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Old 28th November 2010, 11:54 PM   #11
bill_a is offline bill_a  United States
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Hmmm, are we looking at an off-line power supply (no transformer)? Your schematic is not clear about where the AC is coming from. Off-line PS's are creepy, terrible and dangerous.
Bill
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Old 29th November 2010, 03:04 AM   #12
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The power supply also looks like it is a bi-polar design. So there is a very good chance you are exceeding the cathode/filament arc-over voltage when the preamp is first turned on. After warmup there is probably no problem. However, if a tube develops a short due to this issue, quite often it can be intermittant!
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Old 29th November 2010, 03:59 AM   #13
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I don't think it's anything anything related to the signal path -- I'd have to believe in both channels being effected equally at once. On the other hand, both channels are fed by a single psu and both channels share a common grounding scheme.

Last edited by pedroskova; 29th November 2010 at 03:59 AM. Reason: fat fingers
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Old 29th November 2010, 03:06 PM   #14
M Gregg is offline M Gregg  United Kingdom
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spice,

Quote:I hope that made sense, but it must be passing audio if i can hear the music on other (wrong) inputs? very strange! i feel foolish i didn't notice this earlier. I didn't think to use a multimeter on the output of the preamp.

I assume you have tried the pre amp disconnected from the power section? Also that the fault is no different with or without the power amp connected?

Have you tried a temporary power supply from another amp for the preamp section to see if it is the supply section? (with the Pre amp PSU disconnected).

This should give you a starting point ( Is it the PSU or the preamp)

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Old 29th November 2010, 03:49 PM   #15
M Gregg is offline M Gregg  United Kingdom
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spice,


I guess you already know this, however I do not know if this is part of a larger installation that contains a power Tx.

Please note that if you do not have a Tx on the supply any fault current is only limited by the supply. Also the chassis Gnd will carry any leakage current through the caps.

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Old 29th November 2010, 05:51 PM   #16
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Oh my lots of great responses! thanks guys I'll try to answer them all.

First, today is a good day since it has decided to work properly! It may have had something to do with leaving it on for 2 days? But sometimes after i disconnect everything and take it apart and put it back together it works right away after that.

"Hmmm, are we looking at an off-line power supply (no transformer)? Your schematic is not clear about where the AC is coming from. Off-line PS's are creepy, terrible and dangerous.
Bill"

I have a power transformer that supplies 200ac and 12.6v ac to the power supply.

" The power supply also looks like it is a bi-polar design. So there is a very good chance you are exceeding the cathode/filament arc-over voltage when the preamp is first turned on. After warmup there is probably no problem. However, if a tube develops a short due to this issue, quite often it can be intermittant!"

I have replaced the tubes. The unit sometimes works immediately, if it has been disconnected, and is working now after being on for 2 days or so.

" I don't think it's anything anything related to the signal path -- I'd have to believe in both channels being effected equally at once. On the other hand, both channels are fed by a single psu and both channels share a common grounding scheme. "

This is true!

"I take it that you've measured voltages throughout your circuit when it's working properly and when it is on the fritz...and they're the same?"

I will try to do this and post my results. its working right for once so i can get this data.

"I assume you have tried the pre amp disconnected from the power section? Also that the fault is no different with or without the power amp connected?"

I don't have a another power supply to try it with. I ordered another amp to just to be sure, so ill try that as well. I'm going to take the multimeter and measure the output now that it's working.

"Methinks the thing is oscillating up in the RF somewhere. "

If it were oscillating this wouldn't be intermittent?

Thanks for all of your help! I will return with measurements + data. I wish the thing was more consistent with working/not working, hopefully i can trace a cause.
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Old 29th November 2010, 06:25 PM   #17
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It is working:

120vac from mains

I made a mistake the transformer actually supplies 220ac

The outs from the PSU are 222 (-) and 226 (+) also 12.08 for the tubes.

Each tube is getting 12.1 v

the outputs register about 3v

I turned it off, waited til i hear no audio. power back up - still works. tried unplugging it too, still works. tried taking out the tubes and putting them back, still works. tried replacing the tubes with others, still works.

I imagine the only way i could get it 'not work' is to leave it off and cold for an extended period of time... before i test that, is there anything else i should measure while it works?
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Old 29th November 2010, 07:27 PM   #18
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I would check the DC resistance of the grounds of the circuit to whatever is ground in your power supply. Similarly I would check the DC resistance of the B+ and B- points to the outputs of the supply. In all cases this should be nearly zero ohms.

It sounds to me like you have a bad connection somewhere. I am thinking its in the power supply somewhere- maybe the AC to the transformer is getting interrupted?
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Old 29th November 2010, 08:12 PM   #19
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Quote:
Originally Posted by atmasphere View Post
I would check the DC resistance of the grounds of the circuit to whatever is ground in your power supply. Similarly I would check the DC resistance of the B+ and B- points to the outputs of the supply. In all cases this should be nearly zero ohms.

It sounds to me like you have a bad connection somewhere. I am thinking its in the power supply somewhere- maybe the AC to the transformer is getting interrupted?
resistance between psu ground and circuit ground is 0

resistance between chassis gnd and and psu gnd is 33, though idk if that matters.

I have double checked ac - tx connectivity and rewired it, so there shouldn't be any trouble there.

I can't find any info on where the b+ and b- points are

220vac psu input is 0.05 ohms on one side and the other is 80ohms to gnd

the 12.1vac psu output is is 8ohms to gnd

shooting in the dark here, though i suppose these should be all ok since its working? hopefully when its not one of these measurements will be out of wack.
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Old 29th November 2010, 08:16 PM   #20
M Gregg is offline M Gregg  United Kingdom
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spice,

I take it that the heaters are on when it fails? They do not get dim or go off when it fails?

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M. Gregg
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