Hi RAT's:

Below are a few questions that don't seem to be specifically addressed
in the references I have:

1 When using multiple output tubes in parallel on each differential
side with cathode autobias, can you theoretically use one coupling cap,
and/or grid load resistance, and/or one grid stopper resistance for all
of them as a group? Seems likely. If so, would you practically want to,
or not want to, for any reason?

2 Has anybody used the Blumlein "garter" cathode bias method? I've seen
it mentioned but have not seen anyone expound on it. It seems like a
very nice and simple biasing method. Is it not so that once you are in
the general neighborhood of the desired idle current that relative fine
balance between both diff pairs is what it is all about?

3 Can OPT based cathode feedback be used in cathode fixed biased PP
output stages? I have only seen schematics where fixed bias is used and
the transformer leads are connected directly to the cathodes.

"Wessel Dirksen" said:

1 When using multiple output tubes in parallel on each differential
side with cathode autobias, can you theoretically use one coupling cap,
and/or grid load resistance, and/or one grid stopper resistance for all
of them as a group? Seems likely. If so, would you practically want to,
or not want to, for any reason?



Don't do that.
Grid stoppers have to be at the closest proximity of the grid pin.
Further, tubes vary in parameters.
Connecting all the grids in parallel to a single grid grounding
resistor and coupling capacitor will probably work without signal, but
not when driven.

Why? My gut instincts tell me.
I'm just a simple tube hobbyist, not a number cruncher.
There are some "professional engineers" here who can probably tell you
the theoretical ins and outs of this.


2 Has anybody used the Blumlein "garter" cathode bias method? I've seen
it mentioned but have not seen anyone expound on it. It seems like a
very nice and simple biasing method. Is it not so that once you are in
the general neighborhood of the desired idle current that relative fine
balance between both diff pairs is what it is all about?


I haven't the faintest, sorry.


3 Can OPT based cathode feedback be used in cathode fixed biased PP
output stages? I have only seen schematics where fixed bias is used and
the transformer leads are connected directly to the cathodes.


It can.
See e.g. the Quad II circuit, where a single common cathode resistor +
bypass cap are connected to the cold side of the cathode windings to
ground.

--

"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005

Wessel Dirksen wrote:

Hi RAT's:

Below are a few questions that don't seem to be specifically addressed
in the references I have:

1 When using multiple output tubes in parallel on each differential
side with cathode autobias, can you theoretically use one coupling cap,
and/or grid load resistance, and/or one grid stopper resistance for all
of them as a group? Seems likely. If so, would you practically want to,
or not want to, for any reason?

You need separate grid stoppers for each tube.
But nothing wrong with a 2uF cap and a 68k resistor to
bias say 4 parallel EL34. But each EL34 should have its own Rk and Ck
so bias regulation is good for each tube.
In old amps where just one bias Rk and Ck are used the bias currents
can vary by 100%.




2 Has anybody used the Blumlein "garter" cathode bias method? I've seen
it mentioned but have not seen anyone expound on it. It seems like a
very nice and simple biasing method. Is it not so that once you are in
the general neighborhood of the desired idle current that relative fine
balance between both diff pairs is what it is all about?

The sox and trousers method of bias is the one I'd stay with if i were you.

Without the sox and trousers adjusted properly, a lotta instability can
ensue.



3 Can OPT based cathode feedback be used in cathode fixed biased PP
output stages?

No problem, I do it all the time.
Old Quad II amps benefit from having the CFB winding CT grounded,
with 500ohms + 1,000uF as bias networks between the ends of the CFB
windings and the
cathodes.


I have only seen schematics where fixed bias is used and
the transformer leads are connected directly to the cathodes.

Quad II is cathode bias with CFB, but with a common R and C between CFB CT
and 0V.
Its a cheap nasty way of doing things.

Patrick Turner.

Patrick Turner schreef:

Wessel Dirksen wrote:

Hi RAT's:

Below are a few questions that don't seem to be specifically addressed
in the references I have:

1 When using multiple output tubes in parallel on each differential
side with cathode autobias, can you theoretically use one coupling cap,
and/or grid load resistance, and/or one grid stopper resistance for all
of them as a group? Seems likely. If so, would you practically want to,
or not want to, for any reason?

You need separate grid stoppers for each tube.
But nothing wrong with a 2uF cap and a 68k resistor to
bias say 4 parallel EL34. But each EL34 should have its own Rk and Ck
so bias regulation is good for each tube.
In old amps where just one bias Rk and Ck are used the bias currents
can vary by 100%.




2 Has anybody used the Blumlein "garter" cathode bias method? I've seen
it mentioned but have not seen anyone expound on it. It seems like a
very nice and simple biasing method. Is it not so that once you are in
the general neighborhood of the desired idle current that relative fine
balance between both diff pairs is what it is all about?

The sox and trousers method of bias is the one I'd stay with if i were you.

Without the sox and trousers adjusted properly, a lotta instability can
ensue.

Thanks Patrick and Sander,

Patrick,
Sox and trousers? That's funny (Sox=Rk, Trousers=Ck ?)

Let me see if I understand you correctly. Combining the coupling
capacitance and grid leak resistance is OK to multiple output tubes,
but grid stoppers and the autobias cathode parts should kept individual
per tube?

The Blumlein circuit is described here in the middle of the web page.
The other circuits described look good but too complex.

http://www.tubecad.com/2005/May/blog0046.htm

I was hoping to get a more in depth expert opinion on this circuit. It
seems that a normal recommended value of grid leak resistor is fed to
the cathode circuit of the other differential side where it is inserted
between 2 Rk's of normal calculated value in series with each other and
straddled by Ck. I guess this way each diff pair would have twice the
cathode resistance to ground and as mentioned kill some B+. I really
like the "differential auto balance" concept though. Worth it?


3 Can OPT based cathode feedback be used in cathode fixed biased PP
output stages?

No problem, I do it all the time.
Old Quad II amps benefit from having the CFB winding CT grounded,
with 500ohms + 1,000uF as bias networks between the ends of the CFB
windings and the
cathodes.

