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.
18 watt bias drop
Moderators: pompeiisneaks, Colossal
- martin manning
- Posts: 13549
- Joined: Sun Jul 06, 2008 12:43 am
- Location: 39°06' N 84°30' W
Re: 18 watt bias drop
-
- Posts: 4730
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: 18 watt bias drop
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.
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.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: 18 watt bias drop
Re: 18 watt bias drop
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.Stevem wrote: ↑Sun Jan 15, 2023 12:47 pm 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.
Re: 18 watt bias drop
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.
Re: 18 watt bias drop
Have you changed the cathode cap?
Re: 18 watt bias drop
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.Phil_S wrote: ↑Mon Jan 16, 2023 2:03 pm 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’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.
Re: 18 watt bias drop
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 )
Re: 18 watt bias drop
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
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
- Site Admin
- Posts: 4228
- Joined: Sat Jan 14, 2017 4:36 pm
- Location: Washington State, USA
- Contact:
Re: 18 watt bias drop
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.plookit wrote: ↑Fri Mar 24, 2023 3:39 pm 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
~Phil
tUber Nerd!
Re: 18 watt bias drop
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.