Sag Resistor

[Ambugaton]

Hey everyone, a while back I completed the Sluckey Deluxe Lite build. Amp works and sounds great but I ended up with about 460V plate voltage. I want to drop this down to around 400V and I read about a sag resistor to accomplish this (https://robrobinette.com/5e3_Modifications.htm#Simulate_Tube_Rectifier)

Does anyone have any experience with this mod?

The website has a calculator to estimate B+ current and I get:

Calculated voltage at first capacitor (B+): 445.25V. Subtract 6V, if using choke.

Calculated filament current (typically the 6.3v secondary): 1.56A

Calculated current: 106.65mA at 8500R load

I am just having trouble knowing if the estimate is where it should be.

I get 12W 560R (60v drop / .107A) Does this seem correct?

I also had the amp blow 2 fuses... hoping that it is just a bad power tube(s).

Thanks.

[Ambugaton]

CORRECTION (Used the wrong OT primary impedance)

Calculated voltage at first capacitor (B+): 445.25V. Subtract 6V, if using choke.

Calculated filament current (typically the 6.3v secondary): 1.56A

Calculated current: 112.33mA at 8000R load

Seems like a 20W 560R resistor should do the trick.

[12AX7]

560 seems awful high. I experimented with this a few years back and anything over maybe 200R is too much and the amp would just feel/sound mushy to a flawed degree. I think i ended up with a 135R but in the end i still felt the sag was not natural feeling. Just my inexpert but there ya go for what it's worth.

[HotBluePlates]

560 seems awful high. ... anything over maybe 200R is too much and the amp would just feel/sound mushy to a flawed degree. ...

+1.

I'm not sure Rob Robinette has actually tried all the "mods" on his page, or if he's just collected ideas from different places on his site.

If you need to drop "a little voltage" a series resistor between filter caps (or from rectifier output to 1st filter cap) can get it done.

If you want to drop a lot of voltage, you may need a rectifier tube, or put a zener diode between the PT center-tap and ground.

Of course, you'll never know if you like/dislike the impact 12AX7 describes unless you try it out... Maybe it's worthwhile to give it a shot & hear for yourself.

[Ambugaton]

Thanks for the info! The space is very tight (in a stout chassis) so I am trying to weigh my options vs trying to squeeze a rectifier tube in there. I just need to suck it up and just do it. Appreciate it.

[eleventeen]

If you think about a cathode bias output section; you generally see these with a 120-150-180 ohm resistor between ground and the cathodes.


If we assume that 90% of the total current drawn by the amp goes to (and through) the output tubes; and thinking about a 6L6 output stage where we typically see about -50 volts bias (in fixed bias) or see the cathodes (in cathode bias) sitting about 50 volts above ground....then it stands to reason that your sag resistor should be roughly this same value to produce a 50 volt drop.


Just a kind of piece of perspective on sizing this resistor.