Ok, I think I get it. By "CT" you mean grounding the center tap of the
CFB part of the OPT right? If so, then the Blumlein autobias circuit
could theoretically be combined with CFB.

I have only seen schematics where fixed bias is used and
the transformer leads are connected directly to the cathodes.

Quad II is cathode bias with CFB, but with a common R and C between CFB CT
and 0V.
Its a cheap nasty way of doing things.

I tend to like cheap and nasty. Is CFB a good form of local FB? It
would seem to be quite tidy with primarily the reactive parts of the
output tube being in the feedback loop and by stepping up the damping
factor only at the output stage where it suffers the worst impedance
mismatch. The rest of the stages in the chain I am considering have
100 ratio of Zin/Zout thanks to 6H30's.

Patrick Turner.

Wessel Dirksen wrote:

Patrick Turner schreef:

Wessel Dirksen wrote:

Hi RAT's:

Below are a few questions that don't seem to be specifically addressed
in the references I have:

1 When using multiple output tubes in parallel on each differential
side with cathode autobias, can you theoretically use one coupling cap,
and/or grid load resistance, and/or one grid stopper resistance for all
of them as a group? Seems likely. If so, would you practically want to,
or not want to, for any reason?

You need separate grid stoppers for each tube.
But nothing wrong with a 2uF cap and a 68k resistor to
bias say 4 parallel EL34. But each EL34 should have its own Rk and Ck
so bias regulation is good for each tube.
In old amps where just one bias Rk and Ck are used the bias currents
can vary by 100%.




2 Has anybody used the Blumlein "garter" cathode bias method? I've seen
it mentioned but have not seen anyone expound on it. It seems like a
very nice and simple biasing method. Is it not so that once you are in
the general neighborhood of the desired idle current that relative fine
balance between both diff pairs is what it is all about?

The sox and trousers method of bias is the one I'd stay with if i were you.

Without the sox and trousers adjusted properly, a lotta instability can
ensue.

Thanks Patrick and Sander,

Patrick,
Sox and trousers? That's funny (Sox=Rk, Trousers=Ck ?)

Let me see if I understand you correctly. Combining the coupling
capacitance and grid leak resistance is OK to multiple output tubes,
but grid stoppers and the autobias cathode parts should kept individual
per tube?

That's correct.



The Blumlein circuit is described here in the middle of the web page.
The other circuits described look good but too complex.

http://www.tubecad.com/2005/May/blog0046.htm

I was hoping to get a more in depth expert opinion on this circuit.

Blumlien shows genius yet again.

The garter circuit does improve matters of dc balance quite substantially,
and in a worthwhile amount.
But what does one do if you have a dozen output tubes, 6 per side of the PP
circuit?

With two tubes all is ok, but balance is impossible with 12 tubes, so some active
method to
equalise bias current would ahve to be used.

I was just joking about the trousers and sox bit.
But before leaving for the office tomorrow morning,
try cutting one leg off your trousers at the knee, and wearing a long pink sock
on the exposed leg.
It'd be a miracle if your boss doesn't threaten you with the sack during the day.

Conventions are silly eh?

The above website is **very** interesting, and it has some bias circuits using
LTPs with bjts to auto balance the bias of a pair of output tubes.
I have tried almost exactly what they have there with PNP bjts and found they
work
extremly well until one applies loop NFB, and then the circuit becomes impossible

to stabilise at LF.


It
seems that a normal recommended value of grid leak resistor is fed to
the cathode circuit of the other differential side where it is inserted
between 2 Rk's of normal calculated value in series with each other and
straddled by Ck. I guess this way each diff pair would have twice the
cathode resistance to ground and as mentioned kill some B+. I really
like the "differential auto balance" concept though. Worth it?

Yes.






3 Can OPT based cathode feedback be used in cathode fixed biased PP
output stages?

No problem, I do it all the time.
Old Quad II amps benefit from having the CFB winding CT grounded,
with 500ohms + 1,000uF as bias networks between the ends of the CFB
windings and the
cathodes.

Ok, I think I get it. By "CT" you mean grounding the center tap of the
CFB part of the OPT right?

Yes.

If so, then the Blumlein autobias circuit
could theoretically be combined with CFB.

Not really because there is a fairly high signal of opposite
phase at each cathode, so effectively you would at least be
halving the time constant of the CR coupling to the driver stage.
I don't know how exactly the stability would be affected.

But I like a more hands on approach where an LTP using bjt
is used to monitor the Ik of each output tube and if the Ik should drift
appart more than 3mA then an led lights up to tell you which tube is conducting
more current.
There is then a pot where the bias can be slightly adjusted so the two leds near
each output tube
go out when Ik is within 2mA, and swinging the pot will tune the bias balance
even closer.
Ths adjust mechanism tells you when a tube is playing up, wearing out, and if you
cannot
adjust the high Ik tube down with a pot you *know* its stuffed, or there is some
other problem.

Most times where you buy a pair of unmatched tubes you will find that with plain
cathode bias
of individual tubes that the bias current sets itself within 3% balance, which is

quite OK. The sound starts to go to mud when imbalance drifts to 10%,
and the levels are high because the OPT core has its iron µ effectively reduced
so
LF signals tend to saturatae the core and cause intermodulation
with higher F.

Quad II amps are renowned for such problems when the tubes age and many samples
I have serviced have 40mA in one output tube and 90mA in the other which
sometimes has
some red patches on the anode, and the sound becomes bad above 1/2 a watt.
Just placing individual biasing reduces the imbalance to within 7% even with
tubes so far apart
and allows many more years of service from the tubes.
Peter Walker really made a huge mistake in his original design imho by addopting
his use of a single RC for biasing both output tubes.




I have only seen schematics where fixed bias is used and
the transformer leads are connected directly to the cathodes.

Quad II is cathode bias with CFB, but with a common R and C between CFB CT
and 0V.
Its a cheap nasty way of doing things.

I tend to like cheap and nasty. Is CFB a good form of local FB?

It is if well implemented.

