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

[Richard Rasker]

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.

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.

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(!). OK, the sound got slightly more agressive as well -- definitely
more treble in clean mode -- but that didn't matter too much in overdrive
mode (and in clean mode, just small adjustment of the Treble control would
take care of that).

So my question is if anyone can explain why any designer would use such a
ludicrously high grid resistor -- IMHO, there are far better ways to create
a high cut-off circuit instead of this messy, noisy "solution". Or is there
some sort of extra or hidden advantage to this?

Thanks in advance, best regards,

Richard Rasker

 

[Rich Grise]

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.

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.

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(!). OK, the sound got slightly more agressive as well --
definitely more treble in clean mode -- but that didn't matter too much in
overdrive mode (and in clean mode, just small adjustment of the Treble
control would take care of that).

So my question is if anyone can explain why any designer would use such a
ludicrously high grid resistor -- IMHO, there are far better ways to
create a high cut-off circuit instead of this messy, noisy "solution". Or
is there some sort of extra or hidden advantage to this?

Thanks in advance, best regards,

Have you tried contacting Peavey?

Good Luck!
Rich

 

[Richard Rasker]

....

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

 

[Fred Bartoli]

Jan Panteltje a écrit :

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

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

 

[Ross Herbert]

: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
atience 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

 

[Adrian Tuddenham]

[...]

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?

 

[Phil Allison]

"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

 

[Phil Allison]

"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

 

[Phil Allison]

"Chris Quack"

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 .

Almost identical to that used in most Fender models.

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.

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.

Where do trolling fuckwits like Mr C. Quack get their insane shit from ??

And why do they have to spread it around all over the fucking net ???




..... Phil

 

[Phil Allison]

"Chris Quack"

Yes, (yawn) it's long tailed pair with cathode coupling, but the anode
loads are different

** Just as in nearly every Marshall tube amp ever sold.

and there's an added input to the second stage via c51 from next2, which
comes from the first page.

** For the "presence" pot - as used in Marshalls as well.

The asymmetrical anode values may be there just to balance the gain,

** Duhhhhhh ...........


(They are usually the same value)

** 100% BOLLOCKS !!

but who can tell without measuring the voltages under various operating
conditions ?. My guess is that it's an intentional imbalance. 100k vs 82k
?.

** Too hard for you go look at a Marshall schem ??

Or an early Fender model that Marshall copied ??

I do understand you would have to quit masturbating for a couple of
minutes.

Such sacrifices have to be made in the cause of science .....

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.

** But you did anyway - just for the heck of it right ?

You fucking liar.

None, ever, I guess you've seen them all then ?.

** Just about.

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.

That is how I am absolutely sure that YOU are a nothing but a

fucking, trolling BULLSHITTER !!!

Having grown up with 'tubes' and working in pro audio, 1968-1980,

** Doing what ? Cable winding.

I may just remember part of a clue, even now, despite what you may
think...

** You have no clues at all.

Just delusions.



...... Phil

 

[Phil Allison]

"Jan Panteltje"

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.

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 ??

Was you mum a cheap whore who got careless ??



...... Phil

 

[Ralph Barone]

: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
atience 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.

 

[Richard Rasker]

"Richard Rasker"

** 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.

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.

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.

** 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, 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

 

[Phil Allison]

"Chris Quack is a bullshitting LIAR "

And um, what effect is the presence pot having and how does it function in
term of signal path, system gain and frequency response ?.

** Look at the schems yourself - wanker.

Shame you have to give up on fat chick porn for a few minutes.

It's 30+ years since I even saw a Marshall amp with the cover off,

** Schems are readily available on the net - wanker.

Shame you have to give up on fat chick porn for a few minutes.

Really ? when exactly was this ? .

** Done regularly as part of amp testing.

If you've been involved in the business for that long, you must remember a
company called IES London and Mavis mixers ?...

** Don't confuse me with damn pommies.


...... Phil

 

[Phil Allison]

"Jan Panteltje"

( snip piles of putrid crapping on )


** Jan - why were you ever born ??

Was you mum a cheap whore who got careless ??

What a shame this gross mental defective was not aborted.



..... Phil

 

[Phil Allison]

"Richard Rasker"

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.

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.

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

 

[Phil Allison]

"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

 

[Phil Allison]

"christofire"

"Phil Allison" "christofire"

I've seen your photo of the (futuristic?) 120 mm drum elsewhere but I've
never had the opportunity to examine one in detail. Was the rim of the
drum coated with oxide in some way like tape, or was it just a steel drum?

** The Binson drum was made up from several parts.

1. A machined aluminium ring of about 10mm square cross section and 120 mm
in diameter.

2. A thin, *continuous* steel band about 7mm wide, shrink fitted onto the
above ring.

3. A pressed steel disk that held a 12 mm steel centre shaft and the above
ring.

The surface speed of the steel band was about 410 mm per second or 16 ips.



..... Phil

 

[Richard Rasker]

"Richard Rasker"


** No way do I "believe" any such assertions.



** Get a noise gate - fool.

OK, I'll get a noise gate allright -- I believe it's called a "killfile",
and I'm testing it with your messages right now. OK, it seems to work
pretty well. Thanks once again for your helpful insights.

Best regards,

Richard Rasker

 

[Phil Allison]

"Richard Rasker is a TROLL "


** Never come back.



..... Phil