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patrick-turner patrick-turner is offline
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Default Compton reverse log pot, tubed Wien Bridge oscillator

In a previous post about my radical surgery performed on an BWD Electronics 141 oscilator, I mentioned I had troubles with the original Compton pot used
to alter the F within each range. The Compton is a wire wound item with adequate power ratings, and it reputed to work smoothly without Vo jitter or bounce, and with linearlity between DC and at least 1MHz. But I found it performed with jittery Vo for the parts of the ranges where it did work, and for part of each range the oscillations stopped or went wild. I decided the pot was terminally sick, so I replaced it with 12 pos make before break switch.

I re-examined the 2 gang pot, and although it looks impossible to separate the two sealed pots, but the outermost has a metal cover which can be easily prised off. I plotted the R values for each track and sonn found a region where outermost pot had an open connection between wiper and resistance wire track. Both pots measured similar R where the wipers of both did connect to the their tracks.

I was lucky the sick pot was the only one that seemed accessible. I found some grease had been used for lubrication, but no sign of any corrosion or fault in the wiper operation and all mechanical aspects looked brand new. BUT, sure enough, wiper contact wasn't being made at a small length of the track, so I cleaned the track + wiper without damaging the hair thin wire. I thought a bit more use might dislodge the tiny spot of hard oxide that may exist on the resistance wire track. No luck, and I gave up after an hour or two, and felt vindicated that I'd replaced the darn pot with the best thing I had laying around.
So when Phil says the Compton Pot is a blessed marvel that enabled superlative
oscillators to be made, it could never have been alwats true, because pots can become defective, or maybe even be defective when they are sold.
Wire wound pots can be notoriously jerky with Vo levels. But OK when not being moved, when they are then usually noisless.

I'm currently making a tubed version of WB oscillator.

Included in my pile of junk acquired over the last 20 years was a metal box about 400mm x 250mm x 300mm which was amoungst stuff froma retiring ham radio guy. Seems he only wired 3 of 16 tube sockets, maybe some amp for a long lost cause. Anyway, a very suiable box for what I now wanted.

I'm to use FOUR 3 gang pots with each gang having max C = 458pF, and with trommer C the range of C can be 2,770pF to 277pF so the R value for 2MHz operation in a WB oscillator will not be too high, C not too low, giving good HF operation.
For 1Hz to 10Hz, I will use a dual 50k log pot with fixed C about 3.2uF
So 2 dials are needed, but I should easily get 1Hz to 1Mz. So that when the lowest F range is selected, the switch changes to the variable R and fixed C.

The tube amp has 6DJ8+EF80 input diff pair, EL86 gain pentode, then 2 more EL86 used as a white follower. Bandwidth of amp = 3Hz to 3.3Mhz, dead flat between 1Hz and 1MHz. Input C to each input port is low enough. Open loop gain has shelving of gain HF & LF outside the audio band of 20Hz to 20kHz. So far, 1kHz THD of amp 0.03% at 7Vrms output, a bit high, considering the is so much gain reduced from about 1,400 to 3.0 with the FB tried. I think the 6DJ8 CF input is causing excessive and I'm working on a method to apply the NFB to input EF80 cathode, with resistive NFB network allowed to float between 1,100uF output cap after the EL86 buffer and a 8,200uF cap to ground. It means no DC flows in any part of the NFB R divider network, because I have a 13mA CCS sinking cathode Idc to 6BX6. But the bottom R will be about 360r with lamps used, and this acts as local current FB to 6BX6, halvingt its gain, but giving some slight local linearity. Thus the grid may remain at 0V and only terminated by the WB network R values.

I bet nobody in the world followed what I just said, but then my post is aimed at those wanting to making their own Oskar Laytor because they have the patience. Its only when ppl try to make something real that any descriptions around what they do become interesting.

I might find room for a square wave Schmitt Trigger and an additional output buffer. But maybe not with tubes, because getting a decent 1MHz square wave means highest F present should be 10MHz. Maybe I cheat, and use some BJTs, actually easier to do, and for an additional OP buffer after the attenuators, so that Rout of the unit is 50 ohms.

