would need to get to about 15k to use the 25k pot. As soon as I get off the fence and decide which way I am going I will make the change. One thing I see, the higher the pot value adjustment gets jumpy and less smooth. This makes me want to rethink the 50k pot but I am going to put my multimeter to it and see how jumpy the 50k pot is for adjustment. It may work smoothly and I will use it. It has a locking shaft bushing too.

[Enzo]

You started with 39k. If you go to 50k, the bias voltage will rise to even cooler levels. Of course that is only if you have it maxed.

Aside from my zero protection, one point of using the protection resistor is to reduce the pot value so you use more of the pot.

This isn;t rocket science. 39k isn't chiseled in stone. You want to use your 25k pot? First off, measure it. Pots are not precision parts and a 25k pot could be a ways of from that value. It could be 30k inside. But lets say it is right on at 25k, and you want to replace a 39k, and you have 6.8k resistors. Put two of them in series, now you have 13.6k, which is pretty close to 15k. Try that, you can always change it later.

But try this. Just wire up a 50k pot by itself. Turned all the way up, the bias is at its highest voltage, I guess about -56. (close enough for this discussion) Now turn it down while monitoring the tube currents. Turn it down so they are pretty hot. We want to establish a more or less upper limit on current - or lower limit on bias voltage, however you want to look at it.

Now do not turn the pot, disconnect it from the circuit and measure the resistance at that setting. it will be less than 50k of course. The amount less is the approximate value of the pot we really want to use. and the rest woulod be our safety resistor.

In other words, if we found that the only really useful range on the 50k pot was from 50k down to 30k, that means we had a useful range of 20k. With the 50k pot, that 20k is all crammed up at one end. Hard to adjust. SO instead of just a 50k pot, we mount a 20k pot and 30k for safety resistor. Now we have the same 50k, but our control is useful through its entire range so is easier to adjust. Follow me?

[CharlieP]

The other consideration for me is mounting the pot. The weber pot has the typical threaded barrel and I could mount it firmly to the chassis while the Fender pot would be soldered to the board for its only mount. Access is maybe easier with the Weber bias option. I kind of like that better. Again sensitivity of adjustment may be less but tolerable. And I could still use one 6.8k for safety buffer.

[CharlieP]

The other consideration for me is mounting the pot. The weber pot has the typical threaded barrel and I could mount it firmly to the chassis while the Fender pot would be soldered to the board for its only mount. Access is maybe easier with the Weber bias option. I kind of like that better. Again sensitivity of adjustment may be less but tolerable. And I could still use one 6.8k for safety buffer.

OK, my 50k pot came in and measures about 44.7k if memory is correct. I have my 15k resistors too and my Bias Rite is in transit by wed. 6-3. The pot measures smooth adjustment and has a locking collar nut. So I am going with this pot. I will measure across the pot and resistor and set Resistance at 30k before I solder in place. Then I should be somewhere in the ballpark to adjust the bias since a 33k resistor was in it and I will be going hotter anyway. I can get less ohms going clockwise if I use the middle and 1st terminal and ccw using middle and 3rd terminal. With resistance going down the current comes up so I want it cw rotation. Clockwise to raise the current and ccw to lower. It doesn't matter only that it seems logical. I'm from Vulcan I guess.