I am designing a tube based microphone pre-amplifier. At present I am
trying to sort out the HT supply topology. I would prefer a simple RC
network but this reduces the anode volts available to the first stage. I
want to avoid 'anode starvation' and it attendant colouration so my
question is below what voltage would the anode be considered to be
'starved'? The preamp uses 6AU6 tubes.

Cheers

ian

Ian Thompson-Bell wrote:

I am designing a tube based microphone pre-amplifier. At present I am
trying to sort out the HT supply topology. I would prefer a simple RC
network but this reduces the anode volts available to the first stage. I
want to avoid 'anode starvation' and it attendant colouration so my
question is below what voltage would the anode be considered to be
'starved'? The preamp uses 6AU6 tubes.

Cheers

ian

There is nothing wrong with CRCRCRC type filters for preamps.

You need forst to abandon completely trying to copy
what was done in 1955 where preamps rarely had more than 47uF caps.
For good filtering, R had to be high, and thus you got a dc voltage drop
you didn't
want.
AND you also got a preamp very likely to oscilate at LF, ie, motorboat,
because it is a de-facto phase shift oscillator.

But these problems dissapear if you use LARGE value capacitors
which are now quite cheap, such as 470uF x 450V rated.
These are used now abuntanly in SMPS where they work hard, but in your
preamp
they'll be working real easy, and be very effective.
Modern electros are routinely very stable, of high tolerance, and will
bypass
up to a HF.

If you have 330 ohms plus 470uF, the attenuation factor is the same as
using 3,300 ohms plus 47 uF, or 33k and 4.7 uF.

At 100Hz, 470uF has Z = 3.4 ohms, and with 330R the attenuation of
rectifier hum
is around 1/100, and 3 such cascaded RC filters will reduce say 0.2Vrms
of ripple
at C1 by 1/1,000,000 to less than 1.0uV. If Idc was 10mA, the V drop is
only 10Vdc across
3 x 330 ohm R.

Let the moths out of your purse and buy some decent sized capacitors.
Forget the BS that they sound bad; the opposite is true!

Consider learning about high voltage regulation
for the first stage of the preamp at least.
It need only be a shunt reg type, and one using zeners in a string,
say 5 x 75V rated at 5 watts each and with 10mA of current flow.
Trouble is they make noise as they work; lots of noise in fact across a
wide band frequency,
and so then you need to learn how to arrange the zeners to work so their
noise is filtered
away with a series R and added shunt C.
Yet they are able to give good enough regulation of DC to stop motor
boating of slow
low F undulations due to mains voltage shifts.
Don't use zeners if you don't need to; ie, if the preamp output signal
has less than 0.4mV peak of rise and fall
of the 0V trace when full gain is used.
So once the circuit is built, find out if it is stable with volume
turned up full,
and after bursts of gross overload signals. If there are no peaks in the
measured sine wave
response dow to 0.1Hz, and you SHOULD have gear able to read such LF,
then its stable.
Preamps with bass boost adjustment in a tone control stage were renowned
for
becoming LF oscillators when bass was boosted because of the increase in
gain
made stability poor.



Suitable arrangements of coupling cap values and / or shelving networks
will stop most excessive LF transfer.


All electros used for tube stage supply rails need to be bypassed by
plastic polyprop or polyester
caps placed as close as possible to the circuit it bypasses.


In a mike preamp, bass isn't favoured like in a phono amp. But you don't
want to have a preamp
with its 0V signal wobbling up and down like the crude amps made in 1960
all used to to do.

Tube preamps for microphones are damned noisy where the mic signal is
below say 5mV.

So for good SNR, the old practice was to use a step up microphone
transformer.
The mic was very low Z, and its noise much lower than a tube.
When the mic signal was transformed up 10 times, the noise was still
lower than a tube.
And the tube at the input should be a triode; 6AU6 is fine, but betst
srtapped in triode,
or used with the screen as the anode, and anode taken to 0V.
See RDH4 about such antiquated details.

Of course if you used a j-fet for the input you'd not need a
transformer,
and following stage after the fet can be almost any triode.

