[MarkX]

I am a hobby guitar tube amp builder and student of circuit analysis
for tube guitar amps.I need some help with the reason for the
capacitor to ground from the grid of the grounded grid part of the
long tail pair inverter.I have studied quite a bit, but since I don't
work with it regularly I will need a clear explanation,more than one
sentence and not on the level of an electronic engineer.I couldn't
design a long tail pair, but I probably understand about 90% of its
basic functioning. Thanks

[Ian Iveson]

MarkX wrote:

I am a hobby guitar tube amp builder and student of circuit
analysis
for tube guitar amps.I need some help with the reason for
the
capacitor to ground from the grid of the grounded grid
part of the
long tail pair inverter.I have studied quite a bit, but
since I don't
work with it regularly I will need a clear
explanation,more than one
sentence and not on the level of an electronic engineer.I
couldn't
design a long tail pair, but I probably understand about
90% of its
basic functioning. Thanks

To ground the grid for AC, but not for DC, where the DC
voltage of the grid must be other than ground for the
purpose of bias. That's one possible answer.

It's not clear what you think you don't know. There are
umpteen variants of the LTP and, in guitar amps especially,
the "grounded" grid is used to accept feedback from the
transformer secondary, so it's not grounded.

Best thing would be to post a circuit diagram of the
particular LTP you mean. There are many guitar amp schemos
he

http://tdsl.duncanamps.com/schematics.php

Find one like yours (a Marshall JMP would be a pretty
archetypal example) and post a link to it. Then we'll know
what we're discussing.

The related question of why a pure grounded grid needs
grounding isn't as simple as it may seem, maybe. If you
drive a triode from its cathode, and leave the grid floating
(with no connection), what happens? Why and in what way does
the grid follow the signal?

Anyway, a LTP amplifies the difference between the signals
at the two grids, so in order to act just as an inverter,
there should be your signal on one grid, and zero signal on
the other, which means it must be connected to signal
ground.

Ian

[Ian Iveson]

There are many guitar amp schemos he

http://tdsl.duncanamps.com/schematics.php

er...choose "schematics" from he

http://www.duncanamps.com/

Duncan would probably argue that American is the language of
the internet, but the use of American idiom by British folk
really gets up my nose.

So there.

Maybe he's Scottish and doesn't care.

Ian

[Ian Bell[_2_]]

MarkX wrote:
I am a hobby guitar tube amp builder and student of circuit analysis
for tube guitar amps.I need some help with the reason for the
capacitor to ground from the grid of the grounded grid part of the
long tail pair inverter.I have studied quite a bit, but since I don't
work with it regularly I will need a clear explanation,more than one
sentence and not on the level of an electronic engineer.I couldn't
design a long tail pair, but I probably understand about 90% of its
basic functioning. Thanks


The output signals of a long tailed pair (the anodes) are in anti-phase
which is just what you want from a phase splitter. The value of the
output signal is the difference between the input signals (the grids).
Since you often only have one input signal you want to make sure the
other signal input is zero which you do by connecting its grid to ground
via a capacitor.

HTH

Cheers

Ian

[Engineer[_2_]]

On Aug 24, 8:07*am, MarkX wrote:
I am a hobby guitar tube amp builder and student of circuit analysis
for tube guitar amps.I need some help with the reason for the
capacitor to ground from the grid of the grounded grid part of the
long tail pair inverter.I have studied quite a bit, but since I don't
work with it regularly I will need a clear explanation,more than one
sentence and not on the level of an electronic engineer.I couldn't
design a long tail pair, but I probably understand about 90% of its
basic functioning. Thanks

It's to ground the grid to the signal while allowing its DC potential
to be at the higher, shared cathode potential from the large coupling
resistor (or a constant current source if used) and DC coupling from
the prior stage. Thus, this triode section is cathode driven for no
phase inversion, giving P-P drive to the o/p stage.
Cheers,
Roger

[Phil Allison[_3_]]

"MarkX"

I am a hobby guitar tube amp builder and student of circuit analysis
for tube guitar amps.I need some help with the reason for the
capacitor to ground from the grid of the grounded grid part of the
long tail pair inverter.I have studied quite a bit, but since I don't
work with it regularly I will need a clear explanation,more than one
sentence and not on the level of an electronic engineer.


** That capacitor is what " grounds " the undriven grid.

Without it, that grid will have the same audio signal on it as found at the
common cathode bias point - ie where the other end of the grid bias
resistor connects.

This would result in the second half of the LTP being NOT driven with
signal.

Consider a LTP where the common cathode resistor is supplied from a negative
DC rail of say -100 volts. Then there need only be one cathode resistor and
the undriven grid may be connected directly to ground.



