• From: Essex UK
  • Registered: 2010-03-19
  • Posts: 164

Topic: Going thru Pre-amp valves

Hopefully somewone out there can help me,

I have a Marshall DSL 401 valve combo and seem to be going through V1 Pre-amp valves at an alarming rate (about every 3-6 months, they start off really nice and "Bright" sounding, lots of top end and nice crunch, after about 3-6 months the sound has deteriorated and become quite muted and dark, lots of loss on top end and very mushy.

Change the V1 pre amp valve (ECC83S) and the sound is istantly back to a bright crisp and articulate sound with lots of dynamic range.

Anyone got any ideas on what to check?

Thanks in advance
Stewart

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  • From: Detroit
  • Registered: 2006-06-02
  • Posts: 5,807

Re: Going thru Pre-amp valves

Are you using the same brand of preamp tubes? If not you could be hearing a different flavor.
If you are using the same tubes, how many hours would you say they are before you need to change them?


I had a bad preamp tube in my blues jr, and I'm trying to figure out how/why it went bad. People usually tout that preamp tubes last years and years, whereas power tubes deteriorate over time.

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  • From: Essex UK
  • Registered: 2010-03-19
  • Posts: 164

Re: Going thru Pre-amp valves

I am using the same make of pre amp tube and prbably get somewhere in the region of 12-15 hours of "Volume" Playing before the top end drops off again, change it, back to how it should be instantly.

I changed all the power valves a while ago (6 months or so) and I think the problem sorta started after the power valve change, i didn't get the amp re-biased sso maybe thats whats causing the issue, but I would of thought that the lack of a re-bias would affect the power valves not the pre amp?

I hasten to add that untill this proplem started I have had the same pre amp valves in the amp since it was new (2006ish)

Have Fun and laugh like it's your first time! You never know you might get to like it! big_smile
  • Registered: 2009-01-01
  • Posts: 665

Re: Going thru Pre-amp valves

That sounds weird, indeed. Preamp tubes should last much longer, even a lifetime. Biasing is really only required for power tubes, so that is most likely not the problem. Having said that, I would strongly recommend rebiasing your amp anyway. Just buy a bias meter and do it yourself. It's quite easy if the amp has a bias pot.
Bear in mind that V1 is the most important tone-shaping pre-amp tube. I don't claim to be a tube amp expert at all but perhaps the problem lies somewhere else and since V1 is the most senstive tube changing that makes a difference for a short while? PI tube perhaps? Try 1) swapping all the preamp tubes and see if that makes a difference, then 2) see if you have the problem both on the clean and the OD channel and 3) try different power tubes (as the problem started with the new tubes) and *definitely* rebias.
If that doesn't help, a knowledgeable amp tech is in order.
I hate to worry you but I know at least two people who had problems with this amp. In one case it was the OT (this won't be your problem) but it could be the caps or rectifier or poor soldering that became disconnected due to the amp's heat.

  • RickB
  • RickB
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  • From: San Diego
  • Registered: 2009-10-30
  • Posts: 12,198

Re: Going thru Pre-amp valves

Heat is the enemy of electronics. Is it running hotter than usual? If so, the re bias sounds in order. It is very unusual for a low wattage tube to degrade that quickly. If the problem persists, time for the tech to poke around inside as Bluesbreaker suggests. A component in that circuit may have changed value.
Rick

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  • From: Ridgeland, MS
  • Registered: 2009-08-12
  • Posts: 1,248

Re: Going thru Pre-amp valves

Pre-amp tubes do not require any bias adjustments. That only affects the plate voltages of your power tubes. You need an amp tech to look at it. Are you blowing fuses?

7 Reply by StewartW (edited by StewartW 2011-05-13 02:56:19)

  • From: Essex UK
  • Registered: 2010-03-19
  • Posts: 164

Re: Going thru Pre-amp valves

Thanks for the advice Guys in answer to the questions:

1. No it's not running overly hot, well no more than it always has!

2. I will get an Amp Tech to have a look at it for me.

3. I will try a new set of Power valves and see if that has any affect.

4. Never blown a fuse (the amp not me, I have been known to on a few occasions)

Thanks again guts for your wisdom and suggestions, peace to you all!!!

stewart

Have Fun and laugh like it's your first time! You never know you might get to like it! big_smile
  • Registered: 2009-01-01
  • Posts: 665

Re: Going thru Pre-amp valves

Let us know what the problem was. I'm curious.