preamp valve weirdness

[HTH]

Hi all, I've been doing a spot of modding on a Jet City JCA100 I've been gigging for the past year and decided to have a play round with some different 12AX7/ECC83s I have here.

This is where the weirdness starts - if I swap the first preamp valve with any other valve, the amp makes no noise. Obviously this is a concern as valves do wear out.

Any ideas?

Also (problem no.2), when I switch from the lead channel back to the normal channel, the sound seems to fizzle out kinda like the sound of a fuzzbox with a dying battery. It does correct itself, but the problem is getting worse. It only happens on the normal channel too.

I'm thinking that the only possible culprit could be the switching Vacrols, but I've no idea how to test them (never worked with these before). I do have some replacements here, but I'd prefer not to just replace all four Vacrols if I can help it, I'd rather be able to test to see if the existing ones are bad or not.

[daz]

Hi all, I've been doing a spot of modding on a Jet City JCA100 I've been gigging for the past year and decided to have a play round with some different 12AX7/ECC83s I have here.

This is where the weirdness starts - if I swap the first preamp valve with any other valve, the amp makes no noise. Obviously this is a concern as valves do wear out.

Any ideas?

Did you change either the V1A or B grid stoppers? (assuming you have one on the 2nd stage) I had a similar issue except it was only one type of tube, mullard RI which wouldn't work. It would make noise but just crap out. Any other tube worked perfect. Turned out either the input grid stopper or the 2nd stage one (sorry, don't recall which) was causing it. I guess the value just caused problems with the way the rest of the circuit in that area was configured. If you've changed either, change them back one at a time and test. If it's not that then i would start reverting all changes within the first say 3 stages back to stock one at a time checking each.

[g1]

Can you see whether the heaters are lit up or not?

[HTH]

Yep, the heaters are lit up. I'm using NOS Mullard ECC83s all through the amp. I can swap V1 with any of the other valves that are working in the amp, and the amp won't make a sound.

I haven't touched the grid stoppers but as it happens, the normal channel that is affected doesn't have grid stoppers.