b3e5f0fc
From: ruffrecords@yahoo.com
Patrick Turner wrote:
> On Sep 16, 5:34 am, Ian Bell wrote:
>> I am having a strange problem with what appears to be distortion due to
grid current in a 12AX7.
>> It's a CC stage with a 3K Rk bypassed about 120K in the plate and a 300V
supply. Anode sits at just
>> over 200V and cathode just below 2V.
>>
>> I would not expect to see any distortion due to grid current with an input
signal of 250mV rms at
>> 2KHz but I do. There's a 1Meg from grid to ground and the test oscillator
is fed to the grid via a
>> 100K resistor. Analyzer is connected across grid to gnd. Tube off, there
is no distortion with the
>> 100K in cct or shorted. Tube powered up I get about -33dB 2H measured at
the grid, -39dB 3H, -59dB 4H.
>>
>> Short the 100K and the 2H drops to to below -70dB.
>>
>> I tried changing the tube but the result is the same. What is going on?
>>
>> Cheers
>>
>> Ian
>
> In some old AM radio designs where the first audio tube was a high
> gain triode like a 12AX7 the method of biasing was often grid leak
> bias with a 10M Rg and no Rk was used. There is a slight current flow
> along the 10M giving a bias of about -1V and this bias current is
> about proportional to Ia so the more Ia, the more -bias and so the
> triode then self biases OK.
> The distortion generated was ignored and was much less than from other
> causes in most old radios.
>
> The onset of grid current in a 12AX7 is gradual with the actual bias
> voltage value, and Ig begins at some Eg1 well below 0V, and not only
> when Eg1 goes positive with respect to Ek.
>
> 12AX7 are rather a beancounter's favourite tube because you get a load
> of gain. But maybe you'll find that the finite input resistance which
> you are discovering exists also appears with other triodes. The fact
> the distortion dissapears when you short out the series 100k is
> because your source impedance from sig gene falls to maybe600 ohms and
> the grid input resistance is virtually completely shunted, and the Ia
> is then controlled without the non linear grid input resistance
> interfering.
>
Yes, I have come across the same effect in my work with mu followers. I was
just surprised to see it
acting so soon. generally you can allow the signal to take the grid up to -1V
before worrying about
grid current bu that seems not to bee the case with the 12AX7 - today I fed it
with a mere 25mV and
it produced 1% distortion. There's something funny going on here.
> To get the best with 12AX7 performance one should always try to get Ea
> rather high for a given amount of Ia which should be more than 0.6mA,
> so that a decent negative bias voltage to the grid is needed.
Looking at the curves that is going to be quite hard. To get -2V bias at 200V
then Ia needs to be
way below 0.6mA.
Whatever
> is driving the grid of the 'AX7 should have low source resisance.
That is n ot an option in this case as it will be a 100K volume control so
even if that is fed from
zero ohms the worst case source R would be 25K.
Now
> have you tried biasing the 'AX7 grid with say 47k? This is a typical
> value for a phono amp, and could also be good for the input of a power
> amp although you'll commonly see 470k used for power amps and line
> stages.
I am using 1Meg at present - ah maybe that's what is causing the trouble - I
made it that big so as
not to disturb the NFB that will be added later - the 100K would be the input
arm of the NFB - I'll
try smaller grid R and see if that helps - good idea - thanks Patrick.
If you have a source select switch or a switched volume
> control in the grid circuit of some preamp or power amp input tubes
> you can sometimes hear a click each time the source is changed or the
> volume is adjusted and its the altering level of grid Vdc. It only
> takes a jerk of a few mV to make a click sound. To avoid such
> nonsense, one might use say 100k grid bias R with say 0.22uF input
> blocking cap with source select and volume control in front of the cap
> and not able to very slightly adjust whatever slight Vdc may exist at
> the grid. And if you want lowest possible noise generated by the grid
> biasing resistance you need to keep the R value low as possible but
> without being too low a load for the driving stage. If this is a CD
> player with Rout less than 600 ohms you could have Rg = 10k and maybe
> no need for any dc blocker cap.
>
It is a phones amp - bootstrapped 12AX7 CC feeding a pair of 12BH7 arranged as
White follower. rIght
now the White produces less distortion than the 12AX7.
> As tubes age, the grid can act strangely and its slight tendency to be
> always slightly more negative than the bias supply, or say 1V ess than
> 0V where Rg = 10M, becomes less of a tendency, and the bias control
> becomes lost and grid slowly becomes positive and Ia then is turned on
> more so that older tubes can have a positive grid with very low Ea and
> high Ia as a result. This happens in old radios and nobody notices
> until Ea has declined to about +20V only, and distortion is beginning
> to get audible at a percent or more.
>
Don't think that is a problem as I am using new tubes right now.
> Your 12AX7 operating conditions seem about just right, and I guess
> changing samples of 12AX7 should not change the phenomena you see -
> although some well worn 'AX7 samples will probably measure more THD
> than newer ones with a harder vacuum.
>
As I said both ones I tried are new - both by EH.
> Things could be worse - if you used a small BJT with bypassed emitter
> resistance - the THD will be horrendous, but with a much bigger
> difference between having a 100k in series with source and base and
> having no series R.
>
LOL
Cheers
Ian
> Patrick Turner.
>
>
>
>
>
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