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Old 22nd January 2013, 11:38 PM   #1
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Default Red Plate Of Doom

I am working to rebuild a pair of Altec 1570B amps and finally got them going. One amp runs fine with no issues but on the other amp one of the 811A tubes red plates after about 20 seconds. I have ohmed out the primary side of the OPT and nothing is amiss there (96 ohms between either end and 48 ohms from either end to center). The 2 chokes in the amps ohm fine too. I have even ohmed just about every point I can think of and compared the readings of the red plating amp against the readings of the other amp that runs fine and there is negligible difference between them. Am I missing something here?

I have attached the schematic as well.
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File Type: jpg 1570b Schematic.jpg (80.7 KB, 428 views)
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Old 22nd January 2013, 11:42 PM   #2
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BTW I swapped tubes around to eliminate the possibility I just got a bad tube and the same side red plated.
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Old 22nd January 2013, 11:50 PM   #3
MelB is online now MelB  Canada
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Nice amps! Go back in the circuit a bit. It's hard to read but check voltages after C4 and C5???
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Old 23rd January 2013, 04:52 PM   #4
tricomp is offline tricomp  Denmark
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Hi,

Crappy schematic; go here instead:
http://www.dvq.com/hifi/images/1570a.pdf

811A is zero-bias so there's no negative bias necessary (...which is also impossible as the cathodes are at DC-ground and the grids are almost at DC-ground through L1.)
So, what's causing red-plating at zero-signal? OP-transformer shouldn't have any influence at zero-signal.
Obviously bias for the 6W6GT's (-35V) should be checked. If they're running full-out at zero-signal the internal DC-resistance of L1 may be high enough to allow for a positive grid-bias of the 811's.
Obviously L1 may be open.
C4 and/or C5 may be leaky.
Other than that...??

rgds,

/tri-comp

Last edited by tricomp; 23rd January 2013 at 04:57 PM.
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Old 23rd January 2013, 07:08 PM   #5
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C4 and C5 are new orange drops I replaced them last week. L1 is not open as I get the correct ohms across it according to my DVOM...I am really stumped here!
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Old 23rd January 2013, 07:54 PM   #6
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Tricomp, the bias generation of the 6W6GTs seems interesting. Is the negative bias voltage the voltage drop on the 600 Ohms resistor?
There is a voltage (335V) that is made from the 117Vac winding and the two SS diodes. Current from this source flows through both systems of the 6W6GT, L1 and the o.m. 600 Ohm. So higher anode current would automatically make the bias more negative. Stabilizing the bias of the drivers.
As you mentioned, a grossly wrong bias on the driver tubes could also affect the DHTs.
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Old 23rd January 2013, 10:27 PM   #7
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I checked the voltage at the junction of R11 and R12 which is the bias for the 6W6 pair and it is at-31 volts. Pretty close to right on the money. I even "reverse tested" the OPT by applying a bit of AC to the secondary and measuring both sides of the primary to center and both sides measure same. As I expected the voltage between both sides of the secondary is double the voltage from each side to center so I have ruled out the OPT as the cause.
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Old 24th January 2013, 12:18 AM   #8
llwhtt is offline llwhtt  United States
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Did you try swapping the 6W6s? What's the voltage on the cathodes of the 6W6s?

Craig
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Old 24th January 2013, 01:02 AM   #9
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I have swapped the 6W6s and still one of the 811s red plates. The cathode voltage to ground on the 6W6 that drives the red plating 811 is 1.12V and 2.98 on the other one that drives the non red plating 811. This reading is taken with no input signal applied.
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Old 24th January 2013, 01:34 AM   #10
llwhtt is offline llwhtt  United States
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Do those cathode voltages follow the 6W6s?

Craig
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