The idea that Quad used is quite valid.
It allows the full tetrode power of KT66 to be realised but
the tubes act as though Ra was a lot lower, but yet they remain easy to drive.
The more CFB you have, the more grid drive you need, so the more linear your
drive amp has to be.
McIntosh used equal % of P turns in the anode and cathode winding, so
you can have 125Vrms at anode and cathode, and if the tube gain = 15, then
you need 17Vrms between cathode and grid to cause the 250Vrms between anode and
cathode
and so you need 125V + 17V applied to each output grid, and hence the complexity
of the McIntosh driver stage with its extra stage.
The high drive voltage means the thd contribution of the driver becomes greater
than that
produced in the output stage so you would need global loop NFB to reduce it down.

McIntosh used around 20 Db of global and with the 15Db of local NFB in the output
stage
their amps have around the same thd as many other amps which don't use large
amounts of
CFB and just rely on modest global NFB.
But where McI amps shine is with the Rout of thier amps; the total applied NFB is

around 35dB, so Rout is much lower than many other amps; Quad II is about 1 ohm,
but McI is about 0.2ohms.
McI amps allowed one to obtain 50 watts from a pair of 6L6, a huge amount of
power
for 1949, and thd and Rout was quite low due to the mainly class B action of the
output tubes.
Their operation is substantially improved when they are altered for lower B+,
and class A operation, when the Rout is halved and thd reduced to vanishingly
low levels, so that a watt thd 0.01%.

Nevertheless, a good Williamson or class A UL amp will also make around 0.02% at
a watt
and will use one less stage of gain than the McI, and the OPT is a lot simpler.
Hence hardly any DIYers made copies of the McI amp; they voted with their
wallets for the simpler Wiliamson, or the Mullard 520.
Very few have copied the Quad examples.
I believe it to be worth the trouble, and its the only way i make PP amp
but I use a higher % of cfb than Quad ever did.





It would seem to be quite tidy with primarily the reactive parts of the
output tube being in the feedback loop and by stepping up the damping
factor only at the output stage where it suffers the worst impedance
mismatch. The rest of the stages in the chain I am considering have
100 ratio of Zin/Zout thanks to 6H30's.

CFB allows the output tubes to be substantially linearized via a short
feedback loop involving only the OPT and the output tubes.

The reactive elements of the output tube are negligible since the Miller C is
low,
and you only have *one* lot of Miller C, not 3 lots, which is the case with a
3 stage amplifier.

Therefore the use of moderate CFB, say up to 25% of the total signal voltage
between
a and k, is a great way of making the output stage behave well and the global FB
can be reduced
to say between 6 and 12dB which is a low amount.

Quad II amps without their global NFB loop and when using the 8 ohm outlet config
produce
an Rout = 9 ohms. The Rout from pure beam tetrode output stage
with KT66 would be about 132 ohms, so that 10% of CFB Quad uses is *very* useful
and the effect on the thd is also very beneficial.
The global NFB then reduces the Rout to around an ohm for a reasonable damping
factor.

The idea is that the majority of signal error correction by NFB is carried out in
the output stage
and not in the driver amp.

The Quad method of OPT set up is as effective as the use of the 40% of UL
tappings which
has about the same effect as the 10% of CFB, except that perhaps the Quad method
is more efficient because the feedback is mainly applied in the grid-cathode
loop, with also some slight
feedback effect in the grid-screen circuit, wheras the UL circuit has all its
feedback applied
via the anode-screen circuit.
Triode connection of tubes with screens is a case where *all* the anode voltage
is used as feedback
in the screen circuit, ie, the UL tappings are at 100%.

The trouble with triodes is that unless they are pushed into class A2 or AB2, the
output power
is severely limited to half what the tube can deliver, so the UL connection was
tried to allow the tube to approach triode fidelity but without the power loss
penalty.
CFB achieves the same result, only slightly better, ie, Rout and thd
are slightly lower than UL circuits where power output is still near maximum for
pure tetrode
and the output stage sensitivity is about the same; ie, the grid voltage applied
to the output
stage is about the same for the two ways of connecting the tubes.

Quad also liked to be different, to have the best, and they patented their 10% of
CFB,
so everyone else had to make do with something less, but bean counters
loved UL because it was so much easier to make an OPT without a separate tertiary
winding
devoted to CFB, and probably easier to get the OPT to have low leakage inductance
in the right places to
allow good stablity; Leak had only 3P sections and two S sections, and got away
with it.
But of course all the cheap and nasty bean counter driven OPT designs that were
the hallmark
of a lousy minded consumer audio product industry of 1955 can be easily improved
upon,
and its for this reason i regard Leaks and Quads as very mediocre amplifiers,
things that
can and do sound well when properly set up but which all measure quite poorly
by today's standards where we have good sound and less winding losses, better
stability,
and more reliablility, so fewer OPT write offs every time an output tube died in
a vesuvious of
over current excitement.
The Dynaco ST80 was one of the last Dynaco tube amps to be made, and its vastly
superior to its predessor, the ST70, and anything made by Leak, but you'd expect
that after
20 years.
I measured an ST80 they other day, and its OPTs are a lot better than in the
ST70,
and its quite stable, even without any R load, and using just a 0.47uF or 0.22 uF
cap as a load.
Dynaco finally got with it, and did the required homework to get their circuit to
work
properly, after spending the extra $1 for a bit more irn and wire, and $2 for the
extra labour.

Finally, I have cathode bias in my 300 watt amps for the 12 output tubes.
This saves having 24 bias adjustments for 2 channels.

In the latest versions i use a patentable and little understood method of
shunting
excess cathode signal current with a simple two bjt circuit that stops the
change of dc cathode voltage that occurs when you apply high signal levels to a
class AB amp
with cathode bias.
So my 300 watt amps work exactly like fixed bias amps with their lower thd
ability; the cathode bias voltage changes less than 10% between idle and full
power
with a sine wave, and while in class A the ac signal bypass circuit remains
utterly dormant
since it only works when the cathode current change goes over the twice the idle
bias current.
No need to have active biasing of the grid circuits.

