Why are tubes going bad in my Clean module?

[roszatycki]

So I have been using an RM100 for a few months now. I have 6 modules and have no problems with tubes in any of them with the exception of my Clean module. In the last 3 weeks I have had 2 12ax7's fail, both tubes were brand new when I installed them. The first an EH and the latest was a Mullard. I will get output from the module but only 50% or so after the tube fails. The module is basically stock with the exception of 4 cap upgrades to sprague 715's. The tubes have failed in the V2 position. In the V1 position I have been running a 12au7. I believe the plate resistance is much lower on a 12au7 and wonder if that can be changing the circuit enough to cause damage to the other tube in the module. I am still learning pre-amp theory so would take input from anyone that can help. Thanks

 

[Julia]

I've nothing really to add here.... tubes ain't what they used to be. The equipment making them is old. Power tubes can last 2 minutes to 3 months. If they make it to 3 months, they may make it a few years, at which time failure rate starts dramtically increasing.

Preamp tubes? Probably the same except they'll last a lot longer if they make it past the three months. I've had them go out in 2 seconds and another one in 2 months.

Go figure.

I've got a 40 year old used Mullard 12AX7 running in my KH1. and 40 yr old used Mullard and Telefunken running in my Deluxe.

A friend of mine has the original Sovtek 5881s still working in his Soldano after 14 years.

It depends upon how lucky you are.

But if you continue to chew through tubes you might want to look at having the module checked by one of the modders.

 

[GtrGeorge!]

I chatted with Aspen Pittman (of Groove Tubes), a number of years ago...and since then a number of other Tube people in the know..
and the consensus is: Production of Tubes has moved overseas. Here in the US Tubes are considered very old technology and there isnt enough profit to make them like we used to (in great quantities). So Russia and some other countries bought our old machines for manufacturing Tubes.
Now they sell us tubes made on our old machines we sold them. Its kind of odd.
But there is a difference, according to some..the quality control is inconsistant because the 1) the machines are now 75+ years old and 2) the people working the machines are somewhat removed from the origins of "proper" tube manufacturing.
This is what I have heard, but cannot confirm (but I trust people tell me the truth).
Gtrgeorge
(a friend of tubes for some time now)

 

[roszatycki]

From what I am getting for responses it sounds like I may be a going through some bad luck luck verses a trend. I didn't get to concerned when the first tube went bad but when the second went bad it made me think, "I have a problem". I appreciate the feedback, but for now no good tubes in the Clean module. I loved that Mullard 12ax7.

 

[Jaded Faith]

There are occasionally bad batches going around. I had a few TS 12AX7's a while back from a known bad batch.

Ironically I too lost a beloved Mullard 12AX7 this past Tuesday! I was firing up my VHT 50/ST and the Mullard was in V2. Clean channel was good, but no output on the gain channel. Looked in the back and the tube had actually whitened the top and cracked the glass. It was an original factory tube from a 1960's Guild Thunderbass head. She will be missed.

 

[Julia]

We're talking New Sensor Mullards right? Not those sought after $200 per tube UK Mullards, right?

 

[Jaded Faith]

Nope, mine was a real deal 1960's Made in the UK Mullard.

I have a couple more old treasures from the 50's and 60's (Mullard, Telefunkin, etc) that are in AMAZING shape. I pulled them from old tube amps from the era that were collecting dust in buddies attics when they asked me to tune them up to trade them in. That Mullard sounded amazing in that VHT too.