Extending bias range

[Emetal]

Hello people, I can only bias my 6L6GC tubes to 50% that's as far as my trim pot will go.. It sounds good now, but I would just like to hear how would it sound at 55% or 60%. How could I extend bias range? Do I have to change the trim-pot to different value or just some other resistor? Will this in any way be bad for the amp?

[pablogt]

I think you only need to reduce R30. I usually adjust the circuit until I have a Bias voltage about 30% above needed with the pots in full "cold" position.

Pablo

[Emetal]

Ok, so R30 stock is 27K, to what value should I go down to? Any suggestions at least approximately?

[pablogt]

It's quite safe to try different values if you do it without valves and in stand by. Start by maybe halving R30. Or install a temporary pot and dial in the desired negative voltage and measure it later. There is not much current there, any pot rating will do. You could also do the math with Ohm's law (calculate actual current and solve for resistance) but you'll probably get there sooner just trying the values you have at hand.

Pablo

[tubeswell]

I think you only need to reduce R30. I usually adjust the circuit until I have a Bias voltage about 30% above needed with the pots in full "cold" position.

Pablo

Reducing R30 would not increase the range, rather it would keep the same range, but it would be a 'hotter' range. To keep the same range but make the bias cooler, you need more negative voltage, so you would increase R30. But to increase the range itself, you would want a bigger pot.

[Emetal]

I am not shore we are all "on the same page" here. Tubeswell, I want to run my tubes "hotter", they are currently at 50% and I do want them at about 60% at least, so I guess R30 needs to be decreased then, as Pablo sad...

[vibratoking]

I am not shore we are all "on the same page" here. Tubeswell, I want to run my tubes "hotter", they are currently at 50% and I do want them at about 60% at least, so R30 needs to be decreased then right?

I think everyone is on the same page. R30 needs to be decreased so that the bias moves more negative - closer to ground. This will increase the bias current. I'd change it to about 20K. That will probably put you in the ballpark.

Keep in mind that R35 could also be tweaked so that you could switch back and forth between your favorite sets of EL34s and 6L6/5881s without having to changed the pot and rebias.

[tubeswell]

Yes decrease R30 if you want the tubes to run hotter. (But this won't increase the range of the adjustment, it'll only shift it to a relatively hotter range. If you want to increase the bias adjustment range, you'd need a higher resistance pot). Decreasing the R30 resistance lowers the bias voltage, (i.e.; makes it less negative), which will increase the tube current.

R30 needs to be decreased so that the bias moves more negative

That would be 'less negative', right? e.g. -20V is less negative than -30V. When you talk about negative voltage you need to turn your head 'upside down'. Its like me having to think about the northern hemisphere.

[Emetal]

But this won't increase the range of the adjustment, it'll only shift it to a relatively hotter range.

I understand, the range will stay the same it will just shift all together in to "hotter" area. That is completely fine for me The reason is, I have American version of the amp made for 110 V / 60 Hz and I am using it in Europe through step-up transformer but at 50 Hz. Change in frequency made my amp ran really "cold". So I do not need that colder part of the range I had. Same range just "hotter" is perfect solution.

Keep in mind that R35 could also be tweaked so that you could switch back and forth between your favorite sets of EL34s and 6L6/5881s without having to changed the pot and rebias.

How could that save me from future biasing, gotta bias new tubes when you put them in, right? I mean I do have switch on the amp that get's me in the ballpark of EL34 or 6L6/5881 but I gotta bias them when switching, right? And I see that R34 is on the Power Supply side, I am not shore what R35 is doing? Here is bigger schematics...

[Reeltarded]

That would be 'less negative', right? e.g. -20V is less negative than -30V. When you talk about negative voltage you need to turn your head 'upside down'. Its like me having to think about the northern hemisphere.

The man who should know that less IS more. Mr. Up Sidedown. What's it like winter there? Oh my.. TRADE YA spiders and all.

EM, I think what VK meant is other tube types there, for instance 6v6 and needing say -14 to -52 or so to accomodate anything that fits. Short range is fine if you never plan on that though.

[Emetal]

Hmmmm I think I know what Vibratoking meant, looks like R35 is bringing the choice from the EL/6L6 switch (from the back panel of the amp) in to the biasing circuit, but I still do not see how tweaking R35 would enable me to switch between "favorite sets of tubes without biasing"

[tubeswell]

Hmmmm I think I know what Vibratoking meant, looks like R35 is bringing the choice from the EL/6L6 switch (from the back panel of the amp) in to the biasing circuit, but I still do not see how tweaking R35 would enable me to switch between "favorite sets of tubes without biasing"

What you could try instead is this:

Put a (say) 270k resistor in series with a switch, and put the switch and 270k in parallel with the 27k (and you don't need to take the 27k out of the circuit). That way with the switch closed the 270k will be in parallel with the 27k and that makes 24k5, which will give you the hotter range. With the switch open its just the stock 27k in the bias circuit.

[Emetal]

Well solution I was looking for is definitely reducing R30, and since I do not change power tubes for years I am OK. Do not need any more flexibility then that honestly. I was just asking out of the pure curiosity, what did Vibratoking had in mind with R35...

[VacuumVoodoo]

That amp has a bias switch for changing from EL34 to 5881/6L6GC, doesn't it?
The switch places R35 in parallel with R32 for 5881/6L6GC reducing resistance between C18 and C19 from 15k to 10k, thus increasing the magnitude of bias voltage.
So all you need to do is replace R35 (33k) with 68k to make that 12k which i think should be enough to do what you want to do and still use the bias switch to revert back to EL34s.

Or, it might just happen that running 6L6GC with bias switch in EL34 position could let you adjust the bias to where it puts the tubes at 60-70% of max plate dissipation.

[Emetal]

Vacuum Voodoo you are genius as usually