Ok back to being a amp problem

[BlacknWhite]

Just pull some tubes from GC floor models.




Just kidding, don't do that

Tempting but I'm up in Canada. That wouldn't be very nice EH lol

 

[BogeyBass]

Take the plunge and go new matching set.

Don't hurt to measure screen resistors cathode resistors. Look at the board while your there.

Oh wait duh just toob swap. Nevermind

 

[Wasnex]

Yeah I've ordered a set. Figured I'll get them in 3 or 4 days and I'll swap the pair that are not reading. Hopefully that does the trick and if not off to the shop when I get a chance.

You only bought 2 tubes? Even if the new tubes have the same plate current and transconductance ratings as the two working tubes, I suspect the matching will be pretty bad. The operating point of tubes tends to drift a bit as they age. Generally you want the current difference between tubes to be within 15%.

Also you should arrange the tubes so the current is balanced as well as possible on each side of the output transformer. If the current does not balance in the output transformer, the amp will produce a dull hum. When the current is balanced, the hum cancels in the transformer. It's not unusual for the tubes on one side of the transformer to have a bit higher current. This is because the way most transformers are wound, they are unlikely to be perfectly balanced. So put your hottest tubes on the cold side of the transformer.

 

[BlacknWhite]

You only bought 2 tubes? Even if the new tubes have the same plate current and transconductance ratings as the two working tubes, I suspect the matching will be pretty bad. The operating point of tubes tends to drift a bit as they age. Generally you want the current difference between tubes to be within 15%.

Also you should arrange the tubes so the current is balanced as well as possible on each side of the output transformer. If the current does not balance in the output transformer, the amp will produce a dull hum. When the current is balanced, the hum cancels in the transformer. It's not unusual for the tubes on one side of the transformer to have a bit higher current. This is because the way most transformers are wound, they are unlikely to be perfectly balanced. So put your hottest tubes on the cold side of the transformer.

Yeah I emailed them and got them to add 2 more to the order lol. Then I can keep the 2 working ones as back ups.

 

[BassmanPaul]

Yeah I emailed them and got them to add 2 more to the order lol. Then I can keep the 2 working ones as back ups.

Technically that makes more sense.

 

[Josh Pelican]

Tempting but I'm up in Canada. That wouldn't be very nice EH lol

I was trading an amp in at L&M and was VERY tempted to take all my tubes out!

 

[BassmanPaul]

I was trading an amp in at L&M and was VERY tempted to take all my tubes out!

The people who work at Long and McQuade are not stupid y’know!

 

[beans-on-toast]

Not all fixes are easy. The best way to resolve this issue is to diagnose with test equipment. The output power can be measured as well as the voltages within the amp at each stage.

You should get the proper 12AU7 tube for the phase inverter. It provides the required drive to take the power tubes to full power. The power tube bias should be set properly.

If a guitar store close to Sarnia doesn’t have one, this one is close by. Tung-Sol or Mullard reissues are good. Any tested tube in that price range is good.

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[BlacknWhite]

Not all fixes are easy. The best way to resolve this issue is to diagnose with test equipment. The output power can be measured as well as the voltages within the amp at each stage.

You should get the proper 12AU7 tube for the phase inverter. It provides the required drive to take the power tubes to full power. The power tube bias should be set properly.

If a guitar store close to Sarnia doesn’t have one, this one is close by. Tung-Sol or Mullard reissues are good. Any tested tube in that price range is good.

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I did end up replacing the Phase inverter with a Mullard 12Au7. In the end it ended up being a power tube. I've got some replacements on the way.

 

[beans-on-toast]

I did end up replacing the Phase inverter with a Mullard 12Au7. In the end it ended up being a power tube. I've got some replacements on the way.

If one power tube was bad, the amp out have had more hum. Running them too hot shortens their life.

When changing the tubes, check that the tube socket metal contacts are not oxidized. Inspect them with a magnifier or some sort. If the metal is black or appears to be otherwise coated with gunk, they should be carefully cleaned with deoxit.