Lowering bias range

[studiodunn]

If I need to get down from 39mA to 34mA in this circuit would I raise R41?

I currently have a 30k pot and a 1k7 at R44. I’m also sitting at -44 on the grid and think -45 is my target. As I understand, raising R41 should both lower my range of bias and increase my -voltage.

My PT has a 60v Tap feeding this.

88773FB1-85D6-4C5C-B8C0-3DE01CC236D8.png

[Dingleberry]

That would do the trick. Tailor the resistor value to fit your target voltage. And yes it will narrow the effective range of the pot also a bit.

[sluckey]

I prefer to lower the value of R44.

[Stevem]

I agree!

With lowering the value of R44 and the AC voltage it passes to the diode it will be easier to set up the trim pot so that its adjustment range will be better centered for you.

If it turns out that you need to parallel resistors to get your needed value just remmenber that your end value will always be less then what your smallest resistor value of the two are.

Also once you get to millamp goal, please play the amp at the normal playing volume it will likely be used at, and do this for two reasons.

1) so you can recheck the millamp setting as it will likely be pulling more current then what you set it for.

2) so you can then play around with the amp and see if a lower current setting serves you just as well in terms of punch , crossover distortion and where the output stage clips at if your playing the amp that loud.
A lower idle current setting will add life to the tubes.

[roberto]

Please note that R44 was 120k and now is 1k7. R44 has also a limiting effect on the inrush current to charge the capacitors, because often the bias tap has really small currents to keep low the VA of the PT. Going too low could ask too much current at amp startup. What about a FWB? Please check also the admissible maximum voltages of the bias caps.

[studiodunn]

Please note that R44 was 120k and now is 1k7. R44 has also a limiting effect on the inrush current to charge the capacitors, because often the bias tap has really small currents to keep low the VA of the PT. Going too low could ask too much current at amp startup. What about a FWB? Please check also the admissible maximum voltages of the bias caps.

This is why I asked about R41. I assumed I was reaching the lowest safe value for R44 and would need to find adjustment elsewhere.

[sluckey]

I looked at the schematic and just glazed over the part where you said R44 was 1.7K.

R44 was originally 120K? That implies that R44 would have been connected to one of the HT wires, usually 300-400VAC. But now it is 1.7K (odd value)? That implies that R44 is now connected to a bias winding or tap, usually approx 50VAC. What is R44 really connected to?

If R44 is connected to a low AC source then you can safely drop on down to 470Ω.

[studiodunn]

I looked at the schematic and just glazed over the part where you said R44 was 1.7K.

R44 was originally 120K? That implies that R44 would have been connected to one of the HT wires, usually 300-400VAC. But now it is 1.7K (odd value)? That implies that R44 is now connected to a bias winding or tap, usually approx 50VAC. What is R44 really connected to?

If R44 is connected to a low AC source then you can safely drop on down to 470Ω.

The schematic was made with not having a bias tap in mind. The PT I'm using has a 60v bias tap, so that is the reason for my 1k5 value @ R44. Sorry 1k7 is a wrong value/mistake in my original post.

R44 is connected to the 60v tap, so I can safely drop R44 to (if needed)470Ω?

[sluckey]

R44 is connected to the 60v tap, so I can safely drop R44 to (if needed)470Ω?

Lots of Fenders been doing it for 60 years. Fender used a 470Ω 1 watt, but I use a 3 watt metal oxide.

[roberto]

Just consider that the peak charging current will be 50 mA with R44 = 1k7 and 180 mA with R44 = 470R compared to original 700 μA.

[sluckey]

Just consider that the peak charging current will be 50 mA with R44 = 1k7 and 180 mA with R44 = 470R compared to original 700 μA.

Your comparison is flawed. The original 120K was never used with the 60V tap. The 120K only appeared on paper and was meant for a bias circuit that got it's AC input from the PT HT which was probably 300 to 400V. Also, your peak charging current only exists for maybe a couple milliseconds when the charge on the cap is zero volts. Even after 1 RC time constant that peak is greatly reduced. I just don't see it as an issue.

[roberto]

Hi Sluckey,

my point is very simple: instead of having 470R and 15k, you get better results in terms of load and also in terms of ripple filtering if you use two identical higher resistors.
It is also smarter for the BOM and so economical point of view: 4k7 (or 5k6, or 6k8) on both R44 and R42 will do a better job with a lower bias voltage.

[sluckey]

Maybe I misunderstood your point. I was sure you were concerned about "the peak charging current".

[Ten Over]

A 60Vrms tap with the bias circuit the way I understand it should put out way more than -44V. A 60V peak (42.4Vrms) tap might put out the observed voltage.

R41: 47K
R42: 15K
R43: 30K
R44: 1.5K

Where did you get the 30K pot?

[studiodunn]

A 60Vrms tap with the bias circuit the way I understand it should put out way more than -44V. A 60V peak (42.4Vrms) tap might put out the observed voltage.

R41: 47K
R42: 15K
R43: 30K
R44: 1.5K

Where did you get the 30K pot?

You are correct in that after a sorting of the relay supply and short burn in the bias voltage is now sitting at -52v and I have the available adjustment in the bias pot to get down in the 33mA range. All seems stable and withing reason for now. I'm going to continue to run the amp and monitor voltages.

The 30k pot is what I had available.