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Old 14th June 2011, 05:10 AM   #21
PB2 is offline PB2  United States
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Hi Dick,

I've read over your pages and find them very interesting, nice work!
I was wondering if you've seen "Low Distortion Oscillator" by Linsley Hood
that was published in Wireless World 1977? He uses a beta multiplier on
the VAS, in fact it is a MPSA14 darlington and a current source load.
I wondered if you tried a current source or bootstrap for the VAS load
in the IG-18.

Also wondering if you've looked at composite OP amps to use in place
of the Harris for your #3 mod. Here's a Burr Brown app note about it:
focus.ti.com.cn/cn/lit/an/sboa002/sboa002.pdf
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Old 14th June 2011, 07:19 AM   #22
richiem is offline richiem  United States
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@PB2 -- Yes I actually have a copy of that article. In IG-18 #1, I do use a FET for a constant current source load on Q3, and it helped, but beta multiplying caused me problems at higher frequencies, especially near 100kHz; Miller C is a problem. I haven't looked at the Linsley-Hood paper for a while, but I think that he intended that design for maybe 10kHz, tops.

As to composite opamps, I haven't -- thanks for the link, I'll go have a look. One of the appeals of the 239A design is the sparse number of parts. I'm hopeful about the OPA1641....
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Old 14th June 2011, 03:46 PM   #23
PB2 is offline PB2  United States
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Yes the 239A does look quite simple, very nice, and being a production unit it hopefully has all the potential bugs worked out. It seems to do a bit better than the Linear AN-43 Fig. 47 design, yet the Fig. 47 design uses another op amp for common mode suppression for a virtual ground at the midpoint of the bridge. It also uses a composite made up of an LT1115 with an LT1010 output buffer. Looks like Figs. 47 and 48 are swapped in that app note.

Just noticed that you measured far better than spec for the 239A and I wonder if they are all that good.
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Old 14th June 2011, 05:26 PM   #24
PB2 is offline PB2  United States
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Quote:
Originally Posted by jackinnj View Post
Here's the amplifier section of the later version of the Boonton 1120 -- works quite well up to ~140kHz -- you would have to beef up the power supply to use it:

Click the image to open in full size.
Those 44H11s are 10A 50W devices, wow that is nearly a power
amp - the large resistors in the emitter leg certainly limit the current.
I'd think that those outputs would have excessive capacitance being
so large. Drivers are only 100mA max, I'd think it would be better
to use .5 A devices such as MPSA92 and comp.
The output stage is the same topology as the Tiger amps with
output protection.

R31 seems to be missing on the other side.
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Last edited by PB2; 14th June 2011 at 05:39 PM.
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Old 14th June 2011, 08:06 PM   #25
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@PB2 -- The 239A and 339A oscillators are nearly identical -- I have a 239 and a 339, and they are indeed nearly identical in every way, except the 239 has the advantage of a switched ground for floating AC coupled ground or direct chassis ground, which is a big advantage, depending on the other gear being used. I have a friend with a 339, and he gets approximately the same results I do, so I would say "they're all like that."

As to the KH balanced output amp, I have a hard time understanding why that level of overkill was felt to be necessary, but then I don't know what the max output level is on the 1120, nor do i know how these things were actually used in military or industrial applications....
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Old 14th June 2011, 10:31 PM   #26
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Did you consider that all that's needed to run op amps on a single supply is to DC bias the inputs at about half the supply? That could be done in the IG-18 just by leaving in the bias pot circuitry. Essentially just replace all the transistors with say for example the LT1115 and LT1010 in Figure 47 of that App note, or perhaps leave in the emitter follower to buffer the OP amp. Maybe I'll take a look at something like this.

Wondering why the output was biased at 29V and not half of the supply, perhaps to heat the bulb to the proper DC operating point?
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Old 14th June 2011, 11:06 PM   #27
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Well, that still means throwing away a lot of power to get the supply down to where the opamps are happy; I personally like using split supplies for opamps because in my experience they are quieter, and biasing uses more parts. Just a preference.

An easy way to use the Heath transformer is to wire the primary for 240V, which gets the secondary output down where using a FW bridge is a happy thing -- I never did this IG-18 #2 because I used a lower noise toroid transformer for my rebuild, but I will try this with the 239A mod I'm planning for IGT-18 #3 and see if the Heath transformer is quiet enough to let the oscillator do really well. It's an open question at the moment.

In the original, I've found you can't trust Heath's schematic voltages; don't know why. Anyway, the bias pot setting changes the bias point....
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Old 15th June 2011, 12:28 AM   #28
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But the single supply can be the full V+ to V- value so 30 to 44V depending on the part. It is better to go split I'm sure, and that is a neat trick of using the 220V winding. But +/-22V parts should work fine I expect.
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Old 23rd June 2011, 04:19 AM   #29
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Interesting but looks like a lot of work:
Modifications: Heathkit IG-18 Audio Generator

Source for Harris/Intersil HA3-2625:
http://www.newark.com/intersil/ha3-2...p-8/dp/06F5228
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Old 11th July 2011, 08:36 PM   #30
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I dusted off my old IG-18 and recapped it, the only mod being the 100uF upgrade for C5.

When I first looked at the schematics, years ago, a few things jumped out at me.
First the output stage seems to be class B or very low bias class AB.
Second the feedback network, and DC bias network have a very low impedance. This did not make sense at first but then I realized that the lamp has a low impecance and probably needs a good amount of current to warm it up. It seems to me that the output stage should be biased for class A to get the output devices into their best range for beta, Ft, and for the best linearity.

I've always been curious to measure the output stage idle current by way of the drop across R13 and R14. The bias on this unit is set per the manual with 22V at the emitter of Q4. I set the feedback for 10V RMS with the output set full, then put a .1uF cap from the wiper of the feedback pot to ground to stop the oscillation in order to measure the DC operating points. This worked as long as the frequency was set to 10 - 100 KHz. Oddly enough, I measured 62mV across R13 for 4.13 mA and 0.000 V across R14 indicating that the amp is clearly in class B. But the 4.13 mA has to be going somewhere and looking closer at the schematics it can be seen that the square wave generator is DC coupled with a path to ground. The voltage across R15 indicated that it was taking 1.74 mA and the majority of the remaining current must be going through the lamp, feedback pot, and to the bias pot since the base of Q1 and Q2 only require microamps to provide their bias point.

While the unit works, something does not seem right. Looking closer we can see that the base of Q2 is biased from the top of the feedback pot - pin 1 and the base of Q1 from the other side of the lamp L1 by way of RX, RY, and RZ. Ideally, for balance they should be biased to the same DC voltage but here they are not since there is some DC drop through the lamp.

I also measured the collector currents in Q1 and Q2 and they are far off balance.

My gut feeling from looking at these schematics years ago was that while it might work, it looks to be seriously flawed and I think this is confirmation. I don't think that there is a miswire or failed component in this unit.

Perhaps I should start a new thread.
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