BHK Pre Amplifier - Break in

I just got my pre-amp back from repair.
The main board was replaced because of hiss. It’s still there :cry: So in time I have to sell it.

But that’s not the question :blush: I bought the pre-amp used so it was broken in.
The bass is really weak right now. was it also your experience when you first got it?
Anyway, I just have to give it 500 hours. I am using some spare tubes for the break in.

Are you using 6 volt tubes? They are more prone to noise especially if you increased the bias to 5 ma. I prefer 12au7’s at 4 ma.


I’ll second having more noise challenges with 6V tubes. I stuck with 12AU7 and related and had better luck.


I thought 12au7’s were all 6.3volts

The first two numbers of the tube’s descriptions tell you the voltage. 12AU7 Is 12 volts.


Oh, right - I guess I’m thinking about them in parallel

Yes, 12au7 tubes can be run in parallel at 6.3 volts or in series at 12.6 volts, but the circuit has to be set up for it.


Its funny - I am more familiar with my guitar amp tubes(though a dumbass when it comes to understanding them). While I have one that uses 6L6s (assuming 6 volts) I also have one that uses EL34s and another using EL84s… which surely aren’t signifying the voltage

I need to ask, what sonic different is there if any between the two settings? 5ma or 4ma…
I just installed a set of Golden Lions 12au7 tubes and I wondered if there is sonic improvement on way or the other….

The higher bias setting is intended for 6 volt tubes. It will increase the noise if you run it with the 12 volt tubes. Some even run the 6 volt tubes at the lower bias setting to minimize noise which is more present with the 6 volt tubes.


Six pack, thank you for responding back so quickly! After I read your post I opened up my BHKpre and had another close look. What I noticed is that the jumpers are on the 5ma pins. I need to also share with you that I have listened to and hear no noise what so ever emanating from the speakers! Nothing….So I do not know what to make of it! Perhaps I should try the 4ma setting and see what happens.
Any feed back would be appreciated…

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Did you purchase the preamp new? If not the former user may have been running the 6 volt tubes and forgot to change it when the switched out the tubes. I can tell you the high bias may also cause the preamp to sound edgy. Try the 4 ma especially with the Gold Lions as they tend to be a little bright. Your speakers may be lower sensitivity than my B&W 800 matrix which are 93 db per watt. People running horn speakers with 100 db sensitivity are much more aware of the tube rush.

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12AU7 is a 12 volt tube.

Ah sixpack, you just beat me to the punch! I have just finished reading the owners manual and it states that the 12au7 tubes are 12 volt, so the jumpers are factory set at 4 ma.
However I am the original owner and not soon after I purchased the unit I was reading on the PS form about switching the jumpers to 5 ma for a little improvement, which I did! And have left it that way ever since. Not until I purchased the Golden Lions and in conjunction with reading several threads did I started to question which would be sonically better, 4 or 5ma. So the manual was enlightening and cleared up any wonderment I had.
All that said, at the moment I am enjoying the sound of my rig with the jumpers set st 5ma. But I am very interested in switching over to the 4ma to hear the difference if there is any!
I will keep you posted.

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Well I have just finished doing the old-switch-a-roo and in my system I find the 5ma setting to be so much more enjoyable. I think the transparency has improved, there is more air around the notes, the bass seems a tighter and more pronounced. To sum it up, I feel that the sound is fuller and more involving to me as a listener.

So I am leaving it at 5ma. I strongly recommend and suggest that you take a couple of minutes and try the change for your self. Wow the bottom end is more controlled and strong. The mids are clear, strong and transparent. The top end is all around without being tizzy or bright. It floats.

Another comparison would be a tube sound jacked up and less laid-back. Call it a big-tube sound……
But again, don’t take my word, try it for yourself then you will know for sure!

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