patrick-turner;973523 Wrote:
John Stewart recommended we go to a site for HP archived info including
manuals and schematics........
Go to this link- www.hparchive.com
to see the HP 207A Wide Range Oscillator listed under the equipment
heading.
20 Hz to 20 KHz in one twist of the dial. Uses a modified Wein Bridge.
Cheers to All, John Stewart
The 207A is described as an audio sweep oscillator, which I assume means
the F starts at 20Hz and over a time period the F is smoothly changed up
to 20kHz, enabling the audio F response of filters and speakers etc to
- show quoted text -
be seen on a CRO screen. The 207A is at
http://www.hparchive.com/Manuals/HP-207A-Manual.pdf
I have not yet looked closely at the 207A schematic but there is a
typical 4 gang tuning gang in a Wien bridge with each gang having C max
= 500pf.
Its not a wide band instrument because it omits F below 20Hz and above
20kHz which prevents an audio engineer to examine the extremely
important behaviour of audio gear below 20Hz and above 20kHz.
The HP 209A is a more interesting wien bridge oscillator with multi
bands and 4Hz to 2MHz, but it uses all discrete bjts and j-fets. Hm, my
latest little Oskar Layta goes 1Hz to 2MHz, but its oscillator amp is
all tubed. Seems like HP couldn't seem to manage getting down to 1Hz,
but lemme tellya, its dead easy now with discrete parts, and 0.1Hz is
possible, and waves will have less THD than if you use the alterantive
multi-pin signal gene chip which simply uses a pot to control DC on a
pin chip to set the F, as in BWD161 function gene I restored&upgraded
recently. The BWD161 calibration isn't very good. They seek to give too
much too easily, and budget price and hence toylike quality that was
affordable by hobbyists.
Getting down to 0.1Hz with a tubed amp and Wien bridge oscillator
circuit may be difficult because the tube amp needs to be RC coupled,
and hence is bandwidth limited, so perhaps a 0.1Hz-1Hz and 1Hz-10Hz may
be best done with a phase shift oscilator, using R+C parts used in the
biasing of the tubes, as in many guitar amps for tremelo.
Patrick Turner.
All should read the information carefully before submitting a learned
reply. The HP 207A is in fact a sweepable oscillator. Not as one might
think but by mechanical means. Go back to the manual & read carefully.
As for the superior ideas given in Pat's response, it is well known
there is a very long road from a concept to a marketable product that
makes a profit. Lets put your oscillator to that test!
Cheers, John
I have no such intention to test my oscillator construction with a view to making a profit from sales after manufacturing say 1,000 units.
I'm not so sure my response was better ot worse than anyone else's but it seems the 207A was designed so you could manually alter F, and through such a wide range of F that you could gain a quick idea of where a -3dB point was on a filter response. The dial accuracy allowed pinpointing the -3dB point easy.
I made an audio sweep kit which was advertised in Electronics Australia, and it used a 555 chip and other chips to produce a variable sweep speed repeatedly. Unfortunately, it was rather useless because if you had the sweep going and on a CRO and showing the response of a filter, there was no blip marker for frequencie being displayed, so you could see the response, but not see at what F the -3dB pole happened. I rarely ever used that after making it. Its was useless with speakers as well.
But well before such toy type kits there were various types of sweeps, and what were called wobulators, which varied a range of F by perhaps using a tuning cap connected to an electric motor.
Today, its not to hard to make an audio gene with voltage controlled frequency, and apply a saw tooth wave whose rate can be altered with a range of switched F or a pot, and so the sweep rate can be slow or fast, hence response can then be displayed on the CR0, but you need to know the F range and where a certain F occurs when looking at the CRO, and that's the hard part the toy gear doesn't have.
I found pink noise testing and the switched tunable filter for 33 spot frequencies between 20Hz and 20kHz ( Q = 12 ) to be far superior to get meaningful results with speakers. But someting like the oscillator I've just made is far better for filters with R&L&C, and right up to 2MHz.
To get more accurate F readings than a dial can indicate, I use a chip based frequency meter which I also made using an EA kits from 1990's. It does 1Hz to 50MHz, enough for me. BUT, the F readings below 20Hz become unreliable because the digital readouts don't have decimal places for low F, so that 1Hz reading could be between 0.5Hz and 2Hz. so for calibration of 1Hz on my dial after trimming values of Wien netork I must count and time the waves and 10 waves over 10 seconds is 1Hz. The CRO can then be set for 1Wave = full screen width, then when 2 waves ocupy the same distance, you have 2Hz, and so on. By 20Hz, accuracy with decimals isn't important, and if I measure 20Hz without reading jitter down to 19 or up to 21, its near enough to 20Hz, and I am guided sufficiently well to proceed to engineer other stuff which works OK.
There is no need to be obsessive about having what can only be excessive accuracy.
I am building the new oscillator after re-furbishing 2 other BDW olduns purely as an exercize in retro hobby engineering, equivalent to making a decent wooden spoked wagon wheel but there are SFA carts requiring my kind of wheel.
Nobody can make a profit making test gear which uses vacuum tubes. There would be many ways of achieving a Wien bridge oscillator using only solid state, and there is probably an active electronic equivalent version of a wien bridge. I can imagine much brighter young minds than mine could brew up an oscillator making 7Vrms at 50 ohms Rout from 0.1Hz to 10MHz in 8 decade ranges and the electronics on a little board would cost 50c in China to have made. I cannot figure out how to even begin to use a Spice Simulation program, I am dumb as an ox with simulation software, and I could not even figure out how to diplay the functions of a simple RC filter, let alone a single bjt or triode. I tried, but my mind went blank, and the nerds who wrote the "how to use this stuff" section have ZERO communications abilities. But I don't need to CAD because my brain does it all for me in its guess and try method - until it works well and be proved to always work well with wanted stability and no spurious outcomes.
The point is, I happen to like Tubes, even wagon wheels, and i like steam engines, propeller driven aircraft, I like nice women who are faithful and who like to ****, and all manner of old fashioned things that today's youth is trying to leave behind as it too puts on the Agony and Style, just as that old song says. It was written during the Californian Gold Rush of about 1850. Humans get sick&tired of the same old thing, and replace stuff because the new stuff has more features and bells and whistles while costing nothing, weighing nothing, and being transportable and able to hook up to the wireless Internet.
So what chance for a tubed 6 band oscillator? Some day, when the house cleaner-outers clean out my house for an auction sale after I die, someone will see the oscillator and it may fetch $1. Then, if it is sold, it is likely to spend 20 years in some old-bar-studs's garage gathering dust because the bloke said he wanted to study electronics, but he never seemed able to get away from TV, wife, PC, grand kiddies, and 101 other meaningless ways to waste ones end time.
But then, life has no meaning. We like to think is has, when in fact it only may have once, or even twice ot more often when young, but when old, it ceases to seem to have meaning. Life's meaning is that Life Just Is, OK, so enjoy it while ya can, until interuptions force you to stop doing what you like while you still can. Once over 65, and retired, one joins the Army Of Irrellevants. The young have zero need of us at all, so it seems to me. We represent what they don't want to become. They are too occupied repeating our mistakes when we were younger. That's my larger view in stuff, but then there are exceptions to all summations, and we all can point to young ppl who value their elders, and for more than what they are worth in money and inheritances. But my pointing finger looks young because it points to so few exceptions.
I find that Farnarkling around in a shed has inherent sublime peacefullness..
I forgive anyone who uses a tube amp I made to listen to outpourings of one Lou Reed.
Patrick Turner.