Screened grid-wire: shield connection to Plate pin?

[overtone]

I was idly reading a book on the loo by G. Weber and behold, in a piece about stabilizing an Amp against oscillation, he said connect the shield of your screened input-to-grid wire only to the anode/plate pin.
There was no explanation of this "trade secret" and my knowledge is not yet good enough to see if this is absolute humbug or not. I have not found any posts here advocating this technique.
Perhaps I have the Monty Python version of his book, because surely this is just inviting stray capacitance and inductance? Should I really let the anode / plate voltage into the shield of a grid wire?

[Merlinb]

Weber is not known for his electronic expertise. I'm not even sure what he means by that sentence...

[overtone]

I simplified the GW text too much. It took ages to locate it again, I quote:
"Use shielded wire for the input jack to grid, but connect the shielding to the plate of the same tube (and not to ground, as you would normally). Do not hook up the shielding on the input jack side."

Apologies for mentioning GW and I know I really should not read that weirdness. I certainly never saw an idea like that in your book, Merlin.

I guess I could try it out, for the hell of it, and see what happens. Not that I have any oscillation, but maybe I can provoke some...

[Steve Conner]

Well, the full story is:

When you connect the shield to the plate, the capacitance of that length of coax gets magnified by a factor of about 50, because of Miller effect. This rolls off 50 times more high end than it would otherwise.

It also might stabilize the thing, because if it were just capacitance from grid to ground, it could form part of a Colpitts oscillator or something. Magnifying it with Miller effect probably turns it into a dominant pole.

But I still think it's the wrong way to fix the problem. To me, resistors are what you use to damp instability. Capacitors just change the frequency of it, they can't dissipate the unwanted energy.

[dai h.]

you do have to be careful with his info since apparently his background is being a meat salesman and not going to engineer school, but that is just basically talking about using a piece of shielded wire as a (plate to grid capacitor). Marshall did this in some of their MV amps on an input stage. On other amps, you'll find an actual capacitor, or sometimes PCB traces situated to act as a capacitor. I think this is the same thing as on opamps (or transistors) where you'll often find a small cap, say 15-47pF-ish from output to input. You get some signal cancellation on the top since the points are out of phase, and as the output gets larger (signal voltage gets bigger)the effect is magnified.

edit: oops steve beat me to an answer, hope that is at least a somewhat adequate answer, lol

[Steve Conner]

I think this is the same thing as on opamps (or transistors) where you'll often find a small cap, say 15-47pF-ish from output to input.

Yep, it's exactly the same thing. My only misgiving about it in this context is that the surface area of the capacitor is bigger than it needs to be, so it's more likely to pick up hum from heater wires etc. But that's probably no big deal.

[overtone]

thanks for those explanations, that throws light on it. I understand it as a last ditch fix, and certainly not something I want to include in the lead dress from day one.

The fall-out from those texts must drive you guys really crazy, but I really appreciate you clearing that one up.

[dai h.]

thx steve. By "surface area of the capacitor in this context", I take it that the shielded wire "cap" is being referred to.

re: the article, I just looked at it(again), and what is kind of funny is that he goes on in a later paragraph to say, "INSTALL A CAPACITOR FROM PLATE TO GRID OR PLATE TO CATHODE"(as one of his "tricks"), and does mention the high freq. reduction through degenerative feedback happening, but makes no connection/mention to the earlier bit about the "hot shield" hook up. The stabilizing tricks he mentions in the section are, "trade secrets" that took him many years and many hours of exerimenting to perfect", making it sound as the info was black magic from a voodoo witch doctor or something, lol. In hindsight if I had more sense, less ignorance at the time(that I bought the book) I should've just gone to a big library with old electronics texts since those would have been using tubes. Now though, you can get all sorts of specifc info from this site, ax84, amp garage, 18-watt bbs, etc. etc. as well as stacks (or megs and megs) of free books (such as from pete millet's site) and even Radiotron is on the web.

edit:

sorry I should mention merlin's book since that looks like one book worth buying that has solid info--where you don't have to worry about getting advice such as reforming alu electrolytics under a hot lamp...

[Chuck H]

I actually think G Weber gets too much crap. His books were useful to me at a time when other books went over my head and that counts for something IMHO. He also built some outstanding amps early on (and a couple of flops later) and runs a very busy vintage repair and restoration shop. And that counts for something too.

That said...

I do recognise the evangelist/faith healer nature in these books now, but MOST of the actual info is sound, or at least not dangerous. And being that they were practically co-written with Ken Fischer, if you can read between the lines they're a good way to garner some insight into the design philosophy of the most notorious of all boutiquers. How rock solid the actual tech is or at least the way it's put across is sketchy in places but the advice given is usually useful if you want to build good sounding amps.

