High-ohm grid resistor in tube (pre)amp design
in [Design]
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From: Jan Panteltje on 12 Nov 2009 09:09 On a sunny day (Fri, 13 Nov 2009 00:08:45 +1100) it happened "Phil Allison" <phil_a(a)tpg.com.au> wrote in <7m2fn3F3fu8upU1(a)mid.individual.net>: >> So my question is if anyone can explain why any designer would use such a >> ludicrously high grid resistor > > >** Simple - to prevent the previous tube stage from being asymmetrically >loaded by grid current when following stage is overdriven. How much sense does that make? If any grid current flows the wave form will be distorted. I have looked at that diagram, and maybe for a guitarist it is a great diagram. But I would put the thing with the trash if I found it. Even 10 years before it as designed. You can make the same effect with a few diodes.... LOL
From: ChrisQ on 12 Nov 2009 09:13 Phil Allison wrote: > "Chris Quack" >>> Schematic might be in order then... >>> http://www.blueguitar.org/new/schem/peavey/5150evh.pdf >> >> Quite a complex front end and if you take a quick look at the phase >> splitter on page 2, (V4), it looks like they are using some weird >> asymmetrical drive to get core saturation on the output stage. > > ** Nonsense - it is a 100% standard, time honoured circuit . > Yes, (yawn) it's long tailed pair with cathode coupling, but the anode loads are different and there's an added input to the second stage via c51 from next2, which comes from the first page. The asymmetrical anode values may be there just to balance the gain, (They are usually the same value) but who can tell without measuring the voltages under various operating conditions ?. My guess is that it's an intentional imbalance. 100k vs 82k ?. >> I don't know why thay used the 470k to the grid of v1b, but it could be >> somthing to do with the behaviour of the triode under severe overdrive >> conditions, where it may be drawing some grid current. > > ** So you read my post just 10 minutes ago ............. > > Bloody plagiarist. > What makes you think that you are the only one that knows anything about 'tube' amps ? or tube characteristics in general ?. Be assured, I really don't need to plagiarise anything of yours. > >> On Marshall amps, for example, they used to have the output transformers >> wound with the primary centre tap offset to ensure non linearity and core >> saturation at high levels :-)... > > > ** 1000% purest BOLLOCKS !!!! > > Not true of any tube guitar amp ever made. > None, ever, I guess you've seen them all then ?. If you are so clever, why don't you measure the primary inductance of both halves of an early 1970's Marshall OP transformer, or do a turns ratio test, to see for yourself ?. Having grown up with 'tubes' and working in pro audio, 1968-1980, I may just remember part of a clue, even now, despite what you may think... Regards, Chris
From: Ralph Barone on 12 Nov 2009 09:54 In article <g6lnf51p3sgveqtj5cbrcv278qfc6u1up5(a)4ax.com>, Ross Herbert <rherber1(a)bigpond.net.au> wrote: > On Wed, 11 Nov 2009 23:09:56 +0100, Richard Rasker <spamtrap(a)linetec.nl> > wrote: > > :Rich Grise wrote: > : > :> On Wed, 11 Nov 2009 22:54:08 +0100, Richard Rasker wrote: > :> > :>> Hi all, > :>> > :>> I regularly repair tube amplifiers, and there's one thing I occasionally > :>> encounter that doesn't appear to make much sense: a high resistor (> > :>> 100K) in series with one of the first 12AX7 preamp tube grids. > :... > :>> I have here exhibit A, a Peavey 5150 EVH. > :... > : > :> Have you tried contacting Peavey? > : > :No. But I know from experience that it takes a LOT of perseverence and > :patience before you get to speak to someone who remembers "when that guy > :drew up the preamp stage, 25 years ago." Not to mention the fact that > :manufacturers are very rarely inclined to discuss matters of design -- > :especially when it comes to possible shortcomings. > : > :I'm under the impression that I'm dealing with a kind of almost "genetic" > :flaw, where whole schools of Peavey (and other brands') designers simply > :copied the tube preamp stage from one model to the next, without > :questioning it, because "it worked". Never mind the noise, these things are > :meant to be played loud anyway. > : > :So in short, my bets are rather more on this news group than on Peavey. > : > :Richard Rasker > > > Schematic might be in order then... > http://www.blueguitar.org/new/schem/peavey/5150evh.pdf My guess is that it's there to reduce the grid current that flows when you overdrive the tube so badly that the grid goes positive wrt the cathode.
From: ChrisQ on 12 Nov 2009 10:13 Phil Allison wrote: > > ** For the "presence" pot - as used in Marshalls as well. > And um, what effect is the presence pot having and how does it function in term of signal path, system gain and frequency response ?. > > ** Too hard for you go look at a Marshall schem ?? It's 30+ years since I even saw a Marshall amp with the cover off, let alone a schematic and the schematic doesn't define the o/p transformer characteristics or the design rational. > <jealous, bitchy cucumber rant, snipped> >> If you are so clever, why don't you measure the primary inductance of both >> halves of an early 1970's Marshall OP transformer, or do a turns ratio >> test, to see for yourself ?. > > ** Fact is - I have. Really ? when exactly was this ? :-). If you've been involved in the business for that long, you must remember a company called IES London and Mavis mixers ?... Regards, Chris
From: Jan Panteltje on 12 Nov 2009 10:19
On a sunny day (Fri, 13 Nov 2009 01:45:05 +1100) it happened "Phil Allison" <phil_a(a)tpg.com.au> wrote in <7m2lbmF3gnh4cU1(a)mid.individual.net>: > >"Jan Panteltje" > >>>> So my question is if anyone can explain why any designer would use such >>>> a >>>> ludicrously high grid resistor >>> >>> >>>** Simple - to prevent the previous tube stage from being asymmetrically >>>loaded by grid current when following stage is overdriven. >> >> How much sense does that make? > >** Lots actually. > >> If any grid current flows the wave form will be distorted. > > >** But not the one at the previous plate and neither will the coupling cap >charge up much. > > Matters in tube guitar amps. Everything matters in a guitar amp, the guitarist heard a sound that he liked, and that design was his sound from then on. Copied by many. Could have been banging trash cans too, as you know :-) > Little thing sometime called "blocking" . > > >> I have looked at that diagram, and maybe for a guitarist it is a great >> diagram. >> But I would put the thing with the trash if I found it. >> Even 10 years before it as designed. > >** Jan - why were you ever born ?? Well, I had a deep moment of sadness reading the conversation, especially your reply to the OP. But this now really cheers me up, at least you ask solid questions that you need to answer in your life. So WHY were you born? Let's simplify this, I say: to be happy. It is not to reproduce only, and that may be a dead end street anyways, as any dinosaur would be able to tell you. Now the next question, and the important one, is how to always be happy. As the efficiency (of you), in percent, is 100x days_happy / days_lived. There are no official standards yet, many would score really low and have to be ..ahum... And probably there never will be a law about that. But is is beneficially to work on that yourself, introspection, meditation, I recommend it. So how does all this relate to guitar? Well obviously if you are happy and make music, then it will make others happy who listen to it. A sad song will make others sad.. Nature. How, and how much, the thing distorts is not even relevant, although one sort of distortion may sound a lot more pleasant then the other,. But I would have put the tube thing with the trash anyways, because from the above it follows that you can also make nice distortion in many ways with semiconductors... with the same effect on yourself and humanity. And it has been that way since the first Ge transistors came on the market. So, and you were wrong about that load issue, it is the signal that counts. |