[HAM] Capacitors

[Keith H Clark]

No good result!

-----Original Message-----
From: hammond-bounces at zeni.net [mailto:hammond-bounces at zeni.net] On Behalf
Of David Anderson
Sent: Sunday, February 17, 2008 11:13 PM
To: The Hammond Forum
Subject: Re: [HAM] Capacitors


Kon,

When you swap a 1 Meg plate resistor for a 100K plate resistor,  
you're *significantly* changing the operating point of the tube.

In doing so, you might be putting the tube into a non-linear region  
of operation and affecting the frequency response of the circuit, but  
to claim simply that increasing the value of a plate resistor  
increases bass is cutting far more electrical engineering corners  
than I'm comfortable with.

Off the top of my head, I might wager that, rather than increasing  
the bass, you're actually cutting the treble due to increased Miller  
Effect.

Psychoacoustically, it's easy to make the ear think you're increasing  
bass by decreasing the highs and vice-versa.

David

On Feb 17, 2008, at 8:33 PM, Kon Zissis wrote:

>
> I have built a guitar preamp  that uses three 12AX7 valves  and a high
> DC voltage power supply, and I have placed six switches that select
> either 100 K ohms  or 1 Mega ohms  as the plate resistor values of all
> six stages of the 12AX7 valves.
> 100 K ohms is a very commonly used plate resistance value in valve
> circuits  but when  I set the switches to the 1 Mega ohms  settings ,
> this lowers the plate voltage in order to allow an earlier onset of
> distortion  but  the 1 Mega ohms resistances also cause the bass
> response to be warmer and meatier compared to the 100 K ohms settings.
>
> This warmer bass response seems to  be happening because the 1 Mega  
> ohm
> plate resistances  lessen the feedback effect  of the plate resistors
> between the various valve stages .
> Because of this I think that a valve amplifier  would sound warmer and
> fatter if the plate resistor values were very high in order to  
> minimise
> the feedback effect and then have a very high DC voltage  power supply
> so that a proper DC voltage to allow the clean headroom still goes to
> the valve plates even with the very high plate resistor values such  
> as 1
> Mega ohms instead of the more common 100 K ohms.
>
> The 12AU7 valve in the Leslie  122 and 147  uses  56 K plate resistors
> so therefore it would be interesting to see how the bass response is
> affected if the 56 K resistors were replaced with 100 K or 200 K or 1
> Mega ohms values.  Disregarding the fact that the reduced voltage  
> would
> lessen the clean headroom of the 12AU7 , I expect that the higher  
> plate
> resistor values might increase the  bass response to some extent.

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