Mark IV losing treble and gain

[Clairvoyant]

Hi everyone,

I have a Mark IVb which I have owned for almost a year. Since I bought it, I have replaced all of the power tubes and two of the preamp tubes (V1 and V5). Over the last two weeks or so, I have noticed that the amp was losing high end and a little bit of clarity. It is still losing high end, but ver the last few days I noticed that it is losing gain as well. Not a lot of gain, but enough to notice. Does anyone have an idea of what it could be? Just in case it helps, the tubes are: 4 Mesa 6L6s, V1: Tung-sol RI, V2:JJ V3: unmarked, V4: JJ, V5: Mullard RI. I play the amp into a Rectifier 4x12. Any help would be greatly appreciated.

Thank you

 

[bgh]

Do you experience the same loss of gain on each channel, or only one in particular?

 

[Clairvoyant]

It is difficult to tell for channel 2 because I only play it with effects (and it is the least clear of the three channels in my experience), but both channel 1 and channel 3 have experienced these changes.

 

[bgh]

I would take a pre-amp tube that you know to be good, and swap it through v2, v3, and v4 also. V2 seems to impact all channels. V3 and V4 are for the lead channel and for reverb (send and return).

It could be that one of them is going bad.

 

[Clairvoyant]

Thanks for the advice. I was able to borrow a Tung-sol from my friend. I only tried it in V2, but it made a huge difference. It was much more like how I remember the amp sounding before this problem, so it sounds like that tube has gone bad. I guess I will just order some new preamp tubes as soon as I get the chance.