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To all PS Audio BHK preamp owners *** must read ***

Hey everyone

If you have a PS audio BHK preamp you’re going to want to read this. Before I continue I want you all to know I emailed PS Audio to make sure this was completely safe to try and that it wouldn’t hurt anything or void the warranty. They assured me that it would not void the warranty and it’s completely safe to try. 

It has to do with the bias adjustment for the tubes. The bias adjustment For the 12au7 tubes it comes with is factory set for 4mV. They tell you that you can also use certain 6 volt tubes and the correct bias setting for those tubes is the 5mV setting which is adjusted by moving two jumpers. There is a picture in the owners manual with instructions on how to do this. It’s very east to do, all you really need is a good pair of needle nose pliers. 

I wanted to know if there was any benefit of trying the 5mV setting on the 12au7’s? The gentleman on the phone at PS Audio said he had never been asked that question before and that he would check and get back to me. A couple of days had passed and I opened up my email and there was an email from PS Audio. He said he checked with a few people at PS Audio and they all told him that it was ok to try it and he asked me to get back to him with what I had heard. 

So, I took off the cover over the tubes which is held on with two thumb screws (very clever). I switched jumpers J7 and J8 as per the instructions in the owners manual, put the cover back on and put on the exact same cd I was playing before I changed the jumpers. All I have to say is wow! Right away it was better. There was a slight amount of glare that was now gone. Everything was more coherent. Bass was noticeably better, tighter and more solid. Piano had more weight to it. Massed strings were smoother and more realistic and just more beautiful. All from a 1 mV increase in the bias adjustment. If I didn’t hear it myself on my own system, I never would have believed it. Let’s just say it’s not going back to 4mV ever. I just wanted to share that with BHK pre owners. Take care and stay safe.

Scot
scothurwitz
scothurwitz OP
113 posts
 
Tomcy6

It’s cool that you at least tried it. I was using some early 1960 Telefunkens and I heard the improvements right away. I recently had some Amperex nos 7308’s from the early 60’s I’ve been dying to try so I put those in and changed the voltage jumpers to the 6 volt setting and my initial impression of those tubes is very positive. They seem to be a little warmer and a little more full bodied than the Telefunkens.

I normally also do not believe in a quick a/b comparison, it’s just too easy to be fooled into thinking something is “better” when after a few weeks of carefull listening, you realize that the change was just different but not necessarily better. In the case of the 4 to 5mv when I changed the bias jumper and put on the same cd we were listening to when it was set to 4mv we both noticed that the slight glare on the flutes was completely gone. We also both noticed that when the massed strings started playing they sounded more realistic. You know the old saying, ymmv. I would suggest changing it back to the 5mv setting and listening to it for a few weeks, like you said, it’s free and if you decide you don’t like it you could always put it back. 

Scot
noromance's avatar
noromance
9,932 posts
 
Try it with other tubes. Your Teles may be getting tired and the increased bias current is giving them a boost.
earthtones
111 posts
 
I tried it and it’s no better and possibly somewhat worse. I may have detected a slight thinning of the lower mids and flattening of the soundstage. I’m not sure really but I’m keeping at the designed 4mv spec. I don’t understand Bascom’s rationale. For him to have stated a few years ago on the PS Audio forum that you simply chose 4ma for 12v and 5ma for 6-7v, end of story, then to purportedly suggest later on that you should experiment with it makes me wonder what exactly were the design parameters under consideration. In other words, what does he really think about these choices? Also, I would think running higher bias is going to burn out 12v tubes faster.
rodman99999
6,175 posts
 
".....I would think running higher bias is going to burn out 12v tubes faster."                                                                    +1
scothurwitz OP
113 posts
 
Earth tones & rodman99999

I agree with both of you, I think it probably reduce the life expectancy of the 12 volt tubes. My goal was to see if I heard enough improvement in sound quality to warrant leaving it at the 5ma setting and I did. I don’t know how much shorter the tube life will be but the improvement in sound quality, in my system and in my opinion, was worth finding out. 

I also tried a 6 volt tube to hear the difference. I had bought some Amperex nos 7308’s from the early 1960’s and changed the voltage jumpers to the 6 volt setting and left the bias jumpers in the 5ma settings as this is what it calls for in the manual. I have read on several forums that Bascom himself prefers these and feels that his favorite tube is a nos 6 volt tube from the 1960’s. I forgot exactly which one. 

I must admit, I like the 6 volt tube as well. To my ears, the resolution is the same but it’s a little more full bodied and a little warmer. I don’t like to make quick judgements, it’s too easy to be fooled. I’m going to give it a few weeks and go back the the 12au7’s and listen to those again. Either way, I think for the Monet, this preamp is amazing.

Scot
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