My tubes seem to be misbehaving...help

Got a used bargain-price amp a few days ago. For about 90 US$, the used-dealer thought it was solid-state. A discontinued Hughesamp;Kettner quot;Studio 84quot; amp (AKA quot;Tube 20quot;) HOWEVER
When i play bassy notes a bit louder (clean or dirty) I hear an amplified glass rattling sound. I've read that the taller EL84 are prone to rattle. Even with the amp off, if i give a little knock on the top, i can hear the sound of glass (kinda like a tray-full of empty drinking classes being carried down the hall)

It's annoying and pretty loud. But i open things up and carefully had a look-see. Nothing on the valve or its wire-harness seems overtly loose. But being a combo amp, it will vibrate...

I'm going to send it in cause the valves look original (Hamp;K branded), yet the amp must be at least 10 years old.

Any recommendations to me to get the tech to do? What valves should i NOT accept. I'm hoping for a nice warm, sweet amp; articulate vibe. Medium gain. Bluesy rock. should i fear Chinese Valves?

At the moment the overdrive channel is quite extreme-face-melt stuff. If i can influence things with valve choice, I'd like to mellow this out just a bit.

Also the amp makes a intermittent breathing, humming sound, along with the glass-chinking-sound.

This is an exerpt from the manual:
Technical Specifications

TUBES: 2x 12AX7A (ECC 83); 2x 6BQ5 (EL84)

Preamp section: CHANNEL 1, CHANNEL 2 tube channels

INPUT: -10 dBV/ 1 MOhm

FX-RETURN: 0 dBV/ 47 kOhms

FX-SEND: 0 dBV/ 220 Ohms

RED BOX D.I. OUT: - 3 dBV/ 220 Ohms

Power amp section: variable power EL84 tube amp
Output Power: 20 W RMS (into 8 Ohms)

Frequency response (1W): 20 Hz - 25 kHz (into 4 Ohms or 8 Ohms)

Speakers: CELESTION Rock Driver Pro, 12quot;, 8 Ohms
Hope any of you tube-heads can help out....

A good place to start would be to turn the amp on at a moderate volume...say 1/4 of the ay up. get around back of that sucker with a pencil or chopstick of any other similar wood product. LIGHTLY tap each tube and note which ones cause sound to come out of the speaker.

Since it's a channel switching amp it would be a good idea to repeat the process for all the channels. You might find that all the tubes are shot. And if they are ten years old a complete re-tube is probably in order if you want to keep the amp.

As far a tube recomendations there are others far more informed than me. I live in a small city and Groove Tubes are the only thing readily avaliable to me. But I've always been satisfied with them.

What kind of sound am i looking for when i tap the tubes? What do they do if they're good vs bad? Am i looking for trouble simply getting a Pack of Marshall tubes and sticking em in? (obviously the correct type etc)

Any sound produced by a light tap is bad. It means the tubes had gone microphonic and is translating external vibration into sound. This is VERY bad, especially with a combo amp since you can't escape vibrations due to the proxmity to the speaker. Since low notes cause more vibration any note below low A will be especially problematic. If this amp is ment to be a quot;funquot; amp and not your main amp, I'd stick just about anything I could find in there that is the correct type and listen. If it sounded OK I'd call it fixed.

I can't really recommed different tubes to help you quot;voicequot; the amp. To do that you really need to try different types and go with trial and error. Even if you are familar with a tube the circut into which it's installed will have such a big impact on things as to make general assumptions almost meaningless.

Hey, MGTS. I just posted this in another thread so I have it handy. To see comparisons of various different tubes, prices, etc., check this:

thetubestore

Rob

Anytime you buy a used tube amp, it's a good idea to buy a full set of tubes for it, and put the old ones in the boxes as spares. Click the revolution link in my signature, and order JJ's. They're cheap, yet still top of the line.

Those tubes are probably close to shot, but if you ever encounter the glass-rattle noise on a set of good tubes, an easy fix is to put a stripe of silicon glue around the problem tube. I learned that from the owner of Matchless, and the silicon won't burn up. It dampens the vibration.


Originally Posted by GearjoneserAnytime you buy a used tube amp, it's a good idea to buy a full set of tubes for it, and put the old ones in the boxes as spares. Click the revolution link in my signature, and order JJ's. They're cheap, yet still top of the line.

Yep, JJ's make the best modern day EL-84s I've tried.


Originally Posted by Gearjoneser

an easy fix is to put a stripe of silicon glue around the problem tube..

If the rattle persists after it's been for new tubes and a service, i'll give this a try. But am i understanding correctly that the silicon sealer/glue/rubbery-stuff-in-the-shower, around the girth of the glass tube itself? Or quot;aroundquot; it's base on the circuit board?

And another dumb question: Are valves/tubes like halogen lightbulbs, in that they must be handled via a cloth or gloves for fear of greasy fingerprints causing certain expolsion?

Naaa....But you might want to wash your hands if you've just had a Happy Meal.


Originally Posted by More-Gear-Than-SkillAre valves/tubes like halogen lightbulbs, in that they must be handled via a cloth or gloves for fear of greasy fingerprints causing certain expolsion?

Thanks not a dumb question, yeah you should use cloth or gloves when handling tubes.

Regardless, you need new tubes.

As far as the silicon glue trick.....I'd make sure it's a brand that's heat resistant, and then put a stripe of it around the glass part of the tube, closer to the glass end. What happens is that the gray plates inside aren't firmly braced against the glass, so the vibration sends that sound to your speaker. I've had that happen on brand new tubes, although I'd probably return them first.