Squealing Cornell Plexi 7

[TubeyNewbie]

Hello everyone, I just registered and this is my very first post! I look forward to hopefully learning and interacting with other like-minded folks!

-I just have a brief question about a Cornell Plexi 7 amp I picked up on Reverb used. It’s in superb condition, but something strange has been going on with it.
*For some strange reason, when at higher settings on the amp’s volume knob. If I adjust the volume on my guitar down from full up I get loud feedback-like squealing. If I set the guitar back to full up it stops, and I can avoid it if the change is only really slight.
Now, Just to be clear I HAVE totally ruled out the guitar, cables, and all tubes.. I should also mention that this amp has a power scaling feature and is much more problematic at lower output settings. I’ve never run into this before, I’d really like to hear your thoughts!

*Oh, btw when I opened the amp up to troubleshoot, I was pleasantly surprised to find the date stamped inside chassis revealed it’s only ONE month old!!

[TubeyNewbie]

Here’s some photos of the amp and a few gut shots which may be helpful

[Daviedawg]

Welcome to 18 Watt.com. It is super to see your amp which is new to me. It looks really well made. At the moment I have no suggestions for what is causing the oscillation.

There will no doubt be people on here who can help. But I suggest you will get a bigger resource at Wattkins or at Sewatt.com. You will see links to the left side. I do not recall this amp being discussed there so I expect some interest in it.

Dd

[JMPGuitars]

*Oh, btw when I opened the amp up to troubleshoot, I was pleasantly surprised to find the date stamped inside chassis revealed it’s only ONE month old!!

The amp being that young, I would say look into seeing who does warranty service.

If that's not an option, I'm sure we could help with some troubleshooting, especially if you can find a schematic.

[geoff 1965]

nice amp!
do you get the squealing with the volume pot pulled out "EQ cut" as well?
you might struggle with the warranty not being the original buyer but you could contact cornell and see what they say,ask them if they have a schematic.

[geoff 1965]

contacted Cornell UK and asked for a schematic but they refused for copyright reasons! Blackstar is another company that won't give out schematics of their amps,reminds of Tolkiens gollum "this is mine precious"!
shame! i was thinking of an el34 version,notice they use quite a few carbon comp's and i wonder what type of power amp scaling they use?

[TubeyNewbie]

contacted Cornell UK and asked for a schematic but they refused for copyright reasons! Blackstar is another company that won't give out schematics of their amps,reminds of Tolkiens gollum "this is mine precious"!
shame! i was thinking of an el34 version,notice they use quite a few carbon comp's and i wonder what type of power amp scaling they use?

-Well, I’m not surprised to hear that you were denied access to a schematic. Denis Cornell’s amps are proprietary and aren’t based on any prior “classic” designs. The circuits are unique & created by him from the ground up.. Even though the Plexi 7 I own is meant to sound like the classic British amps of the day it really doesn’t share any of the original design concepts that you would find in say a “clone” or boutique repro. There certainly are a lot of carbon comp resistors in this circuit! Far more than I’m used to seeing in modern amp’s at this price point. The thing is, I can’t help but thinking some of the noise & squealing can’t partly be blamed on those carbon comps. Everyone knows they’re inherently disposed to crackling, popping, swings in resistance when exposed to heat ect..
The same thing goes for the power scaling. It’s unique.. not a London Power, or Dana Hall VVR like you have with a Suhr Badger or most of Joe Morgan’s amps (preamp-gain-power). Again it’s his own thing, (4-way rotary switch each w a separate fixed output assigned A-B-C-D) and I can verify that it’s effective but definitely doesn’t operate or behave the same way a Morgan AC-20 or SWR does..

[TubeyNewbie]

*Oh, btw when I opened the amp up to troubleshoot, I was pleasantly surprised to find the date stamped inside chassis revealed it’s only ONE month old!!

The amp being that young, I would say look into seeing who does warranty service.

If that's not an option, I'm sure we could help with some troubleshooting, especially if you can find a schematic.

-Yeah no dice on a service schematic unfortunately.. And being the second owner warranty’s aren’t an option. Not to mention I’m in the states and Cornell is based in London. I hope that someone here can help with suggestions.

[JMPGuitars]

-Yeah no dice on a service schematic unfortunately.. And being the second owner warranty’s aren’t an option. Not to mention I’m in the states and Cornell is based in London. I hope that someone here can help with suggestions.

Ah, that's too bad.

Okay, I don't know your skill level, so... Be careful!! If you're working on a live amp, keep one hand in your pocket or behind your back. Use a chopstick to tap capacitors or move wires ("chopsticking") to test if it has an affect... But first, figure out the ground scheme.

See what wires are ground wires, what they're connected to, and where they're going. Make sure they're all tight, and good quality connections. Do this with the amp off obviously.

Once you've figured out the ground scheme, and made sure the connections are all good quality, and the chassis connections are good and tight, try the amp out again.

Thanks,
Josh

EDIT: I just saw your post in the other topic. You're not quite the newbie your username implies. Do you have an oscilloscope?

[geoff 1965]

Tubeynewbie,
yeah i noticed the absence of the usual VVR components so i presume the rotary switch will have variuos dropping resistors or the PT will have multi taps for the power scaling.
i know you have ruled out your guitar "what guitar? btw" but have you tried a different guitar to double check? the amp has a high input impedence 2.2 meg and did you pull the volume pot "which acts as a boost and bypasses the tonestack" to see if the squealing is still there?