A 2SK369 has at least 1/10 of the noise of a well chosen 1/2 12AX7,
with Id = 5mA, Ed = +8V, supply = +20V and RL = 3.3k, and gain = up to
100 times approx.

A cascode circuit is also very good, see http://www.vacuumstate.com and
Allen Wright's
hybrid stages for phono. They can be used for microphone, without the
RIAA filters.

There are some clues on PSU design at my site,

http://www.turneraudio.com.au

Patrick Turner.

Patrick Turner wrote:

snip

But these problems dissapear if you use LARGE value capacitors
which are now quite cheap, such as 470uF x 450V rated.
These are used now abuntanly in SMPS where they work hard, but in your
preamp
they'll be working real easy, and be very effective.

Excellent idea. To date I had assumed the 100uF 450V electrolytics I
could get from suppliers like Maplin were about all that was available.
It seems both RadioSpares and Farnell do 470uF 450V caps but they are
quite expensive - the cheapest I found was over 4GBP before tax in 10
off quantities. However, Digi-key also seems to do them at 3.40GBP at 10
off quantities but by the time you add in shipping and handling they are
over 5GBP each. So either way it works out at about 5 pounds a pop.


If you have 330 ohms plus 470uF, the attenuation factor is the same as
using 3,300 ohms plus 47 uF, or 33k and 4.7 uF.

At 100Hz, 470uF has Z = 3.4 ohms, and with 330R the attenuation of
rectifier hum
is around 1/100, and 3 such cascaded RC filters will reduce say 0.2Vrms
of ripple
at C1 by 1/1,000,000 to less than 1.0uV. If Idc was 10mA, the V drop is
only 10Vdc across
3 x 330 ohm R.


Idc is a little under 10mA but the basic ripple across the 47uF
reservoir cap measures 1.2V pp which must be about 0.4V rms (can't
remember the crest factor of a sawtooth). This directly feeds the CF
anode and the ripple at its cathode is just 50mV pp so the CF reduces
the ripple by about 20 times which is interesting.

Let the moths out of your purse and buy some decent sized capacitors.

Yes I shall. I think I'll go for 470uF reservoir then two RC stages for
the CF and the gain stage respectively each with 470uF caps.

The current design ends up with 191V (measured) on the CF cathode which
is a bit high so I can afford to drop the CF HT quite a bit. The preamp
is supplied via 22K and 47Uf at present which gives it a 255V (measured)
supply and a 117V (measured) anode voltage.

snip


In a mike preamp, bass isn't favoured like in a phono amp. But you don't
want to have a preamp
with its 0V signal wobbling up and down like the crude amps made in 1960
all used to to do.

Tube preamps for microphones are damned noisy where the mic signal is
below say 5mV.

So for good SNR, the old practice was to use a step up microphone
transformer.
The mic was very low Z, and its noise much lower than a tube.
When the mic signal was transformed up 10 times, the noise was still
lower than a tube.

That's still current practice too. I am using Sowter transformers (10:1)
at the input.

And the tube at the input should be a triode; 6AU6 is fine, but betst
srtapped in triode,
or used with the screen as the anode, and anode taken to 0V.
See RDH4 about such antiquated details.


Yes it is strapped as a triode for the usual reasons. I am using screen
and suppressor strapped to anode as it appears to give slightly better
gm values with the 6AU6 than the screen as anode connection. Heater
induced hum is not an issue as I plan to use a dc heater supply.

Of course if you used a j-fet for the input you'd not need a
transformer,
and following stage after the fet can be almost any triode.


I am trying to avoid semiconductors wherever possible.


A cascode circuit is also very good, see http://www.vacuumstate.com and
Allen Wright's
hybrid stages for phono. They can be used for microphone, without the
RIAA filters.

There are some clues on PSU design at my site,

http://www.turneraudio.com.au

Patrick Turner.

I have printed this email and stuck it in my day book.

Many Thanks

Ian

Patrick Turner wrote:

Ian Thompson-Bell wrote:
I am designing a tube based microphone pre-amplifier. At present I am
trying to sort out the HT supply topology. I would prefer a simple RC
network but this reduces the anode volts available to the first stage. I
want to avoid 'anode starvation' and it attendant colouration so my
question is below what voltage would the anode be considered to be
'starved'? The preamp uses 6AU6 tubes.

