5F10 Build and Bias Question

[Mr Ridesglide]

Love Harvards - I'm sure this will be fantastic

 

[polloelastico]

My name is King Fan and I have a new addiction, simulating these 10,000 maniac bias circuits. Here's the nice solution @Ten Over mentioned above -- as always, if I did it right.



As he predicted, the bias range runs about –19 to -35V. These sims aren't perfect -- but a range anything like that should work well.

The one you made for me the other day saved me tons of time.

I'm am also now hooked on these; as much as I like clipping in different values and experimenting, the simulator is just great.

 

[King Fan]

The one you made for me the other day saved me tons of time.

I'm am also now hooked on these; as much as I like clipping in different values and experimenting, the simulator is just great.

I'm glad that worked for you -- I was grateful to you for your exploration of the original (low) bias voltage on some of these circuits.

 

[mrfitz98]

Thanks for the info. Was that a 68k or a 6.8k before the diode?

Yeah, 6.8K. Typing with one hand post shoulder surgery... ;-)

 

[jsnwhite619]

I didn't make it through all the responses, so I may be repeating someone else here - but I prefer a 10k bias pot over the larger ones like 50k. It's much easier to fine tune the voltage on a smaller value pot. Both rotate 300 degrees, but the 50k is offering 5x the total resistance. Once you land on your fixed resistor value, I've never had one yet that couldn't go from probably 45% to 75% bias adjustment on the 10k pot. The "safe zone" of adjustment is usually a little less than half of the full range of the pot. I even started using a dry erase marker to put dots on the pot so that I know where to keep things. And with dry erase, you can wipe it off and do it again with a tube change.

 

[King Fan]

I didn't make it through all the responses, so I may be repeating someone else here - but I prefer a 10k bias pot over the larger ones like 50k. It's much easier to fine tune the voltage on a smaller value pot. Both rotate 300 degrees, but the 50k is offering 5x the total resistance. Once you land on your fixed resistor value, I've never had one yet that couldn't go from probably 45% to 75% bias adjustment on the 10k pot. The "safe zone" of adjustment is usually a little less than half of the full range of the pot. I even started using a dry erase marker to put dots on the pot so that I know where to keep things. And with dry erase, you can wipe it off and do it again with a tube change.

Well said; an under-appreciated point. Merlin himself notes a 10K pot is ideal for technical reasons and basically always sufficient for range. That's *if* you adjust both head and tail resistance appropriately, and as @Ten Over showed in that recent thread, it can help to choose wisely between variable resistor (2-lug) and true potentiometer (3-lug) wiring.

@Amp vonRorklander , I think you’ll be fine. Lots of smart folks use 25k and 50k. This is especially true in big amps with high plate voltages and the potential to use several output tube types. It was easier for me in my 5G9, though a 10k coulda worked; in that amp I installed 1ohm cathode resistors so I can measure as I turn, so there's no ‘steep slope' issue. As Merlin also notes, there are a thousand ways to build a bias circuit. Some of the 50k folks use multi-turn trim pots to get back the gradual adjustment, though others find *that* tedious.

It's just helpful to dispel the idea you *need* 50k to span a sufficient range.

 

[Scottcurry13]

Agree with @jsnwhite619 . I’ve used 10k in both my builds and they work fine.

 

[Kevin Wolfe]

Another vote for a 10k pot. You won’t have to ”chase“ the bias setting anywhere near as much.

 

[Amp vonRorklander]

Hey Folks,
I’ve finally finished the 5F10. Had to wait for a couple of components and the time to devote to it…
Thanks to all of you and especially TenOver and KingFan for help with the bias. Your assessment was spot on. I used the 15k before and the 20k after the 50k pot and got my -27 Vdc with no problem.
The amp is finally all assembled as of last night. Early reviews are very good. It’s quiet and clean all the way up the dial and sounds very nice. I think the speaker will need some break-in though.
Incidentally, I had to remove the back cover off the speaker to make it all fit…

 

[Kevin Wolfe]

Just out of curiosity, what did your plate dissipation come in at the -27?

 

[Amp vonRorklander]

Just out of curiosity, what did your plate dissipation come in at the -27?

About 23mA.

 

[Kevin Wolfe]

Plate dissipation would be in watts. It’s a function of Ohms law. Multiply your plate voltage by 0.023

 

[Amp vonRorklander]

7.34

 

[King Fan]

If your tubes are dissipating 7.3W, that's about 60% of 12W, a traditional 'safe' max for the 6V6, and 52% of 14W, which many people now take to be a 'useful and safe' max for 6V6s. Since your target is roughly 50-70%, you're cool, but not too cool. And that's cool.

You probably know this, but just as we don't bias to a specific plate current, we definitely don't bias to a specific bias voltage. Especially a Fender vintage bias voltage. Your -27V isn't doing badly, but it wasn't your target, cuz each tube or tube pair will require or react to different values differently.

It’s quiet and clean all the way up the dial and sounds very nice.

Hmm, clean is nice, but is that the sweet spot? *Now* you actually start to use your adjustable bias. Since you're at the low ~50% end of the 14W range, adjust bias "by ear" -- play an E chord several times, maybe part of a simple scale, and adjust bias. Play again. Listen, measure current and *plate voltage*, calculate % max dissipation, and keep going up until you reach 70% or so; then come back down until your ears tell you you've found your best tone.

Let us know if that doesn’t makes sense; depends on how you learned to bias. I use Rob's calculator; extra easy if you have plate current... but do recheck plate voltage, which changes with bias.

https://robrobinette.com/Tube_Bias_Calculator.htm

 

[Amp vonRorklander]

If your tubes are dissipating 7.3W, that's about 60% of 12W, a traditional 'safe' max for the 6V6, and 52% of 14W, which many people now take to be a 'useful and safe' max for 6V6s. Since your target is roughly 50-70%, you're cool, but not too cool. And that's cool.

You probably know this, but just as we don't bias to a specific plate current, we definitely don't bias to a specific bias voltage. Especially a Fender vintage bias voltage. Your -27V isn't doing badly, but it wasn't your target, cuz each tube or tube pair will require or react to different values differently.



Hmm, clean is nice, but is that the sweet spot? *Now* you actually start to use your adjustable bias. Since you're at the low ~50% end of the 14W range, adjust bias "by ear" -- play an E chord several times, maybe part of a simple scale, and adjust bias. Play again. Listen, measure current and *plate voltage*, calculate % max dissipation, and keep going up until you reach 70% or so; then come back down until your ears tell you you've found your best tone.

Let us know if that doesn’t makes sense; depends on how you learned to bias. I use Rob's calculator; extra easy if you have plate current... but do recheck plate voltage, which changes with bias.

https://robrobinette.com/Tube_Bias_Calculator.htm

Well, I finished my fine tuning of the bias (at least for now). I’ve settled on a pretty cool number of 59% (see pics). It sounds good to my ears. I’ll play this for a while and adjust again if I feel like it needs it.
thanks for all your help!