Loop squeal MKIV - also missing eq nobs

[six20aus]

Hmm so I bought a MkIV about a month back...sounds great... been experimenting with my Zoom G7 in the loop. Have it set to +10db which is rack gear level. Everything sounds fine except last 2 weeks when I switch the loop on I am getting this howling feedback that builds up in R1 mostly but also R2 and Lead channels to some extent. If I palm mute the strings or turn the guitar volume down it doesnt stop until I switch the loop off.

Does this mean my preamp tubes are shot ?

Also one of the eq slider nobs is missing is there anywhere someone can point me to to get replacements ?

Thanks

 

[Platypus]

Check your loop preamp tube.

The slider is going to be costly as getting to the area to replace it takes a lot of time from what I've heard.

 

[six20aus]

Okay so I just looked at the manual and it looks like V2 is the tube to replace. Should I go out and buy a set of 12ax7's and replace the lot or just that one ? Not sure if I will hear a difference in replacing the set - no idea how old they all are in there either....it was an ebay score...

The slider pot is fine its just missing the cap on the end...No idea how old my MkIV is but its in otherwise brilliant condition except the missing slider nob which is bugging me as much as the loop squeal...(I'm in Australia its a 240v version so makes getting replacement parts interesting).

 

[ANIMATED SUSPENSION]

Sounds to me like you're overloading the loop somehow, I doubt it's a tube problem but you could try swapping v2 with another one, v4 or v5. The reason I say this is that you say it's worse on R1, that's because the master is set higher than the other channels right ?? That sets the fx loop send level, so try reducing the levels in the Zoom. I assume that model is designed for use in an fx loop btw ?

 

[six20aus]

Ah thanks for this idea I will mess around with the settings.

I used the Zoom G7 in my F50 and it was fine... I have it set for unity gain so it shouldnt be overloading the loop will test it and see what happens.

Also the F50 loop was parrallel - does this make a difference ? Havent seen anywhere whether the MKIV loop is serial/parrallel am assuming its serial since it doesnt have its own wet/dry mix.

 

[six20aus]

After pulling the R1 master down to 3.5 and R2 and L1 to 2.0 the Loop squeal is gone.. I now am running the master output on 4.0 (tweed mode) and all sounds good but am wondering if running it this way am I getting the most out of the preamp section ? I assume so since the R1 gain is on about 6.0, R2 is on 10.0 and L1 is on 8.0

Thanks all for the help