Tube preamp troubleshooting

Hi everyone,

As I posted on the general forum, I have had a tough run of bad luck and mismatches with tube equipment. I just purchased a used Modwright LS100 for the following system:

1. CD - Marantz SA8004
2. Amp - McCormack DNA-125

I had been running it over the last few days with the pre-out on a Musical Fidelity M3i with no problems. Upon putting the Modwright in the system, I got some fairly loud tube rush from both channels. The right channel is a constant sound and the left channel seems to have some variation (much like the sound of wind over a microphone). Turning the volume knob up doesn't cause much change in the right channel. In the left, it makes the sound increase, crackle, whoosh louder while the knob is turning, but once the knob stops it returns to the same level of intermittent tube rush.

I assume the first step is to order new tubes, but if anyone has a suggestion of something else I might try, I'd appreciate it. The Modwright sounds quite good, but the tube rush is a major problem.

While you obviously can use new tubes that are rated as low noise by a reputable dealer, I suspect a major problem for you is the high efficiency of your speakers when driven by a pre-amp with high output (gain) into an amp with a high input sensitivity. With the wrong combo even low noise tubes may not eliminate the noise. Its a guess, but electronically speaking you may find that a SS pre-amp might work much better. You need to rethink you systems synergy and be pre-pared (after you get and listen to your Modwright at its best) for there to be a need for lowering the gain from your pre-amp. You can use inline attenuators which are not very expensive or perhaps getting different speakers which have a much lower efficiency point, i.e. 86dba. Personally I'd get the tubes, then some attenuators (Rothwell), and hope for the best.
I had the exact same problem with mine when I bought it used. It came with a couple sets of tubes, both of which had tube rush. Couldn't believe it. I bought a set of GE 6sn7gtb for around 50 bucks and they took care of the problem. They didn't take very long to break in and they sound pretty darn nice. An excellent tube for little cash.
Newbee - you must have ESP. I have a pair of speakers with a sensitivity 86db on the way! I appreciate the advice, and my current speakers are very high sensitivity (95db).

$2.80 - Thanks for the tube recommendations. I have read up on tube rolling in this amp on audio asylum and have ordered some new tubes. We'll see how things go. At least I know that the GE 6sn7s work in this amp.

Thanks, and I'll keep y'all posted.

Best, Scott
Ok, I just realized a rather serious problem. As I turn the volume up and the left channel has static and crackles, the woofer moves in and out rapidly. I've never seen it move like that. HELP!!
OK, now you need to know whether its the tube(s) or something else. Reverse the tubes in the pre-amp (l to r & r to l) and see if the problem changes channel. If it does you have sourced your problem to a tube in the pre-amp. If it doesn't then reverse your output IC -'s to your amp. If that doesn't do it then reverse you source cables. If nothing changes with the cable/tube reversals you may think that you have a problem with the Modwright itself.
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Just swapped the tubes, and the noise in the left channel is now a very constant and very loud whoosh. The woofer doesn't move in and out, though. The right channel seems about the same, so there's clearly something wrong with the left channel of the preamp. The seller told me that they played the preamp for two days before sending it, and there were no issues.

I give up.
I thought the same thing. Wait until your new tubes arrive before you do anything drastic. When I put the new tubes in, everything was good.
Now that I think about it, I was ready to ditch the LS100 and get a pass labs preamp. Was close to buying one when my tubes arrived. Put them in, and all was well. The LS100 is an excellent piece.
Well, just tried to power it down, and now the power button makes the balance control activate, but the unit won't power down. Clearly a defective preamp. Will be sending it back.
Cdrc, you are entitled to your opinion, but I do think you ramble on a bit too much about euphonic colorations. Especially for someone using Kubala Sosna cables and cords.

So you choose to hard wire your euphonic colorations as opposed to those who choose to use electronics to tailor the sound to their taste. In the end, euphonic colorations are euphonic colorations, no matter how one wishes to install them into their system.
Are your euphonic colorations all over your walls?
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You didn't confuse the balance button with the power button, did you? Been there, done that.
Besides a tube for the crackles it could also be the socket or solder joint. By reversing the tubes you are also re-seating them too.
read my preamp section in my virtual system for a bit of help to your tube questions and concerns
How about a link.

Is a 20amp receptacle code for a 'dedicated' 30amp circuit breaker. Sounds like a potential fire hazard to me. Other than that nice wiring.
friends? Odd way to make friends.
"euphonic (juːˈfɒnɪk) or euphonious
1. denoting or relating to euphony; pleasing to the ear"

It can't be hifi if its pleasing to the ear then?


a specified pervading character or tone of something.
"the productions have taken on a political coloration"

CAn't have that now can we?

Gotta be clean and pure and b/w I suppose, whatever that means.

Why can't it just be nice to listen to and leave it at that?

Since no two ears are the same, one man's colorization may be another's bullseye.

No one should care really what someone else thinks sounds best. It's largely a judgement call based on one's own unique senses and judgement.

OF course that will never happen in the real world. Everyone wants to inform others what the "truth" is.

Hifi is no different.

Good listening to all!
LOL, no I tried both the balance button and the power button, and they both controlled the balance! Anyway, it's going back, and I'm seriously considering a SS preamp.

08-14-15: Cdrc
Keep drinking the kool aid, friends.
On the other hand, though, there are some who would consider using $59,500 worth of cables and power cords in conjunction with $13,500 speakers to be drinking Kool-Aid. Which is not to say, however, that it can't produce fine results. As can a multitude of other approaches, including the converse.

-- Al
On the other hand, though, there are some who would consider using $59,500 worth of cables and power cords in conjunction with $13,500 speakers to be drinking Kool-Aid. Which is not to say, however, that it can't produce fine results. As can a multitude of other approaches, including the converse.
There's more than 1 way to skin a cat!
Kool aid comes in many flavors. Not vanilla though last time I checked.
Smrex13, I'm sorry to hear that. Hope you have no troubles returning it, being used. Some people are hard to work with when things like this happen. "It was working fine when I shipped it. What did you do to it?" Etc., etc. That was what I got when I received mine. It came with 3 sets of tubes, and all of them had issues. I paid extra for the tubes but didn't press the issue when talking with the seller. Just decided to try new tubes and go from there, and this solved my problem. Water under the bridge.
It's too bad this had to happen. The LS-100 is a real nice pre-amp. Good luck with everything.