Class AB cathode bias amps suffer when driven to the point where musical
transients
cause the bias voltage at the cathode to begin undulating around, and the dc
currents
lurch about; but not in my amps.

I did post a circuit of the basic approach at abse about 2 years ago, but nobody
cared, understood, or tried it out, so I won't be giving any other details now,
so
youse can all go and invent the solution to easy biasing and dynamic
regulation on your own, now that I have spelled out the ideas.



Patrick Turner.

Patrick Turner schreef:

Wessel Dirksen wrote:

Patrick Turner schreef:

Wessel Dirksen wrote:

Hi RAT's:

Below are a few questions that don't seem to be specifically addressed
in the references I have:

1 When using multiple output tubes in parallel on each differential
side with cathode autobias, can you theoretically use one coupling cap,
and/or grid load resistance, and/or one grid stopper resistance forall
of them as a group? Seems likely. If so, would you practically wantto,
or not want to, for any reason?

You need separate grid stoppers for each tube.
But nothing wrong with a 2uF cap and a 68k resistor to
bias say 4 parallel EL34. But each EL34 should have its own Rk and Ck
so bias regulation is good for each tube.
In old amps where just one bias Rk and Ck are used the bias currents
can vary by 100%.




2 Has anybody used the Blumlein "garter" cathode bias method? I've seen
it mentioned but have not seen anyone expound on it. It seems like a
very nice and simple biasing method. Is it not so that once you arein
the general neighborhood of the desired idle current that relative fine
balance between both diff pairs is what it is all about?

The sox and trousers method of bias is the one I'd stay with if i were you.

Without the sox and trousers adjusted properly, a lotta instability can
ensue.

Thanks Patrick and Sander,

Patrick,
Sox and trousers? That's funny (Sox=Rk, Trousers=Ck ?)

Let me see if I understand you correctly. Combining the coupling
capacitance and grid leak resistance is OK to multiple output tubes,
but grid stoppers and the autobias cathode parts should kept individual
per tube?

That's correct.



The Blumlein circuit is described here in the middle of the web page.
The other circuits described look good but too complex.

http://www.tubecad.com/2005/May/blog0046.htm

I was hoping to get a more in depth expert opinion on this circuit.

Blumlien shows genius yet again.

The garter circuit does improve matters of dc balance quite substantially,
and in a worthwhile amount.
But what does one do if you have a dozen output tubes, 6 per side of the PP
circuit?

With two tubes all is ok, but balance is impossible with 12 tubes, so some active
method to
equalise bias current would ahve to be used.

I was just joking about the trousers and sox bit.
But before leaving for the office tomorrow morning,
try cutting one leg off your trousers at the knee, and wearing a long pink sock
on the exposed leg.
It'd be a miracle if your boss doesn't threaten you with the sack during the day.

Conventions are silly eh?

The above website is **very** interesting, and it has some bias circuits using
LTPs with bjts to auto balance the bias of a pair of output tubes.
I have tried almost exactly what they have there with PNP bjts and found they
work
extremly well until one applies loop NFB, and then the circuit becomes impossible

to stabilise at LF.


It
seems that a normal recommended value of grid leak resistor is fed to
the cathode circuit of the other differential side where it is inserted
between 2 Rk's of normal calculated value in series with each other and
straddled by Ck. I guess this way each diff pair would have twice the
cathode resistance to ground and as mentioned kill some B+. I really
like the "differential auto balance" concept though. Worth it?

Yes.






3 Can OPT based cathode feedback be used in cathode fixed biased PP
output stages?

No problem, I do it all the time.
Old Quad II amps benefit from having the CFB winding CT grounded,
with 500ohms + 1,000uF as bias networks between the ends of the CFB
windings and the
cathodes.

Ok, I think I get it. By "CT" you mean grounding the center tap of the
CFB part of the OPT right?

Yes.

If so, then the Blumlein autobias circuit
could theoretically be combined with CFB.

Not really because there is a fairly high signal of opposite
phase at each cathode, so effectively you would at least be
halving the time constant of the CR coupling to the driver stage.
I don't know how exactly the stability would be affected.

But I like a more hands on approach where an LTP using bjt
is used to monitor the Ik of each output tube and if the Ik should drift
appart more than 3mA then an led lights up to tell you which tube is conducting
more current.
There is then a pot where the bias can be slightly adjusted so the two leds near
each output tube
go out when Ik is within 2mA, and swinging the pot will tune the bias balance
even closer.
Ths adjust mechanism tells you when a tube is playing up, wearing out, and if you
cannot
adjust the high Ik tube down with a pot you *know* its stuffed, or there is some
other problem.

Most times where you buy a pair of unmatched tubes you will find that with plain
cathode bias
of individual tubes that the bias current sets itself within 3% balance, which is

quite OK. The sound starts to go to mud when imbalance drifts to 10%,
and the levels are high because the OPT core has its iron µ effectivelyreduced
so
LF signals tend to saturatae the core and cause intermodulation
with higher F.

Quad II amps are renowned for such problems when the tubes age and many samples
I have serviced have 40mA in one output tube and 90mA in the other which
sometimes has
some red patches on the anode, and the sound becomes bad above 1/2 a watt.
Just placing individual biasing reduces the imbalance to within 7% even with
tubes so far apart
and allows many more years of service from the tubes.
Peter Walker really made a huge mistake in his original design imho by addopting
his use of a single RC for biasing both output tubes.




I have only seen schematics where fixed bias is used and
the transformer leads are connected directly to the cathodes.

Quad II is cathode bias with CFB, but with a common R and C between CFB CT
and 0V.
Its a cheap nasty way of doing things.

I tend to like cheap and nasty. Is CFB a good form of local FB?

It is if well implemented.

The idea that Quad used is quite valid.
It allows the full tetrode power of KT66 to be realised but
the tubes act as though Ra was a lot lower, but yet they remain easy to drive.
The more CFB you have, the more grid drive you need, so the more linear your
drive amp has to be.
McIntosh used equal % of P turns in the anode and cathode winding, so
you can have 125Vrms at anode and cathode, and if the tube gain = 15, then
you need 17Vrms between cathode and grid to cause the 250Vrms between anode and
cathode
and so you need 125V + 17V applied to each output grid, and hence the complexity
of the McIntosh driver stage with its extra stage.
The high drive voltage means the thd contribution of the driver becomes greater
than that
produced in the output stage so you would need global loop NFB to reduce it down.

