build a dummy load

[Enzo]

You will always find people who recommend some alternative way to do most anything, but for all the many people who want to use crossover distortion, I really think there is a much larger group who prefer to do it the way the factory provided, with the test points.

Note also that that 40Hz thing asks you to adjust for minimum harmonic distortion. Crossover is certainly one distortion but not all. The balance control essentially centers the phase inverter within its range, so they are looking not at crossover so much as they are looking to make waveforms symmetrical. Note that is not the same as setting the bias. The procedure expects you to use a distortion meter.

You could have a way unbalanced phase inverter but no crossover distortion, and likeaise you could have a perfectly balanced PI but tons of crossover.

That 40Hz procedure says crank it to 25vRMS into the load. At 4 ohms I get a little under 160 watts.

[woodyc]

Here's an alternative I find handy...


EDIT: I guess I should have read the thread before I posted. I don't subscribe to crossover-based biasing. Just responding to the "building a dummy load."

[Randall]

The schems I have also provide a volt meter procedure for the 40Hz balance adjustment.

[Leo_Gnardo]

The schems I have also provide a volt meter procedure for the 40Hz balance adjustment.

Stick with the tried n true SVT directions, handily printed on the box.

May not appear full due to settling in shipment. Oh whoops that was the Kelloggs box...

You can confirm AC balance on your scope by driving the amp up to clip. Resistors preferred.

- - - - - -

If you want to really drive yourself koo-koo, keep your milli volt meter plugged into the handy test points, and insert just one output tube at a time, fire up & be astonished (or not) at the difference in current draw with each one. The SVT circuit is fairly forgiving of not-perfectly-matched tube sets, but if you find tubes that are way below or way above average current draw you can eliminate 'em from the lineup, maybe get more lifetime out of a set and less headaches. A favorite way of playing idiots' delight on my workbench.

T'umbs up, dawg!

[bob p]

so where are you guys seeing that procedure being defined for balancing the PI? i've never seen it in the docs that i have. of course, mine may be incomplete. instead of getting a complete service manual, i've got a bunch of fragmented pdf files.

[Leo_Gnardo]

so where are you guys seeing that procedure being defined for balancing the PI? i've never seen it in the docs that i have. of course, mine may be incomplete. instead of getting a complete service manual, i've got a bunch of fragmented pdf files.

This is for the ol' fashion "original" SVT. On mine, front of power amp chassis, printed under the balance control pot, it says "Adjust on oscilloscope for equal clipping on both sides." (Presumably one sends a sine wave in & dials up to clipping.) I have seen others that say "input 40 cps sine wave, set volume so that 25V AC is delivered from the output, and adjust balance trim to read 0.000V between bias test points". Don't have one like that here right now, so the exact language is doubtlessly different but, that's the balance procedure. Also, reading 0.000V on a DVM may not be too easy because we're dealing with a slow wave form. Get it as close as you can, and an old fashioned dial meter may help average things out.

May not apply to newer models. Hope this helps!

[Dave H]

If you bias the amp by looking at idle current, you won't get anywhere close to needing a 400 watt load since this is done with no signal applied. You can use most any crappy speaker laying around that has the correct impedance.

If I'm doing it like that I don't bother with a dummy load resistor. I just use a shorted jack plug.

[Dave H]

Oh wow, between when I started writing and when I posted, three of you also posted, cool.

That happens to me too because I can only type with one finger but you were writing War and Peace

[Tom Phillips]

so where are you guys seeing that procedure being defined for balancing the PI?...

Supplementing what Leo posted, attached is a pic of a 1980's Made in Japan SVT showing where the procedure is silk screened on the SVT PA chassis. I'll keep looking to see if I can find a legible closeup of the wording. I pretty sure it's on the service literature I have in hard copy and was on the earlier SVTs too.

[jbltwin1]

My wife thinks I AM a dummy load!

[g1]

so where are you guys seeing that procedure being defined for balancing the PI? i've never seen it in the docs that i have. of course, mine may be incomplete.

This is for the ol' fashion "original" SVT. On mine, front of power amp chassis, printed under the balance control pot, it says "Adjust on oscilloscope for equal clipping on both sides." (Presumably one sends a sine wave in & dials up to clipping.)

Interesting Leo, I've never seen that about the scope, is that the 6146 version?
The calibration procedure is on the schematic, both the distortion analyzer method and the voltmeter method.
Here is the 6146 version schem, the 6550 schem. has the same notes on it.
ampegsvt6146b.pdf

[Leo_Gnardo]

Interesting Leo, I've never seen that about the scope, is that the 6146 version?
The calibration procedure is on the schematic, both the distortion analyzer method and the voltmeter method.

Nope, this one's an early 70's "aqua" line SVT. Never have seen a 6146 version. I think they made only a couple hundred, and many have been converted to 6550.

[Enzo]

bob, this procedure is in the upper right corner of the power amp schematic for the original SVT. The whole thing is two pages, power amp and preamp. All the SLM stuff is years later.

And those damned Joe Piazza schematics of course do not include it.

[bob p]

g-one, that PDF that you've listed in post # 26 is coming up blank.

I'm familiar with the Piazza schematics -- they say nothing useful. I haven't seen the original SVT documents that Enzo referred to that discusses the procedure. I also haven't seen one of the older SVT chassis that have the silkscreened wording.

I do have a pair of SVT-2 Pro sitting right here, one SLM/USA and one Loud/Korea, and neither the schematics nor the chassis have any such labelling. They both have the idiot light setup for bias adjustment.

I don't know what the current Loud reissue SVT-CL documents or chassis look like, but I think they use the idiot light system as well.

[Enzo]

This is unreadable, but it looks like this:
http://www.drtube.com/schematics/ampeg/v9pwr.gif

Better:
http://www.turretboards.com/layouts_...g_svt6146b.gif
http://www.turretboards.com/layouts_.../ampeg_svt.gif