Cheers

ian

There is nothing wrong with CRCRCRC type filters for preamps.

You need forst to abandon completely trying to copy
what was done in 1955 where preamps rarely had more than 47uF caps.
For good filtering, R had to be high, and thus you got a dc voltage drop
you didn't
want.
AND you also got a preamp very likely to oscilate at LF, ie, motorboat,
because it is a de-facto phase shift oscillator.

But these problems dissapear if you use LARGE value capacitors
which are now quite cheap, such as 470uF x 450V rated.

Patrick, I notice you use these in a number of designs featured on your
web site. Where to you get yours form?

Cheers

Ian

Ian Thompson-Bell wrote:

Patrick Turner wrote:

snip

But these problems dissapear if you use LARGE value capacitors
which are now quite cheap, such as 470uF x 450V rated.
These are used now abuntanly in SMPS where they work hard, but in your
preamp
they'll be working real easy, and be very effective.

Excellent idea. To date I had assumed the 100uF 450V electrolytics I
could get from suppliers like Maplin were about all that was available.
It seems both RadioSpares and Farnell do 470uF 450V caps but they are
quite expensive - the cheapest I found was over 4GBP before tax in 10
off quantities. However, Digi-key also seems to do them at 3.40GBP at 10
off quantities but by the time you add in shipping and handling they are
over 5GBP each. So either way it works out at about 5 pounds a pop.

Yes, but what you are paying per 100 uF isn't too high a price for a
blame free PSU.
Using large 40uF motor start caps would be more expensive, but the C
values are way too low.

Is the price of a 100uF cap 1/5 of the price of a 470uF?

I buy my electros from spare parts suppliers like wescomponents.com
who supply to the repair trades, and never from local retail electronic
places such as Maplin; Jaycar and Dick Smith Electronics are the Oz
equivalent,
and they charge twice the trade price.

I also buy from a mob here who buys job lots of parts left over from
mainstream manufacture runs, rockby.com.au
Sometimes thay have 470uF x 400V caps for $2 each.....

All these caps and things are made in asian factories by thye million,
and the price ex the factory is maybe 20c.




If you have 330 ohms plus 470uF, the attenuation factor is the same as
using 3,300 ohms plus 47 uF, or 33k and 4.7 uF.

At 100Hz, 470uF has Z = 3.4 ohms, and with 330R the attenuation of
rectifier hum
is around 1/100, and 3 such cascaded RC filters will reduce say 0.2Vrms
of ripple
at C1 by 1/1,000,000 to less than 1.0uV. If Idc was 10mA, the V drop is
only 10Vdc across
3 x 330 ohm R.


Idc is a little under 10mA but the basic ripple across the 47uF
reservoir cap measures 1.2V pp which must be about 0.4V rms (can't
remember the crest factor of a sawtooth). This directly feeds the CF
anode and the ripple at its cathode is just 50mV pp so the CF reduces
the ripple by about 20 times which is interesting.

50mV at the CF cathode is bleedin awful!!!!

The reason the ripple at the cathode is 20 times lower at the cathode is
that the
series voltage NFB acting in the CF reduces any noise applied to the
anode,
or applied from an ac heater. Cathode Rout = 500 ohms, anode Rout or its
Ra lokking into the
the anode circuit is say 10k + so the hum at the anode is reduced by
what is effectivly
an active resistance divider with a gain tube and lots of NFB.

So if the filter gave only 0.2mV of ripple at the CF anode, noise at the
cathode
would 10uV, and no more than other parts of the tube will manufacture.

You need far more filtering.


Let the moths out of your purse and buy some decent sized capacitors.

Yes I shall. I think I'll go for 470uF reservoir then two RC stages for
the CF and the gain stage respectively each with 470uF caps.

The current design ends up with 191V (measured) on the CF cathode which
is a bit high so I can afford to drop the CF HT quite a bit. The preamp
is supplied via 22K and 47Uf at present which gives it a 255V (measured)
supply and a 117V (measured) anode voltage.

snip


In a mike preamp, bass isn't favoured like in a phono amp. But you don't
want to have a preamp
with its 0V signal wobbling up and down like the crude amps made in 1960
all used to to do.

