AC-15 tube experts paging

[gorch]

The other day my beloved AC-15 H1TV changed its sound towards more clean tone. It doesn't have that bite anymore. So, I assume that, at least, one of the tubes has given up its live. The question is which one? I'm not an expert in circuits though.

It used to have a noisy tube for quite a while already. It had some blistering here and there. I have opened the cabinet. Power section tubes are gluing. EF86 (V1) is wrapped and cannot be seen gluing. Of them three 12ax7 (V2-V4), V2 seems not be gluing.

According to the scheme, V1 and V4 seem to build up the EF86 circuit, whereas V2 and V3 build up the Top Boost circuit. Concluding, just the Top Boost channel may be affected in case V2 has given up life.

Any suggestions?

[Bloozguy]

Have both channels changed towards more clean or just the Top Boost channel? If only the TB channel has changed then your diagnosis is spot on...V2 is most likely the cause. You could swap V4 and V2 just to check.

[gorch]

Many thanks. Should have been my own idea. I'll go and swap V4 and V2 when back from travel next weekend.

It's got Tungsol in it. Any recommendations for replacements?

Just want to say that it doesn't sound that bad with a bit more headroom. Pretty classic rocking, not that harsh.

[Bloozguy]

If you like the lower gain/more headroom sound on that channel you could try some sort of NOS 5751 in place of V2. Same architecture as a 12AX7 but 30% less gain. Makes a big difference in clean headroom most of the time.