Screen Sag - Resistor in place in Choke

[Littlewyan]

In my JCM800 that I made out of a Pignose G40V I currently have a 1K Resistor instead of a Choke. There isn't really enough room to install a choke so I was thinking of just putting a 390Ohm Resistor in parallel to give me 280Ohms.

Do you guys think this will make much difference tone wise? I don't think ripple should be an issue as the B+ is only 340VDC and I have a 50uF Cap for the screens.

The amp uses a 2 x 6V6 Power Amp by the way. 8K OT. Screen current draw is very minimal.

[Ken Moon]

What's the normal JCM800 choke?

Maybe 100 to 200 ohms?

You could go as low as 100 ohms, just use a power resistor that can handle the average current drawn by the amp (25W should be plenty).

If you use this type you can bolt it directly to the chassis (I've found they don't get very hot at all, so this will be plenty of heat sinking).

http://www.newegg.com/Product/Product.a ... lsrc=aw.ds

If you're nervous, you could go to 50W, but you don't really need it.

When paralleling resistors, don't use one low wattage resistor in parallel with a high watter (duh)

If I were you, I'd buy a few with different values (maybe 100, 180 and 270 ohms) and see if you can hear any difference.

Good luck,

Ken

[Ken Moon]

Looking at your post again, I see you're only using the RC filter to power the power tube screens, PI and preamp. So you don't need much current handling.

But you might want more filtering than a small value resistor and cap provide.

You could determine the response of the LC filter and compare to the RC. If the RC doesn't filter as well as the LC, you could raise the cap value in the RC filter to regain the filtering without increasing the voltage drop.

Cheers

[Littlewyan]

Yeah that was one idea. I did look at a few previous Marshall schematics and see they used a 100Ohm Resistor with 50uf cap on the JCM900 and 6100. So going by that it should be ok. Those were both 100W amps I looked at as well!

The reason for putting the resistor in parallel was A) So I didn't have to unsolder anything to lift the board (I'm using the original power board) and B) To give me a higher wattage rating resistor, altho 7W (what I have on hand) should be plenty for this. Marshall used 4W in the JCM900

What difference do you think it will make? Tighter low end? Harder distortion?

[Stevem]

The answer to that last question is hard to pin down until you can tell us how hard or easy the amp will normally be run!

[Littlewyan]

Its a Marshall Stevem, how do you think? . But seriously I have the gain at 5 and the Master at 8. So the power stage is running near enough flat out.

[Stevem]

Yup, I kinda guessed as much , but did not want to assume !
Well then blast the cap out if it and see if the voltage sag with a resistor goes beyond what you can tolerate !

[Littlewyan]

I vaguely remember that it sags by about 40Vish. I'm just wondering what effect it would have on tone if I lowered the sag?

[Ken Moon]

All else being the same, decreasing the screen voltage by increasing screen sag will make the tubes last longer, and produce lower power output and headroom.

What you are doing is making the screens less able to accelerate electrons toward the plates at high speed (like turning down the turbo boost on a car engine)

The sonic effect is the typical saggy stuff - easier overdrive, less overall volume, and a soft fluffy squishy and more compressed sound.

[Firestorm]

The screen in a 6V6 can't pull all that much current before it melts, so tonal differences from screen sag with varying resistance will likely be small. Tweed amps without chokes typically used 10K there! But the intent wasn't to sag the screens, it was to keep variations in plate supply voltage from affecting the screens and the rest of the amp. You don't want the plate to pull down the screen (or the PI). With no choke and no resistor your 6V6 effectively becomes a triode: lower gain, compression and so on, so the value of the resistor controls how far you go in that direction, but if it's too low, plate voltage variations will impact the rest of the power supply.

[Littlewyan]

I see now.

So as the 6V6s draw a lot less screen current when compared to EL34s this amp will probably have the same amount of screen sag as a stock JCM800.

I'll measure the voltage under load at the weekend and see what I find

[pdf64]

...With no choke and no resistor your 6V6 effectively becomes a triode: lower gain, compression and so on, so the value of the resistor controls how far you go in that direction, but if it's too low, plate voltage variations will impact the rest of the power supply.

My understanding is in opposition to the above.
I can't see how a sag free g2 (eg no screen grid current limiting resistor, low resistance between plate and g2 HT nodes) would be triode like?
Rather 'large signal' compression effects would be reduced.

The overdrive tonal character may be perceived as being 'harder / sharper'.

[Littlewyan]

Harder/sharper is good, the amp is a tad soft sounding. I keep listening to these clips of aggressive sounding JCM800s and can't help but feel that mine is a bit tame.

I looked at my notes for my 67 Marshall and saw that under load the screens node was only 6V below the OT CT. So mine is actually sags quite a lot compared.

Also I suspect that as I have a lower HT (340VDC) my pre amp voltages will be a bit low and if there is a big enough drop across that 1K resistor under load they will be far too low. For this type of amp anyway. I will do some tests over the weekend and shall report back.