BHK Preamp: Low pitched oscillation in left channel?

Over the last few weeks on rare occasions, my audio system’s left channel produces a low-pitched oscillation. This is barely audible, but there, occurs infrequently and after I’ve had the system running for a few hours, and can be corrected by turning the preamp off and back on. (The noise is similar to the muffled sound of a muscle car at idle; as heard from inside your closed house, if that helps give you an idea of the sound produced.)

I wonder if anyone has any thoughts on what may be causing this problem? I am focused on the left channel tube, as I recently installed a brand new pair of Brimar CV4003’s in the preamp, and never experienced anything like this noise prior to the installation of these tubes.

I plan to swap left and right tubes today to see if the noise migrates to the right channel. If so, I will swap out the Brimars for stock Psvanes I own.

Any thoughts or additional comments are welcomed and appreciated.

It sounds like the tubes.

I’ve gotten that kind of sound from inadequately matched tubes (at least I think that was the issue). The same tubes worked fine in other equipment.

Over the years I’ve had tubes develop that sound, kind of like what I call motor boating. Sometimes removing and cleaning the pins and socket helps or sometimes you have to trash them.

1 Like

I swapped left and right tubes a few minutes ago, and will spend the next day or so listening for the ‘motor boating’ noise to appear in the right channel. If it does, I’ll put the other tubes in to replace the Brimars.

I’ll post results later this week. Thanks for your replies.

This is the exact test we’d recommend trying first. Keep us posted Bob.

I’m fairly sure motorboating means something rather different in slang-land, maybe it’s a UK thing. Gave me my second chuckle of the day today anyway :slight_smile:


Ha ha!!! Very appreciated :slight_smile:

1 Like

Indeed! Well played @joma0711

1 Like

Slang is a truly wonderful (and dangerous) thing :slight_smile:

You motorboating son of a bish!! : )

1 Like

Follow up:

After swapping the left and right tubes, in about three hours of playing the right tube (formerly the left) produced the ‘motorboat noise’ in the right channel. So, it’s the tube that’s causing the noise.

I sent an email to the seller (a very well-respected dealer, I might add) asking about a replacement. I don’t know if he will replace the faulty tube, but I’ll let you know.

If he does not replace the faulty Brimar, what’s your opinion: buy another Brimar or go for a new pair of tubes (something other than Brimar)?

Hmmm in my day ('80s), Brimar valves were looked down upon. Is this NOS? I had a lot of faulty Brimar EL34s, EL84s, and I think ECC83 (sorry don’t know the USA equiv numbers), whereas I never had a faulty Mullard example of those (except through wear and age).

I LOVE Mullard tubes, they’re amazing!


Well, guess what? The other Brimar has developed the same intermittent noise after relatively little use (ca. 350 hours). The vendor is not being very helpful in replacing the second tube, so I’m in the market to replace the Brimars and never return to using them. I’m thinking Mullards may be worthy. Anyone have any thoughts on a pair of Mullard CV4003 / 12AU7 ($100) vs. Mullard 12AU7 / ECC82 ($23) tubes?

I’m open to tube suggestions for my BHK preamp, so feel free to add you input.


These are 7 volt. They sound really good, are quiet and reliable.

1 Like

My amps (T+A M40-HV) use 6SN7 tubes. I won’t use NOS simply because the universe of tubes gets worse and worse over time as all the better tubes are tested (are they, really tested?) and sold. The remaining tubes just can’t get “better” than what was once the best and are already sold.

VAC uses NEW manufacture tubes in their very well reviewed all tube amps. These amps are using an updated, modern, manufacture process of tubes that exceeed what was available in the days of NOS.

I’ll NOT be looking at NOS over, like it or not, better testing and consistent “NEW” and not “old” stock tubes.

I have a friend in the Houston Audio Society test so called NOS on a computerized tube tester against new manufacture tubes and ONLY the new manufacture tubes passed the original specs for 6SN7 tubes. Why? Because they are TESTED at the factory and ONLY passing tubes are SOLD. The NOS tubes that were ordered didn’t come close to the “claims” at all.

VAC has to consider that EVERYONE get the same QUALITY. You CAN NOT do this with NOS tubes as the best just keeps getting WORSE over time. New tubes are TESTED to be in the proper spec range, every tube.

This means you can LOVE a specific tube as they will be the same. NOS ? No, every tube is worse and worse assuming they are even TESTED at all. All the NOS tubes aren’t the same “universe” and thus the quality will vary significantly. I’ve seen it over and over.

New maunufacture tubes are TESTED and the UNIVERSE is far, far tighter as the production process prices in the cost of tossing crappy tubes.

I will buy NEW manufacture TUNG-SOL 6SN7 tubes and yes, I’ll still retest to verify the tubes performance falls into the 6SN7 matrix of standards.

Galen Gareis

1 Like

Oh Galen, don’t take all the fun out of it. :wink:

The stock Psvanes (Pavanes) don’t sound bad but they certainly don’t have anything special like some of the very moderately priced NOS 12AU7 labeled for organ companies of days gone by. Baldwin and Conn (mostly Sylvanias I believe)…


I’ve always had a tube tester. This resolves most issues before tubes go in. It’s a B&K 606. Small foot print and easy to use. Cost all of $100.

1 Like


In the day, SPC was a glimmer in the eye of manufacturing. The tubes were so inconsistent that yes, I suppose you could say there is fun in finding good ones!

Modern made tubes are Cpk of 1.0 or better in the process, so not much fun in finding a good one, they are 99.98% good. Just try to find bad ones!

Still, different tubes do sound different, even inside the spec matrix. The trick is to get a BALANCE between tubes so they are closer to the same tube start to finish. Tubes do age differently, so that’s what the tests show over time.


1 Like