....... Phil

[MarkX]

On Aug 24, 7:07�am, MarkX wrote:
I am a hobby guitar tube amp builder and student of circuit analysis
for tube guitar amps.I need some help with the reason for the
capacitor to ground from the grid of the grounded grid part of the
long tail pair inverter.I have studied quite a bit, but since I don't
work with it regularly I will need a clear explanation,more than one
sentence and not on the level of an electronic engineer.I couldn't
design a long tail pair, but I probably understand about 90% of its
basic functioning. Thanks

Thanks for all the excellent answers.That is exactly what I was hoping
for.All were clear and not something out of Radio Designers Handbook,
4th.I was asking about a LTP without neg.feedback.I'll be back with
another question about the neg feedback and will reference a circuit
that can be seen at a website. MarkX

[MarkX]

On Aug 24, 8:14�pm, MarkX wrote:
On Aug 24, 7:07 am, MarkX wrote:

I am a hobby guitar tube amp builder and student of circuit analysis
for tube guitar amps.I need some help with the reason for the
capacitor to ground from the grid of the grounded grid part of the
long tail pair inverter.I have studied quite a bit, but since I don't
work with it regularly I will need a clear explanation,more than one
sentence and not on the level of an electronic engineer.I couldn't
design a long tail pair, but I probably understand about 90% of its
basic functioning. Thanks

Thanks for all the excellent answers.That is exactly what I was hoping
for.All were clear and not something out of Radio Designers Handbook,
4th.I was asking about a LTP without neg.feedback.I'll be back with
another question about the neg feedback and will reference a circuit
that can be seen at a website. �MarkX

If you put a sine wave where it belongs on the 1st section, could you
see a sine wave across the capacitor that gives the grounded grid 2nd
section it's AC ground?

[Phil Allison[_3_]]

"MarkX"

If you put a sine wave where it belongs on the 1st section, could you
see a sine wave across the capacitor that gives the grounded grid 2nd
section it's AC ground?


** Oh dear ...

Anyone see that last Scud missile ???




....... Phil

[Ian Bell[_2_]]

MarkX wrote:
On Aug 24, 8:14�pm, MarkX wrote:
On Aug 24, 7:07 am, MarkX wrote:

I am a hobby guitar tube amp builder and student of circuit analysis
for tube guitar amps.I need some help with the reason for the
capacitor to ground from the grid of the grounded grid part of the
long tail pair inverter.I have studied quite a bit, but since I don't
work with it regularly I will need a clear explanation,more than one
sentence and not on the level of an electronic engineer.I couldn't
design a long tail pair, but I probably understand about 90% of its
basic functioning. Thanks
Thanks for all the excellent answers.That is exactly what I was hoping
for.All were clear and not something out of Radio Designers Handbook,
4th.I was asking about a LTP without neg.feedback.I'll be back with
another question about the neg feedback and will reference a circuit
that can be seen at a website. �MarkX

If you put a sine wave where it belongs on the 1st section, could you
see a sine wave across the capacitor that gives the grounded grid 2nd
section it's AC ground?


No, because it is grounded for ac so there will be (almost) no signal
across it.

Cheers

Ian

[Bret L]

On Aug 25, 6:48*am, MarkX wrote:
On Aug 24, 8:14 pm, MarkX wrote:



On Aug 24, 7:07 am, MarkX wrote:

I am a hobby guitar tube amp builder and student of circuit analysis
for tube guitar amps.I need some help with the reason for the
capacitor to ground from the grid of the grounded grid part of the
long tail pair inverter.I have studied quite a bit, but since I don't
work with it regularly I will need a clear explanation,more than one
sentence and not on the level of an electronic engineer.I couldn't
design a long tail pair, but I probably understand about 90% of its
basic functioning. Thanks

Thanks for all the excellent answers.That is exactly what I was hoping
for.All were clear and not something out of Radio Designers Handbook,
4th.I was asking about a LTP without neg.feedback.I'll be back with
another question about the neg feedback and will reference a circuit
that can be seen at a website. MarkX

If you put a sine wave where it belongs on the 1st section, could you
see a sine wave across the capacitor that gives the grounded grid 2nd
section it's AC ground?

Good question. Why don't you do the experiment and see?

[Ian Iveson]

If you put a sine wave where it belongs on the 1st
section, could you
see a sine wave across the capacitor that gives the
grounded grid 2nd
section it's AC ground?

Still trying to work out exactly what you don't understand.

You've made Phil grumpy, probably because you've already
been given the answer before you asked, assuming you know
what "ground" means. We say that the capacitor is there to
ensure that there is no signal on the grid, and then you ask
if there is a signal on the grid. We wonder quite what part
of "no signal" you don't understand.

Basically, signal ground is the 0V reference from which
signals are measured. A point is considered to be grounded
if it is connected via a very low impedance path to ground
so that in normal circuit operation *no significant voltage
can be present at that point*.

The very low impedance connection in the case of your
imaginary grounded grid stage is the capacitor from grid to
ground. In order to be very low, its impedance at audio
frequencies must much less that the source impedance of the
grid. In most real circuits there will be other stuff
connected to the same grid, reducing the source impedance
seen by the capacitor.

Now, it could be that you don't understand:

* How to calculate the impedance of a capacitor.

* What "ground" means.

* Ohm's law

* What "source impedance" means.