Tube preamps for microphones are damned noisy where the mic signal is
below say 5mV.

So for good SNR, the old practice was to use a step up microphone
transformer.
The mic was very low Z, and its noise much lower than a tube.
When the mic signal was transformed up 10 times, the noise was still
lower than a tube.

That's still current practice too. I am using Sowter transformers (10:1)
at the input.

And the tube at the input should be a triode; 6AU6 is fine, but betst
srtapped in triode,
or used with the screen as the anode, and anode taken to 0V.
See RDH4 about such antiquated details.


Yes it is strapped as a triode for the usual reasons. I am using screen
and suppressor strapped to anode as it appears to give slightly better
gm values with the 6AU6 than the screen as anode connection. Heater
induced hum is not an issue as I plan to use a dc heater supply.

Of course if you used a j-fet for the input you'd not need a
transformer,
and following stage after the fet can be almost any triode.


I am trying to avoid semiconductors wherever possible.


A cascode circuit is also very good, see http://www.vacuumstate.com and
Allen Wright's
hybrid stages for phono. They can be used for microphone, without the
RIAA filters.

There are some clues on PSU design at my site,

http://www.turneraudio.com.au

Patrick Turner.

I have printed this email and stuck it in my day book.

The step up tranny and 6AU6 in triode should be fine if the mic is low
impedance.

In the old days of good recordings, there were no j-fets....

But now a 2SK369 j-fet costing $1.10 from wescomponents.com is cheaper
than a transformer.

A pair used for balanced input also work fine for lower distortion.



Patrick Turner.



Many Thanks

Ian

Ian Thompson-Bell wrote:

Patrick Turner wrote:

Ian Thompson-Bell wrote:
I am designing a tube based microphone pre-amplifier. At present I am
trying to sort out the HT supply topology. I would prefer a simple RC
network but this reduces the anode volts available to the first stage. I
want to avoid 'anode starvation' and it attendant colouration so my
question is below what voltage would the anode be considered to be
'starved'? The preamp uses 6AU6 tubes.

Cheers

ian

There is nothing wrong with CRCRCRC type filters for preamps.

You need forst to abandon completely trying to copy
what was done in 1955 where preamps rarely had more than 47uF caps.
For good filtering, R had to be high, and thus you got a dc voltage drop
you didn't
want.
AND you also got a preamp very likely to oscilate at LF, ie, motorboat,
because it is a de-facto phase shift oscillator.

But these problems dissapear if you use LARGE value capacitors
which are now quite cheap, such as 470uF x 450V rated.

Patrick, I notice you use these in a number of designs featured on your
web site. Where to you get yours form?


wescomponents.com.au

These guys supply trade ppl but are very happy to do mail order;
you send domani, they send the parts a week later.

OS orders might take longer, or maybe they won't do it.
But trade suppliers is where ya go. Every country
has trade suppliers which are usually far cheaper
than places with shopfronts to the general public.

I don't believe BLACK GATE or other fancy shmancy
name brand caps sound any better either.



Patrick Turner.

Cheers

Ian

Patrick Turner wrote:

Ian Thompson-Bell wrote:
Patrick Turner wrote:

snip
But these problems dissapear if you use LARGE value capacitors
which are now quite cheap, such as 470uF x 450V rated.
These are used now abuntanly in SMPS where they work hard, but in your
preamp
they'll be working real easy, and be very effective.
Excellent idea. To date I had assumed the 100uF 450V electrolytics I
could get from suppliers like Maplin were about all that was available.
It seems both RadioSpares and Farnell do 470uF 450V caps but they are
quite expensive - the cheapest I found was over 4GBP before tax in 10
off quantities. However, Digi-key also seems to do them at 3.40GBP at 10
off quantities but by the time you add in shipping and handling they are
over 5GBP each. So either way it works out at about 5 pounds a pop.

Yes, but what you are paying per 100 uF isn't too high a price for a
blame free PSU.
Using large 40uF motor start caps would be more expensive, but the C
values are way too low.