McIntosh used around 20 Db of global and with the 15Db of local NFB in the output
stage
their amps have around the same thd as many other amps which don't use large
amounts of
CFB and just rely on modest global NFB.
But where McI amps shine is with the Rout of thier amps; the total applied NFB is

around 35dB, so Rout is much lower than many other amps; Quad II is about1 ohm,
but McI is about 0.2ohms.
McI amps allowed one to obtain 50 watts from a pair of 6L6, a huge amountof
power
for 1949, and thd and Rout was quite low due to the mainly class B actionof the
output tubes.
Their operation is substantially improved when they are altered for lowerB+,
and class A operation, when the Rout is halved and thd reduced to vanishingly
low levels, so that a watt thd 0.01%.

Nevertheless, a good Williamson or class A UL amp will also make around 0..02% at
a watt
and will use one less stage of gain than the McI, and the OPT is a lot simpler.
Hence hardly any DIYers made copies of the McI amp; they voted with their
wallets for the simpler Wiliamson, or the Mullard 520.
Very few have copied the Quad examples.
I believe it to be worth the trouble, and its the only way i make PP amp
but I use a higher % of cfb than Quad ever did.





It would seem to be quite tidy with primarily the reactive parts of the
output tube being in the feedback loop and by stepping up the damping
factor only at the output stage where it suffers the worst impedance
mismatch. The rest of the stages in the chain I am considering have
100 ratio of Zin/Zout thanks to 6H30's.

CFB allows the output tubes to be substantially linearized via a short
feedback loop involving only the OPT and the output tubes.

The reactive elements of the output tube are negligible since the Miller C is
low,
and you only have *one* lot of Miller C, not 3 lots, which is the case with a
3 stage amplifier.

Therefore the use of moderate CFB, say up to 25% of the total signal voltage
between
a and k, is a great way of making the output stage behave well and the global FB
can be reduced
to say between 6 and 12dB which is a low amount.

Quad II amps without their global NFB loop and when using the 8 ohm outlet config
produce
an Rout = 9 ohms. The Rout from pure beam tetrode output stage
with KT66 would be about 132 ohms, so that 10% of CFB Quad uses is *very*useful
and the effect on the thd is also very beneficial.
The global NFB then reduces the Rout to around an ohm for a reasonable damping
factor.

The idea is that the majority of signal error correction by NFB is carried out in
the output stage
and not in the driver amp.

The Quad method of OPT set up is as effective as the use of the 40% of UL
tappings which
has about the same effect as the 10% of CFB, except that perhaps the Quadmethod
is more efficient because the feedback is mainly applied in the grid-cathode
loop, with also some slight
feedback effect in the grid-screen circuit, wheras the UL circuit has allits
feedback applied
via the anode-screen circuit.
Triode connection of tubes with screens is a case where *all* the anode voltage
is used as feedback
in the screen circuit, ie, the UL tappings are at 100%.

The trouble with triodes is that unless they are pushed into class A2 or AB2, the
output power
is severely limited to half what the tube can deliver, so the UL connection was
tried to allow the tube to approach triode fidelity but without the powerloss
penalty.
CFB achieves the same result, only slightly better, ie, Rout and thd
are slightly lower than UL circuits where power output is still near maximum for
pure tetrode
and the output stage sensitivity is about the same; ie, the grid voltage applied
to the output
stage is about the same for the two ways of connecting the tubes.

Quad also liked to be different, to have the best, and they patented their 10% of
CFB,
so everyone else had to make do with something less, but bean counters
loved UL because it was so much easier to make an OPT without a separate tertiary
winding
devoted to CFB, and probably easier to get the OPT to have low leakage inductance
in the right places to
allow good stablity; Leak had only 3P sections and two S sections, and got away
with it.
But of course all the cheap and nasty bean counter driven OPT designs that were
the hallmark
of a lousy minded consumer audio product industry of 1955 can be easily improved
upon,
and its for this reason i regard Leaks and Quads as very mediocre amplifiers,
things that
can and do sound well when properly set up but which all measure quite poorly
by today's standards where we have good sound and less winding losses, better
stability,
and more reliablility, so fewer OPT write offs every time an output tube died in
a vesuvious of
over current excitement.
The Dynaco ST80 was one of the last Dynaco tube amps to be made, and its vastly
superior to its predessor, the ST70, and anything made by Leak, but you'dexpect
that after
20 years.
I measured an ST80 they other day, and its OPTs are a lot better than in the
ST70,
and its quite stable, even without any R load, and using just a 0.47uF or0.22 uF
cap as a load.
Dynaco finally got with it, and did the required homework to get their circuit to
work
properly, after spending the extra $1 for a bit more irn and wire, and $2for the
extra labour.

Finally, I have cathode bias in my 300 watt amps for the 12 output tubes.
This saves having 24 bias adjustments for 2 channels.

In the latest versions i use a patentable and little understood method of
shunting
excess cathode signal current with a simple two bjt circuit that stops the
change of dc cathode voltage that occurs when you apply high signal levels to a
class AB amp
with cathode bias.
So my 300 watt amps work exactly like fixed bias amps with their lower thd
ability; the cathode bias voltage changes less than 10% between idle and full
power
with a sine wave, and while in class A the ac signal bypass circuit remains
utterly dormant
since it only works when the cathode current change goes over the twice the idle
bias current.
No need to have active biasing of the grid circuits.

Class AB cathode bias amps suffer when driven to the point where musical
transients
cause the bias voltage at the cathode to begin undulating around, and thedc
currents
lurch about; but not in my amps.

I did post a circuit of the basic approach at abse about 2 years ago, butnobody
cared, understood, or tried it out, so I won't be giving any other details now,
so
youse can all go and invent the solution to easy biasing and dynamic
regulation on your own, now that I have spelled out the ideas.



