Voltage divider for screens

[dwinstonwood]

In this RCA Receiving Tube Manual schematic they used a voltage divider consisting of a 2K 20W (R26) and a 12.5K 20W (R27) resistor to lower the screen voltage.

I've never seen this done in a guitar amp. Other than the cost of two resistors, is there a reason why it's not done in guitar amps?

Just wondering. Thanks.

[pdf64]

Perhaps it was only done there to accommodate the 270V screen grid limit of 6L6G?

[shooter]

it's a wash, one R for cap bleeding, one R for screen current, any time you get a 2fir, go for it 

[dwinstonwood]

Thanks y'all.
shooter, I didn't even think about R26 as a drain resistor, or R27 as a screen resistor. But, yeah, double-duty parts.

[PRR]

I don't think there was any adult supervision on those "RCA DIY HiFi" plans. The screen divider either needs bypass caps or be scaled to run HOT; possibly both.

The hottest amp I ever got a blister from was a Fisher 7189 (hi-volt EL84) with screen dividers to get the 400V B+ down to 300V screen rating. Yes it worked, and with mods sounded sweet (like a fine transistor amp) but I could not stand it in summer.

I think some RCA guy had a hobby, a boss heard about it, and commissioned that series of DIY articles. Most of them will work but few are well-balanced design.

[astronomicum]

What pdf64 said +

Assuming the supply was 300V (and lets assume that this remains constant for this example), the voltage divider by itself would consume about 20mA and produce about a 40V voltage drop to the screen node. Lets assume the front end and screens consume about 10mA at idle. This would produce and additional 20V drop. The screen node voltage at idle would then be about 240V (lets assume this was the design target). Now lets assume that at full output, the front end and screens draw about 25mA. The screen voltage at full output would be about 210V. Keep in mind that the difference in voltage drop between idle and full output is being generated across the 2K resistor.

Now lets assume that the designer wanted to maintain the screen node voltage at idle at 240V, but instead used a typical dropping resistor. Without the voltage divider, the front end would be drawing 10mA and the resistor value would need to be 4K. At full output, the voltage drop at 25mA would be 60V and produce a screen node voltage of 180V, 30V lower than using the divider.

The voltage divider would produce less of a voltage drop between idle and full output.

Of course there is no way of knowing what the designer’s intent was, but this example may suggest one possible explanation.

[dwinstonwood]

Thanks for all the input.
So, no wonder it's not seen elsewhere.

[tubeswell]

As well as what the others have said, a voltage divider to supply current ideally needs to be ‘sized’ so that the current at the voltage divider output is a relatively small proportion of the total current flowing down the voltage divider- otherwise, if the amount of current at the output dominates the amount of total current in the divider, the current flow at the output (I.e. to the screens) will affect the output voltage more, causing a noticeable drop in supply voltage when screen current increases. This makes for poor regulation and bad performance. (A voltage divider is a crude ‘shunt regulator’). If you want to regulate screen supply, better to use either a separate HT winding to supply just the screens, or a better type of voltage regulator design (like a tube regulator).

[dwinstonwood]

Thanks for that tubeswell. That shed light on some stuff.

Or, a choke - as shooter always reminds us resists current change - is probably what we should just stick in front of our screen supply. Ah, Leo.

I'm not planning on using any of those bass amp tubes that want 800VDC on the plates and 250VDC on the screens. I was just curious about what that circuit was doing. 

[shooter]

they're called UL OT's, choke build right into the OT, what could be easier 

[sluckey]

There is a lot of wasted current in the voltage divider on the RCA manual schematic. This old Rockola amp uses a similar voltage divider, but puts the divider current to use, powering a field coil speaker. See page 2 of the linked pdf...

     http://sluckeyamps.com/rocky/rocky.pdf