I look back at the HP200A oscillator with a 4 gang cap and I think, "how quaint", and it was said to be a good tool to test audio amps, with THD 1%, and an unneccessarily high 20Vrms+ output. But it only went 30Hz to 35kHz, so how could you test any audio amp? What is wanted is something to look at amp behaviour below 30Hz and above 35kHz, so for me the HP200A would be quite useless!!!!

Patrick Turner.

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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default Compton reverse log pot, tubed Wien Bridge oscillator


"patrick-turner"

In a previous post about my radical surgery performed on an
BWD Electronics 141 oscilator, I mentioned I had troubles
with the original Compton pot used to alter the F within each
range. The Compton is a wire wound item with adequate
power ratings, and it reputed to work smoothly without Vo
jitter or bounce, and with linearlity between DC and at
MHz. But I found it performed with jittery Vo for the parts
of the ranges where it did work, and for part of each range
the oscillations stopped or went wild. I decided the pot was
terminally sick, so I replaced it with 12 pos make before
break switch.

I re-examined the 2 gang pot, and although it looks impossible
to separate the two sealed pots, but the outermost has a metal
which can be easily prised off. I plotted the R values for each
and soon found a region where outermost pot had an open
connection between wiper and resistance wire track. Both pots
measured similar R where the wipers of both did connect to the
their tracks.

I was lucky the sick pot was the only one that seemed accessible
I found some grease had been used for lubrication, but no sign of
any corrosion or fault in the wiper operation and all mechanical
aspects looked brand new. BUT, sure enough, wiper contact
wasn't being made at a small length of the track, so I cleaned the
track + wiper without damaging the hair thin wire. I thought a bit
more use might dislodge the tiny spot of hard oxide that may exist
on the resistance wire track. No luck, and I gave up after an hour
or two, and felt vindicated that I'd replaced the darn pot with the
best thing I had laying around.
So when Phil says the Compton Pot is a blessed marvel that
enabled superlative oscillators to be made, it could never have
been alwats true, because pots can become defective, or maybe
even be defective when they are sold.

** You have seen one faulty WW pot in a BWD141 - probably the only faulty
one that ever existed.

" I was cleaning up my workshop when I discovered I had been given an old
BWD Electronics
141A audio oscillator which had many problems. "

There is a big clue there ....


Wire wound pots can be notoriously jerky with Vo levels.
But OK when not being moved, when they are then usually noisless.

** I have always found WW pots to be highly reliable and long lived.

1. The voltage control on my most used bench PSU has a 10 kohm WW pot made
by IRH in Sydney, Australia - been in daily use for nearly 30 years, still
works perfect.

2. The two fine adjust controls on my Wien bridge notch filter are 200ohm 10
turn WW types - allows up to -100dB notch depth at 66, 996 and 5855 Hz.
They are 30 years old too, got them the same time I bought my Sony CD101
player.

3. I have two Compton dual gang WW pots, same as the one PT is so down on.
One is 40 years old and the other 24 years old. They both work perfectly.

To overcome the issue with fine frequency adjustment, I fitted 500ohm, 10
turn WW pots in series with one of the sections of each Comptom pot - which
is 2 x 15kohms. Makes it easy to set any frequency up to 50kHz, +/- 1Hz.

Stability is good enough to work with the -100dB notch filter, which at 996
Hz has a notch width of about 0.02 Hz.



..... Phil










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GRe GRe is offline
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Default Compton reverse log pot, tubed Wien Bridge oscillator


"Phil Allison" wrote in message
...

[...]

To overcome the issue with fine frequency adjustment, I fitted 500ohm, 10
turn WW pots in series with one of the sections of each Comptom pot -
which is 2 x 15kohms. Makes it easy to set any frequency up to 50kHz, +/-
1Hz.

Stability is good enough to work with the -100dB notch filter, which at
996 Hz has a notch width of about 0.02 Hz.


Stability issues...

The Leader LAG-120A has a sync terminal for synchronisation with a high
stability frequency source, the circuitry where the synchronisation takes
place looks very similar to the corresponding part of the BDW141 schematic.
The manual has a description of the sync option (page 7/8):
http://www.slashdocs.com/kwqytp/lag-...on-manual.html
Unfortunatly, other than the original manual I have, there's no schematic
but with reference to the BDW141 schematic the sync input is simply a
resistor of 10K between the sync terminal and the R4/R5/R54/C14/C17A node.
Component values are very similar too.
For the LAG-120A in practice the sync range is better than the specified
+/-1%, about +/-2.5% at 1kHz (other freq. not measured).
I don't know about possible pitfalls of implementing such a sync input for
the BDW but maybe it's worth looking into it.
BTW, if you have access to ABSE I could post the LAG-120A schematic and/or
manual.

