SVT 3 PRO Bias Question....

[silvertripper]

So before everyone gets on here and tells me to use the search engine I did. And I ALREADY adjusted the bias setting after installing new jj's in my SVT3PRO and the end result is AMAZING!

The problem I am having is my readings have slightly changed over a period of 30 minutes. My averages across resistors are as follows on each bank.

7:30pm
22.25..........22.75

7:40pm
23.75..........23.25

7:50pm
24.50..........24.50


Is this right? I did plug and play for about a minute after I originally set the bias. Unplugged input and output. Took readings.

[gscroggin]

I can give you a non-technical answer; I've observed the same behavior. It was on an older SVT3 Pro (I think I bought it @2000?), and I had just done the same thing as you, changed out the tubes. My friend was over (he builds tube amps) and had his measuring kit jammy, took a few readings while we turned the bias pot, and noticed it changed over the next hour. He said it happens. I recall he said he's seen it way more on full tube amps, like specifically with power tubes, but that it's not out the realm of normality that it would happen on a solid state with the pre tube section.

Again...non-technical answer...from a guy that is as knowledgable about tubes as he is brain surgery :-)

[Foz]

yeah mine drifted a bit as the amp got hotter or cooler... looks like yours are drifting together and you are in the zone. If it sounds good button it up and call it sorted.

my $0.02

[silvertripper]

SWEET! This amp sounds mucho better!! Done!

[john m]

If those numbers are voltages they are most likely increasing because resistance goes up with heat, yielding a greater voltage drop accross that resistance.

Note: current will go down as things heat up

[Jerrold Tiers]

There are two things going on here......

1) the mosfet "on" voltage changes with temperature....

2) the bias temperature compensation is not perfect, AND it is not located "in" a mosfet, but on teh heatsink. So the mosfets and the sensor/compensator have some delay between them, they don't always have exactly the same temperature.

The combination is why I suggest that the amp be allowed to stabilize for 30 min or so before teh final adjustment.

The readings should stabilize, although those numbers look a bit less stable than I'd like..... did you do the bias with NO SPEAKER?

To check or set bias you would unplug the speaker.

[silvertripper]

Originally Posted by Jerrold Tiers

There are two things going on here......

1) the mosfet "on" voltage changes with temperature....

2) the bias temperature compensation is not perfect, AND it is not located "in" a mosfet, but on teh heatsink. So the mosfets and the sensor/compensator have some delay between them, they don't always have exactly the same temperature.

The combination is why I suggest that the amp be allowed to stabilize for 30 min or so before teh final adjustment.

The readings should stabilize, although those numbers look a bit less stable than I'd like..... did you do the bias with NO SPEAKER?

To check or set bias you would unplug the speaker.

I did with no speaker, no input, only the power cord connected. However since my last average reading of 24.5 and 24.5 respectfully, I left the amp on for about 1-1/2 hours then checked readings again and the average is 24.5 and 24. Almost the exact same as my last reading 1-1/2 hours earlier with exception to two resistors had a -1mv reading from previous.

Sound safe and normal to you?

The amp sounds great!

[Jerrold Tiers]

Jah, sehr gut!

You are OK

[silvertripper]

Sweetness! Thanks Jerrold and Foz, your posts a couple years ago helping someone else with proper bias set-up on a SVT3PRO also helped me with mine and I know a ton of others. Thanks again! You guys rock!

[Foz]

all props to JT - he walked me through it and it made a huge difference - that 3pro is nice bit if kit when it is set up proper.