Loud hum in Beepre

[Jeroen]

Hello all,

A couple of days ago I finished my Beepre kit. When turning on the amp it gives a loud pulsating hum on both channels. After 5 minutes the hum fades away and comes back again and so on. The volume of the hum is indepented of the volume meter.
Luckily the amp also does what is it is made for, amplifing music!

All voltages are close to the recommanded values, except 10 and 15; 171 volt,   22 and 25; 196 volt,   31 and 34; 12,3 volt, c3; 4,64 volt, d3 5,14 volt
I did several things to figure out the problem;
-remeasured the (warmed up) amp again.
-checked the contact points of the 300b sockets and rocked the tubes in.
-resolderd a couple of joints.
But nothing worked out.

I'm using the stock tubes.
The amp is in the same wall socket as my Paramount amps.
My previous tube pre amp was dead quiet on the Paramounts.

Has anyone an idea what could be the problem?
Help is very appreciated!

Best Regards,

Jeroen, the Netherlands

[Jeroen]

I just realised that the first time I put power on the amp to test the voltages I didn't put the 300b tubes in the amp.  Could I have damaged a capacitor?

Jeroen

[Paul Birkeland]

I just realised that the first time I put power on the amp to test the voltages I didn't put the 300b tubes in the amp.  Could I have damaged a capacitor?

There is some potential for damage to the TL431 regulators if you run the amplifier for an extended period without 300B tubes.

This is also affirmed by the fact that your regulated high voltage rails aren't being regulated, and that you have hum.  (From the regulator not working)

Are the EL84's glowing? 

For how long was the amp run without 300B's? 

[Jeroen]

Hi Paul,

Both El84 tubes are glowing (as well as the 300b's)

The amp has be running for app. 20 minutes without 300b's.

[Jeroen]

I replaced the LM431 regulators, but the hum is still there. (nothing has changed)

Any other suggestions what could be the problem?

[Paul Birkeland]

Can you post a photo of the wiring around the 9 pin sockets?

[Jeroen]

Hi Paul,

Hereby the photo's.  I hope you can find something I did wrong..

[Paul Birkeland]

Just out of curiosity, what do you see for voltages at OB on both sides?

It's possible that the R1 resistors are switched.  R1 on the "A" side should be about 10 Ohms, while R1 on the "B" side is about 14 Ohms.

-PB

[Jeroen]

Just checked the resistors, both were placed correctly.

The voltage of OB on the A side is 100 V, OB on the B side is 102 V

I remeasured all voltages again, everything is pretty nice, exept for the voltage on terminal 10 and 15, both 171,4 V (should be 186V).                       The voltages on terminals 22,25,40,43 are fluctuating between 195 and 199 volt (instead of 213 V). Could this be the problem of that crappy hum?

(C3 and D3 are a bit on the high side, 4,9 v)

[Paul Birkeland]

This is a very odd situation.  Given more than 160V going into the A side C4S, the hybrid shunt regulator will kick in and regulate the high voltage to 145V.  In order for this to happen, there needs to be sufficient cathode voltage (3+V) on the EL84 to allow the regulator to function, and there needs to be some current flow as well.

You have adequate high voltage for the regulator to work, you have 4.9V going into the regulator, which would allow it to work, and your OA measurements confirm that the current is properly set. 

The next thing I would want to check is the voltage at the "r" pin of each TL-431.  Also, could you carefully unscrew the 4 screws holding down one filament regulator board, tilt it up, and take a shot of one of the power supplies?

We'll narrow this down sooner or later, thanks for your patience!

-PB

[Jeroen]

Hello Paul,

I measured 2,45 volt on pin R of the TL 431 on the left channel, 2,55 v on the right channel.

Hereby the requestef photo of the power supply.

Have a good weekend and best regards,

Jeroen

[Jeroen]

Forgot to post the picture..

[Paul Birkeland]

I'm just reading over this and realized that I had assumed the regulator was not functioning, but never asked you what your IA voltages were?

[Jeroen]

Both IA voltages are 172,5 volt.

[Paul Birkeland]

Can you briefly unplug the 300B's, then measure the OA voltages again?  (A minute or two of running like this will be OK)

-PB