Gio Re







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Default Compton reverse log pot, tubed Wien Bridge oscillator


"GRe"
"Phil Allison"

To overcome the issue with fine frequency adjustment, I fitted 500ohm, 10
turn WW pots in series with one of the sections of each Comptom pot -
which is 2 x 15kohms. Makes it easy to set any frequency up to 50kHz, +/-
1Hz.

Stability is good enough to work with the -100dB notch filter, which at
996 Hz has a notch width of about 0.02 Hz.


Stability issues...

The Leader LAG-120A has a sync terminal for synchronisation with a high
stability frequency source, the circuitry where the synchronisation takes
place looks very similar to the corresponding part of the BDW141
schematic.
The manual has a description of the sync option (page 7/8):

http://www.slashdocs.com/kwqytp/lag-...on-manual.html

Unfortunatly, other than the original manual I have, there's no schematic
but with reference to the BDW141 schematic the sync input is simply a
resistor of 10K between the sync terminal and the R4/R5/R54/C14/C17A node.
Component values are very similar too.
For the LAG-120A in practice the sync range is better than the specified
+/-1%, about +/-2.5% at 1kHz (other freq. not measured).

I don't know about possible pitfalls of implementing such a sync input for
the BDW but maybe it's worth looking into it.


** Thanks for you interest and effort.

One problem is that feeding synch pulses into the bowels a low distortion
oscillator is BOUND to result in contamination of the sine wave signal.
Probably serious contamination.

The smallest things matter when aiming for a 0.001% THD residual !!

The -100dB notch filter I use for THD measuremenents has somewhat better
stability than the oscillator - since it uses MF resistors and polystyrene
caps for the bridge, plus a couple of WW pots made with Ni-Chrome wire.

Luckily, one only needs to achieve full null for a few seconds to get a
reliable reading on a scope or millivolt meter.

This is **** simple for THD percentages up to 0.01 % - but becomes a tad
tedious at 0.001 %.

My best result was circa 0.0005% for my Sony CD101 CD player, playing a test
disk, with a digitally generated sine wave.


.... Phil




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GRe GRe is offline
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Default Compton reverse log pot, tubed Wien Bridge oscillator


"Phil Allison" wrote in message
...

"GRe"
"Phil Allison"

To overcome the issue with fine frequency adjustment, I fitted 500ohm,
10
turn WW pots in series with one of the sections of each Comptom pot -
which is 2 x 15kohms. Makes it easy to set any frequency up to 50kHz,
+/-
1Hz.

Stability is good enough to work with the -100dB notch filter, which at
996 Hz has a notch width of about 0.02 Hz.


Stability issues...

The Leader LAG-120A has a sync terminal for synchronisation with a high
stability frequency source, the circuitry where the synchronisation takes
place looks very similar to the corresponding part of the BDW141
schematic.
The manual has a description of the sync option (page 7/8):

http://www.slashdocs.com/kwqytp/lag-...on-manual.html

Unfortunatly, other than the original manual I have, there's no schematic
but with reference to the BDW141 schematic the sync input is simply a
resistor of 10K between the sync terminal and the R4/R5/R54/C14/C17A
node.
Component values are very similar too.
For the LAG-120A in practice the sync range is better than the specified
+/-1%, about +/-2.5% at 1kHz (other freq. not measured).

I don't know about possible pitfalls of implementing such a sync input
for
the BDW but maybe it's worth looking into it.


** Thanks for you interest and effort.

One problem is that feeding synch pulses into the bowels a low distortion
oscillator is BOUND to result in contamination of the sine wave signal.
Probably serious contamination.

The smallest things matter when aiming for a 0.001% THD residual !!

The -100dB notch filter I use for THD measuremenents has somewhat better
stability than the oscillator - since it uses MF resistors and
polystyrene caps for the bridge, plus a couple of WW pots made with
Ni-Chrome wire.

