Bias Supply Droppers

[FourT6and2]

I suspect Merlin purposely avoided any calculations to determine the value of that resistor. Too involved and the result is often not perfect. Most circuit engineering is done through plagiarizing from reference books. The value of that resistor (often called the bias range resistor) is very quickly and easily determined experimentally. You already know that the higher AC voltage source will require a higher value resistor. It doesn't take much experience to know the resistor will typically be less than 1K when tha AC source is from a bias tap and it will typically be 100K or higher when the AC source is from the HT winding. You can either substitute different resistor values or even use a pot to dial in the proper value to give you the desired bias voltage range. The proper bias voltage depends on the output tube type. Typically for an EL34 a bias range of -30v to -50v will give you the exact bias voltage you need. Your R16 gets you in the ballpark and R20 will help set the amount of swing and the upper/lower limit. You don't even need the output tubes plugged in to get the bias circuit tweaked for proper operation.

Thanks for that!

See that's what I thought. However the brother amp to the one I'm building is spec'd with a PT that has HIGHER voltages on the secondary. Yet the bias supply for that circuit shows 180K and 47K resistors instead of 220K/56K. If the incoming AC voltage is larger, why are these resistors SMALLER in that amp compared to the circuit with lower voltages? Wouldn't it need to be the other way around? Both amps bias up EL34s just fine.

In any event, I already built the turret board with 220K/56K. So I guess I'll see what happens. Thanks for all the info!

[sluckey]

You already know that the higher AC voltage source will require a higher value resistor.

That statement applies ONLY to R16 in your schematic. It does not apply to R 20.

The value of R16 is DIRECTLY PROPORTIONAL to the amount of tube current. IE, Increase the value of R16 will cause the tube current to increase. (Because the bias voltage goes down)

But the value of R20 is INVERSELY PROPORTIONAL to the amount of tube current. IE, increase the value of R20 will cause the tube current to decrease. (Because the bias voltage goes up)

[tubeswell]

... I'm talking about a bias supply fed from the Power Transformer's High-Voltage AC Secondary. Not a separate bias winding. The "dropping resistor" I'm referring to is BEFORE the diode—between it and the PT, which has no bias tap. The below schematic shows the circuit and values from a 50-watt, EL34 amp with 325v PT secondaries. My question is how to figure out a new value for the resistor circled, if I change the PT to a 350v-0-350v that, instead, maybe puts out an actual 365v AC, for example. Doesn't that dropping resistor need to change now?

Not really. See Steve Luckey's reply (above). There is next-to-no-load (a couple of milliamps usually) on your typical bias supply's VAC source. The AC series dropping/limiting resistance is not critical (some amps use like a Pro Reverb, use 1k8, some amps, like Princeton Reverb, use 100k). Anything between 470R (3W) or 100k (1W) resistor will do for the AC side of the bias supply as long as the 'lower leg' of the voltage divider is accordingly sized to get the desired Vout range. Doug Hoffman's page on designing bias supplies is a handy reference http://el34world.com/charts/Biascircuits.htm

Re: Merlin's equation, if you have his power supplies book handy pp176-177 explains how to estimate required bias voltage from the application factor of the tube in question, i.e. with pentodes or tetrodes, the largest bias voltage likely to be needed can be calculated from -Vg2/u*, where u* is the u of the tube if connected in triode mode. Table 8.1 lists common u* values, which is repeated here for convenience:

EL34 10.5
EL84 20.0
KT66 7.5
KT77 11.0
KT88 7.5
2A3 4.2
300B 3.9
5881 8.0
6550 6.8
6L6GC 8.0
6V6GT 9.8

Other than that, a rule-of-thumb for raw negative voltages to aim for 10 percent of the B+ for pentodes and tetrodes.

[FourT6and2]

... I'm talking about a bias supply fed from the Power Transformer's High-Voltage AC Secondary. Not a separate bias winding. The "dropping resistor" I'm referring to is BEFORE the diode—between it and the PT, which has no bias tap. The below schematic shows the circuit and values from a 50-watt, EL34 amp with 325v PT secondaries. My question is how to figure out a new value for the resistor circled, if I change the PT to a 350v-0-350v that, instead, maybe puts out an actual 365v AC, for example. Doesn't that dropping resistor need to change now?

