Screen cap size......

[dogears]

Hi guys,

Just wondering what y'alls take is on screen cap size. In the 90s, some of the HRM amps used the Precision Power card with 40uf or so of screen cap size. Most other amps have either a single 20uf or series 47uf caps.

Anyone messed with this? What difference you notice/feel with the change??

Thanks....

[BobW]

dogears, fwiw, I didn't see any appreciable sag on the B+5 when cranked on the Hybrid REV A using the 100, 100, 20, 20, 20 values. I haven't A/Bd the two circuits yet, to see the sag, but it may be better to trust the ol ears rather than the Oscope.

[jelle]

Dogears,

I'm new to this forum but I've already tested this about a year ago in my 150w plexi and my el-34 100w 90's ODS clone. For both amps I liked low screen grid filtering. The difference is very noticable. I found the high filter values to make the amp very stiff, but that was with a choke!
Via this forum I learned about the choke replacement with a resistor.
I've settled on total 16uF for the plexi with a choke and 50uF for the ODS clone with a 120 Ohm 20w resistor. The resistor takes away the stiffness.

This way it works for me. By the way: I like 110uF total filtering for the plates for the ODS. I play stratocaster and marshall cabs with G12M.

Great Forum!!!

Jelle
The Netherlands

[dogears]

Thanks guys......

So, how do you all feel about the primary caps?? Most people use 2X220uf in series. No problem, except that on a 50 watt amp this is quite high, no?? Biggest I ever saw in a real Dumble was 2X300uf in a 100 watter. That is like 75uf in a 50 watter... I think.

Maybe I should try 2X100uf in my build.

Fwiw, I went back to 40uf screens last night. With the choke replacement resistor in, the tone was not so articulate with the 20 uf in. I prefer the range of the 40uf. It is zingier on top and sustains better (with no choke)

[jelle]

Hi Dogears,

The caps values do not half when the number of output tubes are halved.
Look at the ab763 schematics for the 40 and 100w diode rectified amps.
Please correct me if I'm wrong...

I personally like the Ultra linear bandmaster (2x220uF) into an EV. I don't like the plate voltage to drop below the screen grid voltage, and I do like the headroom supplied by the big primary caps. I like to limit the amount of bass in the preamp.


All I know is that with bigger caps, the bass response is more linear. The power amp has saved more energy to amplify those low notes.

I also like the high filtering for the screen grids with the resistor instead of a choke. What type of poweramp are you using ( 2x6l6?) and what is the size of the resistor you are using?

Jelle,
The Netherlands

[Normster]

I still haven't gotten around to trying 220uF caps but I finally tried the "choke" resistor in my Bassman. I tacked in a 5W/500 Ohm cement and I really like it. The amp finally has that hint of bloom that I heard in my friend's Fuchs/SLX modded Bassman.

BTW, my understanding of the choke resistor is that it's impedence (not resistance) that gives it sag. (Sorry, I'm not a technical guy.) So, would a larger wattage wirewound give it more sag? I saw in the Twreck forum that Ken used a 25W/1k wirewound in place of a choke. Anyone ever experimented with larger values in a Dumble?

Back to the screen cap discussion...what is the impact of larger caps on the OPT? Does that extra stored energy hit the tranny harder or is it just "tighter?"

[Robert]

Seems I'm not the only one who doesn't understand sag! I would love to learn:
1) the technical explanation of sag and how to measure it, therefore no more guessing about its existence,
2) what is the subjective players experience of an amp with sag compared to one without.

Thanks,
Robert

[Funkalicousgroove]

Impedance is the Alternating Current Equivalent of DC Resistance, and changes with frequency. I don't see how the resistor in the power rail could affect Impedance?

Sag is what happens when you push your tubes to draw more current than your filtering setup can supply within a given amount of time, for lack of a better explanation. It's more Jargon than a technical term.

[mlp-mx6]

My understanding -

Sag is when the current demands on the power supply cause the actual power supply voltage to drop - since the power supply itself has some resistance. IOW, if you pull more current through a "choke resistor" Ohm's Law dictates a voltage drop. The same is true when using tube rectifiers - the rectifier itself has an inherent resistance, so changes in current also demonstrate changes in voltage. Higher current draw = lower voltage. (Vdrop across the resistor (or the rectifier) = I * R, where I is the current in Amperes, R is the resistance in Ohms) Solid state rectifiers have FAR less resistance (or forward voltage in rectifier terminology) than tube rectifiers, so they tend to not sag noticeably.

Sag usually makes itself apparent by compression at high levels, OR what is sometimes called "bloom" - where a note is initially compressed due to, say, a picking transient, but then the power supply recovers (meaning the voltage recovers) and the note seems to swell or bloom, when in fact it is just decaying less due to the increased capacity of the power supply.

Hope this makes sense! (Hope it's right too! )

[Funkalicousgroove]

Well said!!



As far as the 200uf Caps go, I've tried both and must say that I like the feel of the 100uf caps even though the bass response is a teeny bit flubbier.

[Robert]

Mlp-mx6 thanks for the explanation!! Earlier I tried measuring the voltage drop on B+2 with a 250 ohm and 500 ohm resistor instead of a choke to see if I could determine which created more sag. With a hard attack, they both dropped ~7 volts, i.e. no difference, although the initial voltages were different. I was unable to measure time to return to initial state but they looked similar.