Luckily, one only needs to achieve full null for a few seconds to get a
reliable reading on a scope or millivolt meter.

This is **** simple for THD percentages up to 0.01 % - but becomes a tad
tedious at 0.001 %.


Until now I have to do with the Leader, with a 0.05%/-66dB THD spec it was
a better than expected when I measured it last year, 0.014%/-77dB at 1kHz.
So, I can't judge "tediousness" at 0.01% but I assume that's true.
Anyway, stuck with a "bad" oscillator the wish arose for a very low THD
oscillator. It's a project I work on, but with my pace because of all other
things to do it will take a year.

My best result was circa 0.0005% for my Sony CD101 CD player, playing a
test disk, with a digitally generated sine wave.


Usually I use the LAG120A un-synchronized as with my bench/instruments setup
it's a PITA to attach the sync cable to the rear panel.
But I remember from some time ago when synchronizing the LAG120A with a sine
from a HP3325A that distortion was not visible on the o'scope, not even when
using a square wave as sync signal.
However, that was an "eyeball measurement" and as such does'nt say anything
about possible increasing THD in sync mode.
Probably you're right about increased THD, now I'm curious how well or bad
behaved it is in sync mode.
I'll do a measurement with the HP334A at 1kHz un-synchronized vs.
synchronized and come back with the results.

Gio Re











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patrick-turner patrick-turner is offline
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Default Compton reverse log pot, tubed Wien Bridge oscillator

Phil replied to my comments with a good post...

** You have seen one faulty WW pot in a BWD141 - probably the only faulty
one that ever existed.

^^^^Well, who is to ever know?

" I was cleaning up my workshop when I discovered I had been given an old
BWD Electronics
141A audio oscillator which had many problems. "

**There is a big clue there ....

Wire wound pots can be notoriously jerky with Vo levels.
But OK when not being moved, when they are then usually noisless.


** I have always found WW pots to be highly reliable and long lived.

^^^^In General, I agree 100%, and for example I use WW pots for biasing amps where failure can lead to smoke. But even then I put a R between wiper and negative end of pot so if the pot fails the grid bias goes more negative.

1. The voltage control on my most used bench PSU has a 10 kohm WW pot made
by IRH in Sydney, Australia - been in daily use for nearly 30 years, still
works perfect.

^^^^Wonderful.

2. The two fine adjust controls on my Wien bridge notch filter are 200ohm 10
turn WW types - allows up to -100dB notch depth at 66, 996 and 5855 Hz.
They are 30 years old too, got them the same time I bought my Sony CD101
player.

^^^^I have a bridged T LC notch filter with two pots, one for course nul adjustment and and the other for fine, and then I have a tuning cap at the 1kHz oscillator to slightly vary the F and it all works well to get good rejection of 1kHz,. After the notch filterI have a bandpass filter with op-amps pass only 2kHz to 11kHz. The carbon pots work fine, and are not surrounded by loops of FB.

3. I have two Compton dual gang WW pots, same as the one PT is so down on.
One is 40 years old and the other 24 years old. They both work perfectly.

^^^^Wonderful. I know why I am down on the Comptons, and why you ain't. And maybe why the guy gave me the BWD 141. Much work had been done on it, to no avail.....

**To overcome the issue with fine frequency adjustment, I fitted 500ohm, 10
turn WW pots in series with one of the sections of each Comptom pot - which
is 2 x 15kohms. Makes it easy to set any frequency up to 50kHz, +/- 1Hz.

^^^^Good practice.

Stability is good enough to work with the -100dB notch filter, which at 996
Hz has a notch width of about 0.02 Hz.

^^^^I don't know how wide my notch filter is at the -3dB point, probably 2kHz, but that's OK, and I don't know what the notch width is at deepest null, maybe it is similar, but for where I have a signal of say 3Vrms and THD 0.001%, then what one cvan see is some dithering of F which seems to be caused by slight noise somewhere and there is some AM and FM going on. But one rarely ever sees less than 0.001% in tube gear becaus4e the noise usually swamps the THD. BUT, in my last trial of a tube amp to use on my latest WB oskar laytar, I DID MEASURE less than 0.001% with signal of 5Vrms. My 1 kHz oscillator makes less than 0.001%. I didn't believe it at first, but then I did the maths on the gain reduction and sure enough the calculated THD was as low as I measured.

