a real challange?

[Yamamaya42]

First smoke test PASS!


As in caps replaced and slow power up with rect tubes installed, nothing caught fire or exploded, 300v out of diode tubes, nice smell of warm 70 year old resistors.
Did not do much else yet, I have a LOT of tube pins and sockets to clean next. :/

[Yamamaya42]

voltages are all wrong, no surprise there with the crazy voltage bleeder used.

routing the chassis ground THROUGH the focus coil and adjustment from the source xformer center tap to the voltage bleeder is... weird.

they also seem to route main bias through horz and vert centering controls to the bleeder,, strange setup.


having dirty tube pins/sockets does not help as this was meant to be loaded.

[Kevin Kuehn]

Originally Posted by Yamamaya42

voltages are all wrong, no surprise there with the crazy voltage bleeder used.

routing the chassis ground THROUGH the focus coil and adjustment from the source xformer center tap to the voltage bleeder is... weird.

they also seem to route main bias through horz and vert centering controls to the bleeder,, strange setup.


having dirty tube pins/sockets does not help as this was meant to be loaded.

B+ center tap passes through the focus coil and a few series resistors which creates a b-(negative) supply which is used for biasing various circuits. Sams often tells you to reference b- when taking voltage measurements. There may be a similar note in Riders, but I have not taken the time to look. It's confusing to look at but the circuits function basically the same as if referenced to ground. The key is to realize the reference is often not chassis ground when taking voltage measurements.

[Kevin Kuehn]

Well now that I looked it does appear both Sams and Riders voltage references are from chassis ground. I do think the b- dropper string is easier to understand looking at the Sams schematic. B+ center tap is connected through the focus coil and 3 series resistors to ground. Because all b+ current passes through those components it creates a negative voltage across it's path to ground. The closer you get to ground, the less negative the voltage becomes. Hope that makes sense?

[Yamamaya42]

Originally Posted by Kevin Kuehn

Well now that I looked it does appear both Sams and Riders voltage references are from chassis ground. I do think the b- dropper string is easier to understand looking at the Sams schematic. B+ center tap is connected through the focus coil and 3 series resistors to ground. Because all b+ current passes through those components it creates a negative voltage across it's path to ground. The closer you get to ground, the less negative the voltage becomes. Hope that makes sense?

right now, all voltages are wrong, which is not surprising, since my tube sockets and pins are very dirty, i need to clean, many would not even light up heaters w/o removing and replacing.

the point that shows 300v on the Riders is at 378v with the variac at 70% and the point that says -100v shows -30v.

the big resistor in the voltage bleeder is getting very warm, burned my finger.
I'm gonna see what happens after i clean the tubes, I'm hoping that after they are clean and the load is more stable, the voltage will be more stable, otherwise, i will have to track down why it is not normal :/

[Kevin Kuehn]

High B+ and low B- would indicate some circuits aren't pulling enough current. As B+ current increases you'll see more voltage dropped across those B- series resistors between the center tap and ground. B- will become more negative and B+ will become less positive. The reason that happens is because as the tubes conduct more they become a greater load on the B+

[Electronic M]

If you are powering up with original caps or you haven't done so yet; check resistance on every section of power supply divider power resistor... They have a tendency to go bad on these.

[Yamamaya42]

Originally Posted by Electronic M

If you are powering up with original caps or you haven't done so yet; check resistance on every section of power supply divider power resistor... They have a tendency to go bad on these.

All new caps, except for the ceramic and mica, the electrolytic & paper have been replaced.
Did a quick check of the power resistors, in-circuit, but not out.
There seems to be a ton of info on how to clean tube pins , but not so clear on how to clean sockets safely.

Going to polish the pins till shiny, using cleaner, buffer.
Some say to use pipe cleaners on sockets, but I doubt there is one small enough for the 7-8 pin minis.
Some say just spray socket with cleaner (deoxit) and stick the tube in and out a few times.

[Kevin Kuehn]

I've never tried it myself, but I hear torch tip cleaners work well.

[Electronic M]

Some real tobacco pipe cleaners (not the dopey (f)arts'n'crafts ones) have a thinner metal core than a 7 pin tube socket.

My default 7/9-pin socket cleaning method is hose tube base pins with contact cleaner till drips hang off the pins, possibly the socket too if I feel it is necessary, then rapidly insert and remove the tube 3-10 cycles.... that usually does the trick except in sockets corroded badly enough that changing them was basically needed anyway and sockets that no longer grip on all pins.

[Yamamaya42]

I just have to wonder WTH was RCA thinking when they came up with this bleeder power design...
By routing main ground (-100) trough the focus coil and the adjustment for it, won't that cause the -18 & -2 to change when you adjust focus? Not to mention reference gnd with respect to the positive bias voltages?
It's hurting my head trying to understand what they ment to do...

[Kevin Kuehn]

Originally Posted by Yamamaya42

I just have to wonder WTH was RCA thinking when they came up with this bleeder power design...
By routing main ground (-100) trough the focus coil and the adjustment for it, won't that cause the -18 & -2 to change when you adjust focus? Not to mention reference gnd with respect to the positive bias voltages?
It's hurting my head trying to understand what they ment to do...

It's a pretty common scheme to obtain negative bias voltages. Putting the focus coil in there actually saves some power vs wiring in to it's own separate voltage dropper off of B+. Don't think of it as ground going through those resistances, because chassis ground is still at 0 volts. What's happening is part of the B+ is shifted to the negative side of the rectifiers because the HV winding center tap is not connected directly to ground. The 5U4 diodes block the positive cycles from going that direction, so it has no choice other than to produce a negative voltage. You are correct that the bias voltages will change with adjustment of focus, but it's a very small amount because those resistances closer to ground are much smaller in proportion. And vacuum tubes are very tolerant to loosely regulated B+

[Yamamaya42]

https://imgur.com/dwrUtJr

same tube, pins cleaned

[Yamamaya42]

part of my voltage problem is /was that the focus control pot was so dirty that the center post was not making contact most of the time, and there was no easy way to clean it w/o taking it apart, could not get cleaner inside it :/

[Yamamaya42]

IT'A ALIVE!! HAHAHAHAHAHA!

https://imgur.com/Dn7mcf3

taken from the plate of the horz osc!