New ST35E build & Weird Sound

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New ST35E build & Weird Sound

Postby MacL » Fri Feb 07, 2014 10:20 am

Hi All,
Speakers installed - no source attached.
Just installed tubes (including el84m) and powered up. After tubes warmed up (10 secs?) I get deep hum then loud feedback whistle...no smoke.
I tried twice and quickly unplugged. I had my meter attached to read bias the second time and noticed the bias voltage go up way pass .35 mv -i was unplugging and it got to .80!!! it might have kept going... I used the zener option - no other mods.
This weekend I will start looking for the simple stuff -but I wondered if anyone had advice where to start? My previous builds have either worked or had no sound- so this is scary and I hope I didn't fry my brand new el84m's. Would a bad 12au7 ( i used a unproven NOS RCA) cause this?
Thanks,
Mark
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Re: New ST35E build & Weird Sound

Postby Writer Frog » Fri Feb 07, 2014 11:46 am

The usual suspect would be the NFB acting as PFB, making the amplifier a rather efficient oscillator.
I would pop the 12AX7s out and power the amplifier up again.

If there is no howling noise, then the next step is to switch the output transformers' secondary wires.

Good luck to you!
/Matt
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Re: New ST35E build & Weird Sound

Postby MacL » Fri Feb 07, 2014 12:48 pm

Thanks Frog-
Pulled the 12ax7 & 12au7-powered up and no sound...
Once again had the meter plugged in for bias readings and it started to spike again-quickly unpluged.
I will take off the bottom panel out and QC the Edcor output wiring to junction blocks tomorrow.
Can some comment on the bias reading spiking!
Thanks,
mark
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Re: New ST35E build & Weird Sound

Postby Shannon Parks » Fri Feb 07, 2014 4:24 pm

Mark,

1) Disconnect J3-3 and J4-3, as that disconnects the feedback as Writer Frog suggested. It could be PFB instead of NFB.
2) Make sure you have your pots fully cranked open at R19,R22,R25,& R28. The bottom of the zener to ground should be 110 ohms.
3) Make sure you didn't populate R17, R20, R23 & R26.

Shannon
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Re: New ST35E build & Weird Sound

Postby MacL » Sat Feb 08, 2014 10:42 am

Great advice, Shannon.
First off I did populate R17, R20, R23, & R26 - my dumb mistake - didn't follow checklist - followed schematic -However, I can see others doing this...

Bias didn't spike -still got feedback whistle.

Disconnected J3-3 & J4-4.


Amplifier now works! What do I need to do to get NFB to work correctly?

Note on Bias reading-Shannon you had ask for feedback on this... took reading after amp was on 10 minutes.
Used brand new Sovtek el84m's - I only had one valve I could get below .350 (.348). Had two that could only go down to .356 and one that would go below .365.
Unfortunately, the lowest and highest were on the same channel... going to try and swap tube combination. I don't want to tear into the PCB. Should I bias all 4 tubes to the same high setting (maybe .365) or only be concerned about biasing each each channels tubes to the lowest matching bias reading? I am not the expert you guys are but I have to think .365 v on a el84m can't be all that bad for hi-fi- is this a correct assumption.

I would appreciate the advice from anyone- I want to start listening to this amp!!!
Thanks for the great support!
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Re: New ST35E build & Weird Sound

Postby Writer Frog » Sun Feb 09, 2014 8:13 am

MacL wrote:Great advice, Shannon.
Amplifier now works! What do I need to do to get NFB to work correctly?

Congratulations!
You are almost there.

I do not have the Rev. E board, so this suggestion is strictly from reading the manual and not
from an actual experience.

The problem you have is that, due to the output transformer's phasing issue, your NFB loop
became a positive feedback one.

To correct the situation you will have to swap the relative phasing of the primary windings of the
output transformers as follows:

1) Swap the wires to J2:1 and J3:1.
2) Swap the wires to J2:2 and J3:2
3) Swap the wires to J4:1 and J5:1
4) Swap the wires to J4:2 and J5:2

That should do it if I am not mistaken.

