18 watt bias drop

[sluckey]

About the only thing left to do (and should have suggested this sooner) is measure resistance from pin 3 to chassis. If less than 120Ω replace the cathode capacitor.

Thank you. I’ll try that in the morning. Appreciate it.

Probably best to just go ahead and replace the capacitor. If it's dried up the cathode voltage will be low and the cathode resistor will still check OK.

[martin manning]

About the only thing left to do (and should have suggested this sooner) is measure resistance from pin 3 to chassis. If less than 120Ω replace the cathode capacitor.

Thank you. I’ll try that in the morning. Appreciate it.

You can disconnect one end of the cathode cap and see if that brings the bias back in line, then you can also measure the resistor to see if it is still within spec.

[Stevem]

Just because you got new tubes is no guarantee there any better then what you took out unless your using high end well tested tubes like apex , Groovetubes or Euro tubes.

Even with Groovetubes for example there 1 to 10 numbering system covers a 10% change in output power, and of course that will show up in the bias voltage.

[plook]

About the only thing left to do (and should have suggested this sooner) is measure resistance from pin 3 to chassis. If less than 120Ω replace the cathode capacitor.

Thank you. I’ll try that in the morning. Appreciate it.

Measured p3 to ground, 121 ohm.

I really have no idea what this is. I noticed the voltages have swung up again too. v1 getting over 200v

[plook]

Just because you got new tubes is no guarantee there any better then what you took out unless your using high end well tested tubes like apex , Groovetubes or Euro tubes.

Even with Groovetubes for example there 1 to 10 numbering system covers a 10% change in output power, and of course that will show up in the bias voltage.

I have about 10 sets of EL84 tubes, tried them all with no readings above 6.2v on p3. The brand new JJ tubes measured lower than those in the amp, so I believe what you are saying.

[Phil_S]

Please indulge my question. I'm trying to be helpful. When you built the amp, did you actually measure 10V? You imply this, but I don't see a clear statement, or I missed it. I ask because 6.2V is clearly not tube selection related and I've started to wonder if the amp has always been like this. If that is so, then we are looking for a different kind of problem.

[sluckey]

Have you changed the cathode cap?

[plook]

Please indulge my question. I'm trying to be helpful. When you built the amp, did you actually measure 10V? You imply this, but I don't see a clear statement, or I missed it. I ask because 6.2V is clearly not tube selection related and I've started to wonder if the amp has always been like this. If that is so, then we are looking for a different kind of problem.

I built it a long time ago and the voltages were all good. It did come back to me after a drink spill which required the mains transformer being changed and I recorded all voltages as correct before I let it go.

I’ve been working away this week so not had a chance to get back to it, but will next week.

No, I have not changed the cathode cap but that’s my first plan.

Do appreciate the questions and help.

[Phil_S]

Uh huh! Makes me wonder what else was compromised with the drink spill. Maybe a slow burn on the OT? Something else? I think I'd start by putting every part on a meter. In some cases, it will be necessary to lift one leg to get a proper reading. (No dog pee jokes )

[plookit]

Sorry it took me so long to come back to this but I could not log in because I had not recorded my password and about 50 attempts to reset it and contact admin resulted in ZERO replies, so basically I was locked out until now - when I used an old Mac which seemed to know the password. Unfortunately I can't reset it without knowing g toehold one.

Good news though. I finally got to the bottom of that bias drop problem. The original mains transformer did not have a secondary CT and I had the 1ohm resistors in there. When I changed the MT last year after the drinks spill the new MT had a CT and I forgot to remove the 1ohm resistors and the valves were not getting 6.3v. It sounded terrible for ages. Now it sounds great. Thanks for all for your help.

Is there any way to get my account password reset without knowing the old one?

Tom

[pompeiisneaks]

Sorry it took me so long to come back to this but I could not log in because I had not recorded my password and about 50 attempts to reset it and contact admin resulted in ZERO replies, so basically I was locked out until now - when I used an old Mac which seemed to know the password. Unfortunately I can't reset it without knowing g toehold one.

Good news though. I finally got to the bottom of that bias drop problem. The original mains transformer did not have a secondary CT and I had the 1ohm resistors in there. When I changed the MT last year after the drinks spill the new MT had a CT and I forgot to remove the 1ohm resistors and the valves were not getting 6.3v. It sounded terrible for ages. Now it sounds great. Thanks for all for your help.

Is there any way to get my account password reset without knowing the old one?

Tom

Sorry I'm the admin, but the forum and domain owner won't let me fix email. I don't get most of the admin email likely. This hosting providers entire IP address space has been on multiple blacklists. Ours domain is not. I can't fix it, due to lack of dns control so basically I don't know I'm not getting your email. I know I do periodically get email... but that doesn't help much.

~Phil

[plookit]

Phil, thanks a lot for your help, really appreciated. All sorted and have a record of new password; should know better as a former IT teacher.