Is the price of a 100uF cap 1/5 of the price of a 470uF?


Roughly.

I buy my electros from spare parts suppliers like wescomponents.com
who supply to the repair trades, and never from local retail electronic
places such as Maplin; Jaycar and Dick Smith Electronics are the Oz
equivalent,
and they charge twice the trade price.


Interesting. Not sure what the equivalents are in the UK. Don't they
have rather large minimum order quantities or values??

I also buy from a mob here who buys job lots of parts left over from
mainstream manufacture runs, rockby.com.au
Sometimes thay have 470uF x 400V caps for $2 each.....

All these caps and things are made in asian factories by thye million,
and the price ex the factory is maybe 20c.


Guess I need to investigate the Pommie equivalents.



If you have 330 ohms plus 470uF, the attenuation factor is the same as
using 3,300 ohms plus 47 uF, or 33k and 4.7 uF.

At 100Hz, 470uF has Z = 3.4 ohms, and with 330R the attenuation of
rectifier hum
is around 1/100, and 3 such cascaded RC filters will reduce say 0.2Vrms
of ripple
at C1 by 1/1,000,000 to less than 1.0uV. If Idc was 10mA, the V drop is
only 10Vdc across
3 x 330 ohm R.

Idc is a little under 10mA but the basic ripple across the 47uF
reservoir cap measures 1.2V pp which must be about 0.4V rms (can't
remember the crest factor of a sawtooth). This directly feeds the CF
anode and the ripple at its cathode is just 50mV pp so the CF reduces
the ripple by about 20 times which is interesting.

50mV at the CF cathode is bleedin awful!!!!

The reason the ripple at the cathode is 20 times lower at the cathode is
that the
series voltage NFB acting in the CF reduces any noise applied to the
anode,
or applied from an ac heater. Cathode Rout = 500 ohms, anode Rout or its
Ra lokking into the
the anode circuit is say 10k + so the hum at the anode is reduced by
what is effectivly
an active resistance divider with a gain tube and lots of NFB.

So if the filter gave only 0.2mV of ripple at the CF anode, noise at the
cathode
would 10uV, and no more than other parts of the tube will manufacture.

You need far more filtering.


Indeed I do. That is the main lesson I have just learned.

Let the moths out of your purse and buy some decent sized capacitors.
Yes I shall. I think I'll go for 470uF reservoir then two RC stages for
the CF and the gain stage respectively each with 470uF caps.

The current design ends up with 191V (measured) on the CF cathode which
is a bit high so I can afford to drop the CF HT quite a bit. The preamp
is supplied via 22K and 47Uf at present which gives it a 255V (measured)
supply and a 117V (measured) anode voltage.

snip

In a mike preamp, bass isn't favoured like in a phono amp. But you don't
want to have a preamp
with its 0V signal wobbling up and down like the crude amps made in 1960
all used to to do.

Tube preamps for microphones are damned noisy where the mic signal is
below say 5mV.

So for good SNR, the old practice was to use a step up microphone
transformer.
The mic was very low Z, and its noise much lower than a tube.
When the mic signal was transformed up 10 times, the noise was still
lower than a tube.
That's still current practice too. I am using Sowter transformers (10:1)
at the input.

And the tube at the input should be a triode; 6AU6 is fine, but betst
srtapped in triode,
or used with the screen as the anode, and anode taken to 0V.
See RDH4 about such antiquated details.

Yes it is strapped as a triode for the usual reasons. I am using screen
and suppressor strapped to anode as it appears to give slightly better
gm values with the 6AU6 than the screen as anode connection. Heater
induced hum is not an issue as I plan to use a dc heater supply.

Of course if you used a j-fet for the input you'd not need a
transformer,
and following stage after the fet can be almost any triode.

I am trying to avoid semiconductors wherever possible.

A cascode circuit is also very good, see http://www.vacuumstate.com and
Allen Wright's
hybrid stages for phono. They can be used for microphone, without the
RIAA filters.

There are some clues on PSU design at my site,

http://www.turneraudio.com.au

Patrick Turner.
I have printed this email and stuck it in my day book.