Good luck to you, and enjoy! :))
/Matt
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Re: New ST35E build & Weird Sound

Postby MacL » Sun Feb 09, 2014 9:47 am

Thanks Writer Frog!
Swapped the wire and no feedback squeal... but now it sounds like the music is "underwater"...

Any ideas on that?

Played the unit last night for two hours without feedback hooked up and it sounded real good... I thought the NFB loop would put it over the top...

Also, checked bias again today (after playing for a while) and bias was way down so I adjusted back up to spec... is this something you can expect after new burning tubes for a couple hours? I could hardly get bias low enough at first power up - now I have plenty of pot to adjust.

Thanks for everyone's help and any input is valued
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Re: New ST35E build & Weird Sound

Postby MacL » Sun Feb 09, 2014 10:28 am

UPDATE
Everything is good.
I came from the guitar amp (all but one project was cathode bias) world over to HI-FI tube world and never experienced this before.
Major bias tube drift is what caused the "underwater" sound. The bias readings on a couple tube changed as much .356 mv to .290's mv over a couple hours of playing-and it sounded good when I shut things down last night.
I tried music again after bias re-adjust and it is back to good sound. Going to let it run a few hours today and check again.
I will post again after a week of playing and include photo.
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Re: New ST35E build & Weird Sound

Postby MacL » Sun Feb 09, 2014 6:02 pm

When to measure bias?
Just played the system for a couple hours - sounding great! i didn't think the NFB made a difference at first, but now I can tell the real tight bass and clear projected vocals ( I have good ears but not the golden ones some have)

doing hourly bias readings-unplug inputs & take reading.
I am reading .330's now- although I think volume is lower (because of bias being lower)-maybe.
Checked my input voltage and running thru variac to keep base of 122 volts as I started with today.

When do you dial in the bias? Trying not to obsess-but when do you pro's check?
I want to move off the bench and put on my living room shelf. I wonder want I should do with my Budgie SE? (b)
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Re: New ST35E build & Weird Sound

Postby Writer Frog » Mon Feb 10, 2014 6:30 am

MacL wrote:When to measure bias?
...
I am reading .330's now- although I think volume is lower (because of bias being lower)-maybe.


For a given value of the plate voltage, the gain of the stage stays more or less constant
regardless of the bias, although the bias will affect how the signal might clip.

I assume you are measuring the bias voltage across the 10 ohm resister with the input terminals shorted out.
And I also assume that you are using the 100 ohm bias pots in place of 10K ohm bias pots.

You should not have to keep adjusting the bias if you have the Zener based bias scheme.
The Zener diode would automatically keep the reverse bias voltage across its terminals
at 10.0V +- 2%.

Although I am not sure whether the plus-or-minus 2% value refers to manufacturing tolerances or thermal
stability, considering all other variables such as line voltage fluctuations, thermal drifts of
power rail components, the bias current drift from 35mA to 33mA sounds quite reasonable.

I would not worry about it.

I usually check bias once every few months, first at ~20 minutes after power up, and again at ~60 min.
/Matt
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Re: New ST35E build & Weird Sound

Postby MacL » Thu Aug 14, 2014 7:27 am

Hi All,
Just wanted to post a photo of my completed ST35 -hope I didn't already post but found photo on my phone and thought I would share.
RCA's in back also. 16"x8" chassis was tight.
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Re: New ST35E build & Weird Sound

Postby chrx » Fri May 08, 2020 12:49 am

Writer Frog wrote:The problem you have is that, due to the output transformer's phasing issue, your NFB loop
became a positive feedback one.

To correct the situation you will have to swap the relative phasing of the primary windings of the
output transformers as follows:

1) Swap the wires to J2:1 and J3:1.
2) Swap the wires to J2:2 and J3:2
3) Swap the wires to J4:1 and J5:1
4) Swap the wires to J4:2 and J5:2


I am using the Edcor transformers as well and also got the loud hum on the first run. Following this procedure above resolved the issue.

I also installed the Zener mod and had to ultimately go with 13-volt Zeners and 200-ohm trimmer pots, to get my EH EL84s within range.

Thank you.
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