A
amdx
Guest
On 9/30/2022 9:32 AM, John Larkin wrote:
don\'t expect much
in the way of a circuit required. I was disappointed it doesn\'t come in
a quad.
 If there is a second round I might try it.
                    Mikek
 The H11F1M has been used, but I didn\'t get a schematic for it. IOn Fri, 30 Sep 2022 08:29:07 -0500, amdx <amdx@knology.net> wrote:
On 9/30/2022 12:01 AM, John Larkin wrote:
On Thu, 29 Sep 2022 23:23:18 -0500, amdx <amdx@knology.net> wrote:
On 9/29/2022 5:42 PM, John Larkin wrote:
On Tue, 27 Sep 2022 12:46:28 -0500, amdx <amdx@knology.net> wrote:
On 9/27/2022 3:59 AM, piglet wrote:
On 24/09/2022 13:44, amdx wrote:
On 9/24/2022 6:47 AM, piglet wrote:
On 24/09/2022 11:58, amdx wrote:
On 9/24/2022 1:11 AM, piglet wrote:
On 24/09/2022 00:33, amdx wrote:
  On one of the groups I monitor, it was ask if the tube circuit
shown below could be bootstrapped to raise the input impedance,
especially at the higher frequencies where it drops pretty low.
(purpose is to reduce loading on the LC being measured--Q meter)
  Can you give a little detail on how you would implement it? And
what happens to gain and frequency response?
When was bootstrapping introduced, the word started in the 1880s,
but when was it first used in an electrical circuit?
I did manage to find a bootstrapped tube circuit, but it didn\'t
use a 100M? grid resistor, Is that a detriment to adding
bootstrapping?
  I made my proposed idea on the right side.
Hmm, the problem is more the tube input than the 100M?, does
bootstrapping help that?
  Might be barking up the wrong tree.
Â
https://www.dropbox.com/s/in6no3nvw0qnie7/Boonton%20260A%20Tube%20bootstrap%20question.jpg?dl=0
                            Thanks, Mikek
Did you see the value of C302? I think V301 is being used in the
so called \"infinite impedance\" detector configuration. The cathode
voltage is not the buffered grid ac input but instead is a dc
level representing something between the peak and average ac
input. So AC bootstrapping that circuit not possible. I guess you
could insert a whole new hi-z buffer between tuned circuit under
test and detector but in practice it would be difficult to get it
to be an improvement?
piglet
  I thought I\'d seen it referred to as an \"infinite impedance
detector\", but don\'t find it in the manual.
I do see this line in the manual and don\'t understand how transit
time loss in the voltmeter
tube causes a shunt resistance across the measuring circuit at High
Frequencies.
Can someone provide a simple explanation?
\"QVOLTMETERCONDUCTANCE.
Anotherinternalparameterwhich causestheindi-
catedQtodeviatefromeffectiveQ,atbothvery low
andveryhighfrequencies, is theinputconductanceof
theQvoltmeter circuit.Atvery low frequenciesthis
conductance consistsofa 100megohmgridleak resistor
inparallelwiththeinternallossesofthevacuum tube.
Atveryhighfrequenciesthetransittime lossinthevolt-
metertubeshuntstheresonatingcapacitorandintro-
duces ashuntresistance acrossthemeasuring circuit\"
Someone has developed a single fet circuit that does an OK job.
I always wanted to try this circuit,
https://www.dropbox.com/s/wvwkw2iapf4ufd7/High%20impedance%20input.jpg?dl=0
  But have not got to it yet, to much computer time and not enough
bench time.
                                      Mikek
The transit time part comes into it like this: due to the electron
travel time there is a short delay before grid voltage changes
during the positive peaks get to influence the cathode voltage. At
very high frequencies this increased loading at the peaks gets more
significant.
The 535A tube is quite large and looks like it was built for low
inter-electrode capacity and low leakage. For VHF use a smaller tube
like an acorn device may have had shorter delay times.
Looks to me like that bootstrap all nodes fet buffer came straight
out of a late 1960s-early 70s NatSemi app note.
piglet
  Yes, it did, but, the question is will it work in place of the tube?
                                             Thanks, Mikek
The touted capacitance of the bootstrapped fet is 0.25pF which at
50MHz is reactance of 13kohm.
 I would think any capacitance just adds to the resonating capacitance,
so just need to calculate in the loss of the capacitance, not the
capacitance.
Since your main interest is around 1.5MHz you perhaps needn\'t worry a
lot about behavior at 50MHz?
 That is very true, but as the much maligned John Larkin said, \"this is
a discussion group\" so, I\'m having the topic discussed!
Mikek
I\'m not maligned by anybody that matters. That\'s by definition.
 Agreed and, I looked it up before I used it, I thought it fit well.
\"If you describe someone or something as much-maligned,
you mean that *they are often criticized by people,
but you think the criticism is unfair or exaggerated because they have
good qualities too*.\"
 The people that criticize you have a political opinion and they have
to go after you personally
rather than argue their point. That don\'t like it when you have a good
point, so they malign you.
                                            Mikek
Because they can\'t design electronics.
Neither can I, but I\'m not maligning you!
 btw, I got the 1000 Hz bandpass to work, it was tuned to 1008 Hz.
I wanted it to use to set the focal point of a parabolic mic.
 I could not get a clean voltage measurement because of all the other
environmental sound. I added the 1008 Hz bandpass filter between my
preamp and amp, then, with an app on my phone that I put across the room,
I generated a 1008Hz sine wave, and had a very clean,
stable waveform on my scope. Easy peasy, I moved the mic about 3\"
further away from where I thought the focus was, and doubled the Voltage
output.
 4 hrs building the bandpass filter for a 3 minute adjustment!
 Also, I have never seen this on a parabolic mic, I put a small funnel
on the mic
this also increased my output voltage.
 I used a sensitive, low noise AOM-PU5024 mic,
https://api.puiaudio.com/file/8a1a7f37-cd74-49b0-81f3-f74090655669.pdf
 A low noise TS-472 preamp,
https://tinyurl.com/56zv2czx
and this \'very neat\' little headphone amp to drive the headphones. 3.7\"
x 2\" x 0.5\".
https://www.amazon.com/dp/B07MHRBMBY?psc=1&ref=ppx_yo2ov_dt_b_product_details
Which I was able to tap into the power switch to drive the preamp.
 I used a $19 (slightly scratched, returned) squirrel baffle (16\") as
the parabolic reflector, I can always upgrade to
a $135 22\" parabolic dish, but no.
 Totally worthless project, but it was fun to figure out.
 Next up, a photochopper replacement for my HP3400A, I finally got a
schematic, it uses a DG403 and 555 for 90Hz drive signal.
                              Mikek
There are some great optocoupled solid-state relays around. The input
side is LEDs and the output is mosfets, zero offset voltage.
don\'t expect much
in the way of a circuit required. I was disappointed it doesn\'t come in
a quad.
 If there is a second round I might try it.
                    Mikek