^^^^Electronics World had an article on measuring THD with two cascaded notch filters with very slightly different F for the notch null, so that the only control needed for measuring THD at 1kHz was slight F adjstement which wasn't much used. But my own THD checker works fine for me. The beauty of a bridged T LC notch filter is that its passive, and presents a 5ko load to a power amp, and you can put in 200Vrms without smoke.

^^^^I have a high Z input buffer but that's all solid state and protected against any Vin above about 8Vrms.

^^^^Getting the WB oscillator with tubes to oscillate properly between 500kHz and 1Mhz has been difficult because of unpredictable bothers with biasing and gain at such high F, and with stray C, and the minimum C needed in the WB network to overcome stray C elsewhere is about 130pF. This means that a tuning gang to cover 100kHz to 1MHz must vary from 1,300pF to 130pF.

^^^^But I have 4 x triple gang caps with each gang 400pF to 13pF. The C value can be 2,400pF to 102pF. But to the mimum C value, enough trimmer C must be added to so that minimum total C including stray C and tube C is around 250pF, and maximum C is then 2,500pF, and then the R value is trimmed, but for 100kHz to 1MHz, the R = 636r, and then Zin for the network at Fo = 1,347r, and if the Vo = 7Vrms, then I in max = 5.2mA. The NFB network with lamps seems to work OK so far because caps give smooth F change, but load is 1k2, so Iin = 5.8mA, and then I have a 1k0 pot to power so another 7mA, so total I needed = 18mA, plus there will be a feed to a Schmitt trigger, so say 20mArms.
So load to be powered by the oscillator = 7 / 0.02 = 350r.

^^^^But the White follower I made with two EL86 in triode have Ea = 130V, Iadc = 53mA, and do give very fine perfomance with loads as low as 200R.
I found changing load from 400r to 200r caused only 1mV of Vo drop because of the NFB.

^^^^Perhaps the Technics output transformerless output stage with a 3rd EL86 triode may make a better output follower buffer because the drive to lower SET is not derived from current flow in top tube, but both tubes always get the same Vg-k regardless of load.

^^^^I have some work to do to finalise circuits and prepare the webpage to explain myself better than time allows here with mere words which are not enough because a schematic is needed.

Patrick Turner.


..... Phil

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Default Compton reverse log pot, tubed Wien Bridge oscillator


"patrick-turner"

** aka " The Turneroid Public Menace "


^^^^Wonderful. I know why I am down on the Comptons, and why you ain't.


** No you don't.

You are just a rabid, raving nut case.

As any reader here can plainly see.

Funny how there are so few.

Or not.



..... Phil


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Default Compton reverse log pot, tubed Wien Bridge oscillator

Phil gave a not so hot reply this time......

Phil Allison Sep 25
Other recipients:
"patrick-turner" ** aka " The Turneroid Public Menace "
"patrick-turner"
** aka " The Turneroid Public Menace "
^^^^Wonderful. I know why I am down on the Comptons, and why you ain't.
** No you don't.
You are just a rabid, raving nut case.
As any reader here can plainly see.
Funny how there are so few. Or not.
..... Phil

Phil has no useful online website based contributions to the detailed construction and understanding of Wien bridge oscillators. I am preparing my page on the subject and I do not really have to worry much about what Phil thinks of me personally.
Patrick Turner.

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Default The Turneroid Public Menace


"patrick-turner"

** aka " The Turneroid Public Menace "


^^^^Wonderful. I know why I am down on the Comptons, and why you ain't.


** No you don't.

You are just a rabid, raving nut case.

As any reader here can plainly see.

Funny how there are so few.

Or not.



..... Phil




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GRe GRe is offline
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Default Synchronizing a Wien Bridge, was: Compton reverse log pot, tubed Wien Bridge oscillator


"GRe" wrote in message
l...

"Phil Allison" wrote in message
...

"GRe"
"Phil Allison"

To overcome the issue with fine frequency adjustment, I fitted 500ohm,
10
turn WW pots in series with one of the sections of each Comptom pot -
which is 2 x 15kohms. Makes it easy to set any frequency up to 50kHz,
+/-
1Hz.

Stability is good enough to work with the -100dB notch filter, which at
996 Hz has a notch width of about 0.02 Hz.

