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Old 21st May 2012, 12:14 PM   #1871
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I had a diode fail in a hybrid bridge recently. worth checking

andy
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Old 21st May 2012, 12:18 PM   #1872
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Hi William, nothing about your construction stands out as being hum-inducing.

Maybe we are down to some feature of the layout. First, may we assume that the rectifier heater wires are twisted, and don't pass to close to other parts? Rectifier heater connected to HT+ or GND?

Is the first 330uF in the PSU chassis? (should be).

How does the PSU chassis connect to the amp chassis, and how does it connect to mains safety earth? How/where does the PSU B+ negative connect to chassis?

Layout photos can often reveal odd problems.
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Old 21st May 2012, 12:46 PM   #1873
hihopes is offline hihopes  South Africa
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Rectifier heater wires are twisted. I tried it floating. It is connected to ground currently (with external DC sujpply).

I did test the diodes. I can't remember but I think I replaced them at one point. I did find snubbing them helped a bit, but not much. Maybe I should replace them again, just to be sure?

All smoothing is in PSU chassis, with extra decoupling caps where B+ enters amp and again right at plate load R's. Decoupling caps at input are right at input socket, between B+ and 0v. Decoupling caps at Plate R's connect to star earth (copper plated board) which connects to 0v by wire to socket. Heater supply is not connected to directly to 0v but connects via cathode biasing (as per Rod's instruction sheet).
I currently have signal 0v of input sockets connected directly to 0v of ouput sockets and to star earth board. Have tried some variatioons of this connection scheme without noticeable results.

Amp chassis is mostly wooden with an alu plate for tube sockets and heatsink for BJT's on heater boards, and it is currently not connected to earth at all. PSU chassis has alu top plate, which is connected by a thick wire to 0v star point (1st capacitor 0v) and from there a short wire runs to IEC plug's earth pin.
Unfortunately, I don't have a camera and my cellphone can't download pics to my computer. (Takes lousy pics anyway).
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Old 21st May 2012, 01:37 PM   #1874
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Hi William,

Quote:
Originally Posted by hihopes View Post
Amp chassis is mostly wooden with an alu plate for tube sockets and heatsink for BJT's on heater boards, and it is currently not connected to earth at all.
Connect the aluminum plate to signal ground. If that doesn't help also connect signal ground to safety earth.

Best regards

Thomas
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Old 21st May 2012, 01:45 PM   #1875
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Correction: Actually all metal parts of the chassis should be connected to safety earth. In addition you should also connect signal ground to saftey earth at one point in the system. Connect the alu plate to safety earth first, then experiment with signal grounding

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Old 21st May 2012, 02:10 PM   #1876
hihopes is offline hihopes  South Africa
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Thanks Thomas. I only need 1 wire to connect alu plate to star ground. Signal and power grounds are connected to star already. I understand the safety aspect, but not how this could remove hum?
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Old 21st May 2012, 02:52 PM   #1877
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Hi!

The safety aspect has nothing to do with hum. You should connect all metal parts which are touchable to saftey ground.

Thomas
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Old 21st May 2012, 03:05 PM   #1878
hihopes is offline hihopes  South Africa
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OK, the connection is done and the amp is now safer.

I have a tapping in the transformer for a lower HT - 140v if I remember correctly. I had it wound that way in case I needed to resort to SS rectification. I would prefer to stick with valve rectification if possible, but it is there if nothing else works. I have been struggling with this amp for a long time and am feeling quite disheartened.
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Old 21st May 2012, 03:21 PM   #1879
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Hi!

How about the signal ground connection? is it also connected to the chassis?
Did it solve the hum problem?

Thomas
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Old 21st May 2012, 03:25 PM   #1880
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Hi again,

if the ground connection doesn't help, some systematic search is due. Measure the ripple in the B+ and across the filaments. PSUD is fine for theoretical analysis, but if something is wired wrongly, or a part is faulty, reality can be very different.
How is the grid conencted? To a pot? connect the grid directly to ground and check for hum.

No reason to be disheartened. This should be fun! Once you solved such a problem you will have learned a lot from that

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