50Hz hum

[dai h.]

I agree about the jack statement. I prefer to use isolation washers and make a dedicated wire chassis connection.

doing that and just having the chassis as shield and safety ground connection makes sense, but have wondered sometimes if the older style could be safer (many redundant connections to chassis). Say, something comes loose, and you experience hum buzz, etc., from the grounding scheme getting screwed up or something that should be grounded floating, so you could get more noise, but you still have lots of redundant connections to the chassis (safety ground). If you have just one circuit ground to chassis(safety ground) connection that would seem to make the integrity much more important (since the operator, i.e. guitarist is most always touching circuit ground).

[dai h.]

I never addressed this on the scheme, (but I have in other forums) I do both, I use both the non insulated jack as the main connection, and I usually use, a safety net connection right directly underneath with as little lead length as possible in case the jack comes loose or corrodes.

Right. Makes perfect sense.

To be honest with you every old amp, and even new ones I work on (you would be surprised at how little grounding practices have changed!), I just put in the extra work......

It makes sense to change them (most of the rationale probably applies when it comes to interfacing with other AC-powered gear?--i.e. to avoid ground loops and end up using non-safe solutions such as lifting safety grounds), but I'm surprised the owners of vintage amps would let you do that. For example when you recommended the 'SL' temp. coefficient Murata ceramic (on another bbs--which probably would make a good sub. for N750 since a) it's a leaded thru-hole part, b) in-production c) inexpensive, d) offered in voltages high enough to be a sub, and e) temp. compensating type, which I would estimate would sound the same), I wonder how many would reject the choice because it wasn't "exact vintage" or because it was blue, etc.? (I shouldn't criticize too much because I kind of understand but sometimes the "vintage" pursuit starts looking a bit flaky lol.)

You have to deal with the multican scenario often, but alot of those amps are lower in gain levels so some routing is not as critical.

My understanding is that since the frequencies bypassed (coupled) are low (audio), that gives leeway in placement of the filter (i.e. you can get away with putting it further away than directly across plate and cathode). Oh, and also the lower gain levels you mention probably help make things viable. A document I read recently on the yahoo micbuilders group (I'm not a 'mic builder' really, just tinker w/easy mods) on designing for low noise suggested against using oversized caps since this would contribute to making signal loops bigger increasing susceptibility to noise, so by inference for high gain stuff appears best to be more stringent and keep loops small.

[txstrat]

...and all ground connections are *not* drawn in in the diagram).

That's right. I didn't complete the drawing because we had to deal with the power amp only. The preamp was disconnected and the hum stayed the same - so it had to come from the PS.

[dai h.]

in general, I think it helps to make things more clear (for assembly, troubleshooting, etc.) to draw how the grounds are connected instead of just "the triangle" or "lines making a triangle" symbol (since it can make a difference in noise and stability).

That's right. I didn't complete the drawing because we had to deal with the power amp only. The preamp was disconnected and the hum stayed the same - so it had to come from the PS.

there could be a situation where you solve the hum in the power amp section on its own, but hook up the preamp section (which may still have a problem or create one due to the way the grounds are wired) and get a hum problem again

[txstrat]

in general, I think it helps to make things more clear (for assembly, troubleshooting, etc.) to draw how the grounds are connected instead of just "the triangle" or "lines making a triangle" symbol (since it can make a difference in noise and stability).

I totally agree. With all the amps I've built so far they got (more or less) more quiet with every build because I was following advices from books or here in the MEF and also I made my personal experiences. It could have been way easier if it had been the way you describe.
I'll deal with the preamp in a couple weeks and see what it does.

[txstrat]

Second day after my vacation I've rewired the amp following the layout modified by dai, as well as keeping the layout of defaced/Joey close by.
The amp is dead quiet now and I've learned a whole lot from this thead.
Thanks to all who contributed and helped me getting a little further in my understanding of grounding tube amps.

Matt

[dai h.]

glad to hear that. (Whether you use an individual bolt or existing) make sure that extra chassis connection (which connects to chassis and thus to safety ground ) is secure (clean, tight connection that can't come loose).

[txstrat]

Actually I've two bolts each with a self-locking nut. One is for the circuit ground, the other for the mains ground. Nevertheless thanks for reminding me.

[dai h.]

(Something I meant to add: ) I don't know if you made a new hole or used an existing bolt, but if you did use an existing bolt (say, for a choke or whatever) and for some reason (repair, replacement, etc.) remove the connection, REMEMBER TO PUT IT BACK ON (maybe even write a note to yourself inside the amp or even just some mark such as a "schematic ground mark", etc.). Since the pots are connected to the chassis (and you are using the backs for part of the ground line IIRC), the amp should work, but pots can come loose so they aren't as reliable as a bolt w/toothed washer, nylock nut, (plus locktite, paint), etc.(which is the reason for the redundant connection). (I think) this sort of train of thought is the same for the reason you see the AC safety ground have its own individual connection (less likely to be "messed with" with its own connection as opposed to sharing a bolt with, say a power transformer, where someone may forget to re-install the safety ground connection). Also, if it has its own dedicated spot on the chassis (sometimes there is a sticker by it in production amps) it may be more likely to be noticed if the connection is missing from the chassis.

[defaced]

Cool. Glad to hear you got it working/quiet.