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Old 10th March 2017, 07:29 PM   #11
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I am not really a car guy (I like two wheels) but those are beautiful.
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Old 12th March 2017, 07:00 PM   #12
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OK, I replaced C4, C5 and R30 (with the diode) and the checkout numbers look good. B+ is a little high from previous but OK. I am getting:
304-307 B+
Neg across R7 -162
5842 came right up and settled in 175v
bias for output tubes all works good set at 28mA
My problem is the choke now hums. If I turn down the output tube bias (45's) the hum drops accordingly. I am using an ElectraPrint 10-15 uH choke, this amp was dead tomb quiet before.
Two changes since building this board are I changed R14 and R25 to 20K instead of 30K and I installed a LED pilot lamp inline with the back of the IEC plug for the power cord to the on/off switch. I forgot one other thing is I used a 4.7uF cap for C4 instead of a 20uF cap used previously.
I have not hooked up to speakers yet, the choke can be heard humming away and can be felt from the board when adjusting the output tube bias through the screw driver used to adjust the pot.
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Old 13th March 2017, 02:34 AM   #13
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I went through and made sure of solder connections, removed the choke and pulled off the bell covers and nothing looked smoked or hot. I soldered the leads for the choke in opposite holes from the firs time. Set bias and rechecked 5842 voltage. I used a cheater plug and make sure I did not have any current leaking to ground. The amp was quiet.

Brought it upstairs and made music. Eventually the hum returned, I tapped the choke and hum went away. This happened a number of times until the hum came and stayed.

I read another post where someone swapped the power transformer secondaries and hum went away.

This hum is audible during quiet passages and no music playing. You do not have to strain to hear it.
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Old 13th March 2017, 04:04 PM   #14
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I removed the choke and installed a 150ohm resistor and hum still persists. Will hum with the 5842 tubes removed. I do not know why tapping on the choke made the hum go away before unless that tap exposed a problem elsewhere like the power transformer or a cold solder joint.
All the secondaries on the pwr transformer are correct and I talked to Edcor last week and they indicated it would be highly unusual for a bad prw transformer it the secondaries are good.

Getting frustrated and way above my head.
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Old 13th March 2017, 06:08 PM   #15
w5jag is offline w5jag  United States
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Have you added any new components in the system? Maybe you have a ( dreaded ) ground loop that has started.

It doesn't surprise me it would hum with a resistor in place of the choke.

It's possible you replaced bad capacitors with bad capacitors. I've done that before. It's also possible that choke was ( is ) bad. I'm not sure how to check for a bad choke, other than to sub in a known good one. Maybe someone else could give some insight.

Win W5AJG
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Old 13th March 2017, 09:39 PM   #16
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Hi Win,
It's not that it hums with the resistor, it has not seemed to change. I assumed it was the choke since on and around the choke at one time made the hum go away (not now).

I am back thinking power transformer because of the clicking I used to hear. Adding a CL-140 termistor stopped the clicking because the slow startup of HV secondary.

I already eliminated ground loops as a problem.

I talked to Jack at Electra Print and he gave instructions to check the choke and everything seems good there.

Jack said to take readings on the pwr transformer secondaries while the amp is under load. Just got back from the gym and store and prepping for the snow tomorrow.
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Old 13th March 2017, 11:55 PM   #17
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Make sure your choke wires are away from input/output wires.
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Old 14th March 2017, 12:11 AM   #18
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What's your source? I had trouble with hum that turned out to be a phono cartridge that was too near the power transformer of my amp. It took me a while to figure it out. Rearranging the equipment made it go away.

Just a thought
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Old 14th March 2017, 02:32 AM   #19
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HI all,
The choke wires are away from input/output. Same setup I had before except with this new board I have hum, this same amp was dead quiet.

The source is not a factor and neither is ground loops. Hums with nothing connected to input - both channels. Again hum is there without the 5842's connected.

On the previous board I used Riken-Ohm carbon film caps for grid stoppers for the 5842's and 45's. This amp I am using Ohmite carbon comps on the 5842 and Dale RN60's (metal film) on the 45's.
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Old 14th March 2017, 06:18 AM   #20
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Spent time with the amp downstairs with the amp cleaning up wiring and reflowing again.
I measured the secondary's under load and the only alarming number was the 6.3 volt tap is reading 7.2 volts. I did not check the 5842's but the 45 heater voltage is spot on.

With the amp on the bench without any input connected I am getting 24-25 mV (.024 volt a/c measured on my DVM) out of both channels. I am running the amp into 8 ohm power resistors. It is enough that the sound can be heard coming from the Electra Print output transformers. No vibration can be felt but I can hear the transformers singing quietly.
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