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Old 26th Jan 2018, 12:37 am   #81
Bazz4CQJ
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Default Re: The Best GDO Ever Made?

I don't have any silver plating kit but I might manage to nickel plate it. I'm told that small workshops doing plating will sometimes throw small jobs in with big ones for a small fee.
B
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Old 26th Jan 2018, 9:34 pm   #82
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Arrow Re: The Best GDO Ever Made?

Reference posts 2 and 3: The reason why a choke is sometimes inserted into the cathode cct. of an oscillator. I did a bit of research & analysis on this and came up with the following reasons.

The attached drawings should be referred to.
Figure 1: this is the actual, physical cct.; its equivalent cct. is figure 2.
Stray capacitances are shown by dotted lines.

The idea of the choke in the cathode cct. is to introduce a very high impedance between the cathode and R.F. ground. To achieve this, the choke is made resonant with its own capacitance at the desired freq. of operation: a parallel tuned cct. of high Z is the result. The ratio of grid-cathode voltage to anode-cathode voltage is determined by the ratio (Ca-k / Cg-k)* of the inter-electrode capacitances of the valve, so it is relatively independent of the stray capacitance C1 and C2 to ground. Hence, the feedback is determined by the valve and not by capacity coupling of the valve itself to ground - and couplings to other parts of the circuit. The magnitude of the previously mentioned voltage ratio can be varied by de-tuning the choke from resonance.

Now all of that is fine for an oscillator that is required to operate on a specific freq. - or a over a narrow range of freqs. When a wide sweep in freq. is required with a given L and C, the choke is usually made resonant at the H.F. of the freq. range, since the cct. stray capacitances have a greater frequency-determining effect at H.F. than at lower frequencies. An alternative is to have switched L and / or C for a number of freq. ranges. In that case, different cathode chokes can be switched in accordingly, as per the selected freq. range.

* The voltage is proportional to the reactance, Xc; Xc is inversely proportional to capacitance, C. Hence, not (Cg-k / Ca-k) as might be expected on first reading.

Al.
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Old 26th Jan 2018, 11:02 pm   #83
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Default Re: The Best GDO Ever Made?

Many thanks for that Al. Did you note my comments in post #8 of the thread regarding the deliberate inclusion of a resistor of 10-20k in parallel with the cathode choke, and if so, any comment on that? Much of this finer stuff is "above my pay grade" (comment on BBC pay grades, avoided).

Do you have any comment re my guess that the RFC in the picture (a couple of posts ago) of the Model 59 head would be 200-300uH?

Bits are arriving for my M59-Homage project. As a minimum, I simply want that to work from 5-150MHz, so should be straight forward. The split-stator tuning cap I have have as a swing of ~ 4-45pF, a bit smaller than ideal (the M59 runs to 110pF) but apart from the swing, it's a nice component for this specific task and, crunching the numbers in Excel, looks like there need not be too many coils for my needs.

My numerous medical appointments wiped out the last week, but my root canal now seems fine, so I should be firing on at least three cylinders this coming week (don't ask if it's a 4 or a 6). A visit to Redditch is pencilled in - where is that Kevlar vest?

B
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Old 26th Jan 2018, 11:53 pm   #84
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Arrow Re: The Best GDO Ever Made?

Any resistor in parallel with the RFC in the cathode produces a parallel tuned circuit whose bandwidth is made wider by that R by lowering the Q of that tuned cct. A resistor in series has much the same effect, but that also changes the bias arrangement for the valve. So in either case, for an oscillator covering a wide range, there is an obvious trade-off. My analysis above refers.

In the cathode of the Model 59, I would expect that the inductance of the choke to be approximately determined by noting the highest freq. that the GDO goes to and estimating (or by measurement) the self-capacity of that choke, plus a small allowance for stray C at that point in the cct. The classic formula for a tuned cct. applies.

Above, you refer to a GDO tuning from 5-150MHz. In an earlier post, I made reference to a GDO that I built many years ago to a design by G3TYJ (featured in Practical Wireless). Haven't used it for years, but from memory, it uses 7 plug-in coils, a split-stator capacitor (25 pF + 25 pF ?) and an EF91. Tunes from < 1.8 MHz → 150 MHz+. I'll see if I can dig it out from the archive - and be a bit more specific. I used it extensively during the mid-1970s when building home-brew TX gear for 144 MHz and later for 432 MHz.

Al.
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Old 27th Jan 2018, 4:30 am   #85
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Default Re: The Best GDO Ever Made?

For the G3TYJ design (post #84), which was in the July 1966 issue of Practical Wireless see:

https://www.vintage-radio.net/forum/...9&d=1514811964

73 John
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Old 27th Jan 2018, 7:02 am   #86
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Default Re: The Best GDO Ever Made?

