BHK Preamp.....Afraid to try 7308’s


I am on my second BHK pre. First one reacted badly to my NOS 7308 tubes. The preamp is just fabulous with Mazda 12au7’s (imo better than my previous Virgo ii) but these 7308’s are just dangling in front of me, and I so much want to put them in. Please convince me to proceed!


There are some members of the forum running Mullard 7308’s with great results. I have a set on order and I have no concerns about using them in the BHK Pre or BHK 300’s . Your problem was most likely a tube with a short not the 7308. You should get a tube tester. I have a B&K 747B solid state and it tests for shorts effectively. To state the obvious, did you have the correct jumper settings for a 6 Volt tube?


Yes…everything was in the right place. Tubes came from Brent Jesse (NOS Siemens) sent them back to him for a re-evaluation and they test fine. I felt lucky that PSA sent me a new preamp no q asked, but don’t want to press my luck. Not to mention the terribly loud thumping my Apogee speakers fortunately were able to endure. Switched the tubes l/r channel and same channel was bad.


Wow, with such a potential downside that you described, ruining your BHK, what could you possibly expect from the upside?
It reminds me of the guy who goes to his doctor and says “doc, it hurts when I do this”. “So don’t do that”.


I get it…that’s my dilemna. However in light of the potential improvements (or not) and the fact that I paid $350.00 smackaroos for the tubes, mixed in with that they are supposed to work, what’s a guy to do? Are you suggesting that I not use these particular 7308’s, or any?


I’ve never heard of a blanket problem with 7308’s. You would have had the forum members here being very vocal about this problem. I think Jefferybehr runs them in his BHK 300’s and they are his tube of choice. If they work in an amp they’ll work in a Preamp.


think I’ll buy some.


Have you considered asking PSA, if you could send them in to be tested on their BHK? If they are willing, you would then know if it is safe. Would only cost you shipping.
If there is a compatibility issue, they then could advise against using them.
Did you buy direct, or from a dealer? If a dealer, take them to the dealer.

Just a suggestion…


Call me cynical but No dealer will admit to a tube that trashed a preamp. 7308’s do not have a problem that I’ve found with a quick search.


I have read about issues related to swapping out 6 or 7 volt tubes on the BHK preamp mostly on this forum, but elsewhere too. I think I am going to see if I can exchange the 7308’s I have for an alternative to my Mazda 12au7. As I mentioned yesterday I think the Mazda is fantastic, and there are likely other, perhaps better 12’s. out there for me. I am going to play it safe for now.


Just wondering how are you able to use a 12v tube on a circuit that designed for 6/7v (6922 / 6DJ8 / 7308 / PCC88) ?


There’s a jumper for each tube. Refer to the manual. There’s also a PSA video.


There’s actually two changes that can be made in the preamp one for 6-12 Volt the other adjusts 4-5 mamp setting. In theory a 7 volt tube is under less stress in a 6 volt circuit, so I’m sceptical.


Something else just dawned on me. Bascom King likes the PCC88 Tungsram’s and they are a 7 Volt tube. I doubt that with the knowledge Bascom Has, that he would recommend a tube that was prone to hardware damage.


I don’t have the preamp was just wondering. I thought the jumper was to go from 6v to 7v - didn’t know you could jump to 12. Either way you’re only talking 2 tubes - might make your life a lot easier if you go with Upscale Audio. Been dealing with them for many years and have never gotten a bad tube. It’s worth the extra couple of bucks to know you’re getting a quality tested tube. Their support is great also.


The jumper is between 6v and 12v. 7v tubes work at the 6v setting.