Capacitor limits after tube rect ?

[Rattler66]

I believe on a 5AR4 the capacitor directly following it shouldn't be higher than 60uf. My amp is setup with rect-standby switch-40u-20u (power tube B+) then two RC filters for B+'s. I'm getting hum with the PI tube out, I need to AC bypass the grids to see if I'm picking up hum there. But when I turn the amp on standby the hum goes immediately (so I think PS). So if the hum is through the power tube B+ can I add any more filtering for the first node?

thanks

[Enzo]

You didn't mention the amp, but I will assume it is push pull. A push pull output stage cancels plate ripple. That depends though on the tubes being more or less balanced. Mismatched tubes will not cancel as well and will allow more hum. Likewise mismatched bias levels.

While we often get better filtration than this, I see maybe 30v of ripple on the plate supply and it is normal. it will cancel out. On the other hand, the bias supply MUST be clean DC, otherwise its ripple will be injected as signal.

With the PI out any hum you get is coming from the power tube stage. You could easily clip a largish cap to ground from the grids - a 0.047 for example or a 0.1 - if it diminishes the hum, then it is indeed coming in the grids. But first thing I'd do would be check the bias supply for ripple. Scope it or set your meter to AC volts and measure it. I'd check the plate supply for excess ripple, anything over 5% of the B+ voltage would be a reasonable standard.

Then I'd be checking things that balance. Measure current through the tubes individually - are all similar? Check the voltage at each screen grid - is there B+ at EVERY tube screen? No screen voltage, no tube conduction, and that is the ultimate unbalance. Is the DC voltage at all the grids the same?

Megafiltering the B+ supply would indeed reduce hum, but it would just be masking the real problem. Unless the filter is bad, I don't think more filters is the answer.

[Amp Kat]

You did try changing the 5AR4 didn't you ?

[Rattler66]

You did try changing the 5AR4 didn't you ?

actually, i didn't but if that's a possibility i'll do it. it was a newish rect tube though.

[Rattler66]

You didn't mention the amp, but I will assume it is push pull. A push pull output stage cancels plate ripple. That depends though on the tubes being more or less balanced. Mismatched tubes will not cancel as well and will allow more hum. Likewise mismatched bias levels.

While we often get better filtration than this, I see maybe 30v of ripple on the plate supply and it is normal. it will cancel out. On the other hand, the bias supply MUST be clean DC, otherwise its ripple will be injected as signal.

With the PI out any hum you get is coming from the power tube stage. You could easily clip a largish cap to ground from the grids - a 0.047 for example or a 0.1 - if it diminishes the hum, then it is indeed coming in the grids. But first thing I'd do would be check the bias supply for ripple. Scope it or set your meter to AC volts and measure it. I'd check the plate supply for excess ripple, anything over 5% of the B+ voltage would be a reasonable standard.

Then I'd be checking things that balance. Measure current through the tubes individually - are all similar? Check the voltage at each screen grid - is there B+ at EVERY tube screen? No screen voltage, no tube conduction, and that is the ultimate unbalance. Is the DC voltage at all the grids the same?

Megafiltering the B+ supply would indeed reduce hum, but it would just be masking the real problem. Unless the filter is bad, I don't think more filters is the answer.

thanks Enzo..........it is a push pull no NFB............i will check what you said. i do need a better meter because it doesn't ever seem to measure the ripple. i get very erratic numbers.

BTW...........what is a recommended meter (hopefully reasonably priced)

thanks

[Alex R]

BTW...........what is a recommended meter (hopefully reasonably priced)

thanks

I got a new-in-box ex-UK-military Fluke 25 off Ebay for I forget but about £25, after Enzo pointed out that a diode test function is quite a helpful thing. Don;t really know now how I managed without. It is generally an excellent meter. Accurate down to fractions of an ohm, will go up to 10 amps, diode and continuity tests, handy little thing all round & a lot cheaper than the general run of Flukes.

[Rattler66]

i'll look into it Alex, my meter doesn't like to measure ac ripple

[Enzo]

REally cheap meters will read just rectify the AC and read it as DC internally, and adds a correction factor. SO it cannot ignore DC offset. Good meters isolate any DC away from the AC component.

You can beat this. Add a cap in series with your probe tip. SOme film cap like a 0,047 would work. That will block DC, pass AC, and then your meter will have no choice but to read the ripple.

[Rattler66]

ok i ac grounded the grids and the hum went away............

i also put a .047 orange drop cap in series with the probe and it showed -00.3 volts (on the 200 vac range). i got this same reading without the cap.

so i know it's coming into the grids but not sure from where.

my bias ckt is using 10uf caps to filter....should i bump em up.

btw i get hum without any preamp or pi tubes installed

thanks

[Enzo]

Then either it is bias ripple or it is coming through from the PI circuit. You can disconnect the coupling caps from the PI to verify that as a source. If the hum remains, it must be the bias.

There is about zero current through the bias supply so the filters don't need to be huge. 10uf ought to be OK, but it is a simple matter to clip a larger cap in parallel with the second filter and see. Having said that, I usually use 100uf caps. Then again I have a huge [pile of them. Unless your cap has failed, I don't see brute force filtering as the answer. Yuo could have a grounding problem in that area for that matter.

[Rattler66]

alright tonight i'll disconnect the coupling caps and look into my pi ground. we'll see what happens, thanks alot, i finally feel like i'm getting to the bottom of this.

[Rattler66]

update

if i ac ground the grids, the hum goes away (there still is a very very slight bit, but practically nothing)

if i disconnect the coupling caps at the marked points the hum remains

if i disconnect only the upper leg (82k) and the 91k is connected the hum almost completely goes

if i disconnect only the lower leg 991k) and the 82k is connected the hum remains

[Jag]

Are the caps in the bias supply okay?
Are they both installed in the correct orientation (+ to ground)?


What about the rest of the bias circuit?

[Rattler66]

Are the caps in the bias supply okay?
Are they both installed in the correct orientation (+ to ground)?


What about the rest of the bias circuit?

as far as i know they are good, i have placed good ones in parallel with no change. caps are in proper orientation. other parts good as far as i know, i'll double check

[Enzo]

The 82k has an effect on the hum even with the couplers lifted???

If the bias supply is smooth and ripple free, and lifting the couplers leaves the hum still there, and both tubes are conducting more or less the same current, then I would be looking at layout issues.

Where exactly are the two power tube pins 8 grounded? Where exactly are the two bias filter caps grounded? And the bias trim pot?

For example, if the bias filters are grounded through the same wire as the B+ filters, all their ripple current shares he same ground path as the bias, and it will impress that ripple on the bias supply. Bias filters will not affect that. The main B+ filters shoul;d ground right to the point where the high voltage winding CT on the PT is grounded. The bias filters should have their OWN ground wire to that point. So should the power tubes.

That is called a star ground.