Stability issues...

The Leader LAG-120A has a sync terminal for synchronisation with a high
stability frequency source, the circuitry where the synchronisation
takes
place looks very similar to the corresponding part of the BDW141
schematic.
The manual has a description of the sync option (page 7/8):

http://www.slashdocs.com/kwqytp/lag-...on-manual.html

Unfortunatly, other than the original manual I have, there's no
schematic
but with reference to the BDW141 schematic the sync input is simply a
resistor of 10K between the sync terminal and the R4/R5/R54/C14/C17A
node.
Component values are very similar too.
For the LAG-120A in practice the sync range is better than the specified
+/-1%, about +/-2.5% at 1kHz (other freq. not measured).

I don't know about possible pitfalls of implementing such a sync input
for
the BDW but maybe it's worth looking into it.


** Thanks for you interest and effort.

One problem is that feeding synch pulses into the bowels a low distortion
oscillator is BOUND to result in contamination of the sine wave signal.
Probably serious contamination.

The smallest things matter when aiming for a 0.001% THD residual !!

The -100dB notch filter I use for THD measuremenents has somewhat better
stability than the oscillator - since it uses MF resistors and
polystyrene caps for the bridge, plus a couple of WW pots made with
Ni-Chrome wire.

Luckily, one only needs to achieve full null for a few seconds to get a
reliable reading on a scope or millivolt meter.

This is **** simple for THD percentages up to 0.01 % - but becomes a
tad
tedious at 0.001 %.


Until now I have to do with the Leader, with a 0.05%/-66dB THD spec it
was
a better than expected when I measured it last year, 0.014%/-77dB at 1kHz.
So, I can't judge "tediousness" at 0.01% but I assume that's true.
Anyway, stuck with a "bad" oscillator the wish arose for a very low THD
oscillator. It's a project I work on, but with my pace because of all
other
things to do it will take a year.

My best result was circa 0.0005% for my Sony CD101 CD player, playing a
test disk, with a digitally generated sine wave.


Usually I use the LAG120A un-synchronized as with my bench/instruments
setup
it's a PITA to attach the sync cable to the rear panel.
But I remember from some time ago when synchronizing the LAG120A with a
sine
from a HP3325A that distortion was not visible on the o'scope, not even
when
using a square wave as sync signal.
However, that was an "eyeball measurement" and as such does'nt say
anything about possible increasing THD in sync mode.
Probably you're right about increased THD, now I'm curious how well or bad
behaved it is in sync mode.
I'll do a measurement with the HP334A at 1kHz un-synchronized vs.
synchronized and come back with the results.


Here they are.

THD of LAG120A sync & un-sync, range=1kHz x dial=1(!)
Sync signal is 1Vrms/1000Hz/sine/0.05%THD.
0.0149% THD, un-synced
0.0146% THD, synced

The numbers are an average of 4 measurements each, individual readings are
taken from an (averaging) mV-meter connected to the 334A output.
No significant THD difference between synced and un-synced, yet in sync
mode, consistently for all readings, THD is a fraction better.
For the time being I attribute that on absence of short term frequency
instability, any other thoughts?

(!)When experimenting there's always a surprise.
THD of LAG120A @ f=1000Hz BUT range=100Hz x dial=10.
Again sync signal is 1Vrms/1000Hz/sine/0.05%THD.
0.0198% THD, un-synced
0.0189% THD, synced
WTF could be the surprise after unwrapping it?

So, how bad behaved is the generator when provoked?
0.059% THD, synced with ~0.98Vrms/1000Hz/sine/2.4%THD
0.064% THD, synced with 0.1Vpp/1000Hz/square
0.62% THD, synced with 1Vpp/1000Hz/square
Considering the amount of crap injected into the bowels not bad I guess.

Gio Re








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Default Compton reverse log pot, tubed Wien Bridge oscillator

Gee, we seem to have drifted way off dicussions about the Compton pot,
but that's always to be expected, but then GRe's latest post is somewhat incomprehensible to most readers...........

See my notes at bottom of quoted text from GRe

"GRe" wrote in message
l...


"Phil Allison" wrote in message
...