Patrick Turner.

Patrick,

Thanks for the valuable info. This is exactly what I asked for and a
great read with some history. I've saved it.

You mentioned UL. Perhaps a dumb question since I have never seen this,
but why can't UL leads be used as CFB; perhaps with some attenuation?
Or is the phase wrong at 43%. Does this imply that a CFB coil has an
equal # of turns as the primary?

BTW: With this continuing first PP EL34 project of mine, I don't like
UL at all compared to triode no matter how the output has been driven.
I can't imagine ever going back (at least not in hi-fi) despite the
power penalty.

Wessel

Wessel Dirksen wrote:

Patrick Turner schreef:

Wessel Dirksen wrote:

Patrick Turner schreef:

Wessel Dirksen wrote:

Hi RAT's:

Below are a few questions that don't seem to be specifically addressed
in the references I have:

1 When using multiple output tubes in parallel on each differential
side with cathode autobias, can you theoretically use one coupling cap,
and/or grid load resistance, and/or one grid stopper resistance for all
of them as a group? Seems likely. If so, would you practically want to,
or not want to, for any reason?

You need separate grid stoppers for each tube.
But nothing wrong with a 2uF cap and a 68k resistor to
bias say 4 parallel EL34. But each EL34 should have its own Rk and Ck
so bias regulation is good for each tube.
In old amps where just one bias Rk and Ck are used the bias currents
can vary by 100%.




2 Has anybody used the Blumlein "garter" cathode bias method? I've seen
it mentioned but have not seen anyone expound on it. It seems like a
very nice and simple biasing method. Is it not so that once you are in
the general neighborhood of the desired idle current that relative fine
balance between both diff pairs is what it is all about?

The sox and trousers method of bias is the one I'd stay with if i were you.

Without the sox and trousers adjusted properly, a lotta instability can
ensue.

Thanks Patrick and Sander,

Patrick,
Sox and trousers? That's funny (Sox=Rk, Trousers=Ck ?)

Let me see if I understand you correctly. Combining the coupling
capacitance and grid leak resistance is OK to multiple output tubes,
but grid stoppers and the autobias cathode parts should kept individual
per tube?

That's correct.



The Blumlein circuit is described here in the middle of the web page.
The other circuits described look good but too complex.

http://www.tubecad.com/2005/May/blog0046.htm

I was hoping to get a more in depth expert opinion on this circuit.

Blumlien shows genius yet again.

The garter circuit does improve matters of dc balance quite substantially,
and in a worthwhile amount.
But what does one do if you have a dozen output tubes, 6 per side of the PP
circuit?

With two tubes all is ok, but balance is impossible with 12 tubes, so some active
method to
equalise bias current would ahve to be used.

I was just joking about the trousers and sox bit.
But before leaving for the office tomorrow morning,
try cutting one leg off your trousers at the knee, and wearing a long pink sock
on the exposed leg.
It'd be a miracle if your boss doesn't threaten you with the sack during the day.

Conventions are silly eh?

The above website is **very** interesting, and it has some bias circuits using
LTPs with bjts to auto balance the bias of a pair of output tubes.
I have tried almost exactly what they have there with PNP bjts and found they
work
extremly well until one applies loop NFB, and then the circuit becomes impossible

to stabilise at LF.


It
seems that a normal recommended value of grid leak resistor is fed to
the cathode circuit of the other differential side where it is inserted
between 2 Rk's of normal calculated value in series with each other and
straddled by Ck. I guess this way each diff pair would have twice the
cathode resistance to ground and as mentioned kill some B+. I really
like the "differential auto balance" concept though. Worth it?

Yes.






3 Can OPT based cathode feedback be used in cathode fixed biased PP
output stages?

No problem, I do it all the time.
Old Quad II amps benefit from having the CFB winding CT grounded,
with 500ohms + 1,000uF as bias networks between the ends of the CFB
windings and the
cathodes.

Ok, I think I get it. By "CT" you mean grounding the center tap of the
CFB part of the OPT right?

Yes.

If so, then the Blumlein autobias circuit
could theoretically be combined with CFB.

Not really because there is a fairly high signal of opposite
phase at each cathode, so effectively you would at least be
halving the time constant of the CR coupling to the driver stage.
I don't know how exactly the stability would be affected.

But I like a more hands on approach where an LTP using bjt
is used to monitor the Ik of each output tube and if the Ik should drift
appart more than 3mA then an led lights up to tell you which tube is conducting
more current.
There is then a pot where the bias can be slightly adjusted so the two leds near
each output tube
go out when Ik is within 2mA, and swinging the pot will tune the bias balance
even closer.
Ths adjust mechanism tells you when a tube is playing up, wearing out, and if you
cannot
adjust the high Ik tube down with a pot you *know* its stuffed, or there is some
other problem.

Most times where you buy a pair of unmatched tubes you will find that with plain
cathode bias
of individual tubes that the bias current sets itself within 3% balance, which is

quite OK. The sound starts to go to mud when imbalance drifts to 10%,
and the levels are high because the OPT core has its iron µ effectively reduced
so
LF signals tend to saturatae the core and cause intermodulation
with higher F.

Quad II amps are renowned for such problems when the tubes age and many samples
I have serviced have 40mA in one output tube and 90mA in the other which
sometimes has
some red patches on the anode, and the sound becomes bad above 1/2 a watt.
Just placing individual biasing reduces the imbalance to within 7% even with
tubes so far apart
and allows many more years of service from the tubes.
Peter Walker really made a huge mistake in his original design imho by addopting
his use of a single RC for biasing both output tubes.




I have only seen schematics where fixed bias is used and
the transformer leads are connected directly to the cathodes.

Quad II is cathode bias with CFB, but with a common R and C between CFB CT
and 0V.
Its a cheap nasty way of doing things.

I tend to like cheap and nasty. Is CFB a good form of local FB?

It is if well implemented.

