Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: isaac_teller on August 06, 2020, 09:46:14 am

Title: squeal
Post by: isaac_teller on August 06, 2020, 09:46:14 am

Hello. I built a DIY clone of a Fender Deluxe reverb. after powering up there was a high-pitched squeal. I switched tubes, reversed output leads but the squeal persists. I removed V1 and the squeal is still there. If I turn the master down(Frondelli master volume) it cuts off the squeal. Any suggestions of what to try next?

Title: Re: squeal
Post by: SILVERGUN on August 06, 2020, 10:18:29 am

https://el34world.com/Forum/index.php?topic=17701.0 (https://el34world.com/Forum/index.php?topic=17701.0)

Title: Re: squeal
Post by: tubenit on August 06, 2020, 10:28:48 am

When you say you switched tubes,  did you change ALL the tubes for known good ones?

What are the voltages?  What do you have the tubes biased at? 

Can you post a hi-rez picture of the chassis interior?

Did you try disconnecting the NFB?

Did you try pulling one tube at a time and also jumpering over gain stages?

with respect, Tubenit

Title: Re: squeal
Post by: isaac_teller on August 06, 2020, 11:06:54 am

Yes, I changed all the tubes for known good ones. I will check voltages and post later. As for a picture, I don't have a smartphone, I'll have to ask a friend to photograph.

Title: Re: squeal
Post by: shooter on August 06, 2020, 11:12:29 am

does the amp have NFB?, if so, disconnect and see

Title: Re: squeal
Post by: isaac_teller on August 06, 2020, 11:49:41 am

I also disconnected the NFB. Didn't help.

Title: Re: squeal
Post by: sluckey on August 06, 2020, 12:11:35 pm

Quote from: isaac_teller on August 06, 2020, 09:46:14 am

Hello. I built a DIY clone of a Fender Deluxe reverb. after powering up there was a high-pitched squeal. I switched tubes, reversed output leads but the squeal persists. I removed V1 and the squeal is still there. If I turn the master down(Frondelli master volume) it cuts off the squeal. Any suggestions of what to try next?

Please provide a link to the schematic/layout you used. Also provide some info on the frondelli master volume. Maybe even remove that frondelli to see if it is the problem.

Pull the PI tube. It's the 12AT7 closest to the 6V6s. Squeal stop?

Title: Re: squeal
Post by: isaac_teller on August 06, 2020, 03:37:39 pm

https://www.tdpri.com/threads/removed-the-master-volume-on-my-81-vibrolux.735213/ (https://www.tdpri.com/threads/removed-the-master-volume-on-my-81-vibrolux.735213/)
Post #5 has the Frondelli schematic. When I get home I'll pull the PI tube.
http://www.prowessamplifiers.com/schematics/fender/deluxe_ab763_schem.pdf (http://www.prowessamplifiers.com/schematics/fender/deluxe_ab763_schem.pdf)
Here's the schematic