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Author Topic: SB-200 grid current goes negative when driven by T4X-C  (Read 3407 times)
K1HL
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Posts: 2




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« on: September 10, 2012, 08:47:39 PM »

SB-200 QUESTION:  I am the original owner/builder back in 1971.  Amp worked perfectly with same tubes until just two years ago.   Got some HV glitches which ultimately took out the tubes and sent me, for the first time, to the SB-200 forums.   Read about the Harbach replacement HV power supply module and figured even though my existing PS was OK, for $90, change it out.

I noticed during the switch-out that one of the 33 ohm resistors to the grid of one of the 572B’s had burned in half.  Replaced that.

Added a “glitch resistor” in series with HV.

Constructed two new parasitic suppressors (using original coils, new resistors).  Then installed two new tubes.

Turned unit on and measured -135V bias at both tubes.  Normal.  HV about 2300-2350V, confirming that Harbach PS works and was built correctly.

Connected exciter and went into “key down”.  With key down and with NO DRIVE relay engages, grid voltage goes to -2.5V keeping 572B’s shut down.  Idle plate current 90 – 100 ma – perfect.

As exciter drive is raised, grid meter on SB-200 goes NEGATIVE, below scale.   Tuning and loading capacitors have no effect at all.  Plate current goes very high – 600-700 ma, no RF output, and red plates.  Shut down immediately.

When I bought my 572B's, I decided to get four of them and I've already tried swapping them out.  I would assume many responses would have included the suggestion to check my tubes, but that's already been tried.  Unfortunately we're going to have to be more imaginative than to swap tubes.  Sorry. 

Any ideas?  This thing has been perfect for 40 years.

Harry, K1HL
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K1ZJH
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Posts: 949




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« Reply #1 on: September 10, 2012, 09:29:53 PM »

If you installed the PM-200 board, make sure you didn't reverse the meter leads for the meter that reads grid current.  It looks like the RF drive is increasing the plate current.

Pete
« Last Edit: September 10, 2012, 09:32:03 PM by K1ZJH » Logged
W1QJ
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Posts: 1444




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« Reply #2 on: September 11, 2012, 05:32:43 AM »

Did you check the other 33 ohm resistor on the other tube?  Check that on also.  Normally negative going grid current would indicate a bad tube with a possible G-F short, but since you swapped the tubes out it must be something else.  I would check the 1.5 ohm grid resistor that is underneath the chassis on a terminal strip.  The grid current meter gets switched across this resistor to read grid currnet.  If this resistor went open you would probably get a negative reading.  Please check out that 1.5 ohm resistor carefully, you may have to lift one leg to be sure.  Lou
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G3RZP
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« Reply #3 on: September 11, 2012, 08:37:33 AM »

What is it like with the original 572s? I had 4 brand new Chinese ones. Two of them developed grid filament shorts immediately they were switched on (mounted horizontally), one had the plate come adrift, and the other was good.
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AD4U
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« Reply #4 on: September 11, 2012, 09:17:42 AM »

What is it like with the original 572s? I had 4 brand new Chinese ones. Two of them developed grid filament shorts immediately they were switched on (mounted horizontally), one had the plate come adrift, and the other was good.

I hope the Chinese rockets and space capsules are of better quality than their vacuum tubes.

Dick  AD4U
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K1HL
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« Reply #5 on: September 12, 2012, 03:34:41 PM »

I have actually considered the bizarre possibility that two of the four new tubes are bad and that by chance one or more bad ones are finding their way back in the sockets.  Maybe I'll play with changing them out a few more times.  Lou's suggestion that I check the other 33 ohm grid resistor was great idea; but already tried that days ago.  I honestly can't remember whether I already checked the 1.5 ohm resistor in series with the grid supply.  Will check again - just takes a couple of seconds.  I've also considered calling RF Parts as they sold me the four tubes.  Maybe some tech there has an idea.

G3RZP:  A filament to grid short is easily confirmed with an ohm meter. No short.   I had such a short a while back and the -135V grid supply makes the 6V filament light up like a light bulb.  So, that isn't it.

Best bet is my call to RF parts to see if they've got ideas.

Thanks all for the ideas.

Harry K1HL
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W1QJ
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« Reply #6 on: September 12, 2012, 09:12:45 PM »

Most G-F shorts CAN NOT be measured with an ohm meter.  The short only shows up when the filament is lighted, so don't think the tube doesn't have a G-F short because you do not see it with an ohm meter.
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G3RZP
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« Reply #7 on: September 13, 2012, 02:47:27 AM »

1QJ,

you beat me to it! I've found that in general with filament tubes, you need the filament alight to see a short.
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W1QJ
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Posts: 1444




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« Reply #8 on: September 13, 2012, 03:29:16 AM »

1QJ,

you beat me to it! I've found that in general with filament tubes, you need the filament alight to see a short.
True, in most cases.  Sometimes you can be rarely.
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VU2PTT
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Posts: 50




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« Reply #9 on: October 05, 2012, 05:30:45 AM »

Most G-F shorts CAN NOT be measured with an ohm meter.  The short only shows up when the filament is lighted, so don't think the tube doesn't have a G-F short because you do not see it with an ohm meter.

I just received a supposedly matched pair of 572B's from QMall in China for my SB-200. Just popped them into the amp with the plate caps disconnected. After a few minutes, I heard a click like a relay engaging and the grid current meter indication got pegged the wrong way.

Suspected a G-F short and that was what it was- one of the tubes had a G-F short though it does not show up on the multi-meter. It happens only when the culprit is plugged in and filaments lit up for a few seconds. Then CLICK!

QMall has  30 day return policy but not sure if it is worth pursuing by shipping both of these tubes back for a replacement matched pair - India to China and back again. Or cut my losses and keep one good one for a rainy day. I am going to drop them a note today to see what happens.

73 de Prasad VU2PTT
« Last Edit: October 05, 2012, 05:38:39 AM by VU2PTT » Logged
G3RZP
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« Reply #10 on: October 05, 2012, 05:34:44 AM »

I had 572Bs from RF Parts that did the same.
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W1QJ
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Posts: 1444




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« Reply #11 on: October 05, 2012, 09:46:59 AM »

I had 572Bs from RF Parts that did the same.
Really doesn't matter where you buy Chinese tubes from.  I think there is only 2 factories over there that make them with equal quality.  You'll pay more from RF Parts as they tend to offer a good warranty and tend to test them first before going out.  Although they test them, some tubes die after an hour or so.  It's a crap shoot.  Thing is, if you do get a good set, they work great as far as performance.  When I get good ones, they work great even on 6 meters.
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