Low noise triode mode for pentodes
Preamp
I bought on eBay and noticed that they give full details on using a
6AU6 pentode in both triode mode, and low noise triode mode (where the
anode is grounded and G2 is used as the anode. I recall Patrick
mentioned this mode earlier.
G2 dissipation is 0.65 watt. Basic circuit values are given for a
range of supply voltages. Gains from 14 to 25. Seems a useful tool.
They also give some example circuits in the back, and one is a 25 watt
PP amp that uses 2 stages of 6AU6 in this low noise mode, then 2 6AU6s
as phase splitters, and then 2 807s as outputs.
Certainly an unusual lineup, but I've always like 6AU6's :-)
Preamp
As some interest has been expressed, I have scanned the circuit. It is
here - http://hotbottles.freeservers.com/images/6au6807.jpg
Patrick Turner
Preamp wrote:
> I was just browsing through an old AWV Radiotron Valve data book that
> I bought on eBay and noticed that they give full details on using a
> 6AU6 pentode in both triode mode, and low noise triode mode (where the
> anode is grounded and G2 is used as the anode. I recall Patrick
> mentioned this mode earlier.
The ideas I mentioned are in RDH4.I dunno anyone who has tried the
grounded plate idea out,
except me, and too briefly to be conclusive.
> G2 dissipation is 0.65 watt. Basic circuit values are given for a
> range of supply voltages. Gains from 14 to 25. Seems a useful tool.
Could be, so why don't you take a try of the idea?
> They also give some example circuits in the back, and one is a 25 watt
> PP amp that uses 2 stages of 6AU6 in this low noise mode, then 2 6AU6s
> as phase splitters, and then 2 807s as outputs.
>
> Certainly an unusual lineup, but I've always like 6AU6's :-)
I have 6AU6 drivers in a pair of SE 6BQ5 output stage amps in my
re-engineeredold Trio AM / FM stereo reciever. They work well, and only
have to produce
a few volts drive to the output tubes.
Patrick Turner.
David Crittle
<http://www.retrovox.com.au/6AU6pAWV.pdf>
David Crittle
da...@retrovox.com.au
--
David Crittle
da...@retrovox.com.au
Preamp
Thanks Dave, very interesting reading....
Preamp
recent triode vs pentode discussions...
They say...
"Triode connection is desirable for a preamplifier because triodes are
inherently less noisy then pentodes, firstly because of their higher
gm/sqr(Ib) ratio, and secondly because there is no noise due to random
division of current between plate and screen"
Patrick Turner
Preamp wrote:
On page 783 of RDH4 triode shot effect noise is determined by the
equivalent noise resistance,which is approx 2.5 divided by Gm in ma per
volt. So 6DJ8 shoud be less noisy than 12AX7.
But in practice there are wide variations.
Pentodes produce "partition" noise as well and noise will be 3 to 5 times
a triode's noise for the same
amplification.
From pages 782 to 793 there is quite a bit on preamps, and noise.
But on page 938 there is a chart with the noise reistances of a lot of
tubes,
and triodes are best with a 6J6 at 470 ohms, then the lowest pentode, a
6AC7 is 720 ohms.
frequency converters are just plain noisy by comparison.
Noise depends on the bandwidth
of the circuit and this is all covered in the dear old bookie.
One reason why the RIAA phono amp is so quiet is that the noise
is attenuated by an effective first order low pass filter acting above
the pole at 50 Hz.
So a microphone amp might be noiser than a phono amp.
Patrick Turner.
Patrick Turner
David Crittle wrote:
> Here's the full AMW Circuit Lab. Report: (275K Acrobat file)
>
> <http://www.retrovox.com.au/6AU6pAWV.pdf>
Not bad explanation here of the grounded plate triode using the
6AU6.The curves show linearity as entirely adequate for low
level.
The -60 Db of noise levels was good for those days.
I just get -66 Db, unweighted with my phono 12AX7 preamp.
One thing not mentioned was the use of DC for the heaters,
which was avoided back then because of suitable cheap reliable
parts to do it.
It is now routine practice to reduce possible hum with DC
supplies.
There were no silicon diodes, and electro caps were a bit dodgy.
The currents in all most old tube gear were only heavy
at the signal output to speakers, and even then, most speakers
were 16 ohms.
I think Sovtek now make 12AX7 with helical heater coils which
have less hum.
Am I right?
Patrick Turner.
Preamp
>I think Sovtek now make 12AX7 with helical heater coils which
>have less hum.
>Am I right?
That might be the 12AX7LPS ?
TubeGarden
Dunno the jibberish technoid obfuscation.
I am listening thru 12AX7LPS in my hot rod Harmon-Kardon Citation IV. Their
sound is clean and full, if you go for that sort of nonsense.
Happy Ears!
Al B^}
Alan J. Marcy
Phoenix, AZ
PWC/mystic/Earhead