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

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From: Richard Rasker on 11 Nov 2009 16:54

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
--
http://www.linetec.nl

From: Rich Grise on 11 Nov 2009 16:56

From: Richard Rasker on 11 Nov 2009 17:09

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
--
http://www.linetec.nl

From: Dave Platt on 11 Nov 2009 19:32

In article <4afb3280$0$729$7ade8c0d(a)textreader.nntp.internl.net>,
Richard Rasker <spamtrap(a)linetec.nl> wrote:

>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?

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: http://en.wikipedia.org/wiki/Madman_Muntz)

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

--
Dave Platt <dplatt(a)radagast.org> 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!

From: Tim Wescott on 11 Nov 2009 20:19

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.
>
> 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,

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

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