Bottlehead Forum
Bottlehead Kits => BeePre => Topic started by: Jeroen on June 23, 2014, 09:28:16 AM
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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
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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
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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?
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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.
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I replaced the LM431 regulators, but the hum is still there. (nothing has changed)
Any other suggestions what could be the problem?
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Can you post a photo of the wiring around the 9 pin sockets?
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Hi Paul,
Hereby the photo's. I hope you can find something I did wrong..
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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
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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)
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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
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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
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Forgot to post the picture..
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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?
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Both IA voltages are 172,5 volt.
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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
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Hello Paul,
Without the tubes I measure 172,7 v on the left side and 175,2 v on the richt side.
Best Regards,
Jeroen
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Ignore my last post, those were the OA voltages.
(it's getting late here in Holland and I'm getting sleepy)
The IA voltages are; 177,7 v on the right side, 177,8 on the left side.
Cheers,
Jeroen
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There is a fundamental issue here that does not seem to have been addressed. From the photos we can see that this preamp is not stock. It looks like the pot or pots and the jacks have all been changed from stock. Every one of those parts has a connection to ground. Anywhere that a ground has been incorrectly connected there is the possibility of a hum issue. Without us being able to know how the non stock parts have been connected it is not really possible to offer a solution based upon our experience.
My suggestion is to reconfigure the preamp into its stock configuration. You may find that the hum issue will be solved. From there you can methodically change one thing at a time to be sure your changes are not adversely affecting the performance of the circuit.
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Hello Doc,
Thank you for your response and advice.
The hum problem has likely been solved.
It wasn't a ground hum, so the problem wasn't in the little ajustments I did nor in the ground wiring.
The amp was getting not enough AC voltage sometimes. The AC voltages at our home are varying between 223 and 225 volt. With 225 volt the amp works fine without hum, but as soon as the AC voltage drops the amp produces a loud hum. The filament regulator boards were getting not enough power to regulate the current propperly.
I will wire a power transformator (between AC outlet and amp imput) to raise the imput voltage with 12 volt.
Hopefully that will do the job.
Keep you updated!
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A couple of months ago I finished the Beepre amp, wow what a great amp! Incredible soundstage and a lovely smooth sound, I love it!
Before I could play music with the amp I had to raise the AC voltage to let the regulator work properly. (see previous posts).
By the time I finished the amp the AC voltage at our home was somewhere between 223 and 225 volt. I raised the voltage going to the amp by 12 volt, to get 235, 237 volt. This way the amp works well, although the amp is getting hot.
Lately the AC voltage at out home sometimes drops to 218 volt. When this happens the amp starts to hum very loud. I'm thinking of raising the voltage by 20 volt to tackele this problem. But when the AC voltage at our home is 225 again the amp will get 245 volt.
Could I hurt the Beepre with 245 volt?
Best Regards,
Jeroen
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There are what is called a constant voltage transformer. It includes a tuned circuit, lots of reactance that keeps the voltage at the specified voltage.
You could get one of those or a power regenerator and set it to 240V. Either way the voltage to your stereo will not vary.
Sorry, but neither answer is inexpensive. Well, not to me.
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245V is not an issue on the 240V transformer.
-PB
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Hi Paul and Grainger,
Thanks you for you response.
I'm a bit reserved using a constantant voltage transformator. I tried a isolating transformer on the Beepre to filter out trash coming with the AC current. It made the sound completely dead.
Glad to hear I can use 245 volt. Could you give an indicating how much power the Beepre is using? It helps me picking a suitable transfomer.
Thanks in advance.
Best Regards,
Jeroen
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I suspect that the BeePre is somewhere around 60VA, as always, leave a bit of headroom (so maybe 70-100VA).
-PB
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Dear Paul,
Thank you for your response. I've just found a transformer that will do the job.
I will post some pictures of the finished Beepre soon.
Regards,
Jeroen