Adding Filter Nodes?

[dankinzelman]

Hi everyone,
I just received new can caps for my FBT 500R, which has a bit more hum than I'd like. It works out that I have an extra 100uf section and I'm wondering whether I can use it to further reduce the noise.
I was considering two options: either paralleling the unused section directly with the first 200uf cap (so I'd essentially have a 300uf reservoir), or adding a resistor for an additional stage of filtering and taking the B+ from the new node (see attached schematics). If I were to go with the second option, could I just divide the 1k resistor into two 470 ohm resistors and put one before the new 100uf cap and one after? Or should I divide up the resistance differently?
I've attached a schematic of the stock configuration and my proposed mod. Any advice or feedback is appreciated (explanations are welcome, I'm new at this and eager to learn).

[tubeswell]

The input stage is the most noise-sensitive, so if it was me, I'd use the separate filter cap for a dedicated preamp filter section (even for the first couple of pre-amp stages), and run that cap's ground lead to the input jack's sleeve terminal

[dankinzelman]

Thanks tubeswell. That's a cool idea, but the input jack is in a separate chassis, so it would be pretty tricky to rig that up (I'd need to figure out how to add a conductor to the umbilical, which means a new plug and socket). I failed to mention the noise is there even if the volume on both channels is all the way down. If I crank the bias up, the noise goes down a lot, but it starts to sound like crap.
I guess I need to start by figuring out exactly where the noise is coming from. Would it be worth disconnecting the grids of the power tubes to see if the noise is still there, or is there an easier way with an oscilloscope? I just presumed it was coming from the B+ (about 2v p-p on the power tube plates). I was thinking that since each node cascades from the preceding one, any cleanup in the early stages will likewise cascade down to the following stages. Am I on the wrong track here?

[shooter]

easier way with an oscilloscope

as long as you know what normal is, abnormal is easy to spot 
since it's hum, identify the frequency, 60hz, 120hz, some odd harmonic....
the quick n dirty;
pull tube ONE at a time starting with the input, stopping AT the PI

[Williamblake]

You could also use it for filtering a dc reference for the filament ...

[dankinzelman]

easier way with an oscilloscope

as long as you know what normal is, abnormal is easy to spot 
since it's hum, identify the frequency, 60hz, 120hz, some odd harmonic....
the quick n dirty;
pull tube ONE at a time starting with the input, stopping AT the PI

That is, I should NOT pull the PI, correct?

[tubeswell]

Thanks tubeswell. That's a cool idea, but the input jack is in a separate chassis, so it would be pretty tricky to rig that up (I'd need to figure out how to add a conductor to the umbilical, which means a new plug and socket).

Okay - I see now that it isn't a separate filter cap, just a spare 100uF cap can section.


But if you had a separate filter cap, it would be pretty easy to rig it to the pre-amp. All the pre-amp tubes are in the top chassis, along with the input jacks and the controls, so there must already be a B+ wire and a ground return wire between the main chassis and the top chassis

[dankinzelman]

So removing the preamp tubes did not resolve it. It is 100hz hum, and I can see a nice 2V p-p 100Hz sawtooth at the output transformer center tap, so I presume that's where the noise is coming from. Cranking up the bias shuts it down (although I guess that's pretty obvious).
I tried jumpering the extra 100uf section in parallel (so I would have 300uf at the first node) in and it made a barely perceptible difference...

[tubeswell]

... It is 100hz hum, and I can see a nice 2V p-p 100Hz sawtooth at the output transformer center tap, ...

What does the signal ground connection look like?

[sluckey]

It is 100hz hum, and I can see a nice 2V p-p 100Hz sawtooth at the output transformer center tap

That's very typical for the first filter cap node. Even 5V_pp or higher is common.

[dankinzelman]

... It is 100hz hum, and I can see a nice 2V p-p 100Hz sawtooth at the output transformer center tap, ...

What does the signal ground connection look like?

How would I observe it? Wouldn't it be the same as chassis ground?

[tubeswell]

... It is 100hz hum, and I can see a nice 2V p-p 100Hz sawtooth at the output transformer center tap, ...

What does the signal ground connection look like?

How would I observe it? Wouldn't it be the same as chassis ground?

I mean, what condition is it in? and how is it arranged?
By 'signal ground', I mean every ground on the secondary side of the PT (including the grounds for the output stage). These all have the potential to feed back noise into the ground return that affects the signal

[dankinzelman]

Regarding grounds, there are several grounding points in the main chassis, and ground is carried to the preamp chassis via a cord in the umbilical. Filter caps are grounded to the chassis. Is there something specific I should be looking for, or any kind of tests I can run to see if things are functioning as expected?

[tubeswell]

The more ground return points you have, the more ground loops you create, and the more hum you get. Cap cans, on the other hand, can also beget hum due to the way they internally aggregate the ground returns of the different cap sections, and depending on whether you use the same cap can to filter the more noise-sensitive input stages alongside the more current-generating output stages (hence I suggested a separate cap for the preamp). Look at Merlin Blencowe’s Grounding article for good advice on how to run relatively hum-free signal grounds with a ‘galactic’ ground return system.


Also, how well have you soldered in the cap can? Did you use a really hot (like 150W+) flat tip iron with ample solder flux paste for the chassis connection, or did you attempt to solder with a smaller iron that can’t get a good hot temperature and no flux?

[shooter]

any kind of tests I can run to see if things are functioning as expected?

assuming the amp works;
put a 1khz ~~ 150mV signal in
put scope on speaker and compare to input

[dankinzelman]

Thanks everyone for your help so far!
I need to make up some cables to hook my signal generator in and rig up a dummy load, which I will do soon.
In the meantime, and following some more probing, I found an error in the schematic, which indicates the PI being powered from the C node (325V at ~7mv p-p ripple), but in reality it is taking power from the B node (356V at 140mv p-p ripple). The wiring is pretty neatly done and looks factory to me, so I presume this is the way the amp was built.
In any case, the ripple is entering the signal path in the phase inverter: at pin 2 on the phase inverter (first grid), there is no ripple, whereas on the first plate (pin 1), the ripple is 40mv p-p, and this carries through obviously to the second grid and thus to the second plate.
I tried jumpering in the extra 100uf section in parallel on the B node, which brought the ripple down to about 45mv p-p at the node (making it ~20mv at PI pin 1) and lowered hum significantly. So I have three questions now:

1. Should I try moving the PI B+ to the C tap, as indicated in the schematic?
2. Will the additional 100uf section at the B (or C, if I move the PI to the C node) node cause any problems?
3. Can I add another smaller electrolytic to ground directly at pin 1 to further clean things up, or will that simply short all the signal to ground? If it were feasible, would its effect be negated by the ripple on the phase inverter's second plate, and if so, would there be any advantage to doing the same at the second PI plate? If this does make sense, what size capacitor would be appropriate?

[dankinzelman]

Update: I shifted the phase inverter to the C tap as shown in the schematic and connected the extra 100uf in parallel on the same node. This has essentially resolved the hum issue (although when the high gain channel and reverb are cranked I still get a bit of hum).

Next step is examine the Paul C mod for the phase inverter...I bought the resistors today but need to plan how to lay it out.