Thanks for that; '3TYJ certainly was another subscriber to the "Big Coil" school. I'll study his article before going much further, though I will be sticking with my Acorns . I recall that '3TYJ was a regular PW contributor and his various designs seemed popular - wonder if he is still around?

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Old 27th Jan 2018, 7:13 am   #87
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Default Re: The Best GDO Ever Made?

Nuvistors are also there.

There's no law which says that a GDO has to be based on a single device oscillator. I built one with an ALC-controlled oscillator, monitoring the gain control voltage to see the dip (well, peak in this case) as the oscillator fought the power loss.

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Old 27th Jan 2018, 7:50 am   #88
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Default Re: The Best GDO Ever Made?

Yes, I think it was an article on a Nuvistor GDO, in which he acknowledged Acorns, which first aroused my interest in the '59. I'm also curious as to what something like Wendy's analyser will do (that must have a few devices in it!). But currently, I want to pay simple homage to the 59 with a close derivative. And perhaps it has something to do that I've never built anything with an Acorn...and time may be running out . I think some of the most recent GDO designs (some of the recent kits?) may have "stretched" the concept quite a lot

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Old 27th Jan 2018, 2:38 pm   #89
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Arrow Re: The Best GDO Ever Made?

Ref. post #85: thank you, John.

That PW article covers much of the guessing that I made in my post about features in the one that I built. To mine, I did introduce one small change: every coil (except the VHF hairspring one) had a link winding that via the coil plug & socket connection, is connected to a BNC plug mounted on the case. To that, I attach a loop probe. That makes usage of this GDO a lot easier. A minor re-calibration of the frequency scale was necessary to accommodate the Z thrown across each coil on account of that modification.
However, for freqs. above 100 MHz approx., I would suggest that a separate VHF GDO would be a good idea. And building one for UHF would be a real challenge!

Al.
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Old 27th Jan 2018, 2:44 pm   #90
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Question Re: The Best GDO Ever Made?

Quote:
Originally Posted by Bazz4CQJ View Post
I think some of the most recent GDO designs (some of the recent kits?) may have "stretched" the concept quite a lot
Well the 'principle' of a GDO is really simplicity in itself - or so it seems to me. When you say 'concept', are you equating the two words?

Al.
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Old 28th Jan 2018, 6:15 am   #91
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Default Re: The Best GDO Ever Made?

To be quite honest, I have not taken a close interest in GDO's designed in the last 20-30 years, and so I am speculating about the extent to which people may have "evolved them". And of course, we now have things like antenna analysers, probably pushed to the fore by companies like MFJ and I've not taken a great interest in those. So, I guess mostly, the designs I have noted have been fairly bog-standard "Vintage" GDO's, even if they have had a FET, Nuvistor or a tunnel diode in them.

For myself, I am quite convinced now that the M59 was "the best GDO ever made ...at least up until the year 2000 (arbitrary)". Others are free to disagree , I'm not inclined to debate it further at this late stage. One single suggestion was made that the American PMR-10 was up with the best, but that is actually a M59 with bells on; a single Acorn triode with a moving coil meter and exact same tuning cap. But if we at least say that the 59 was exceptionally good, I'm still picking through bones to try pin down those factors which made it so. Some seem fairly apparent, other aspects may still be 'TBC'. When you look at pictures of it, the purpose-made tuning cap with the Acorn almost inside it, the folks who designed it must be on 10/10 ? Using modern components, and cramming in a DFC (DFM), the 'Homage' (M59-H) should be 'pleasing' and a memorial to the people at Measurements Corp..

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Old 29th Jan 2018, 4:47 pm   #92
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Arrow Re: The Best GDO Ever Made?

Boonton M59 G.D.O. To supplement the attachment in post #1, here is the operator's manual. Includes the full cct. diag.

Al.
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Old 29th Jan 2018, 7:34 pm   #93
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Default Re: The Best GDO Ever Made?

Thanks for that input Al, and I've had a related PM from John KC0G. Let me mess things up a bit for you both . Viist the blogpage http://blog.aa6e.net/2009/10/my-new-...el-59-gdo.html

The owner, Martin, gives links to manuals at two different locations. First link brings up a PDF of a "Measurements Corp" manual with a green cover (some had white, all seem to be dated 1947); go to the back and there are two schematics (one was probably taped in at some stage), both are labelled as Page 18. The first circuit has an identifier 'X-434-E' and this shows the 6k8 pairs (with an errata note permitting the use of single resistor of 15k value). When you look at this, you need to look at the value of R3 in the meter circuit is 270k.

