High-ohm grid resistor in tube (pre)amp design

Have you tried contacting Peavey?

Good Luck! Rich

On a sunny day (Wed, 11 Nov 2009 22:54:08 +0100) it happened Richard Rasker wrote in :

And then also explain why the moron used tubes, and not a good opamp and / or semiconductors.

...

...

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

--
http://www.linetec.nl

At a guess, it's done because it's simple and cheap, and "good enough" for the specific application in which the product is used. The design was Muntzed.

(ref:

Or, possibly, the designer was drunk or insane at the time :-)

--
Dave Platt                                    AE6EO
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
  I do _not_ wish to receive unsolicited commercial email, and I will
     boycott any company which has the gall to send me such ads!

Is the resistor in the path from grid to ground?

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).

Could that be it? If so, shorting out the resistor would overbias the tube and cause excessive current. Shorting the resistor out with a cap should cause the same effect in the audio (if it's a big enough cap).

Such a resistor would also tend to reduce the bias on the tube in the event that it gets over-driven - as soon as the grid starts to conduct the stage would get biased for less plate current, gain, output, etc.

--
www.wescottdesign.com

Jan Panteltje a écrit :

semiconductors.

Sure! How could they use a grid resistor with opamps?

--
Thanks,
Fred.

: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...

[...]

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

"Richard Rasker"

** 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.

** Not true, in normal use.

** 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

"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

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

On a sunny day (Fri, 13 Nov 2009 00:08:45 +1100) it happened "Phil Allison" wrote in :

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

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.

On a sunny day (Fri, 13 Nov 2009 01:45:05 +1100) it happened "Phil Allison" wrote in :

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 :-)

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.

semiconductors.

In fact, it's very difficult to get the required musical characteristics from a solid state amp because of the hard limiting with a typical transformerless output stage. A valve preamp and output stage gives a softer saturation characteristic. I guess the closest you could get to a solid state version would be to use power mosfets driving a transformer, but valves are very rugged as well in terms of short and medium term gross overload and a power mosfet design would need to have quite a bit of protection to handle shorted output, open circuit speaker lead at max volume etc.

No idea what the current state of art is now, but even in the mid to late 70's, it was getting difficult to find good quality output valves like 6L6 and EL34's. Most if not all the western manufacturers were out of the valve business and the russian and bulgarian substitutes were poor quality and rarely lasted for more than a show or two. The Marshall amps of 1979 timeframe overdrove the anode voltage at no load (~500v dc, iirc) as well and the valves blue glowed from new. Failure was a red hot anode and subsequent blown ht fuse. But there again, the sound was perfect for the job :-)...

Regards,

Chris

semiconductors.

Real men know how to shape clipping so that it is "soft", yet maintain output damping ;-)

Real men don't use valves ;-)

Real men don't use transformers ;-)

Did that shit when I was 16-18 years old... I'll never go back ;-)

...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

The Democrat theory is that spend.. spend.. spend.. and government
jobs will bolster the economy and employment will improve. If that
were actually true, wouldn\'t France have the world\'s best economy?

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.

I measured this with the "pre" gain turned 1/4 up and the "post" gain halfway up. In a silent environment, it makes the difference between audible noise and almost total silence. OK, at normal playing volume, it's hardly noticeable, but during quiet passages, it's rather annoying -- especially in a studio environment.

That seems a bit far-fetched in this case, considering the placing in the circuit -- it's R82, behind the very first triode, with some filtering and the volume control in between.

Anyway, I'll do some more measuring, experimenting and listening; perhaps the best thing to do is to decrease the resistor value to 100K or thereabouts.

Thanks everyone for their input; I've learned a few new things about tube amp design :-)

Richard Rasker

--
http://www.linetec.nl

Or eat quiche, from what I understand :-).

Not even ferrite ?. I might, even now, though the rest of it would be solid state. Not such a new idea either. They were using some transistorised plug in tube replacements for 12ax7 etc in some amps as far back as the 70's, though tubes and transformers always in the output. I guess you could consider the amp itself as part of the musical instrument. Thus, almost everything is subjective.

Go back to what, Jim :-). Was all part of the ongoing book of the progress of electronics. Look back at it as a great time to be working in the art, with the techniques of the day having something to teach us even now...

Regards,

On a sunny day (Thu, 12 Nov 2009 17:26:28 +0000) it happened ChrisQ wrote in :

semiconductors.

It all depends. If we leave the combination speaker-amplifier out of the picture for a short moment, I will tell you what happened to me some time ago. I was listening to some old recording on the PC, and that was an old analog recording. And I thought: 'Hey analog sounded better'. Now I caught myself thinking that, and realized that I was listening to, YES, an mp3 version, digitally processed, made from a wave recording of the old analog tape... So the essence is of course that if you have an amplifier with sufficient low distortion, or amplifier-speaker combination with the same, then you can *reproduce* any sound. Somehow you seem to think, well that seems to me at least, that the sound or specific distortion so to speak ( :-) ) needs to be created in the output stages?

Yes I have build a nice tube amp for a band, and the lead guitarist liked the distortion when overdriving it :-)

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. But with all those pots it would be difficult. But then, listening to Jimmy Hendrix in *my* opinion it would not really matter if it was a few percent off.

Regards Jan