Confirms that it is the tube bias...

Posted by Al Sekela (A) on January 18, 2008 at 12:41:11

In Reply to: More info...... posted by lektrik on January 17, 2008 at 20:44:24:

Your observation,

"Another bit of information to add is that when turned on, the PT has a very faint hum, but when the KT-88s are on for around 30 seconds and are heated enough to produce music, then the PT gets louder. When I install 6V6s, there is no increase in PT loudness once they're warmed up."

confirms that it is the B+ current that is overloading the power transformer. It takes about 30 seconds for the tubes to warm up enough to draw their full plate current.

It is not safe to use such tubes with your amp. Either the transformer has something wrong with it, the bias circuit is not adjusted properly, or the power transformer is simply too small. I would look into the bias settings and consult with the factory.

Sorry that I don't have experience with the 6V6 to say whether it runs hot. It may be the case that the auto bias is too high for that tube type. Generally, running tubes at high bias improves the sound at the expense of tube life. The tube is only rated for 14 watts of plate dissipation, so it may well be overloaded in your amp. Yes, the smaller envelope is part of the power limitation in the tube design.

This post is made possible by the generous support of people like you and our sponsors:

Follow Ups Full Thread

Topic - Power transformer hum.....how to get trid of it??? - lektrik 16:13:34 01/13/08 (22)

You may be overloading the transformer. - Al Sekela 15:08:53 01/15/08 (9)

RE: You may be overloading the transformer. - lektrik 17:31:20 01/15/08 (6)

KT88s draw more filament current than EL34s. - Al Sekela 10:57:30 01/17/08 (5)

More info...... - lektrik 20:44:24 01/17/08 (4)

Confirms that it is the tube bias... - Al Sekela 12:41:11 01/18/08 (3)

More info, again............ - lektrik 10:22:01 01/19/08 (2)

Checked the bias......... - lektrik 16:19:21 01/19/08 (1)

OK for the tubes, perhaps too much for the power transformer. - Al Sekela 12:01:05 01/20/08 (0)

How are your filter caps these days? Nt - bluelobster 17:10:51 01/15/08 (1)

RE: How are your filter caps these days? Nt - lektrik 17:55:08 01/15/08 (0)

If it is a mechanical noise from transformer itself. - Ron Oehlert 13:17:34 01/15/08 (1)

Re: If it is a mechanical noise from transformer itself. - lektrik 17:10:17 01/15/08 (0)

Is it mechanical noise from the iron or hum thru the loudspeakers? (nt) - cheap-Jack 11:19:07 01/15/08 (9)

It is mechanical............ - lektrik 17:05:27 01/15/08 (8)

RE: It is mechanical............ - kenster 17:31:23 01/15/08 (7)

How come "DC ridng on AC line"? (nt) - cheap-Jack 11:26:57 01/16/08 (0)

DC on the AC line? - lektrik 18:10:36 01/15/08 (1)

RE: DC on the AC line? - kenster 18:25:56 01/15/08 (0)

What would be a good solution if that's the problem? - lektrik 18:02:43 01/15/08 (3)

RE: What would be a good solution if that's the problem? - and this - HumanMedia 05:06:08 01/17/08 (1)

Now that's what I'm looking for.......... - lektrik 06:28:37 01/17/08 (0)

RE: What would be a good solution if that's the problem? - kenster 18:14:23 01/15/08 (0)

Follow Ups

Confirms that it is the tube bias... - Al Sekela 12:41:11 01/18/08 (3)

More info, again............ - lektrik 10:22:01 01/19/08 (2)

Checked the bias......... - lektrik 16:19:21 01/19/08 (1)

OK for the tubes, perhaps too much for the power transformer. - Al Sekela 12:01:05 01/20/08 (0)

Confirms that it is the tube bias...

Post a Message!