Teahead
01-22-2008, 12:08 PM
Not a very snappy title, I know, but this was tough to sum up in one line. I bought a Cornell Plexi used on ebay, when it arrived the preamp valves had been damaged in transit, no big deal I thought, they were only a GT and a no name, I pulled a known good pair of NOS Sylvania Black Plates that I had used in an AC30 and also used NOS Sylvania EL84s.
The amp sounded sensational, everything I had hoped for and more. However, when it was pushed and on certain notes it gave this awful "woosh" type noise that robbed the sustain and sounded nasty. I thought power valves and swapped them to no avail, I stupidly thought the preamp valves couldn't be to blame.
After opening the amp up and finding a toasted resistor, I had it replaced after consulting with Denis Cornell and hoped for the best. Unfortunately the noise remained and now we get to my question ...
Why is it that these NOS valves worked fine with the Vox but are so incredibly microphonic with the Plexi?
I tap them and they ring loud and clear, in fact I tap the back of the amp and the whole thing rings out. Plus the awful noise they introduce at times when playing. These valves have only a few dozen hours of use on them at the most.
I found an old pair of rubbish preamp valves and put them in, they don't ring when tapped and the noise disappeared completely. On the downside the idle noise increased and the amp doesn't sound quite so sweet, but it's infinitely more playable now the irritating noise has gone.
I would like to buy some decent valves but have no idea what to look for and how to avoid buying something that will cause the same problem the Sylvanias did. Apologies for the very long winded post and thanks for any help and advice you can offer.
Tea.
The amp sounded sensational, everything I had hoped for and more. However, when it was pushed and on certain notes it gave this awful "woosh" type noise that robbed the sustain and sounded nasty. I thought power valves and swapped them to no avail, I stupidly thought the preamp valves couldn't be to blame.
After opening the amp up and finding a toasted resistor, I had it replaced after consulting with Denis Cornell and hoped for the best. Unfortunately the noise remained and now we get to my question ...
Why is it that these NOS valves worked fine with the Vox but are so incredibly microphonic with the Plexi?
I tap them and they ring loud and clear, in fact I tap the back of the amp and the whole thing rings out. Plus the awful noise they introduce at times when playing. These valves have only a few dozen hours of use on them at the most.
I found an old pair of rubbish preamp valves and put them in, they don't ring when tapped and the noise disappeared completely. On the downside the idle noise increased and the amp doesn't sound quite so sweet, but it's infinitely more playable now the irritating noise has gone.
I would like to buy some decent valves but have no idea what to look for and how to avoid buying something that will cause the same problem the Sylvanias did. Apologies for the very long winded post and thanks for any help and advice you can offer.
Tea.