Tube Amp Archive

Lars, The customer had been doing two things and neither caused any squealing or howling. in one case, he was using a 16 ohm cabinet with the selector on 16. Then, he was using an 8 ohm cabinet and the selector was on 8 and sometimes on 16 (he would forget to change the setting), and neither caused ...

Lars (and all), Yes, it was the OT primary leads that were reversed. I guess I knew that all along, but what I couldn't understand was that this amp was used for over 2 years, into various speaker cabinets/loads, and it didn't exhibit any negative characteristics related to squealing or howling. Onl...

Lars,

Yeah, that will be the first thing I do tonight when I get off of work.

I will post the results tomorrow morning, or late tonight

Thanks.

Mike

Shielded cable on the wires running from the pots to the PI and also used shielded cable on each wire from the pots to each power tube (to the 1.5K resistors on each tube). All are grounded to the bias connection of the pots. I really don't think there are any lead-dress issues in this amp. That's a...

I have an Express clone that I built and sold a few years ago and the customer asked me to install a PPIMV. So, I did that. Amp now squeals like the OT primary leads are reversed, but it only does that when the Impedance selector is set to 8 ohms (into an 8 ohm cabinet). When I change the selector t...

Martin, I moved the 220K resistors in the PI to the bottom of the board, and the 100K/82K PI plate resistors at the bottom of the board to the top of the board, and their corresponding wires. I lengthened the plate wires for the PI so they would reach the top of the board, and then I shortened the g...

Guys, the Vibrolux Reverb amp has been fixed. Really just one thing cured the problem, and that was what I posted previously - I shortened the grid wires to the power tubes by moving the resistors in the PI. Even with the scope, I could never determine what, if anything, had changed in the amp to ca...

Phil, I certainly wasn't complaining about your suggestion for a scope. It was much appreciated! Heck, this new scope may turn out to be more than I'll ever really need. The HP scope was actually overkill. It had features that I never used, it was VERY LARGE and took up a lot of real estate, and you...

Firestorm, I will check out the PI tail and bias resistors later today and see if lifting them off the board and isolating them helps. I did lift the entire board and check for anything weird beneath it, but I didn't find any wire clippings or any obvious problems. There were a couple of solder blob...

Guys, I have made some positive progress. It's not 100% better - maybe 90% - but the problem has been significantly reduced. I switched the 220K power tube bias supply resistors and the PI plate resistors from the top of the board to the bottom, and vice-versa. This way, I was able to shorten the po...

Firestorm, I disconnected the NFB and it had no impact. It didn't make the symptoms worse, nor did they get better. Same amount of distortion... I switched the PI outputs and the bias symptoms moved to the other power tube - now the 6L6 closest to the PI tube is changing bias wildly - pulling up to ...

No, I didn't change out the PT. I realize the frequency range is right to possibly contribute to the problem, but I have measured voltages under load and the PT seems to be holding up. However, like all of you, I've had stranger things happen, so, I'll change it out - I should have one in the shop s...

I will try disconnecting the NFB at the speaker jack on Monday afternoon. Good suggestion... Regarding the scope, thanks for the tip - I will check it out tonight. The scope I had was really high-end, but old. It died a horrible death - while rearranging the shop, moving some benches, my helper move...

I put the suppression caps back on - no change in the symptoms. I disconnected the vibrato from the circuit - no change in the symptoms. On Sunday, I am going to remove the PI tube socket, replace it and all the wiring attached to it, and see what happens. If this causes no change, I am going to cal...

Martin, My scope is down - it's old and too expensive to repair. So, unfortunately, I'm limited in the types of signal tracing I can perform. Since the issue occurs using either input stage (Normal or Vibrato), I thought I would try to make sure that something wasn't happening in the reverb section ...