High-ohm grid resistor in tube (pre)amp design
in [Design]
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From: Phil Allison on 12 Nov 2009 18:23 "Richard Rasker" > Phil Allison wrote: > >> ** Not true, when it is in a later stage of the circuit than the input >> stage. > > OK, but in this case, the resistor is found in series with the grid of the > second triode -- and believe me, it's responsible for a lot of noise. ** No way do I "believe" any such assertions. >> ** Not true, in normal use. > > OK, at normal playing volume, it's > hardly noticeable, but during quiet passages, it's rather annoying -- > especially in a studio environment. ** Get a noise gate - fool. >>> 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. > > That seems a bit far-fetched in this case, ** You are completely missing the point. Grid current causes the coupling cap to become charged and when the signal stops it will bias off the following triode stage until it discharges. Something best avoided in a guitar amp. ...... Phil
From: christofire on 12 Nov 2009 20:36 "ChrisQ" <meru(a)devnull.com> wrote in message news:CF%Km.9817$RG.3911(a)newsfe20.ams2... > Jim Thompson wrote: > >> >> No transformers in the signal path. >> > > Like all engineering, it's what gets the job done in the most effective > way and a transformer is a perfectly respectable component to use for such > an application, especially considering it's history of getting the job > done well at low cost. It even adds weight to the box, so the roadies > think it's been properly screwed together :-). Such stuff matters and you > build the product, imperfections and all, to sell into that market. I got > into micros in the late 70's, so you can see where my head was going. > There's only so much you can do with a class ab amplifier and it was time > to move on. > > If you could accurately model a Marshall 100 or Watkins AC30 or similar > and then dsp the resulting signal chain, you can bet that some muso's > would still say they could still tell the difference and to be honest, > they probably could :-)... > > Regards, > > Chris What's a 'Watkins AC30'? The well-known AC30 certainly wasn't one of Charlie Watkins's creations. Chris(a different one)tofire
From: christofire on 12 Nov 2009 20:46 "John Walliker" <jrwalliker(a)gmail.com> wrote in message news:c35f8d6c-f5dc-4916-9093-7c453e7cdfb5(a)r24g2000yqd.googlegroups.com... > On 12 Nov, 19:12, Jan Panteltje <pNaonStpealm...(a)yahoo.com> wrote: > >> But really, with modern processing techniques you can make a lot more, >> and perhaps >> better, effects at the low signal level and then play it via a good solid >> state amp and high >> quality speakers... >> Probably with more volume too... >> It would be interesting to somehow characterise the transfer curves of >> some of those old tube things, >> so it can be mimicked in FPGA or with a DSP for example. > > Many years ago (probably 1977 or 1978) Charlie Watkins of WEM gave a > talk to my university audio society. One of the questions he answered > was "why not use non-linear networks to simulate the valve sound" in > guitar amps. > > The answer was that WEM had tried hard to do this, as there would have > been considerable robustness and cost advantages. Unfortunately, they > could not get the sound right, so they decided to carry on making > valve amps. I think the problem was not just getting the right > transfer function, but also the time dependencies like power supply > sag and even valve microphonics that affect the overall sound under > severe overload. > > John The requisite DSP techniques hadn't been rummaged by 1978. WEM carried on making the 'Copycat' with a tape loop and an AC induction motor long after bucket-brigade devices had become available, and then started using them after the rest of the industry had gone digital. Line 6, for one, have made a reasonable job of simulating 'valve sound' in DSP, but that's only my personal opinion. As noted in another thread in this Usenet group, it's believed that Line 6 upsample before applying a non-linear function (e.g. tanh) so the highest of the resulting harmonics don't create noticeable enharmonic aliases when they beat with the sampling frequency. Chris(tofire)
From: christofire on 12 Nov 2009 20:57 "christofire" <christofire(a)btinternet.com> wrote in message news:YdednTxZ3IMLJ2HXnZ2dnUVZ8v-dnZ2d(a)bt.com... > > "John Walliker" <jrwalliker(a)gmail.com> wrote in message > news:c35f8d6c-f5dc-4916-9093-7c453e7cdfb5(a)r24g2000yqd.googlegroups.com... >> On 12 Nov, 19:12, Jan Panteltje <pNaonStpealm...(a)yahoo.com> wrote: >> >>> But really, with modern processing techniques you can make a lot more, >>> and perhaps >>> better, effects at the low signal level and then play it via a good >>> solid state amp and high >>> quality speakers... >>> Probably with more volume too... >>> It would be interesting to somehow characterise the transfer curves of >>> some of those old tube things, >>> so it can be mimicked in FPGA or with a DSP for example. >> >> Many years ago (probably 1977 or 1978) Charlie Watkins of WEM gave a >> talk to my university audio society. One of the questions he answered >> was "why not use non-linear networks to simulate the valve sound" in >> guitar amps. >> >> The answer was that WEM had tried hard to do this, as there would have >> been considerable robustness and cost advantages. Unfortunately, they >> could not get the sound right, so they decided to carry on making >> valve amps. I think the problem was not just getting the right >> transfer function, but also the time dependencies like power supply >> sag and even valve microphonics that affect the overall sound under >> severe overload. >> >> John > > > The requisite DSP techniques hadn't been rummaged by 1978. WEM carried on > making the 'Copycat' with a tape loop and an AC induction motor long after > bucket-brigade devices had become available, and then started using them > after the rest of the industry had gone digital. Line 6, for one, have > made a reasonable job of simulating 'valve sound' in DSP, but that's only > my personal opinion. As noted in another thread in this Usenet group, > it's believed that Line 6 upsample before applying a non-linear function > (e.g. tanh) so the highest of the resulting harmonics don't create > noticeable enharmonic aliases when they beat with the sampling frequency. > > Chris(tofire) PS: of course that should have been 'Copicat'.
From: Phil Allison on 12 Nov 2009 21:25
"christofire" > WEM carried on making the 'Copycat' with a tape loop and an AC induction > motor long after bucket-brigade devices had become available, ** The original valve WEM Copicat was never more than a cheap and cheerful device for impoverished guitarists - subsequent WEM models were only marginally better. Much better preforming but more expensive units existed, like the Klempt and the famous Binson Ecorec - the latter using a rim driven drum which eliminated most of the horrible problems experienced with tape loops. There are still no readily available digital units that produce the same kinds of sound effects possible with these mechanical units. ..... Phil |