Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: gerry113377 on August 18, 2021, 09:33:39 pm

Title: 5e3 hum and squeal at high volume
Post by: gerry113377 on August 18, 2021, 09:33:39 pm

I recently built a robrob deluxe 5e3 and have a squeal at high volume only on the normal channel. The lead channel is nice and quiet. I have chop sticked and swapped different tubes, cleaned the pots already. Not too sure where to look next. Made a video of the issue here. Thanks.

Title: Re: 5e3 hum and squeal at high volume
Post by: PRR on August 18, 2021, 10:15:03 pm

Move wires while squealing. ANY change, even for the worse, is a clue.

Also do it with a "lead" pencil. I once noted squeal got worse with a pencil between two wires. More conductor (carbon) between these wire makes more squeal? Let's try VERY far-apart. That was the fix.

This was on a very mild Champ with a copy of Fender's layout at hand. So just being close to known-good may not be good enough.

Title: Re: 5e3 hum and squeal at high volume
Post by: sluckey on August 18, 2021, 10:19:44 pm

Rob has a lot of bells and whistles available for his 5E3. Do you have a schematic (not layout) for your build? What's the knob on the left?

Title: Re: 5e3 hum and squeal at high volume
Post by: gerry113377 on August 18, 2021, 11:08:28 pm

Sorry, there is no schematic on the entire amp that I know of. Only of individual sections. I followed this component layout. The pot on the left is the master volume.

Title: Re: 5e3 hum and squeal at high volume
Post by: pdf64 on August 19, 2021, 04:14:41 am

Putting a master volume in close proximity to input jacks is just bad.
Why site it in about the worst conceivable position?
There’s inevitably going to be a strong positive feedback loop.
Get rid of the stupid thing and re-assess.

Title: Re: 5e3 hum and squeal at high volume
Post by: sluckey on August 19, 2021, 04:47:29 am

He didn't put it next to the input jacks. It's at the other end of the chassis, next to the fuse holder!

Title: Re: 5e3 hum and squeal at high volume
Post by: gerry113377 on August 19, 2021, 05:02:53 am

I'm pretty sure the noise has nothing to do with the master volume because it works perfectly on the lead channel. I'm just going to rewire and change all the components on the normal channel. I'm sure the issue is in the that circuit somewhere.

Title: Re: 5e3 hum and squeal at high volume
Post by: sluckey on August 19, 2021, 05:15:53 am

Does it squeal if you disconnect the 56K NFB resistor?

Title: Re: 5e3 hum and squeal at high volume
Post by: gerry113377 on August 19, 2021, 12:07:48 pm

Sorry. I didn't mention. I already swapped the brown and blue output transformer wires when I first finished the amp because the amp squealed badly and they were reversed, so I know they are right. I'm going to check if some plate and grid wires are too close to each other and the lead dressing  today.