Berman 76 preamp schematics

[arnoldc]

i think it appeared in issue #13 of sound practices which i do not have.

can anybody share the schematics, please?

 

[Algar_emi]

Here the Preamp, external Supply schematics and the parts list. By the way, this preamp is very close to the famous Korneff 76 Preamp

 

[mthoi]

I build this preamp and followed the specifications and values closely per the article. Unfortunately, I have a hum problem which I believe is due to grounding. Can someone share their experience in resolving this issue.

It is in a two chassis - power supply and preamp is connected via screened 5-way umbilical power chord.

Tx in advance.

 

[tcqanh]

I build this preamp and followed the specifications and values closely per the article. Unfortunately, I have a hum problem which I believe is due to grounding. Can someone share their experience in resolving this issue.

It is in a two chassis - power supply and preamp is connected via screened 5-way umbilical power chord.

Tx in advance.

Do you try heater floating. It is really good for both tubes.

If I build this schema, I will use AC heater with floating.

 

[6L6]

Can you shed some more light on exactly what your problem is? And is it signal hum or PSU hum? I.E., 60hz or 120hz? (possibly 50hz or 100hz if your AC mains are 50hz)

Please post some photos of your build and an exact schematic.

 

[mthoi]

Hi Tcqanh, Tx for the response. Heater floating would help the life of the tubes. Would it address the hum/grounding issue?

I measured the B+ values after the CLCLCLC on load to be 310VDC using a 5u4g rectifier, unloaded the volatage is 410VDC. The filament voltage is 6.25VDC (loaded).

There is a low-pitch hum that is that is annoyingly evident in-between tracks.

 

[mthoi]

Hi 6L6, it is a 50Hz hum. I would get take some pics and post them this evening.

The schematics is per posting #2 by Algar_emi. I did not use a separate filament transformer for the build.

 

[6L6]

Then it's most likely a signal grounding issue, instead of PSU noise. If it were PSU, the hum would be 100hz.

 

[mthoi]

I solved the hum problem after reading the very informative article by David Davenport on Audio component grounding and interconnect in the diyaudio articles archive.

I may have created a ground loop by having both chassis earth-grounded, and also earth-ground the power supply-ground at each of the chassis.

I removed the power supply-ground connection to earth-ground on the power supply chassis, and the hum disappears.

 

[6L6]

That's simple to fix - you need a 'ground loop breaker' in-between the PSU ground and Chassis earth. This is a device that has a few ohms resistance, enough to break the ground loop, but still be ground when it needs to be.

The trick, however, is that you need a device that can stay intact if you have a genuine AC fault - this requires something that can handle real current.

Look at this bit that I took from a tubecad.com product manual -

If you use the resistor as shown above, it needs to be at least 5w dissipation.

A large diode bridge works very well, seen in this schematic -

You can also use a CL-60 thermistor as shown here -

The green is the AC safety earth connected to the chassis, the black is the PSU ground, which finds the safety earth through the CL-60,

 

[Burnedfingers]

So why couldn't you use a 56 in place of a 76? With the exception of the heater voltage the tube specs look the same.

 

[Palustris]

You can use a 27, 37, 56, 76 and 6P5 just ensure the heater voltage and socket are correct.

 

[tboll]

I realized the preamplifier Berman. Results listening were not good at all. I changed the power supply with the self first. I am now with capacitor enpremier.
Have you had similar problems?