Capacitor limits after tube rect ?

[Alex R]

- or it could still be an imbalance issue. Are the tubes passing similar currents (if you have a way of measuring this)? Is the bias voltage on each grid the same?

[Enzo]

He can ground the grids with a cap and make the hum diminish. That is a clue of some sort.

[Alex R]

...because imbalance hum comes via the plate supply, I see. So forget imbalance till the grounds and layout are eliminated.

[Enzo]

Still, he SHOULD verify both sides are conducting similarly, you are right.

[Rattler66]

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

If I disconnect both caps the hum is there, if i reconnect only the 91k the hum diminishes. If in turn i only reconnect the 82k the hum returns. I am lifting the connection after each cap only, not before. Should i be disconnecting before the cap or after?

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.

I'm not sure about the bias supply, I connected an .1uf orange drop cap in series and i still get the same -00.5 reading on my meter. Both tubes are close in bias.

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

power tubes are grounded to the chassis at the bolt that holds the socket it. the two bias filters and trim pot share the same wire going to the center tap ground connection. it is a star scheme. i am sure i have tried in the past to connect the power tubes ground to the same star point but without change. i'll try again tonight

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 should 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.

BTW I am using a jj cap can 40,20,20,20

thanks

[Rattler66]

- or it could still be an imbalance issue. Are the tubes passing similar currents (if you have a way of measuring this)? Is the bias voltage on each grid the same?

bias voltage is very close to each other, i'd have to retest for exact numbers, but i remember being close

[Rattler66]

i'll try to post some pics later, i got to get my camera up


my camera died.........i may not be able to post pics yet.

[MWJB]

"BTW I am using a jj cap can 40,20,20,20", preamp and PI filters should not be grounded to the same point as the main & screen filters. PI filter's ground wire should be grounded to the buss wire/brass plate around halfway along the front panel, or at the input jack. Preamp filter should be grounded to the input jack.

Amps that have all filters grounded to the same point often hum.

[MWJB]

Voltages are a little hard to read, both plates are 467v? Bias voltage at pin 5 is <50v?

What is your plate current in mA or mV?

[Rattler66]

MJWB............those voltages are not my current voltages, I will measure and repost

[Rattler66]

"BTW I am using a jj cap can 40,20,20,20", preamp and PI filters should not be grounded to the same point as the main & screen filters. PI filter's ground wire should be grounded to the buss wire/brass plate around halfway along the front panel, or at the input jack. Preamp filter should be grounded to the input jack.

Amps that have all filters grounded to the same point often hum.

i will redo the ground scheme, can you give me a basic setup for all points, thanks

[Alex R]

Still, he SHOULD verify both sides are conducting similarly, you are right.

- yeah could be just a bad tube, or even a really unbalanced pair. Rattler, a quick way would be to try pulling one tube at a time. Then it should hum ok! No hum would mean a bad tube.

[Rattler66]

- yeah could be just a bad tube, or even a really unbalanced pair. Rattler, a quick way would be to try pulling one tube at a time. Then it should hum ok! No hum would mean a bad tube.

ok.........i'll check an post. i want to go through some of the suggestions. thanks

[Rattler66]

ok guys here's a pic of the innards, it should be clear enough to follow the runs......i also marked out what my new grounding scheme will be and where it corresponds on the schem

[Rattler66]

i did a little more investigating and i think the problem lies in B+ lines running too close to the grids, i disconnected all the B+ but the one going to the CT on OT. i wonder can it be a problem if i have the B+1 twisted with the two screen leads. i messed around a little in there and there seem to be somewhat of an effect.