1

MP566-II Tube microphone preamplifier with HPF / Re: Signal gets heavy distorted and very quiet.

« on: August 21, 2026, 04:13:11 PM »

I can measure an AC Voltage on all TP's except TP2 and 3. I applied a 1 kHz sin wave and I can hear it ok. I can hear the changes in gain and distortion and I see it on the frequency analyser. But there is no measurable AC voltage on TP 2 and 3. Although I can see it on the oscilloscope feature of the multi meter. The signal is there but quite low compared to TP 4 for example, but it seems to have some kind of a DC offset - it is not on the 0 line of the oscilloscope. All other signals on the other TP's are at the 0 line. I don't know if this is normal, as my testing right now with mic, line and HiZ inputs was all ok....

2

MP566-II Tube microphone preamplifier with HPF / Signal gets heavy distorted and very quiet.

« on: August 20, 2026, 07:06:17 PM »

Intermittendly the signal gets distorted and very quiet. Sometimes it disappears and comes back after a while and sounds ok. This happens on and on. It seems to be contact problem (?) - I was thinking of the tube circuit and soldered the connections of the tube pcb again but this does not help. It happens on both inputs line and mic.
Any hints?
Thank you in advance!
Guenter

3

MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« on: August 07, 2026, 12:59:14 PM »

After reflowing the solder joints it seems to work now. This is a bit of a surprise to me because measuring the connections alway showed that everything is fine. Thank you for your help!

4

MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« on: August 06, 2026, 07:08:42 AM »

what should I check next? 

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MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« on: August 03, 2026, 06:35:16 PM »

the connections are all ok

6

MP566-II Tube microphone preamplifier with HPF / Re: MP566-II Phantom Power problem - signal fades to zero

« on: August 03, 2026, 06:00:31 PM »

So I guess the reason was a somehow defective microphone that finally died during the testing phase. Thank you for your support and fast feedback!

7

MP566-II Tube microphone preamplifier with HPF / Re: MP566-II Phantom Power problem - signal fades to zero

« on: August 02, 2026, 11:48:44 AM »

i fear my mic died during testing. I tried another one and it seems to work. is it possible that i have destroyed the mic while testing/ measuring? What could be the cause? I just want to be certain that I do not destroy the other mic as well.

8

MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« on: August 02, 2026, 10:26:54 AM »

it gives also 1 Meg in the circuit

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MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« on: July 30, 2026, 07:46:55 PM »

U4 against 0V
Pin 1   -16,54V
Pin 2      3,59V
Pin 3     10mV approximately
Pin 5     10mV approximately
Pin 6     3,26V
Pin 7    -16,55V
Pin 4     16,62V
Pin 11   -16,55V

Both Out and Gain at 0, no Signal applied

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MP566-II Tube microphone preamplifier with HPF / Re: MP566-II Phantom Power problem - signal fades to zero

« on: July 30, 2026, 07:37:30 PM »

XLR Pin 2 and 3 on the cable 29,4 Ohm, on the 500 Rack Slot 28,94 Ohm
Pin 2 on the cable to 0V on the board 22 MOhm
Pin 3 on the cable to 0V on the board 22 MOhm
both slowly increasing

Mic Cable is ok. I checked with a different cable - same behaviour 

11

MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« on: July 30, 2026, 02:16:15 PM »

Voltage on U4 Pin 3 and 5 are the same
DMM Diode Test shows that both sides (red and green) are working.
What I see on Pin 6 of U4: with Gain at 0 and Out at 0: 3,26V. Turning Out up to max the Voltage on Pin 6 stays at 3,26 V. When backing off the Out a bit the Voltage goes up to 15V, the red LED is on, the Voltage fades down and the green LED comes on for a while and fades to off. Voltage gets down to 3,26 V again.

12

MP566-II Tube microphone preamplifier with HPF / Re: MP566-II Phantom Power problem - signal fades to zero

« on: July 30, 2026, 01:54:25 PM »

Phantom Power on Pin 15 is stable (49,45 V), Voltage on R3 is 49,45V on one and 47,90 V on the other side, C1 populated again.

13

MP566-II Tube microphone preamplifier with HPF / MP566-II Phantom Power problem - signal fades to zero

« on: July 29, 2026, 03:18:34 PM »

Hi all, following my previous post about LD2 there is another problem that came up. When using Phantom Power, Everything works fine for a short while and than the signal fades away quite rapidly. The Voltage on IN+ and - is down to 28V when the 48V switch is engaged. What confuses me a bit is that on the kit C1 (100u, 63V) is packed but the partslist with C1 on is crossed out and on the new one C1 is no longer there. I have put C1 in initially and thought that this might cause the problem and so I desoldered it, but the behaviour is the same.
Any help highly appreciated!
Best regards,
Guenter

14

MP566-II Tube microphone preamplifier with HPF / Green LED (LD2) does not light

« on: July 29, 2026, 09:58:29 AM »

Hi all, just finished the MP566-II and everything seems to work ok, but LD2 does not light green when a signal is processed (it stays dark). When saturating LD2 does light red, so this way of LD2 is working. Testing the LED shows that both directions (red and green) are ok. What I have seen is that U4.2 Pin 7 stays at -16 V whether a signal is applied or not. Does this mean that the Op Amp is dead - at least the one for the green path of LD2? I have also checked the connections and Diode-orientation of the R's, C's and D's that are components in this part of the circuitry and this also seems to be fine.
Any help is appreciated!
Thank you in advance,
Guenter

15

True-Grit Tube saturator / Re: True Grit (Kit) not working

« on: July 02, 2026, 08:52:08 AM »

I measured my way through the circuitry and discovered a broken link between C13 and R12. After fixing this everything seems to work fine. 

All the best
Guenter

16

True-Grit Tube saturator / True Grit (Kit) not working

« on: July 01, 2026, 05:23:50 PM »

Hi all,
I just finished the assembling of the true grit and unfortunately it is not working. The only thing that is working is the IN switch and the Blend Poti. All other switches and Potis have no function. I could measure the + and - 16 V on V+and V-. Also the tube is heating up.

Since I have not found any troubleshooting infos, I do hope to get support in the forum.

Any help is highly appreciated!!

Thank you in advance,
Guenter