The idea that Quad used is quite valid.
It allows the full tetrode power of KT66 to be realised but
the tubes act as though Ra was a lot lower, but yet they remain easy to drive.
The more CFB you have, the more grid drive you need, so the more linear your
drive amp has to be.
McIntosh used equal % of P turns in the anode and cathode winding, so
you can have 125Vrms at anode and cathode, and if the tube gain = 15, then
you need 17Vrms between cathode and grid to cause the 250Vrms between anode and
cathode
and so you need 125V + 17V applied to each output grid, and hence the complexity
of the McIntosh driver stage with its extra stage.
The high drive voltage means the thd contribution of the driver becomes greater
than that
produced in the output stage so you would need global loop NFB to reduce it down.

McIntosh used around 20 Db of global and with the 15Db of local NFB in the output
stage
their amps have around the same thd as many other amps which don't use large
amounts of
CFB and just rely on modest global NFB.
But where McI amps shine is with the Rout of thier amps; the total applied NFB is

around 35dB, so Rout is much lower than many other amps; Quad II is about 1 ohm,
but McI is about 0.2ohms.
McI amps allowed one to obtain 50 watts from a pair of 6L6, a huge amount of
power
for 1949, and thd and Rout was quite low due to the mainly class B action of the
output tubes.
Their operation is substantially improved when they are altered for lower B+,
and class A operation, when the Rout is halved and thd reduced to vanishingly
low levels, so that a watt thd 0.01%.

Nevertheless, a good Williamson or class A UL amp will also make around 0.02% at
a watt
and will use one less stage of gain than the McI, and the OPT is a lot simpler.
Hence hardly any DIYers made copies of the McI amp; they voted with their
wallets for the simpler Wiliamson, or the Mullard 520.
Very few have copied the Quad examples.
I believe it to be worth the trouble, and its the only way i make PP amp
but I use a higher % of cfb than Quad ever did.





It would seem to be quite tidy with primarily the reactive parts of the
output tube being in the feedback loop and by stepping up the damping
factor only at the output stage where it suffers the worst impedance
mismatch. The rest of the stages in the chain I am considering have
100 ratio of Zin/Zout thanks to 6H30's.

CFB allows the output tubes to be substantially linearized via a short
feedback loop involving only the OPT and the output tubes.

The reactive elements of the output tube are negligible since the Miller C is
low,
and you only have *one* lot of Miller C, not 3 lots, which is the case with a
3 stage amplifier.

Therefore the use of moderate CFB, say up to 25% of the total signal voltage
between
a and k, is a great way of making the output stage behave well and the global FB
can be reduced
to say between 6 and 12dB which is a low amount.

Quad II amps without their global NFB loop and when using the 8 ohm outlet config
produce
an Rout = 9 ohms. The Rout from pure beam tetrode output stage
with KT66 would be about 132 ohms, so that 10% of CFB Quad uses is *very* useful
and the effect on the thd is also very beneficial.
The global NFB then reduces the Rout to around an ohm for a reasonable damping
factor.

The idea is that the majority of signal error correction by NFB is carried out in
the output stage
and not in the driver amp.

The Quad method of OPT set up is as effective as the use of the 40% of UL
tappings which
has about the same effect as the 10% of CFB, except that perhaps the Quad method
is more efficient because the feedback is mainly applied in the grid-cathode
loop, with also some slight
feedback effect in the grid-screen circuit, wheras the UL circuit has all its
feedback applied
via the anode-screen circuit.
Triode connection of tubes with screens is a case where *all* the anode voltage
is used as feedback
in the screen circuit, ie, the UL tappings are at 100%.

The trouble with triodes is that unless they are pushed into class A2 or AB2, the
output power
is severely limited to half what the tube can deliver, so the UL connection was
tried to allow the tube to approach triode fidelity but without the power loss
penalty.
CFB achieves the same result, only slightly better, ie, Rout and thd
are slightly lower than UL circuits where power output is still near maximum for
pure tetrode
and the output stage sensitivity is about the same; ie, the grid voltage applied
to the output
stage is about the same for the two ways of connecting the tubes.

Quad also liked to be different, to have the best, and they patented their 10% of
CFB,
so everyone else had to make do with something less, but bean counters
loved UL because it was so much easier to make an OPT without a separate tertiary
winding
devoted to CFB, and probably easier to get the OPT to have low leakage inductance
in the right places to
allow good stablity; Leak had only 3P sections and two S sections, and got away
with it.
But of course all the cheap and nasty bean counter driven OPT designs that were
the hallmark
of a lousy minded consumer audio product industry of 1955 can be easily improved
upon,
and its for this reason i regard Leaks and Quads as very mediocre amplifiers,
things that
can and do sound well when properly set up but which all measure quite poorly
by today's standards where we have good sound and less winding losses, better
stability,
and more reliablility, so fewer OPT write offs every time an output tube died in
a vesuvious of
over current excitement.
The Dynaco ST80 was one of the last Dynaco tube amps to be made, and its vastly
superior to its predessor, the ST70, and anything made by Leak, but you'd expect
that after
20 years.
I measured an ST80 they other day, and its OPTs are a lot better than in the
ST70,
and its quite stable, even without any R load, and using just a 0.47uF or 0.22 uF
cap as a load.
Dynaco finally got with it, and did the required homework to get their circuit to
work
properly, after spending the extra $1 for a bit more irn and wire, and $2 for the
extra labour.

Finally, I have cathode bias in my 300 watt amps for the 12 output tubes.
This saves having 24 bias adjustments for 2 channels.

In the latest versions i use a patentable and little understood method of
shunting
excess cathode signal current with a simple two bjt circuit that stops the
change of dc cathode voltage that occurs when you apply high signal levels to a
class AB amp
with cathode bias.
So my 300 watt amps work exactly like fixed bias amps with their lower thd
ability; the cathode bias voltage changes less than 10% between idle and full
power
with a sine wave, and while in class A the ac signal bypass circuit remains
utterly dormant
since it only works when the cathode current change goes over the twice the idle
bias current.
No need to have active biasing of the grid circuits.

