Can you guys look at this.....

[C Moore]

A friend of mine bought a Marshall 1986 clone. It was a disaster.....so I gutted the chassis and rewired everything. Some of you guys helped me with a phase problem.
Anyway.....Do you see the first PS Cap.? On the schem it is 50-50/500. This guy used a single 100.
But look at the pic, he has a 220k resistor connecting the +/- of the cap.
What is that for.?
Thank You
http://www.drtube.com/schematics/marshall/1986jp.gif

[martin manning]

It's a bleeder to drain the cap when the standby is open.

[C Moore]

Yeah OK...that make sense.
There is High Voltage with both Power and SB in the off position. That is relative to the OFF of the panel.

How do you figure that value.?
So that it will be high enough to be inconsequential when the amp is being played..
Thanks

[Leo_Gnardo]

Do you see the first PS Cap.? On the schem it is 50-50/500. This guy used a single 100.

Horrors! Heavens to Betsy ! ! ! What sacrilege ! ! ! ! !

Just kidding. Nothing wrong with that.

[C Moore]

Do you see the first PS Cap.? On the schem it is 50-50/500. This guy used a single 100.

Horrors! Heavens to Betsy ! ! ! What sacrilege ! ! ! ! !

Just kidding. Nothing wrong with that.

I was not slighting him for that.
Probably not available in 1968.

[Tillydog]

How do you figure that value.?
So that it will be high enough to be inconsequential when the amp is being played..

^ Spot-on!



(Actually you can get all technical about leakage currents, but that's as good an explanation as I've ever heard, and is probably The Truth )

[C Moore]

How do you figure that value.?
So that it will be high enough to be inconsequential when the amp is being played..

^ Spot-on!



(Actually you can get all technical about leakage currents, but that's as good an explanation as I've ever heard, and is probably The Truth )

I cannot tell if you were making a joke or not.
What I SHOULD have written was.....HOW do you figure for a resistor with a value high enough to render it inconsequential while the amp is being played.?

[Smitty]

http://en.wikipedia.org/wiki/Bleeder_resistor

[C Moore]

Geez...even Wiki knew what it was for.
Does not talk about calculations, but it is a good explanation of function.
Thank You

[rdjones]

Geez...even Wiki knew what it was for.
Does not talk about calculations, but it is a good explanation of function.
Thank You

IIRC the US codes call for 50 volts or less within 5 minutes.
Other areas are more strict, AUS are EU are 50 volts or less within 1 minute.

Calculation for a tube circuit is complicated by the fact that the tubes will drain some voltage until they cool enough to stop conducting.

If a bleeder draws 10% or less of total current at full signal it can be considered "negligible".
2x220k in series (as main supply cap balance resistors) is common and serve as bleeders.

rd

[martin manning]

Nice little worksheet and good info RD, thanks!

[C Moore]

That sure is an impressive worksheet....!
Thank You

Yes...I think that Wiki link talks about 50V residual at the One Minute mark.
I never would have thought 10% would be considered acceptable.....very interesting.
Though I could swear I have never seen an amp that did not drain down by itself...IF the tubes have gotten hot, and there was no open SB Switch involved.
Seems like all tube guitar amps, that I have seen, drain themselves down to about 12-15 VDC in a hurry.
Not arguing against a bleeder resistor(s).....just commenting on what I have experienced. YOU guys have seen a lot more amps than I have.
I can remember a video that Bill M did a few years back (probably with a Blues Jr) with a DMM connected to the B+ of an amp.....then watching the caps drain with the power off.
I would have to look at a B Jr schem to see if that amp has bleeders.
Anyway.....Thanks Again

[Paultergeist]

Now you have me wondering.....on my amps without bleeder resistors......do those filter caps hold a charge when powered down, and if so, for how long? I had always just assumed that there might be residual charge, and treated them accordingly (discharged with a snubber stick). Good practice regardless, but now I am wondering.......

[C Moore]

Well.....throw your meter on them and see. But I would bet that they drain down pretty fast.
If your amp has an SB Switch, that has to be off of course.....otherwise, the caps will stay charged.
Will be interesting to hear what you observe.
best

[jelle]

The meter might speed up the discharging process.