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Clipperton V




 
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Author Topic: Clipperton V  (Read 833 times)
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KD1SH
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« on: September 15, 2021, 09:50:25 AM »

Picked up a Dentron Clipperton V at Moxboro. Never owned any Dentron gear before, but I can't say I'm terribly impressed by the build quality, or the schematic. Check out this snippet of the schematic. How can this work? Unless I'm missing something, there's no return path to ground for the filament current. C31 is 200pf; filament ac comes up through RFC3 but can't get back to ground through RFC2 with C31 in the way. I'm thinking C31 was intended to be a bypass but got drawn as a series element instead.


* Clipperton V PA.png (22.74 KB, 331x458 - viewed 133 times.)
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KA2PTE
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« Reply #1 on: September 16, 2021, 10:28:13 PM »

Does it match whats physicly in place in the chassis?
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KD1SH
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« Reply #2 on: September 17, 2021, 08:01:26 AM »

No. Confirmed by ohmmeter check rather than visually because getting access to the underside of the RF deck requires significant disassembly. The schematic is generally quirky: parts not shown, shown but not included in the parts list, etc.
My consulting the schematic came about as a result of not being able to adjust the idle bias to the value called for in the manual: manual says 50ma, but can only get about 15ma on the meter. Got B+; got screen V; got filament V; got bias V, changed out tube; but only 15ma with bias set to -38 V.
Testing reveals that the metering circuit is fibbing, and tube exhaust air warms up with idle bias, so it's definitely pulling idle current.
Eimac 4CX250B specs say 100ma idle for AB1 - Dentron says 50ma - and I'm not sure if the bias adjustment could even get to 100ma as it is; I haven't worked through the curves to figure it out. My plan, anyway, is to replace the 4CX250B with a 4CX400A to get better plate-diss headroom for AM use (the amp is converted from 2 meters to 6 meters).

Does it match whats physicly in place in the chassis?
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KD1SH
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« Reply #3 on: September 26, 2021, 06:36:51 PM »

   I corrected the plate current inaccuracy by simply padding the current shunt until the meter agreed with my accurate current meter - problem solved. Still couldn't get near the specified idle plate current even with -38v on the grid. Aha! - Dentron, by design, only puts 200v on the screen, and in reality that turned out to be only 190v. Thus starved for screen voltage, there was no way in heck that 4CX250 was going to give me 50ma plate current at idle with the bias voltage available. Makes me wonder how it ever worked, or why the original owner(s) never questioned it. Replacing the screen zeners with a pair of 160v jobs and stiffening up the dropping resistor corrected that, and now the bias adjustment will give me from cutoff  to over 100ma with no problem.
   But now here's another puzzle: why would Dentron design their bias circuit like this? (see picture inserted)
   Note that the relay contacts, K2A, are shown in the receive - cutoff - position, and switches to put the zener across the grid on transmit.
   The zener is unmarked and doesn't appear on the parts list, but it appears to be somewhere around 38v since that's what I get with the pot, VR1, at full crank. But, that design is weird to me - adjusting VR1 essentially just varies the current through the zener, so through at least 3/4 of VR1's range the zener doesn't draw enough current to regulate, thus the bias voltage soars up to the output of the rectifier as if the zener weren't even there. With VR1's wiper at the high end of the pot, the zener gets into its knee and regulates at -38v. But, since I need -56v bias to get 50ma idle plate current, I'm in effect setting VR1 so that the zener is barely operating, giving me a poorly regulated bias voltage that's very twitchy to set with the pot. Maybe I'm missing something here, but this bias circuit seems like a real JS design. What was Dentron thinking?






* Clipperton V Bias.png (8.78 KB, 439x108 - viewed 53 times.)
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KD6VXI
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« Reply #4 on: September 27, 2021, 08:29:58 PM »

I've always run my 250b tubes at 300 or more on the screen.

Adjust bias as needed for standby current. But that usually falls near 65 negative.

250R / 8930 falls around - 80 or more negative.

--Shane
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KD1SH
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« Reply #5 on: September 28, 2021, 09:13:59 AM »

I'm getting 50ma idle plate current with -56v on the grid; pretty close to Eimac specs. I don't get why Dentron put only 200v on the screen - Eimac doesn't even publish curves for that low of a screen voltage; you'd have to extrapolate from existing data to get numbers for that.  Eimac also states 100ma idle for AB1, so Dentron must have been aiming more for AB2. The amp doesn't have a grid current meter, so maybe I'll patch one in to see where it's going.

I've always run my 250b tubes at 300 or more on the screen.

Adjust bias as needed for standby current. But that usually falls near 65 negative.

250R / 8930 falls around - 80 or more negative.

--Shane
KD6VXI
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