Bleed Resistors

[skynyrd]

Been wondering, have read alot about various circuits in a amp and they talk about the bleed resistors diverting excess signal to ground. Now I understand part of that is so it doesn`t overload the Op Amp and the stage after it. Now say you have a lead channel circuit and all the resistors on the pre-gain path are 1.2K or larger, then you have 2 bleed resistors that are 470 ohm going to ground? That would be dumping alot of signal out of the circuit at least in my line of reasoning. How would it work out to bump the bleeds up to 1K ? or 800 ohms so its not quite doubling the bleed blockage? Has anyone experimented with this? My buddy has a Peavey Heritage and on the lead channel and Saturation channel there are several low ohmage bleeds, when you turn the saturation all the way off it has very little pre amp gain, about like putting a normal amp on 5-6 pre gain wise. Would closing up the bleeds a LITTLE help with boosting the pre amp a little?

[tubeswell]

Are you talking about voltage dividers in the signal path? In some circuits they are put there to attenuate the signal so it doesn't sound too mushy.

[skynyrd]

Ok, that makes more sense. I am trying to figure out circuits a little bit at a time and read 1 article where the author ( unknown ) talked about bleeder resistors that in the signal path they ran to ground and bleed off alot of the signal depending on their value, he stated by upping their value you would get better signal flow.

[HTH]

The most common use of the term 'bleeder resistor' that I'm aware of is when you have a resistor (typically 220k) to ground in a valve amp in the HT line. This allows the stored voltage in the caps to bleed off when the amp is off so you don't get zapped when working on the amp.

[Bruce / Mission Amps]

I think the real reason the balancing resistors (sometimes, and I think in error, called bleeder resistors) are put across the two series caps is to swamp the capacitive reactance of the two different caps so the ripple voltage is more evenly divided across them.
Bleeding off the high voltage after the B+ is removed is probably a minor or at least a secondary plus... although a good one.

[Steve Conner]

No, it's to swamp the leakage current so the DC voltage is evenly divided across them.

Bleeding and equalizing are two different functions. The resistors placed across series cap strings do both.

[jmaf]

I think the real reason the balancing resistors (sometimes, and I think in error, called bleeder resistors) are put across the two series caps is to swamp the capacitive reactance of the two different caps so the ripple voltage is more evenly divided across them.
Bleeding off the high voltage after the B+ is removed is probably a minor or at least a secondary plus... although a good one.

I think the OP's talking about grid leak resistors, not the PSU's power cap leaks. He's worried that a small value may be shunting too much signal to ground at the inputs.

[Steve Conner]

He also mentioned op-amps, and resistors in the 1k range. I have no idea what he's talking about

[Enzo]

No grid leaks, it is a solid state amp except for the power stage.

The circuit is below. I THINK he is talking about resistors like R4, R16. They are scattered about the circuit in both channels.

[mooreamps]

No, it's to swamp the leakage current so the DC voltage is evenly divided across them.

I would vote for Steve on this one. It would be a secondary benefit about them bleeding off the charge after power is removed.


-g

[tubeswell]

Hey - who're U callin' a grid leak?

I guess we'll have to wait til he clarifies what little bleeders he is talking about

[skynyrd]

Enzo is correct, I was reading an article where the unknown author was talking about them in the input stages and saying that changing them to a higher value would increase input signal as well as correct the bleed off of highs and upper level mids. I just was curious how much of that was truth and if they would impact anything down the line as far as sending TOO MUCH signal into it?

[tubeswell]

Ah they look like bias resistors e.g.; (I think) R3 and R4 form a fixed bias divider to bias the base (off the collector)

Types of bias

Somebody will correct me if I'm wrong

[jmaf]

Ah they look like bias resistors e.g.; (I think) R3 and R4 form a fixed bias divider to bias the base (off the collector)

Types of bias

Somebody will correct me if I'm wrong

I *think* R3 and R4 form a BIAS for the diodes, setting the clipping point at (R4 * I + 0,7V). Or less than 0,7 for germanium diodes.

Hey - who're U callin' a grid leak?

LOL! We havin' pun yet?

[skynyrd]

So if they were changed out to say 5k the diodes would clip harder? The owner of the Heritage is wanting to get a little more distortion out of it, I had thought about lowering the value of R1, after I read the article, which I will paste a link to at the bottom, I think the author refers to what I am calling bleed resistors as Rg. I thought maybe doubling their value might help increase the signal and give him a little more distortion.

Heres the article.
http://www.regiscoyne.com/tech/preamp_mods/