MKIII noise issues

for Dynaco Mark II/III/IV and DIY PP monoblocks

MKIII noise issues

Postby justinsweber » Thu Aug 28, 2014 12:27 am

I hooked up the MKIII to my scope and test load. The input consisted of my generator through a volume pot into the input jack. I turned on the amp and let the tubes warm up a few minutes, then I adjust the bias slowly up to 1.4V. This setup is NOT using the balance pot and a single 10 ohm resistor for both tubes like the original. With the scope in the 20mV per division (P-P) I get about 18 mV wide band noise. The scope can not hone in on one frequency, when I shift the time base to look at the higher frequencies, the scope starts to pick out 50kHz through 80kHz noise. I installed an input jumper on the RCA jack and hooked up the amp to a speaker. I powered on and let the amp sit a few minutes to warm up and I verified the bias was 1.4V and I hear nothing at the speaker. I put my ear up to it and its pretty quiet. I took the cover off of the speaker to feel the woofer and there is no movement. I checked my connections to make sure everything was hooked up properly and it was. When I turned off the amp I heard a small pop out of the speaker so it was hooked up correctly. My question is, should I worry about this high frequency noise if it is inaudible? Is there anything you suggest I try short of desoldering the driver or power supply board? Thanks.

BTW this unit has the Bal omitted and is using the C. Chong power supply PCB.
MKIII-Cap-Board.pdf
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Re: MKIII noise issues

Postby Geek » Thu Aug 28, 2014 1:33 am

justinsweber wrote:....the scope starts to pick out 50kHz through 80kHz noise.


Pickup from a CFL bulb? (???)
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Re: MKIII noise issues

Postby justinsweber » Thu Aug 28, 2014 4:21 am

No idea... sure could use some help to figure out.
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Re: MKIII noise issues

Postby Shannon Parks » Thu Aug 28, 2014 2:00 pm

Use 100mV, 1kHz output from your generator and then use a plain old DMM. Start measuring from the input, to the grid of V1, to the plate of V1, to the grid of V2, to the plate of V2, etc.

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Re: MKIII noise issues

Postby justinsweber » Sun Aug 31, 2014 7:30 pm

Ok. Found the issue...

My good buddy uses a Metcal soldering station which is apparently a UF amplifier... supposed to be an amazing soldering iron.
When we were bench testing it was on... it was injecting HR noise. with it off, all noise went away.
The amp is crazy powerful and quite... quite enough for use with my La Scalas and a passive pre :-).
I'm getting 35watts RMS from 20hz-20khz at -1db... pretty freaking amazing.
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Re: MKIII noise issues

Postby Shannon Parks » Mon Sep 01, 2014 2:00 pm

Cool! Glad you found the culprit, Justin. The noisiest item on my bench is my Variac, with a nice radiated 60Hz hum. I always have to turn it off during noise measurements.

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