So here's an example question:

** Let's say you drive the cathode of your "grounded grid"
stage with a 1kHz signal such that the grid, when not
connected to anything, develops a voltage of 1V. Assuming
the grid itself has an impedance of 1 Megohm, what voltage
will be on the grid when it is connected to ground with a
10nF capacitor? What will it be if the experiment is
repeated at 50Hz? What value of capacitor would you choose
for this application?

Somebody's going to tell me that's a stupid question but I
don't care as long as it's answerable.

Ian

[MarkX]

On Aug 24, 7:07�am, MarkX wrote:
I am a hobby guitar tube amp builder and student of circuit analysis
for tube guitar amps.I need some help with the reason for the
capacitor to ground from the grid of the grounded grid part of the
long tail pair inverter.I have studied quite a bit, but since I don't
work with it regularly I will need a clear explanation,more than one
sentence and not on the level of an electronic engineer.I couldn't
design a long tail pair, but I probably understand about 90% of its
basic functioning. Thanks

I had to seek medical attention from Phil's right cross but he is very
knowledgable as are all of you, and my follow up question was such
that it looked like I learned nothing from the first round of
answers.I was trying to see if even a trace of signal could be on the
grounded grid even though the A/C ground was there .I do understand
Xc .The most recent amp I built was modeled on a Fender 1955 Pro 5E5,
allegedly used by Buddy Holly.That was built three years ago, so I
haven't worked with or thought about electronics for a long time and
it shows.As I recall,that amp has a concertina inverter/cathodyne
maybe it's called so I couldn't experiment with it.I very much
appreciate all the information,including the follow up information.I
really did learn a lot from each person contributing.

[Patrick Turner]

On Aug 26, 9:16*pm, MarkX wrote:
On Aug 24, 7:07 am, MarkX wrote:

I am a hobby guitar tube amp builder and student of circuit analysis
for tube guitar amps.I need some help with the reason for the
capacitor to ground from the grid of the grounded grid part of the
long tail pair inverter.I have studied quite a bit, but since I don't
work with it regularly I will need a clear explanation,more than one
sentence and not on the level of an electronic engineer.I couldn't
design a long tail pair, but I probably understand about 90% of its
basic functioning. Thanks

I had to seek medical attention from Phil's right cross but he is very
knowledgable as are all of you, and my follow up question was such
that it looked like I learned nothing from the first round of
answers.I was trying to see if even a trace of signal could be on the
grounded grid even though the A/C ground was there .I do understand
Xc .The most recent amp I built was modeled on a Fender 1955 Pro 5E5,
allegedly used by Buddy Holly.That was built three years ago, so I
haven't worked with or thought about electronics for a long time and
it shows.As I recall,that amp has a concertina inverter/cathodyne
maybe it's called so I couldn't experiment with it.I very much
appreciate all the information,including the follow up information.I
really did learn a lot from each person contributing.

There are many schematics of power amps using LTP stages which have
one live grid input, and two anode outputs with opposite phase. See my
website at http://www.turneraudio.com.au

Now there are a variety of ways of setting up up the LTP and most
involve two triodes and a single B+ supply of say +400V. The anodes of
the two triodes will be at about +250V, and the commoned cathodes at
about 100V. Each grid will have the same Vdc bias of slightly lower
Vdc than the common Ek Vdc.

One grid has a cap to 0V which effectively grounds this grid at audio
frequencies, but not at dc voltages.

But you could have both grids biased at 0V and have a negative B-
voltage supply for connection to the common cathode resistance. Or the
common cathode R could be replaced by an active circuit arrangement
called a constant current sink, CCS, which offers the common cathodes
a constant flow of current regardless to cathode voltage changes. I
prefer this method of LTP set up.

Consider the LTP with two triodes drawn side by side with the left one
being V1 and with an active grid input and the right one being V2 and
with its grid grounded by a cap to 0V.

The action of the LTP is as follows:-

Consider a positive going voltage input signal applied to the live V1
grid input.

This turns on more current in V1, and the anode voltage becomes lower,
or more negative, because more current flows in the RL.

The cathode voltage of both triodes V1 and V2 will rise slightly, but
always less than the applied grid voltage to V1.

The rise in cathode voltages causes the relative voltage between
cathodes and V2 grid to become more negative, ie, the action is as if
a negative going grid voltage was applied to V2. So less current flows
in V2. Therefore the anode voltage of V2 rises, ie, becomes more
positive.

Now thus you have two phases of signals with similar amplitude being
produced by the ONE input signal. And there is considerable voltage
gain and good cancelation of 2H distortion so the LTP offers a way to
make a stage offering good voltage gain and low THD.

If a CCS is used as the common source of current to both V1 and V2
cathodes, the two anode output voltages will be equal in amplitude.

However, there are some finer points to remember when using the normal
LTP which usually uses a simple common cathode resistor. In this case
the anode load of V1 must be adjusted to a lower value than the anode
load for V2 or else you will get more anode output voltage at V1 anode
than at V2 anode.
This is because the current change in V1 equals the current change in
V2 *plus* the current change across the common cathode Rk.

The higher the gain of the triodes used and the larger the value of
the common Rk, the better the natural balance becomes of output
voltages and with 12AX7 to get accurate balance the anode R load
values need not be very different.

Patrick Turner.