Heath IG-18 IG-5218

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#21
I received an old (really old!) IG-18 today. The lamp itself is marked SYL 90V. I was able to cross-ref to a Sylvania 90MB, which is a T2 bayonet-mount bulb with the right envelope shape, 90 volts at 30mA, giving a hot resistance of 3k ohms. I pulled one lead of the lamp and its cold resistance is around 280 ohms. I say around because even the small current from the ohmmeter raised it to 290 ohms while I was measuring. This all seems to be in the right ballpark to me, even though the cold R seems low.
 
#22
I decided to take the old eBay IG-18 and revisit Reg Williamson's "greening." The results are here: IG-18 #11. Had a couple of surprises.

A second IG-18 arrived, and one or the other of the two will be the platform for using Bob Cordell's great state-variable oscillator circuitry, for which I did a board layout. Don't have a timetable for that yet.

Best,
Dick Moore
 
#23
Regarding your problem with the darlington modification, Williamson retracted that change in the fourth issue of 1971. He recommended going back to a single transistor. Other letters in the Audio Amateur in subsequent issues confirmed the Darlington problem.
 
#26
Well, audiohead, you've got the stuff. How do the HP239 and the SG505 compare, when measure with the AP1? I've never had either one. Interesting variation there -- the SG505 is a "phase shift" oscillator, i.e. a state-variable design like the Cordell. The HP239 is a Bridged-Tee very much like the IG-18, except the Bridged-Tee filter has range caps in a 100:1 ratio, compared to the 10:1 ratio of the IG-18. This means that the filter selectivity is much higher in the 239, which I'm sure helps distortion and stability. And the 239 has a JFET AGC control very similar to those in Cordell's design and in the SG505. It would be fairly straightforward to convert the IG-18 to a 239 using the resistor frequency switches and dual pot, and re wiring the range switch to use 8 caps instead of 5. Then redo the power supply for +/- 15V and use a 5534 for the oscillator device and for an output buffer amp, and maybe a couple of 5534s or OPA134s or LM318s for the AGC loop amps/integrators.

Not long ago, I sold most of my gear including a Cordell oscillator and THD analyzer, and those boxes were just very good, with residuals less than 5ppm at 1kHz. Then I got to missing them and decided to get another IG-18 to use as the base for the Cordell oscillator as I had done before. Then I got the second IG-18 fairly cheaply, so decided to re-visit the greening. There's still some playing to do there. The attraction of these oldies but goodies are the switches, and precision caps and resistors that make it easy to do what starting from scratch these days is not, given the relative absence of mechanical parts out there.
 
#27
IG-18 never worked right for me since I bought it in mid 70's while in college. Never bothered to do the mods in TAA.

505 and 239 work every time.
I use them quite often.

I still have a sound tech 1700B, I grew up with it. The generator takes too long to settle in low distortion mode.

SG-505 with its own mainframe TM-502 is too deep, at times I do not have enough room on my work bench to use it. The second one shares the TM-504 mainframe with AA-501.

My favorite AF gen is Leader LAG-125B. It is small and reliable, though the THD+N is only -70 dB.
 
#40
FYI Just got some 90MB lamps in from Allied.

As expected the resistance did change as a result of test current. The Fluke 8050's 2K Ohm range uses a 1 mA current which is sufficient to cause a positive TCR.

Though it provided less display precision I used the 20K scale where the test current is ~100 uA. This didn't seem to shift the resistance appreciably.

The first 90MB measured ~365 Ohms cold on the 20K scale.

I measured 5 units total with the range being 354 - 366 Ohms.
 
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