Keeping in the spirit of this post I will recall one GW shinannagin. In one of the books he is interviewed about his development of thier Blackframe speaker that uses a convex bell shaped cone and takes full credit for the idea. Now, this is a guy who has seen a lot of vintage speakers. I have an old convex bell cone Jensen P12R in a box. I called him on it (hey, I have this bell cone P12R, you know anything about that?) but he never responded

Chuck

[dai h.]

I learned stuff too but at the same time it's filled with what appears to be self-serving BS such the whole PCB "co-planar" traces and thus PCBs sounding bad, which if in fact he was spending a bunch of time phoning up people like Andy Marshall (later competitor also building PCB-based Bassman clones) and Ken Fischer and getting good free technical consults/help looks even worse. I agree (and I've said before) that the amps looked really well made, but if you read stuff like this (dunno how much is exactly true or not, but) :

The Amp Garage :: View topic - Mr. Gerald Weber almost left us...LOL

maybe he really wasn't that great himself at building stuff, so that could just be a misperception (that HE made the nice amps). Also, he cloned Fenders, so the work was mostly already done for him in a sense (as long as he followed things closely enough--plus as was revealed another professional company did the trafos and not him--apparently wasn't capable of doing that himself either according to a personal friend of the owner of the transformer company, Mike W. who posted here a bit some time ago). Also, it's not like his info was free (VG mag or his books I paid for) so was it just innocent oversight or did he feel he could just get away with whatever he could get away with (as far as suspect or incorrect, not checked by qualified personnel info.)? There were people complaining about technical issues on the VG boards but these would be deleted (not good for the aura of guruness/infallibility? or something...). Very problematic when you have people who really should not be there (IMO) being put on a pedestal by virtue of being in a magazine. Then I've seen things posted such as one guy who took his amp that had a ghosting prob. and apparently GW's outfit added a bunch of filter capacitance but yet ended up not solving the problem and charging what sounded like not a small sum. So... things like that don't sound good at all. So yeah... learned some things but later a sort of Wizard of Oz style disappointment, lol.

[Bruce / Mission Amps]

The Amp Garage :: View topic - Mr. Gerald Weber almost left us...LOL

Oh brother... Gerry is actually a very funny guy to hang out and knows his limits.
That whole scene was just a joke, you know, for fun... it's called educational entertainment.
It was to lighten up his video..... all he did was open up the "switch" part of the input jack with his finger tip, with the volume up so it would buzz badly at 60Hz.

Yeah, he's not an electronics engineer. Neither am I and with a few obvious exceptions, I doubt there are very many here that are and even those might not have the worldly experiences that is really needed to turn electronics into music art.
Yup, he says lots of opinionated things and miss speaks about stuff... so do I.
He is quite the bloviator, but he a great story teller if you ever meet the guy... non of that means he has no idea what he is talking about.
He gets way too much negative press over the silliest things.. like hiring people to design and build his stuff... why would that anger anyone?

[Chuck H]

Wow + LOL

But keep in mind that the actual severity of Geralds exploits is probably exaggerated by a pi$$ed off guy. And there was some defense for Gerald at a later point in the very long thread. As you stated. How much is true is questionable, but the fact that it's written at all is telling. I realized it too. Thus my eangalist/faith healer reference. Maybe I just sift through bull$h!t better than average but I did learn quite a lot from the books that Gerald? wrote. Also, the charts and schems in the books weren't as redily available on line when I bought the books, so that was useful.

Fun attachement. Gerald the shyster. I kinda knew, or could tell, so it was OK

Chuck

[dai h.]

Bruce, that part is not very important. It's the later stuff in the thread that is quite.. entertaining (at least) lol (again, maybe there is a bunch of BS, I can't say for sure).

I guess it just depends on how you look at it. Maybe he is this clever, not formally taught(at least in electronics but perhaps in salesmanship??), personable, avuncular, self-made character of a guy who outwitted (employees?), (customers?), and outcompeted formally trained amp makers (who on the surface might appear to easily make amps more popular than anything he could make).

OTOH, something else to other people and especially ones that he really pissed off (maybe all that friendly nature looks and feels insincere to those people), hence the vitriol of the guy such as in the thread as well as others I've seen posting things about him.

Chuck I think I get what you are saying. You go as a tourist in some places and there might be someone that tries to sell you something or a service that seems like BS, but if they are perhaps friendly, charming, and funny enough you'll play along and happily let them defraud you for 15 bucks (but probably not for 5000, lol).

[clyde1]

Bruce Zinky did this very thing on the Vibro King he designed for Fender.

[Chuck H]

Oh no!?! I love Bruce's stuff. Other than a few pops and a little extra noise, the Prosonic's tone impressed me so much early on that I went over Zinky designs again and again to try to garner insight.

OK, I'll bite. ????????????

Chuck