Not really. See Steve Luckey's reply (above). There is next-to-no-load (a couple of milliamps usually) on your typical bias supply's VAC source. The AC series dropping/limiting resistance is not critical (some amps use like a Pro Reverb, use 1k8, some amps, like Princeton Reverb, use 100k). Anything between 470R (3W) or 100k (1W) resistor will do for the AC side of the bias supply as long as the 'lower leg' of the voltage divider is accordingly sized to get the desired Vout range. Doug Hoffman's page on designing bias supplies is a handy reference http://el34world.com/charts/Biascircuits.htm

Re: Merlin's equation, if you have his power supplies book handy pp176-177 explains how to estimate required bias voltage from the application factor of the tube in question, i.e. with pentodes or tetrodes, the largest bias voltage likely to be needed can be calculated from -Vg2/u*, where u* is the u of the tube if connected in triode mode. Table 8.1 lists common u* values, which is repeated here for convenience:

EL34 10.5
EL84 20.0
KT66 7.5
KT77 11.0
KT88 7.5
2A3 4.2
300B 3.9
5881 8.0
6550 6.8
6L6GC 8.0
6V6GT 9.8

Other than that, a rule-of-thumb for raw negative voltages to aim for 10 percent of the B+ for pentodes and tetrodes.

I think I got it sorted out. Thanks for the help

The main source of my confusion is that Marshall was not very consistent back in the day. And I totally overlooked the fact that some of the schematics I am looking at are actually for amps that had 6550s and not EL34s. Not all of the schematics are labeled. Some had high-voltage PTs, some did not. Some had 6550s, some did not. Some had bias windings on the PT, some did not. Once I figured all of this out, it made more sense.

220K/56K combo usually was for 50-watt amps with no bias winding running EL34
150K/47K combo usually was for 50-watt amps with no bias winding running 6550
Lower value dropper for amps WITH a bias winding (usually 100-watt amps)
The dropper's value on 50-watt amps changed depending on the PT used.

So... looks like the two circuits I'm building: 1 was designed around EL34s (220K/56K) and one around 6550s (180K/47K). But the latter is still spec'd for EL34s and seems to bias up fine. Not sure why the builder chose to do it this way. But that is why I was so confused.

[angelodp]

I suspect Merlin purposely avoided any calculations to determine the value of that resistor. Too involved and the result is often not perfect. Most circuit engineering is done through plagiarizing from reference books. The value of that resistor (often called the bias range resistor) is very quickly and easily determined experimentally. You already know that the higher AC voltage source will require a higher value resistor. It doesn't take much experience to know the resistor will typically be less than 1K when tha AC source is from a bias tap and it will typically be 100K or higher when the AC source is from the HT winding. You can either substitute different resistor values or even use a pot to dial in the proper value to give you the desired bias voltage range. The proper bias voltage depends on the output tube type. Typically for an EL34 a bias range of -30v to -50v will give you the exact bias voltage you need. Your R16 gets you in the ballpark and R20 will help set the amount of swing and the upper/lower limit. You don't even need the output tubes plugged in to get the bias circuit tweaked for proper operation.

Thanks for that!

See that's what I thought. However the brother amp to the one I'm building is spec'd with a PT that has HIGHER voltages on the secondary. Yet the bias supply for that circuit shows 180K and 47K resistors instead of 220K/56K. If the incoming AC voltage is larger, why are these resistors SMALLER in that amp compared to the circuit with lower voltages? Wouldn't it need to be the other way around? Both amps bias up EL34s just fine.

In any event, I already built the turret board with 220K/56K. So I guess I'll see what happens. Thanks for all the info!

Steve, can you expand on the comment about not needing the output tubes plugged in to set bias. No load does not effect the bias set?

[sluckey]

Steve, can you expand on the comment about not needing the output tubes plugged in to set bias. No load does not effect the bias set?

I was not talking about setting the bias for tube current. I was talking about getting the proper negative bias voltage range that will in turn allow you to set the desired tube current.

[angelodp]

Steve, ah I see now. Thanks.