vox heritage ac30 problem

[thenorm]

Hi first posting here. I have a vox hertiage ac30 that I have owned for about a year. It has newer tubes and is a great amp. I was playing it at lower volumes last night and it has an underlying distortion that was not there before. Every note I hit will ring clean but has a distotion underlying the clean note. It has never done this before. I hoping someone can help me out before retubing or taking it to get serviced. Sorry for sounding like such a novice, but I am.
Tha

[thenorm]

Thanks for the help.

[Robert M. Martinelli]

Hi,
Well, maybe I'm just shooting in the dark, because it's hard to make the correct statement without having the chance to physically "hear the problem", but my advice would be to check the EF86 tube in the preamp section - your amp, unlike the more common "standard" AC30/6, sports one of these tubes, just like the first AC30 developed by Dick Denney ( the AC15/AC30 designer ) back in 1959-60.

EF86s are great sounding tubes indeed, with plenty of gain, but they tend to develop a behavior called "microphonics", because they' re very fragile mechanically. This means the mechanical vibrations induced on the grids by the amp sounding are amplified by the tube, so what you perceive like distortion could be actually a sort of "rattling" coming from that tube ( usually in sync with the note you're playing, and more pronounced on bass ones ).

To check for this, with the amp on and the volume up, knock on the amp to hear if your knocking makes it to the speakers; if so, you have at least one tube that has developed a "microphonic" behavior.

If you have the skills ( be aware that the voltages inside your amp are LETHAL - and I mean that - LETHAL ) to open your amp, you could try to identify which tube has gone "microphonic" by tapping on each tube with a plastic pen or with a pencil.

If this turns out to be your problem, you will probably have to find a very good quality replacement EF86 ( if you manage to find a NOS Special Quality tube like a Philips "E86F" it will well be worth the price ) otherwise your problem is likely to re-occur in the future.

I would like to add a footnote on how the manufacturers and the marketing guys sometimes manage to revive even some design flaws/problems in the name of this "vintage" craze ( and the resulting price tag ).

Back in the early 60s, Dick Denney recognized the use of EF86s to be a problem in amps that were supposed to be gigged with and transported in vans and trunks, and fixed this by using ECC83s in all the following issues of the AC30. ECC83s are more rugged and less expensive tubes, and, even if you usually need two to get the same gain a single EF86 is capable of, they' re less prone to failure and less likely to give rise to reliability problems. I think EF86s to be OK only in a home Hi-Fi equipment, which is unlikely to be moved or treated roughly.

Hope this helps

Best regards

Bob

[52 Bill]

Something else to check are the speakers.

Make sure that you know if the problem is in the amp or the speakers before trouble shooting one or the other.