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Old 27th June 2012, 01:43 PM   #1
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Unhappy 845 Heater Power supply problems

Hi
I have an 845 Valve amp that has some interesting heater voltages. Normally the 845 tube has 10v on the heaters. Mine are showing 7.5v and 10.7v! I have swapped tubes around, and no difference. The 845 heater circuit is a basic bridge capacitor circuit. One bridge per channel, and separate connections for the AC. Both AC inputs to the rectifiers are showing the same input voltage.

Is this likely a Bridge or CAP issue? The amp is less than 5 years old.

Any input gratefully received.

Cheers
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Old 27th June 2012, 02:37 PM   #2
Elvee is offline Elvee  Belgium
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Cap dried out?
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Old 27th June 2012, 02:54 PM   #3
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Sounds like an open cap.
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Old 27th June 2012, 02:59 PM   #4
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Cheers fellas. Valve amps do get warm, and due to cost constraints, I can understand that not the "best" caps in the world are used....

I will check these.
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Old 27th June 2012, 08:26 PM   #5
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In an 845 filament, the rms ripple current in the capacitor may well be over 5A!
5 years of this is a heavy burden for any electrolytic, and replacing them is well-advised. Please check the ripple-handling of proposed alternatives, and use parallel multiples if needed.

Failure of one diode in the bridge would also answer to the symptoms, and the stress on these is also very high. Replacing with schottkys is worth the trouble: MBR1045 diodes recommended for this application.
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Old 27th June 2012, 09:17 PM   #6
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Excellent. Thank you for the information Rod. I found out what the problem was. Mastersound had connected the wires leading to the PSU capacitors to a dead part of the main PCB. The part it SHOULD have been connected to was fresh as a flower and untouched

I have now connected the correct part of the circuit up, and now although high at 10.7v. I now have both channels the same heater voltage.

You were all right about CAPS!
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Old 27th June 2012, 09:24 PM   #7
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10.7V is enough to shorten the life of the 845s. Much better service can be had if 0.1R WW resistors are added in series (or a 5A choke with 0.1 .. 0.2R dcR) to get the voltage to 9.9V at nominal mains input.
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Old 27th June 2012, 09:45 PM   #8
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Hi Rod. The 0.1R WW would that be placed in series on the + heater rail DC? 5 or 10 Watt should be OK at that voltage drop.
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Old 28th June 2012, 07:09 AM   #9
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To ride first class, use 2x 0.1R 5W, one in + line, one in the -.
Then, Connect another capacitor, eg 1000uF 35V Panasonic FC series across the filament. This will help absorb some of the rectifier recovery current-pulses - which are HUGE in this situation, and will harm the sound considerably!

BTW, to improve the sound dramatically, I offer a self-assembly kit to solve this problem completely - search for "Rod Coleman regulator" to find more about it.
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Old 28th June 2012, 12:10 PM   #10
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Rod. I had a look around and did not find anything definate. Do you have a web page or information on the regulator?

BTW: Cheers for the other info. I have ordered 4 12Watt non inductive resistors for the job.
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