High-ohm grid resistor in tube (pre)amp design
in [Design]
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From: Ross Herbert on 12 Nov 2009 04:26 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
From: Adrian Tuddenham on 12 Nov 2009 07:45 Tim Wescott <tim(a)seemywebsite.com> wrote: [...] > Tubes exhibit an effect where the electrons boiling off the cathode want > to settle on the grid. Make the path to ground with a high enough DC > bias and the tube will self-bias. The resistor is then called a "grid > leak" > > (and before all you old tubies jump on me -- no, I don't remember if 470k > is a high enough resistance for this to work, or if you need multi- > megohms). It's usually 10 megohms and develops about 1 to 2 volts of negative bias. If the valve becomes the slightest bit 'soft', the grid can pick up ions and go positive, which leads to runaway if there is no current lmiting factor elsewhere in the circuit. Because of this, this method of biassing is only really suitable for low level signal stages with high value anode resistors, which will work adequately with such low and unpredicatable levels of negative bias. Even 470k can sometimes be too high a grid leak for power stages, even with normal cathode bias, which might become slightly soft due to the high temperatures out-gassing the anode structure. The series resistor described by the original poster is called a 'grid stopper' (at least, it is on this side of the Pond), but I have never seen one as high as 470k. Usually they are in the range 10k to 47k. I don't suppose the OP has mis-read the colour code or measured one that is on its way to going O/C? -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
From: Phil Allison on 12 Nov 2009 08:08 "Richard Rasker" > > 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. > > OK, I fully understand that this resistor, together with the Miller > capacitance between grid and anode, can act as a high cut-off filter, and > that this can prevent HF oscillations and the likes, but it also > introduces > noise -- a LOT of noise. ** Not true, when it is in a later stage of the circuit than the input stage. Cos the noise generated by the first tube stage swamps that resistor's noise. BTW: 470kohms has a self noise of about 10uV in the audio band - much less in the narrower band guitar amps use. The audio band INPUT noise level of a typical 12AX7 triode section is 3 V - multiplied by the gain of 50 or more at the plate. > I have here exhibit A, a Peavey 5150 EVH. Shorting the aforementioned 470K > grid resistor in the second preamp triode cut the noise level by as much > as > 15dB(!). ** Not true, in normal use. > 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. Peavey's clearly has a delicate touch with tube guitar circuits. ...... Phil
From: Phil Allison on 12 Nov 2009 08:16 "Phil Allison" ** Hmmm.... two typos crept in: " The audio band INPUT noise level of a typical 12AX7 triode section is 3 uV - multiplied by the gain of 50 or more at the plate." " Peavey's designer clearly has a delicate touch with tube guitar circuits." ..... Phil
From: ChrisQ on 12 Nov 2009 08:19
Ross Herbert wrote: > > > 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 assymetrical drive to get core saturation on the output stage. It's all about the sound on valve guitar amps and it doesn't always follow the usual expected rules of electronics. 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. Whether the design engineer realised this at the time, or whether it was just a hack to stop some squegging problem on the prototype, who knows. 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 :-)... Regards, Chris |