Now the second schematic in the same manual is distinguished by the fact that somebody seems to have “highlighted” it with a 2” paint brush. This shows only lone resistors of 7k5 on both grid and anode pins. The value of R3 in the meter circuit goes to 100R. Now, take a look at the pictures on the head in Martin's blog; single resistor of green–blue-red (5k6) is visible. I've seen pictures of a 59 in the UK, and could only see 5k6's.

Note that in this Blog, the M59 GDO is referred to as the “Boonton”, which as an earlier American poster to this thread pointed out, is incorrect.

Now going on to the Manual which was printed by the Army for their use and looks totally different from the Measurements Corp. document. The Army versions shows only pairs of 6k8 and R3 is 270k.

You know, forum member Wendy has a signature “Must get out more” – and that bothers me some times; the word "anorak" hangs in the air .

Any profound insights on these issues will be most welcome.

B
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Old 30th Jan 2018, 12:06 am   #94
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Default Re: The Best GDO Ever Made?

My manual for serial # 7782, copyright 1947, fourth printing 1956, shows 6k8 resistors in series, and R3 as 270k. The drawing is identified as B-1978-Q, and has annotations re. default powers of resistors etc.

The army manual dated January 1959 shows 6k8 resistors in series. This is a nice manual, with lots of repair information.

The manual at:
https://www.pa3esy.nl/meetinstrument...rements59.pdfi s copyright 1947, but has no other dates. It is a later manual as the front cover states that Measurements is a McGraw-Edision Division. The drawing is again identified as B-1978-Q. The last page of the scan is for an errata dated 1960/08/19 which shows that the series resistors were changed to 5k6.

My guess is that the drawings found by Bazz4CQJ were from earlier versions of the instrument. I fully reserve the right to be wrong. The options seem to range from 7k5 single to 6k8 in series to 15k single.

I looked at the pictures at A6EE's blog, and saw one 5k6 resistor in the picture with the acorn valve on top. I am not convinced that it is a single resistor. I opened up the head on my unit. The connection between the two resistors is in mid-air. On mine that joint would not be visible with that particular camera angle.

I hope this helps.

These are really nice GDO's. As others have noted they have a large meter, regulated power supply, go to 400MHz, and have smooth tuning. They are also very nicely put together.

Geoff Arnold wrote a short (2 and 2/3 pages) article on the Measurements 59 for the Aug/Sept 1995 issue (no. 36) of Radio Bygones. He noted that the Model 59, and very similar successor, the Model 159 were produced from the late 1940's to 1985. In a January 1984 price list, the 2-400MHz head plus power supply cost $1819. All three heads plus a power supply cost $3493, and the carrying case another $284. In January 1984 the sterling dollar exchange rate was about $1.40 to the GBP. It fell to a minimum of $1.042 to the GBP on Feb. 26th 1985. These instruments were not cheap! BTW the schematic in the article shows 5k6 resistors in series.

There is a fairly steady stream of these instruments on that well-known auction site here in the USA. They can appear with the title "Magacycle meter" or "Measurements 59" or "Measurements 159". The quality varies enormously. Ideally the power supply and head should have the same serial no. I found one locally a few years ago. It came from an estate and had been treasured. They is hardly a mark on it. It is a keeper

73 John

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Old 30th Jan 2018, 1:53 am   #95
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Default Re: The Best GDO Ever Made?

Many thanks for that info John. So, it looks like I would do well to use a pair of 5k6 on the anode and grid of the Homage?

I'll see what I kind find on the 159; that had escaped me.

Non-believers may challenge the idea that these were the best GDO's ever made, but it's looking likely that they were the most expensive . The budget for the Homage is about GBP20, and much of that will go on the frequency counter.

B
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Old 30th Jan 2018, 3:17 am   #96
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Default Re: The Best GDO Ever Made?

Re. posts #93, 94 and 95, I have just noticed that the schematic with the single 7k5 resistors and R3 = 100 ohms has a 500uA meter. All of the other schematics have 200uA meters. My guess is that the one with the 7k5 resistors and 500uA was the earliest of those under discussion.

Good luck with the search for information on the 159. If you find anything, please report back.

73 John
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Old 30th Jan 2018, 5:02 pm   #97
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Default Re: The Best GDO Ever Made?

I just spent a short time looking for the 159; there's one in NX4Y's collection, with pictures, but I did not find any more technical data. I'm tempted to guess that it could have been the same device in a more modern box; black wrinkle paint gives way to instrument grey. Actually, it looked a bit like one of the Boonton RF Millivoltmeters.

I did try contact NX4Y via his website before Xmas but no reply. The last update on that site looks like 2008 and I think it mostly goes back some years prior to that.

B
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