Super Comp - Microphonic Squeal

[manco53]

Hello all,

New to the forum.  I'm fairly active over at the Agile guitar forum, same handle.  Couple of the guys there recommend I ask some questions here as well, in light of a few issues I've been having.

Just some background, I am currently running the Super Comp with a 4x12 with G12T-75's and I also run a Mesa Triple Rectifier with a Mesa 4x12.  I run them both together, split my signal equally into both amps.  I stumbled on the Splawn in a search for a JCM800.  Seemed like a no brainer to go with the Splawn versus the Marshall in light of the price (got a great deal, hope it's not because the amp is screwed up), features, quality, etc.

I purchased a used Splawn Super Comp off reverb.com back in July.  Immediately when I plugged in I noticed a very high squeal.  It wasn't so much a harmonic squeal, like if your guitar is too close to a high gain amp, but upon further research, appeared to be preamp tube related.  The amp was usable when I changed modes/OD's on the footswitch, and if the gain knob was turned past 1 o'clock; however, it was still frustrating to not have a "fully functional" amp (at least my initial thoughts).

After some research, I changed out the preamp tubes to Tung Sol 12AX7's.  It seemed the issue went away; however, I didn't mess around with the footswitch settings too much.  I generally do not use the amp footswitch, even on my Mesa.  I more or less like to find a channel/setting, and then use my volume knob/pedal for controls. 

Last Friday at a gig, I had the volume on the amp turned up around 9:30/10 oclock, louder than normal as usually our cabs are mic'd and this venue did not.  I generally keep the volume netween 8 and 9 oclock.  After a few songs in, the amp started squealing again.  I didn't have my footswitch with me to make adjustments.  However, it was only noticeable in between tunes, with my input volume rolled on or off. 

This past Saturday, it happened again after our first song.  This time, it almost overtook the amp and could even be heard noticeably during our second tune.  I tapped a few things on the footswitch, made some adjustments with the volume and gain knobs on the amp, and was able to get it to go away. 

I set my stuff back up this morning to try and replicate the problem.  This time, to eliminate any pickup issues, I turned the amp on without a signal at all.  It started squealing. I then started tapping the preamp tubes with a wooden pencil to see if I could notice any ringing. I thought the V1 may have been a bit chimey when I tapped it, but not quite sure. It was slightly chimey. I then plugged in the footswitch to try and do some additional troubleshooting and came to the following conclusion, not sure what it means:

Trying to post pics I took of footswitch for better reference, but not working....
1. Mode Off/OD 1? (no LED's on) - amp squeals after gain goes past 1 oclock, volume knob does not cause an issue

2. Mode On (LED)/OD 1 - amp squeals after gain goes past 1 oclock, volume knob does not cause an issue

3. Mode Off/OD2 (LED on) - no squeal present despite altering gain knob and/or volume

4. Mode On (LED on)/OD2 (LED on) - no squeal present despite altering gain knob and/or volume

Here is the other issue, I'm pretty sure, the problems I experienced at the past two gigs, the settings were on #4 listed above, but I cannot recall for certain.  So trying to replicatethe problem this morning, I could not with setting #4. 

If any of you have any other troubleshooting recommendations, let me know.  Inevitably, I hope to avoid bringing it in the to shop for expensive repairs if I can avoid it. 

Thanks in advance 

[Tempest]

Sounds like it's only doing it on od1. Ask anyone you know with a valve preamp if you can borrow a tube you KNOW is good, and replace V1, just to be sure it's not faulty, as you have already tried a tube change. I have had the same problem in a marshall before, It usually came down to one of 3 things.

1. Microscopic crack in a power tube
2. underspec bridge rectifier and/or resistor cooking
3. preamp tube due to #2.

I've never had a problem with any components in my Nitro, they're very well made. In saying that I use a noise gate because it squeals like a pig with the volume between 10-12 o'clock and the gain passed 1:30-2 o'clock.

[manco53]

I have another head I'm selling that I can pull tubes out of to check again.

Just to confirm, your Nitro squeals as well with your gain past 2 o'clock? It doesn't if you noise gate it? Do you noise gate in front or in the loop?

[Tempest]

Yep, it's definitely pickup squeal though. At bedroom level it's fine, but with the volume at 10-12 o'clock, the Nitro drowns out a whole band (it's what I set the solo level to when playing live). So at that volume any high gain amp will squeal. ISP Decimator in front of the amp fixes this, but you shouldn't need one at home levels. It sounds like a different problem if your Triple Rec is fine but just the Splawn is doing it, just on OD1.

[manco53]

Yes I've only ever had to turn the Super Comp up to about 9:30 for any gig or rehearsal, it is freakin loud!!

I also keep my Mesa Triple Recs output (basically a master) around 8/9 o'clock.

Should have brought the super comp home from rehearsal last night to keep testing it. Going to have to wait a bit til I get back to our studio to test with the other tubes.

Thanks

[sonofiam]

WARNING....HIGH VOLTAGE INSIDE AMPS, DO NOT ATTEMPT ANY REPAIR WITHOUT PROPER DISCHARGING OF CAPACITORS OR IF YOU DON'T KNOW WHAT THIS MEANS! THE VOLTAGE CAN/WILL KILL YOU!!!
Don't know if you've figured out your problem but I had a similar one with my Competition. It would squeal in 3rd gear if I had the gain over 12:00 and occasionally in 2nd gear above 2:00. It turned out to be the shield from the 2 conductor wire that is connected to the Treble control pot. It had a small solder contact to the ground, approximately 1/8", but another 1/8" to 1/4" of the shield was bare. I soldered the bare shield to the ground and have not had any problems since.