"GRe"
"Phil Allison"


To overcome the issue with fine frequency adjustment, I fitted 500ohm,
10
turn WW pots in series with one of the sections of each Comptom pot -
which is 2 x 15kohms. Makes it easy to set any frequency up to 50kHz,
+/-
1Hz.


Stability is good enough to work with the -100dB notch filter, which at
996 Hz has a notch width of about 0.02 Hz.


- show quoted text -
Stability issues...

The Leader LAG-120A has a sync terminal for synchronisation with a high
stability frequency source, the circuitry where the synchronisation
takes
place looks very similar to the corresponding part of the BDW141
schematic.
The manual has a description of the sync option (page 7/8):

http://www.slashdocs.com/kwqytp/lag-...on-manual.html

Unfortunatly, other than the original manual I have, there's no
schematic
but with reference to the BDW141 schematic the sync input is simply a
resistor of 10K between the sync terminal and the R4/R5/R54/C14/C17A
node.
Component values are very similar too.
For the LAG-120A in practice the sync range is better than the specified
+/-1%, about +/-2.5% at 1kHz (other freq. not measured).

I don't know about possible pitfalls of implementing such a sync input
for
the BDW but maybe it's worth looking into it.


** Thanks for you interest and effort.

One problem is that feeding synch pulses into the bowels a low distortion
oscillator is BOUND to result in contamination of the sine wave signal.
Probably serious contamination.

The smallest things matter when aiming for a 0.001% THD residual !!

The -100dB notch filter I use for THD measuremenents has somewhat better
stability than the oscillator - since it uses MF resistors and
polystyrene caps for the bridge, plus a couple of WW pots made with
Ni-Chrome wire.

Luckily, one only needs to achieve full null for a few seconds to get a
reliable reading on a scope or millivolt meter.

This is **** simple for THD percentages up to 0.01 % - but becomes a
tad
tedious at 0.001 %.


Until now I have to do with the Leader, with a 0.05%/-66dB THD spec it
was
a better than expected when I measured it last year, 0.014%/-77dB at 1kHz..
So, I can't judge "tediousness" at 0.01% but I assume that's true.
Anyway, stuck with a "bad" oscillator the wish arose for a very low THD
oscillator. It's a project I work on, but with my pace because of all
other
things to do it will take a year.

My best result was circa 0.0005% for my Sony CD101 CD player, playing a
test disk, with a digitally generated sine wave.


Usually I use the LAG120A un-synchronized as with my bench/instruments
setup
it's a PITA to attach the sync cable to the rear panel.
But I remember from some time ago when synchronizing the LAG120A with a
sine
from a HP3325A that distortion was not visible on the o'scope, not even
when
using a square wave as sync signal.
However, that was an "eyeball measurement" and as such does'nt say
anything about possible increasing THD in sync mode.
Probably you're right about increased THD, now I'm curious how well or bad
behaved it is in sync mode.
I'll do a measurement with the HP334A at 1kHz un-synchronized vs.
synchronized and come back with the results.



Here they are.

THD of LAG120A sync & un-sync, range=1kHz x dial=1(!)
Sync signal is 1Vrms/1000Hz/sine/0.05%THD.
0.0149% THD, un-synced
0.0146% THD, synced

The numbers are an average of 4 measurements each, individual readings are
taken from an (averaging) mV-meter connected to the 334A output.
No significant THD difference between synced and un-synced, yet in sync
mode, consistently for all readings, THD is a fraction better.
For the time being I attribute that on absence of short term frequency
instability, any other thoughts?

(!)When experimenting there's always a surprise.
THD of LAG120A @ f=1000Hz BUT range=100Hz x dial=10.
Again sync signal is 1Vrms/1000Hz/sine/0.05%THD.
0.0198% THD, un-synced
0.0189% THD, synced
WTF could be the surprise after unwrapping it?

So, how bad behaved is the generator when provoked?
0.059% THD, synced with ~0.98Vrms/1000Hz/sine/2.4%THD
0.064% THD, synced with 0.1Vpp/1000Hz/square
0.62% THD, synced with 1Vpp/1000Hz/square
Considering the amount of crap injected into the bowels not bad I guess.

Well, I have lost all track of the significance of GRe's figures. Phil's figures seem sensible about the F setting and he describes the well known method of how he fine tunes each Compton pot section with a smaller adjustable resistance to get 50kHz, +/- 1Hz, not too bad.