The step up tranny and 6AU6 in triode should be fine if the mic is low
impedance.


It is.

In the old days of good recordings, there were no j-fets....

But now a 2SK369 j-fet costing $1.10 from wescomponents.com is cheaper
than a transformer.

A pair used for balanced input also work fine for lower distortion.



How true, but I have designed semiconductor audio stuff for years
(actually decades). Now I want to design/build some (pure) tube gear.

Ian

Patrick Turner.


Many Thanks

Ian

Ian Thompson-Bell wrote:

Patrick Turner wrote:

Ian Thompson-Bell wrote:
Patrick Turner wrote:

snip
But these problems dissapear if you use LARGE value capacitors
which are now quite cheap, such as 470uF x 450V rated.
These are used now abuntanly in SMPS where they work hard, but in your
preamp
they'll be working real easy, and be very effective.
Excellent idea. To date I had assumed the 100uF 450V electrolytics I
could get from suppliers like Maplin were about all that was available.
It seems both RadioSpares and Farnell do 470uF 450V caps but they are
quite expensive - the cheapest I found was over 4GBP before tax in 10
off quantities. However, Digi-key also seems to do them at 3.40GBP at 10
off quantities but by the time you add in shipping and handling they are
over 5GBP each. So either way it works out at about 5 pounds a pop.

Yes, but what you are paying per 100 uF isn't too high a price for a
blame free PSU.
Using large 40uF motor start caps would be more expensive, but the C
values are way too low.

Is the price of a 100uF cap 1/5 of the price of a 470uF?


Roughly.

I buy my electros from spare parts suppliers like wescomponents.com
who supply to the repair trades, and never from local retail electronic
places such as Maplin; Jaycar and Dick Smith Electronics are the Oz
equivalent,
and they charge twice the trade price.


Interesting. Not sure what the equivalents are in the UK. Don't they
have rather large minimum order quantities or values??

Serch for them.

You won't know about mimimium orders until you ask them.

There is RS and Farmells, but they try to cater for engineers with deep
pockets.

Most trade repair supply companies deal with repair ppl trying to fix TV
sets
for a low price to maximise earnings from pplo who hate having to have
anything repaired. The repair guy might only want ONE cap today,
but in a year will use 10, so he buys 10 to begin with, and so do I;
maybe I by 50 pcs.

I also buy from a mob here who buys job lots of parts left over from
mainstream manufacture runs, rockby.com.au
Sometimes thay have 470uF x 400V caps for $2 each.....

All these caps and things are made in asian factories by thye million,
and the price ex the factory is maybe 20c.


Guess I need to investigate the Pommie equivalents.



If you have 330 ohms plus 470uF, the attenuation factor is the same as
using 3,300 ohms plus 47 uF, or 33k and 4.7 uF.

At 100Hz, 470uF has Z = 3.4 ohms, and with 330R the attenuation of
rectifier hum
is around 1/100, and 3 such cascaded RC filters will reduce say 0.2Vrms
of ripple
at C1 by 1/1,000,000 to less than 1.0uV. If Idc was 10mA, the V drop is
only 10Vdc across
3 x 330 ohm R.

Idc is a little under 10mA but the basic ripple across the 47uF
reservoir cap measures 1.2V pp which must be about 0.4V rms (can't
remember the crest factor of a sawtooth). This directly feeds the CF
anode and the ripple at its cathode is just 50mV pp so the CF reduces
the ripple by about 20 times which is interesting.

50mV at the CF cathode is bleedin awful!!!!

The reason the ripple at the cathode is 20 times lower at the cathode is
that the
series voltage NFB acting in the CF reduces any noise applied to the
anode,
or applied from an ac heater. Cathode Rout = 500 ohms, anode Rout or its
Ra lokking into the
the anode circuit is say 10k + so the hum at the anode is reduced by
what is effectivly
an active resistance divider with a gain tube and lots of NFB.

So if the filter gave only 0.2mV of ripple at the CF anode, noise at the
cathode
would 10uV, and no more than other parts of the tube will manufacture.

You need far more filtering.


Indeed I do. That is the main lesson I have just learned.

