Bassman AB165 huge hum

[guitician]

That cap decouples the plates AC voltage from getting into later stages and causing oscillations. 16uf would work the same, but would give slightly less high frequency response. I'll be checking email til about 3:00pm PST.

[rpbale]

Found first cause.

I check the big caps on the bench. Sure enough I fried the second on in the dog house. The first one is still good and spot on at 100mfd. Service me right for putting it in backwards. I have to wait until tomrrow to get another one.

I will pull the diodes to make sure they're all good. Hopefully I don't have an OT problem also. One fix at a time so I can tell all of you which was the cause.

[rpbale]

Fixed the Hum!

Thanks for all the help. The problem is fixed. I simple killed the filter cap when I put it in backwards. After replacing there is virtually no hum and sounds great.

9 watts is the bias setting that gives me the lowest noise and least crossover on the scope. Bothe power tubes are set at 9 watts according to the SRS bias tool. I thought I would be setting it a bit higher than that.

Thoughts?

[Enzo]

Try ignoring crossover and go for tone. This is not a hifi. You may find it sounds better at a hotter setting.

Remember, the guitar amp is part of hte instrument, it contributes to the tone. Unlike a hifi, the guitar amp is NOT designed to accurately reproduce its input.

[trem]

Try ignoring crossover and go for tone. This is not a hifi. You may find it sounds better at a hotter setting.

Remember, the guitar amp is part of hte instrument, it contributes to the tone. Unlike a hifi, the guitar amp is NOT designed to accurately reproduce its input.

No kidding, and it is easy to forget that. They are tone generators as much as they are amplifiers.

[rpbale]

Good point. I was pretty happy with the tone. I left the bias at 9 watts and packed it up.

I've moved on to fixing a '67 Pro Reverb and a '66 Super Reverb. Both are missing their rectifier tubes so I need to wait for them to arrive.

Thanks for all the help.

Rick

[rpbale]

Heater balance

One cquestion I'm not sure I saw an answer for was about whether it make a difference when I added the two 100 ohm balance resistors to the heater taps. I was trying to ensure my hum wasn't coming from imbalanced voltage. The problem being that I mis-interpreted the ground leads on the PT. It already had a CT for the heaters.

I'm thinking that by adding another CT through the resistors I've in advertenly given it another path to ground and that would reduce the voltage getting to the tube heaters. Is that logic correct? What would running cooler tubes do to the tone and would I be able to measure the effect on the other pins or see it on the scope as reduced gain? The curiosity is killing me. Obviously I'm an ameture hobbiest.

[Enzo]

ALl you have done is add 100 ohms to each side of hte thing - and that is no load at all. But the resistors also don;t do anything useful.