So without being able to QUICKLY decipher WTF GRe was saying, and without a simple summary from him, I'm lost about his figures.

Meanwhile, I have nearly got my newly made tubed Wien bridge oscillator completed and I have settled on 2 cascaded stages using 6BX6 each with Ia at 7mA, and with a following White follower using trioded EL86 with Ia at 53mA.
Stability was difficult to obtain with so much maximum possible gain.
But shelving networks to keep open loop phase shift less than 135 degrees where NFB gain reduction 1 seemed to work fine and there is quite low THD at between 100Hz and 10kHz. There are 5 F ranges possible using 4 available paralleled tuning caps which have C from about 260pF to 2,600pF.
Bottom F is 20Hz, with fixed R at 2.8Meg ohms. When I tried to have a 6th range with 28Meg ohms to make 2Hz to 20Hz, the amp became quite unstable below about 6Hz because of grid current biasing setting itself up because the grid resistance is so high. I have used a dual gang 25k log pot plus 1k2 in series to get the bottom range which can be 1Hz to 20Hz.
The highest range goes to 2Mhz, at the 7.3Vrms output. Final max output level after a buffer stage with white follower might be 5Vrms, or more with a simple mosfet complementary pair source follower stage and 47r to OP terminals.
So far so good.

The NFB network is 4 x 12V/50mA lamps in series plus variable R. Each lamp is about 30r cold and 45r with normal operation, so NFB network is approximately 360r + 180r, but the ac cathode current of V1 also flows in 180r of the lamps.
The rest is far too difficult to describe and you have to wait a week or 3 before I publish my circuit which includes the necessary gain shelving in the amp and the practical means to get good unconditional stability andf natural DC stability with R&C coupled stages. It has been quite a lot more difficult to get 1Hz to 2MHz with tubes compared to using a discrete component amp with bjts and mosfet input diff pair.
In my tubed version I found using a 6DJ8 as a differential input pair did not give the gain or the stability at HF I wanted, compared to using a pentode input stage with 6BX6, and with PFB applied to the grid, and NFB applied to the cathode which acts as a diff pair, although technically it isn't, because there's only one tube, not two. So my circuit shares a fundamental feature in common with HP's original 200 oscillator - NFB to cathode. But the figure of merit for the tubes I have used which were made post 1950 allow a huge bandwith and give much lower THD at 1kHz than HP's original contraption.

I almost despaired entirely whe trying to make the output voltage stable when adjusting the F from say 200Hz to 2,000Hz. When I'd get to about 1.4kHz, and the C in WB network is getting small, the Vo began to become very bouncy and at over 1.4kHz the Vo would become like an amplitude modulated wave. So, I put a +/-12V diode+zener diode clamp on output, but AM continued, and then I made a voltage clamp with 22 x 1N914, with 11 + 11 diodes in series, but facing opposite directions, and this had the magical effect of very gently limiting Vo after optimising the signal an an F where thre AM didn't occur.
Probably most of the THD is caused by the genteel limiting action, but the price one pays for a wide bandwidth variable F oscilator is that Vo level control and stability mean some voltage limiting and thus THD. There may be other methods, but all would be far more complex than what I have used.
The THD is quite low enough for testing amplifier F response. For testing amplifier THD, a better WB oscillator can be made using switched R for say 40Hz, 220Hz, 1kHz, 5kHz 10kHz, whatever. But my low THD 1kHz oscillator makes about 6Vrms at 0.004% with op-amp and lamp reg. A following op-amp plus R&C feedback filtering filter stage reduces that THD to 0.0004% before it is applied to any amp. There'd be very little point in tring to make my variable F oscillator produce such low THD.

So, I can ask you all what are the best N and P general purpose TO220 package mosfets suitable for a commplementary pair source follower buffer, operating with +/- 15Vdc, and with about 200mA idle current, to give fairly linear class A operation? With a 47r output R, a shorted output means max current = 14V across 47r, so +/- 280Ma pk which won't cook the mfets. The 47r ouput R will cop 2Watts, OK if it is rated for 1/8 of a watt, so it fuses open and costs little to replace. The inside end of the 47r will have diodes to +/- 15V to limit back voltages.
Patrick Turner.
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