Class AB cathode bias amps suffer when driven to the point where musical
transients
cause the bias voltage at the cathode to begin undulating around, and the dc
currents
lurch about; but not in my amps.

I did post a circuit of the basic approach at abse about 2 years ago, but nobody
cared, understood, or tried it out, so I won't be giving any other details now,
so
youse can all go and invent the solution to easy biasing and dynamic
regulation on your own, now that I have spelled out the ideas.



Patrick Turner.

Patrick,

Thanks for the valuable info. This is exactly what I asked for and a
great read with some history. I've saved it.

You mentioned UL. Perhaps a dumb question since I have never seen this,
but why can't UL leads be used as CFB; perhaps with some attenuation?
Or is the phase wrong at 43%. Does this imply that a CFB coil has an
equal # of turns as the primary?

The CFB *is part* of the primary.

Only in McIntosh amp does the turns used in the cathode windings equal
those in the anode windings.

You can use a UL OPT so that the 43% tapps are electrolytic cap coupled to the cathodes

which are each grounded through a separate choke with CT.
Say each 1/2 primary is part A and part B, each with opposite phased signals.
Then UL tap from A goes to the screen in the tube with its anode to B, and UL
tap from B goes to screen in the tube with its anode to A.
So its cross coupling; the phases are right for CFB but its not as
wonderful as it may look because you have a very high amount of CFB,
say -70Vrms at the cathode and +168Vrms at the anode, so if you need 16V between
g1 and k to get the 238 total a to k signal volts then you need 86vrms of drive to the
grid.




BTW: With this continuing first PP EL34 project of mine, I don't like
UL at all compared to triode no matter how the output has been driven.
I can't imagine ever going back (at least not in hi-fi) despite the
power penalty.

I don't know anyone who has been able to detect a difference in the sound
between using triode or UL connected tubes where the amount of FB was
such that the Ro of the amps was the same.
Its all good.

Using 12.5% CFB with a pair of 6550 give an Ro without global FB of about 3 ohms,
somewhat lower than 40% UL which would give about 6 ohms using the same
turn ratio and a-a loading.
Global NFB is then required.

Pure triode without global FB will give Ro = about 1.5 to 2 ohms, and that's a bit
high,
so I like to use 12 dB of global FB, and all this became patently clear
when i tried to use EL34 in a pair of Quad II amps in triode.
The OPT winding losses in 8 ohm config is 17%, so Rout due to winding resistance
is 1.4 ohms eeven if the ra of the tubes was zero, so Rout with triodes fitted to Quad
II amps
without FB is dissapointing, bearing in mind the impedance ration of 4k : 8 ohms, so
the Ra-a
of the 2.5 k of ra-a of the tubes reduced to 4.8 ohms, to which we have to add the 1.4
ohms so
final Rout without any FB = a massive 6.2 ohms. This is reduced somewhat by the CFB,
but triode gain is low, so the amount of CFB with CFB = 10% is only around 3dB,
so Rout with CFB and no glbal FB is about 5 ohms, so the CFB merely compensates for the

winding resistance of the OPT.
With only 6dB of global, the bass was woolly, dynamics poor in comparison the the other
amp the guy owned
which ran smaller 6GW8 UL with 18 db FB.
The little 6GW8 10 watter was definately better than the 13 watt EL34 triode amps.
When i replaced the EL34 with Sovtek KT88 in triode and applied 12 dB of fb to allow
20 watts in class AB triode from these Quad amps they began to sing as well as the
smaller amp,
and the owner then also enjoyed the better headroom with the larger tubes.
The load for triodes is critical, and the 4k a-a for triode EL34 is too low;
one should have 10k.
But with KT88, 4k is OK.
Rout is below 1 ohm, just, and thd lower than the original Quad II.
The driver amp is all different, using a pair of twin triodes with SET input + LTP
rather than the pair of EF86.
I have since revised my assumption that triode drivers are always better and the last
pair of Quad II retained the two EF86, but were arranged with a long tail taken to
-400V
with a resistor, so the pair are operated as a true LTP rather than a floating
paraphase, and I gote extremely nice
sound using the original config of KT66, but with a **much** better power supply
plus the separate cathode biasing and bias balance adjustment.

You gotta weigh things up without too much prejudice, but
good sound can be had from triode amps but the best of it
is the result of hard work.

Patrick Turner.








Wessel

Wessel Dirksen wrote:

Hi RAT's:

Below are a few questions that don't seem to be specifically addressed
in the references I have:

1 When using multiple output tubes in parallel on each differential
side with cathode autobias, can you theoretically use one coupling cap,
and/or grid load resistance, and/or one grid stopper resistance for all
of them as a group? Seems likely. If so, would you practically want to,
or not want to, for any reason?

2 Has anybody used the Blumlein "garter" cathode bias method? I've seen
it mentioned but have not seen anyone expound on it. It seems like a
very nice and simple biasing method. Is it not so that once you are in
the general neighborhood of the desired idle current that relative fine
balance between both diff pairs is what it is all about?

3 Can OPT based cathode feedback be used in cathode fixed biased PP
output stages? I have only seen schematics where fixed bias is used and
the transformer leads are connected directly to the cathodes.

The one to 10 K, 1/2 watt resistors are in the grid circuits to prevent
parasitic oscillation at RF, since most power tubes still have lots of gain
at several MHZ.
The stray C & L in the wiring form resonant circuits at frequencies well
into the RF. The R is in the circuit so that Q is lowered & oscillation
prevented (we hope).

Cheers, John Stewart

Patrick,

Aha, I would like to try this because my OPT has 33% UL taps (as it
turns out) which are not being used. After giving this some thought
while waiting for your response, I had partly figured out that
decoupling and attenuation of FB would be necessary. But I am not sure
what you mean about the whole business of grounding with a choke
between the cathodes and CT.

1 Undoubtedly I'm overlooking something and am anxious to learn why,
but why is a choked connection to ground needed via the CT?
2 I understand the crossed UL leads and the decoupling bit, but how do
the cathodes, chokes, CT, and ground connect together in this setup
with UL leads as CFB?

Thanks again,

Wessel