Let the moths out of your purse and buy some decent sized capacitors.
Yes I shall. I think I'll go for 470uF reservoir then two RC stages for
the CF and the gain stage respectively each with 470uF caps.

The current design ends up with 191V (measured) on the CF cathode which
is a bit high so I can afford to drop the CF HT quite a bit. The preamp
is supplied via 22K and 47Uf at present which gives it a 255V (measured)
supply and a 117V (measured) anode voltage.

snip

In a mike preamp, bass isn't favoured like in a phono amp. But you don't
want to have a preamp
with its 0V signal wobbling up and down like the crude amps made in 1960
all used to to do.

Tube preamps for microphones are damned noisy where the mic signal is
below say 5mV.

So for good SNR, the old practice was to use a step up microphone
transformer.
The mic was very low Z, and its noise much lower than a tube.
When the mic signal was transformed up 10 times, the noise was still
lower than a tube.
That's still current practice too. I am using Sowter transformers (10:1)
at the input.

And the tube at the input should be a triode; 6AU6 is fine, but betst
srtapped in triode,
or used with the screen as the anode, and anode taken to 0V.
See RDH4 about such antiquated details.

Yes it is strapped as a triode for the usual reasons. I am using screen
and suppressor strapped to anode as it appears to give slightly better
gm values with the 6AU6 than the screen as anode connection. Heater
induced hum is not an issue as I plan to use a dc heater supply.

Of course if you used a j-fet for the input you'd not need a
transformer,
and following stage after the fet can be almost any triode.

I am trying to avoid semiconductors wherever possible.

A cascode circuit is also very good, see http://www.vacuumstate.com and
Allen Wright's
hybrid stages for phono. They can be used for microphone, without the
RIAA filters.

There are some clues on PSU design at my site,

http://www.turneraudio.com.au

Patrick Turner.
I have printed this email and stuck it in my day book.

The step up tranny and 6AU6 in triode should be fine if the mic is low
impedance.


It is.

In the old days of good recordings, there were no j-fets....

But now a 2SK369 j-fet costing $1.10 from wescomponents.com is cheaper
than a transformer.

A pair used for balanced input also work fine for lower distortion.



How true, but I have designed semiconductor audio stuff for years
(actually decades). Now I want to design/build some (pure) tube gear.

Filtering is just as important in SS circuits....

Patrick Turner.

Ian

Patrick Turner.


Many Thanks

Ian

On Sun, 25 Nov 2007 01:44:55 GMT, Patrick Turner
wrote:

How true, but I have designed semiconductor audio stuff for years
(actually decades). Now I want to design/build some (pure) tube gear.

Filtering is just as important in SS circuits....

Patrick Turner.

No it isn't. SS circuits are generally built fully balanced, using
current sources instead of collector load resistors etc, which gives
them many dBs more common mode and power supply rejection than you see
in a valve circuit. You can get away with much poorer supply filtering
in SS.

Now as for valves, don't just slap in big capacitors; think. This is a
filter you are designing, for a specific purpose. So design it. Decide
how many dB rejection you need at 100 (120) Hz and do the sums. That
will give you a set of resistor and capacitor values you can use as
your minimums, Check that the spec is still met when you include the
parasitics of internal resistance in a cap; there is no point trying
to cure hum by making a cap bigger when it is the ESR causing the poor
performance.

d

--
Pearce Consulting
http://www.pearce.uk.com

On Sun, 25 Nov 2007 01:44:55 +0000, Patrick Turner wrote:

snip

There is RS and Farmells, but they try to cater for engineers with deep
pockets.

snip

Farnell are rather expensive, but carry a range of caps of the same
value. You'll find that you have a choice with varying ESR values,
ranging from "not cheap" to "ludicrously expensive". :-)

Ian: remember that it can be quite a bit cheaper to parallel several high
ESR caps of lower value than fit a single cap with low ESR - if you have
the space and don't mind it looking a bit odd. You get the same low ESR
and the same final value but with a little extra work.

Rapid Electronics are good, but they don't do much in the way of caps
over 400V. They do carry some axial lead 450V ones up to 100uF though.
*Much* cheaper than Farnell or RS.

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info http://mixpix.batcave.net