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MP566-II Tube microphone preamplifier with HPF / Green LED blinks when there is no signal

« Last post by JPK on Today at 09:20:41 AM »

This problem arose when we started using a new batch of TL074. The solution is simple:
- Replace R44 (1M) by a 100k 1%-1/4W.
- Replace C19 (100nF) by a 1uF-50V ceramic.

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MP566-II Tube microphone preamplifier with HPF / Re: Possible ICs fried MP566-II

« Last post by JPK on May 08, 2026, 08:46:01 AM »

Can you tell me which pins were shorted?
What results do you get from the voltages checks in the test guide steps 2 to 5?

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MP566-II Tube microphone preamplifier with HPF / Possible ICs fried MP566-II

« Last post by spurton@gmail.com on May 07, 2026, 09:10:04 PM »

I recently built the MP566-II Tube pre and when I first powered it on I could not get any sound out of it. So I removed it and discovered I had a solder bridge on the back of the header pins to the tube PCB. I fixed the bridge and ran it again. It puts out sound but very quiet, not enough to register signal but I can hear it faintly. It also distorts heavily when I turn up the gain knob or output knob. I know shorting leads can fry the ICs. Does this sound like what I did? If so I will order new ones. Thank you. 

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Discussions about the products / Re: LA502T Tube Compressor Setup

« Last post by UrbanOsterman on May 01, 2026, 10:23:48 AM »

Thanks,
Later I looked closer and then I realised it was The SKTPS that was meant.

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Discussions about the products / Re: LA502T Tube Compressor Setup

« Last post by JPK on April 30, 2026, 07:58:00 PM »

Sorry for the mistake, it is not U5 but U9, the large préassembled module.

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Discussions about the products / LA502T Tube Compressor Setup

« Last post by UrbanOsterman on April 30, 2026, 05:05:24 PM »

I have just finnished building the LA502T Compressor.
And I am trying to set it up.
But I dont understand 
point 3 Section 'High voltage check' and 'Heater voltage check'

High Voltage check:
  It says I should check the voltage Between V- an B+ pin on U5 
  (which is a TL431 chip).
  the Setup paper say it will start at 30V and rise up to about +225V.
  But I measure just 2,48V?

And in section
Heater voltage check 
it says  I shuld meassuer between H- and H+ on U5 (same TL431) ... it should go from 7 Volt up to 12 Volt.

I assume some thing is very strange here?

Urban Osterman

 

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MU524 Vari-Mu Tube Compressor / pair of MU524 - crackling/noise/hiss

« Last post by herwig.scheck on April 30, 2026, 11:37:30 AM »

Hi,

I have a pair of MU524's. One has a cracklinkg/noise/hissing problem. When I turn down the gain (in as well as out) the noise goes down with it. After some swapping out of the tube and op-amps between the units the problem stayed. 

Until I swapped out the U6. They're different for both units and the problem moved with the U6. The "small" one behaves while the "tall" one is the noisy one.
When I touch/tap the small daughter board I can hear that in the circuit.


Is there something I can do on my end or should I contact support for this?


TIA,
Herwig

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MU524 Vari-Mu Tube Compressor / Re: Low output voltage step 6

« Last post by JPK on April 29, 2026, 09:13:34 AM »

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MU524 Vari-Mu Tube Compressor / Re: Low output voltage step 6

« Last post by Ben on April 29, 2026, 05:29:16 AM »

I found it at last! there was a tiny solder bridge between pins 3 and 4 on U2. Thank you so much for your help and patience?
Cheers!
-Ben

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MU524 Vari-Mu Tube Compressor / Re: Low output voltage step 6

« Last post by JPK on April 28, 2026, 09:07:39 AM »

7  0.05vdc
This should be 0.00VDC.
Maybe something prevents the output from changing.
Check that there is not a short circuit between TP4 (output of U1.2) and 0V or an other point around J1-pin17-18, C12, C9, R15.