Just a quick follow-up...

With the help of interactive and archived RAT advice, I've been tinkering
with my Heathkit aa-151 for months now. When I last asked about adding a
choke, Patrick Turner suggested (among other things) to run the pre-amp
6AU6 pentodes in triode mode.

Well... I found out how easy enough through the RAT Archive, and even
hooked it up to a switch, so I can switch between pentode and triode mode
for these tubes. Big waste of time!! See.. I can't imagine EVER running
them in pentode mode again!

The results? The sound is a bit "wetter", which I expected. The gain is a
bit less, which I expected.. but wow.. in triode mode these things are MUCH
more quiet!

It was never a "noisy" amp, with no signal ii did have a sort of "white
noise" though, not really a hum. With this mod it's now dead silent with
nothing playing... wow (lemme spell it backwards.. wow!).

Anyrate.. if you have a Heathkit with 6AU6's in the preamp... I'd HIGHLY
recommend you just go ahead with this mod (forget the switch!).

BTW.. NOS Amperex and Mullard 6AU6's can be had for about $5US... not a bad
psuedo-triode eh!

Thanks again all,
EC

EC wrote:

Just a quick follow-up...

With the help of interactive and archived RAT advice, I've been tinkering
with my Heathkit aa-151 for months now. When I last asked about adding a
choke, Patrick Turner suggested (among other things) to run the pre-amp
6AU6 pentodes in triode mode.

Well... I found out how easy enough through the RAT Archive, and even
hooked it up to a switch, so I can switch between pentode and triode mode
for these tubes. Big waste of time!! See.. I can't imagine EVER running
them in pentode mode again!

The results? The sound is a bit "wetter", which I expected. The gain is a
bit less, which I expected.. but wow.. in triode mode these things are MUCH
more quiet!

It was never a "noisy" amp, with no signal ii did have a sort of "white
noise" though, not really a hum. With this mod it's now dead silent with
nothing playing... wow (lemme spell it backwards.. wow!).

Anyrate.. if you have a Heathkit with 6AU6's in the preamp... I'd HIGHLY
recommend you just go ahead with this mod (forget the switch!).

BTW.. NOS Amperex and Mullard 6AU6's can be had for about $5US... not a bad
psuedo-triode eh!

Thanks again all,
EC

Triode connected pentodes like the 6AU6
are marginally quieter than when pentode mode is used.
Many preamps from the 1950s used EF86, and some used 6AU6,
and usually, they were run in pentode, with feedback loops
to make the gains of two channels equal, and to reduce
the output resistance, noise, and distortion of the circuit.

The Quad22 preamp has EF86 as the phono amp,
and it has a combination of passive RIAA eq and feedback eq.
With high output MM, it is fine.
Triodes may or may not be quieter, since tubes vary quite a lot,
but generally, theory suggests triodes with high gm have the least
input noise.

I have not seen your amp schematic, so exactly what the engineering
benefits are I don't know.

Long ago, pplo used to use the 6AU6 with its anode grounded, and the screen as
a triode anode, at maybe 0.5mA "anode current", and this was supposed to be a
very quiet way of building a microphone amp, since shot noise was minimal,
ie the noise of electrons whacking into the anode metal.
That still leaves the grid noise, so I dunno exactly what
the benefits of the screen for anode connection may be.
But at least the "partition" noise, ie, the noise caused by electrons
trying to get past the screen is reduced with triode connection.

Noise in old preamp line stages, whether triode or pentode
is quite common, because the tube was often a high mu triode, with high Ra,
or a pentode, and the following passive tone controls used pots of say 2 meg,
and all the R elements were high value, and all this junk was in series
with the power amp.
To enjoy really low noise, use medium mu troides with low Ra,
and low impedance R in the tone, balance and volume circuits, and
so a line stage could be 1/2 a 6CG7/12AU7/6AU6(triode) followed by a 100k
volume pot.
The dc carrying RL to the tube might be say 68k,
so the total load seen by the tube is 41k.
This is a fair load for such triodes, and the thd will remain below 0.1%,
and be nearly all 2H, and the sound can finally be tailored by swapping
brands of tubes.

The best tone controls, IMHO, are feedback types,
so another tube, which can be almost anything one likes,
since whatever gain it has open loop is reduced to less than unity
with the FB elements in a "Baxandal" shunt FB loop.
I have never found any versions of these to be coloured.
They do need pots of 500k, preferably, but at least they are lot easier
to set up, and trim for a flat response in the mid position of the pots because
the
pots are all linear types, and more likely to track equally between channels.

For more food for thought, try the circuit at
http://www.turneraudio.com.au/htmlwe...tubepreamp.htm

I now used an upgraded version of this amp, with
better PS, and a more linear version of the mu follower,
and a 2SK369 fet in the phono stage, since these j-fets have
a whopping 40 mA/V gm at only 5 mA.
2SK147 is also usable, and has the same specs.
It seems to me the family of j-fets which have
such high gm, like a mosfet, or a bjt, is a very small family.

One of these days, I shall upgrade the website
to include the last year's learnings.

With noise, the more R , the more noise.
At room temp, a 2kohm R makes about 1 uV of noise.
100k makes about 7uV, and 1 meg, about 22uV.
If the gain of a tube is 20, and all you have is a 1 meg bias resistor
att he grid, then the R will input 22 uV, which is 440 uV at the output,
and if that is amplified 20 times by a power amp, there is 8.8 mV at the power
amp
output.
This is theoretical, and the noise is considered to be 20 kHz wide in BW.
Hench the feint shshshshshsh that you hear, maybe with some hum,
since that "gets in" when things are made messily.

But in practice, other noises, like splotchy plops are also heard, which is
"flutter", or "flicker" noises.
These are dreadful in a phono amp, where the LF gain is very high.
Old worn and gassy tubes are the noisiest, and often the most
microphonic.

Patrick Turner.