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Author Topic: Power ratings on 1960s transceivers  (Read 17682 times)
W8JI
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« Reply #30 on: September 06, 2011, 09:14:48 AM »

The RCA Transmitting Tubes handbook has four 811 amp in it with 220pF capacitors from grid to ground BUT it does have neutralisation. But I do think they are too small.

I found that scnematic in TT-5, dated 1962. It is schematic 5-13. It does not appear in TT-4. The amp is for 80-10, so the caps may be adequate.

220 pF is 206 ohms on 80 meters. The grid impedance is less than 50 ohms when conducting heavily, and higher with less G-K voltage.

How could placing 200 ohms of reactance in series with a time-varying resistance that gets less than 50 ohms over part of the cycle be adequate bypassing?

High reactance grid bypasses add a level of non-linearity to the system. Grid impedance varies greatly over the RF cycle, voltage division is obviously not constant over the RF cycle. It is also not constant with frequency. It is certainly not negative feedback like Collins and Orr said.

It is the polar opposite of what we actually want. I think it is an inexcusable design error. They acted like the tube has infinite grid-cathode impedance. Obviously they forgot it was a sub-2 class that draws grid current over a large portion of the RF cycle, with a wildly varying G-K impedance. 

73 Tom
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N2EY
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« Reply #31 on: September 06, 2011, 09:46:07 AM »

The RCA Transmitting Tubes handbook has four 811 amp in it with 220pF capacitors from grid to ground BUT it does have neutralisation. But I do think they are too small.

I found that scnematic in TT-5, dated 1962. It is schematic 5-13. It does not appear in TT-4. The amp is for 80-10, so the caps may be adequate.

220 pF is 206 ohms on 80 meters. The grid impedance is less than 50 ohms when conducting heavily, and higher with less G-K voltage.

How could placing 200 ohms of reactance in series with a time-varying resistance that gets less than 50 ohms over part of the cycle be adequate bypassing?

Ya didn't read far enough, Tom.

The schematic I found in TT-5 shows .01 uF grid bypasses, not 220 pF. That's less than 5 ohms per bypass, and there are four of them, one for each grid.

The TT-5 schematic is identical to a 1961 QST article by K6SNO.

High reactance grid bypasses add a level of non-linearity to the system. Grid impedance varies greatly over the RF cycle, voltage division is obviously not constant over the RF cycle. It is also not constant with frequency. It is certainly not negative feedback like Collins and Orr said.

It is the polar opposite of what we actually want. I think it is an inexcusable design error. They acted like the tube has infinite grid-cathode impedance. Obviously they forgot it was a sub-2 class that draws grid current over a large portion of the RF cycle, with a wildly varying G-K impedance. 

I agree that 220 pF is too small, but that's not what was used.

73 de Jim, N2EY
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G3RZP
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« Reply #32 on: September 07, 2011, 12:52:26 AM »

220pF on each grid is what is used in the RCA book that I have. Don't know which one it is, as it is about 700 miles away at the moment - I'm at a meeting in Switzerland.
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W8JI
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« Reply #33 on: September 07, 2011, 02:49:13 AM »

I was referring to the 220 pF. I read the .01 uF.

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G3RZP
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« Reply #34 on: September 07, 2011, 06:21:02 AM »

I wonder if this was a copy of a copy of a circuit diagram and although the 220pF was supposed to be changed, it didn't actually happen. It is not unknown for the published circuit diagram to not match the equipment because of upgrades, or even errors in the original.

I'm not totally convinced that 0.01 mFd is as good as a very short piece of copper tape...
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W8JI
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« Reply #35 on: September 07, 2011, 01:06:08 PM »

I wonder if this was a copy of a copy of a circuit diagram and although the 220pF was supposed to be changed, it didn't actually happen. It is not unknown for the published circuit diagram to not match the equipment because of upgrades, or even errors in the original.

I'm not totally convinced that 0.01 mFd is as good as a very short piece of copper tape...

A short piece of copper tape would be good, but there is sooo much thin lead from the socket up to the actual grid the overall fifference is pretty small. The only problem with small caps is at lower HF, at 20 MHz and above nothing can change very much. The long thin lead to the grid dominates.

I tried to get manufacturers to change how the grid lead is configured in both the 811 and 572. They really stink for RF. The Chinese didn't understand the idea, and the Russians were not even making the tubes any longer. Everything was existing stock.
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N2EY
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« Reply #36 on: September 07, 2011, 02:47:27 PM »

Tom & Peter,

Well, there's a mystery! Because the original schematic and all the reprints I can find show .01 uF from each grid to ground. Maybe there was a typo in early TT-5s and they fixed it; I think the Transmitting Tube Manuals went through several printings each.

Oddly enough, the SB-200 schematic I have shows 220 pF from each 572B to ground! And a little RF choke in series.

The design has the power supply right on the same chassis (866As and choke-input filter; bet that thing was heavy!) so there's no reason not to just ground the grids directly and change the metering circuits slightly.

----

Too bad they wouldn't listen to you, Tom. I can think of several tube designs that would be a lot better with "disc seal" bases (or whatever the proper name is).

For example the 807 is IMHO a great lower-power tube except for the very long grid, screen and cathode leads. (Yes, the 807W/5933 is an improvement, but they could have used an octal base and made it really sweet.

And it's not like this stuff was unknown; the 2E26 and 6146 have much shorter leads and multiple cathode connections.

Could it be that the serious development work was going into external-anode tubes for VHF/UHF? One impression I get from my limited reading of RF stuff from the late 1960s onward is that the focus mostly seemed to be on VHF/UHF, with HF being a sort of backwater.   

73 de Jim, N2EY
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G3RZP
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« Reply #37 on: September 08, 2011, 12:28:51 AM »

Jim,

The 807 was a 'high frequency' tube for its time. In fact, there was an RAF aircraft tx that used a selected version, the 8018, at 120 Mc/s. Class C, but not very, with grid modulation of the previous stage, which probably explains why to this day, aircraft comms always sound horribly distorted to me! The tx got replaced fairly quickly: the new one had the design taken to the US where they probably had two fits, and built the SCR522 instead - a far superior device. Although the slide bars for doing the tuning were the same.The 807 did come out in 1936 or so, too.....
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G3RZP
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« Reply #38 on: September 08, 2011, 01:32:40 AM »

There was an 807 family on a pressed glass Loctal base, with much shorter leads, made in the UK. The basic 807 equivalent was the STC 5B/254M: a wire ended version was the 5B/254G, while a single ended version was the 5B/255M (effectively a 6L6 on a Loctal base) . The 5B/257M was a 5B/255M with a 12.6 volt heater, and the 5B/258M was a 19 volt 0.3 amp heater version of the 5B/254M.

Strangely, in spite of the much shorter internal leads, it was still rated to 60MHz for full power and 125MHz at reduced power.

It first appeared in the early 1950s.
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K4EJQ
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« Reply #39 on: September 13, 2011, 10:45:52 AM »

Jim, Tom and Peter; I must say I have enjoyed reading your "drifty" thread. Thanks for sharing it with me. 73, Bunky, K4EJQ
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