EQ1A Tube Tech - Noise Problem

[Axel.Thoma]

Hi Folks,

I have a problem with a EQ1A. Actually I bought two used devices and had to change the tubes because some of them didnt survive the transit. Because of this I dont know exactly how the sounded before the new tubes.
The problem is that both of them generate noise with a fundamental at 50 Hz (net frequ.) plus harmonics with a amplitude of about -60 dbfs which is too much for the purpose I bought them. I attach  a picture of the frequency content of the output signal to this post please keep in mind that ist somehow weighted (the ampltitude axis isnt linear in the picture!).  I have got some questions now and maybe someone has a clue.

First of all is there anyone who has a schematic?
Can it be that it is not the PSU? - I adjusted all parameters on the amp + psu board, DC voltage and the Offset + basic Gain.
If you think the problem comes from the PSU what can I do to fix it. I thought about changing the Caps in the next step since they seem to be old and not of good quality.

Waht do you think? Thanks a lot and regards,
Axel

 

[CJ]

probably the pwr supply, do you have 50 hz power where you live?

that unit might have DC for heating the tubes,

if so, then you have 2 possible sources of 50 hz hum, the b+ supply and the heater supply,

you can check the caps by putting a big filter cap across the supply lines and seeing if there is a hum reduction,

i believe Tube Tech might not be happy about schematics getting posted,

but maybe someone could email you a schematic,

 

[MHanson]

Where Are You Located?

Are you 50 or 60Hz power?

Let's start there.

 

[ruffrecords]

B+ hum tends to start with the second harmonic (100Hz). You have significant 50Hz content which implies there is a magnetic coupling between a mains transformer and an inut or output transformer; either that or maybe you have a hum loop.

How did you measure the output noise. Did you terminate the input with a 200 ohm resistor?

Cheers

Ian

 

[pucho812]

how old are the units. might need to recap the psu

 

[ruffrecords]

Looking at the manual for this I see it uses tube op amps. These usually involve at least one tube operating with its cathode at an elevated voltage relative to ground. If any such tube has some grid to cathode leakage then cathode elevation will show it up as hum.

Cheers

Ian

 

[outoftune]

i've repaired a few tube tech units.

depending on the age they generally need a recap of the PSU. other than that replacing the tubes has usually fixed most problems i've seen.

 

[mjrippe]

You can PM me your email address for a copy of the schematics.  It is a large PDF - about 10.5MB.

 

[Axel.Thoma]

Thank you so much for all of your replies!

I am located in Germany, 50 Hz is the AC Frequency.

I measured it in the studio connected to other devices in bypass mode. Therefore the input is directly connected to the soundcard output and must "see" the output impedance. As soon as I get new capacitors (any recommondations like brand maybe higher value?) I'll take them home and observe the frequency content of the signal with my scope and 200 ohm between the inputs.
Regards

 

[Niketouille]

Repaired several tubetech, eletro caps are definitely the weak point.

 

[Axel.Thoma]

With different input configurations (high impedance, 200 ohm, soundcard) its always the same situation at the output.

 

[Niketouille]

Also I remember the wiring was not that good in some tubetech gear. Check ground soldering point and wire connection.

 

[Axel.Thoma]

Thanks again to everybody, a change of the main capacitors solved the problem. I took twice as large caps for b+ voltage and 1,5 for the +-150 V furthermore twice the capacity for 12V. regards